7/29/2019 Aplicaciones Valvulas de Control
1/45
ISA is the international society for measurement and controlISA is the international society for measurement and control
Volume EMC 21.01
Control Valve ApplicationsHerbert L. Miller
Application Categories
Valves in Parallel and Series
Frequent Application Problems
Taken from the Practical Guide Series book: Control Valves
7/29/2019 Aplicaciones Valvulas de Control
2/45
Notice
The information presented in this publication is for the general education of the reader. Because neither the
authors nor the publisher have any control over the use of the information by the reader, both the authors and
the publisher disclaim any and all liability of any kind arising out of such use. The reader is expected to
exercise sound professional judgment in using any of the information presented in a particular application.
Additionally, neither the authors nor the publisher have investigated or considered the effect of any patents on
the ability of the reader to use any of the information in a particular application. The reader is responsible for
reviewing any possible patents that may affect any particular use of the information presented.
Any references to commercial products in the work are cited as examples only. Neither the authors nor the
publisher endorse any referenced commercial product. Any trademarks or tradenames referenced belong to the
respective owner of the mark or name. Neither the authors nor the publisher make any representation regarding
the availability of any referenced commercial product at any time. The manufacturers instructions on use of
any commercial product must be followed at all times, even if in conflict with the information in this
publication.
Copyright 2000 Instrument Society of America.
All rights reserved.
Printed in the United States of America.
No part of this publication may be reproduced, stored in retrieval system, or transmitted, in any form or by any
means, electronic, mechanical, photocopying, recording or otherwise, without the prior written permission of
the publisher.
ISA
67 Alexander Drive
P.O. Box 12277
Research Triangle Park
North Carolina 27709
7/29/2019 Aplicaciones Valvulas de Control
3/45
Editors Introduction
This mini-book is available both in downloadable form, as part of the ISA Encyclopedia of
Measurement and Control, and bound in a print format.
Mini-books are small, unified volumes, from 25 to 100 pages long, drawn from the ISA catalog of
reference and technical books. ISA makes mini-books available to readers who need narrowly focused
information on particular subjects rather than a broad-ranging text that provides an overview of the entire
subject. Each provides the most recent version of the materialin some cases including revisions that havenot yet been incorporated in the larger parent volume. Each has been re-indexed and renumbered so it can
be used independently of the parent volume. Other mini-books on related subjects are available.
The material in this mini-book was drawn from the following ISA titles:
Control Valves, a Practical Guide Series book edited by Guy Borden, Jr. and Paul G. Friedmann,
Chapter 12. Order Number: 1-55617-565-5
To order: Internet: www.isa.org
Phone: 919/549-8411
Fax: 919/549-8288
Email: [email protected]
7/29/2019 Aplicaciones Valvulas de Control
4/45
7/29/2019 Aplicaciones Valvulas de Control
5/45
v
Table of Contents
Chapter 12 CONTROL VALVE APPLICATIONS 1by Herbert L. Miller
The Application Categories, 2
Valves in Parallel, 14
Valves in Series, 23
Frequent Application Problems, 25
References, 37
INDEX 39
7/29/2019 Aplicaciones Valvulas de Control
6/45
7/29/2019 Aplicaciones Valvulas de Control
7/45
1
12
Control Valve
Applications
The purpose of this chapter is to present the information that will help processengineers specify control valves properly. It will show that not all of the control
valves attributes are uniquely required for the process design. There are many
common requirements for the categories of applications, regardless of the industry
or the process. By recognizing the common requirements and their importance,the designer can specify the most economical design that meets performance
needs.
Many books on control valves concentrate on the design aspects of the valve,
and little information is included on the subject of applications. An exception to
this is Reference 1, which provides the practical design input associated with a
good valve application. This reference provides many examples of control valve
applications and illustrates the importance of knowing the control characteristics
dictated by the process. Another source of good application knowledge is the
control valve manufacturers. Many have prepared specific application brochures
and guidelines for experiences they have frequently encountered. These brochures
have a wealth of background information because they integrate the experience of
many customers involving the same applications and the experience gained across
different industries encountering similar applications. Many practical guidelines
regarding selection and installation practices are also provided in Reference 2.
In the discussion in this chapter all applications of control valves are classified
into four categories. In describing the applications, the attributes that will be
emphasized are those important in each of the four categories, independent of the
many different application names used from industry to industry. Valve types or
materials will not be discussed unless they are key to a successful application.
Selection among valve types is covered in Chapter 14 of the Practical Guide
Series bookControl Valves; selection among materials was discussed in Chapter
11 ofControl Valves. There are exceptions within every category because of the
needs unique to a specific process within an industry. An example of this would bea pump recirculation valve that would normally fail open to protect the pump but,
in some nuclear power plants, fails closed by design.
It is assumed in the following discussion that the valves have been sized
correctly. A correctly sized valve is a major factor in a successful application. The
use of the ISA data sheet, Reference 3, ensures that all the pertinent information is
available for the sizing decisions. It is recognized that many companies have their
own valve data sheets, but many contain omissions that result in important
information not being passed onto the valve designer. The ISA data sheet standard
provides a good checklist of key information that may impact the users specific
7/29/2019 Aplicaciones Valvulas de Control
8/45
Control Valve Applications
2
application. Chapter 6 ofControl Valves discusses sizing; Chapter 22 ofControl
Valves describes a valve sizing computer program.
In the discussion in this chapter emphasis is placed on the attributes that are
unique to the application and not so much on problems that occur due to poor
selection. Of course, being aware of these attributes can help assure that you make
a proper valve design choice.
The application categories are as follows:
Process control/feed regulation Continuous letdown Intermittent letdown Recirculation
In addition to these four categories, a separate discussion is provided for valves
that are installed in parallel or series applications. These configurations occur in
many processes so a specific section is devoted to them. Also discussed in this
chapter are common valve application problems caused by improper specification
or installation practices.
The definitions of terms used in this chapter are discussed in Chapter 3 of
Control Valves and are based on the ISA standard S75.05, titled Control Valve
Terminology.
The Application Categories
Process Control/Feed Regulation
In this category, a control loop or system has been pressurized and has a valve
that is controlling the feed to a process in response to a control signal. The control
signal could be based on a need for flow or used to maintain a pressure or
temperature to the process or to maintain a fluid level. This setup would typically
require continuous valve operation. Frequently, there are parallel control valves to
aid in start-up or shutdown when wide rangeability is required.
To provide proper valve control trim characterization is usually needed. Thisvalve trim is required for process startup and shutdown, which, of course, cannot
be avoided even though it may not be frequent. When the system is operating at
low loads both the flow rate and system pressure drop are low, but valve inlet
pressure can be high due to pump runback. This results in high pressure drop
across the control valve at low flow. At full flow (full load) conditions, the system
pressure loss has increased to cause lower pressure drop across the control valve,
as shown in Figure 12-1. These conditions point to the need for good valve
rangeability, which usually results in reciprocating stem-type valves with equal
percentage trim characteristic. With parallel start-up valves and/or low-pressure
processes, this regulation may be possible using a rotary-type valve with a
modified ball design.
Figure 12-1 shows a schematic of the situation covered by this application.Also shown is a frequently encountered pressure-versus-load curve for the
application. As seen at the low-load condition, the pressure drop across the valve
is very high. This is the difference between the pressure developed by a pump or
compressor upstream and the pressure drop through the system.
The names assigned to the valves in this application are numerous. Some of
them are as follows:
Flow control Level control Pressure control
7/29/2019 Aplicaciones Valvulas de Control
9/45
The Application Categories
3
Pressure reduction
Regulator (flow)
Throttle
Figure 12-1. Process Control/Feed Regulation Application.
Key attributes for this application category are as follows:
1. Accuracy of control. The output of this valve affects almost all of the
downstream functions in the process. If a steady feed rate cannot be
maintained, then all of the pressures, temperatures, and flows will becontinuously changing, sometimes with an increased gain or error signal
multiplication. Thus, the quality of the process output, whether it is
electric power output, clean gas, paper, chemical product, or other, is
impacted by the ability of this valve to maintain an accurate output. In
some processes, the variation of an inaccurate valve output may not be
noticed at the process output because of a damping due to long residence
times. In these cases, however, the continuous variation may lead to long
period oscillations and fatigue in process equipment near the valve. Small
signal response is often important to optimize efficiency or process
7/29/2019 Aplicaciones Valvulas de Control
10/45
Control Valve Applications
4
output.
2. Rangeability. This is the second most important attribute for this valve
because the process must be controlled during start-up and shutdown.
Also, during operation, abnormal conditions may frequently exist that
create the need to operate at reduced loads. In the electric power
generation field, it would be necessary to reduce load depending upon
whether the load dispatcher responded to diminished use of electric
power. In the chemical process field it may be a breakdown or
maintenance of a parallel train in the system that calls for the process to
operate at reduced load.
To handle the start-up and shutdown conditions, it has been traditional
to use two valves in parallel to achieve the needed rangeability. There are,
however, valve designs today that can handle this function in one valve
body (see the discussion on parallel valves later in this chapter). To
achieve good rangeability and linearity of the valve-flow-to-stem position
(installed linearity), the valve trim must be characterized. Thus, at the
low-load conditions, a valve capacity versus position curve
(characterization) will look like that shown in Figure 12-2. This
characterization not only provides better rangeability and control but alsoassures that the valve closure member does not operate near the seat and
thus minimizes damage to the critical seating surfaces as a result of
excessive fluid velocities. Depending upon the low-load pressure drop,
other measures may be necessary to limit high fluid velocity erosion such
as the use of multi-stage trim designs. These designs become a
consideration when pressure ratios, p1/p2, across the valve exceed three;
otherwise pressure drop across the valve could result in cavitation or
excessive noise. Cavitation (see Chapter 7 ofControl Valves) and
excessive noise (see Chapter 8 ofControl Valves) can occur at pressure
drops as low as 30 psi (0.2 MPa) for some fluid conditions. The
characterization and trim-erosion considerations are very important
because they contribute significantly to the most vital function of thisvalve and that is accuracy of feed control. In this application, other
attributes are usually secondary. These are as follows:
7/29/2019 Aplicaciones Valvulas de Control
11/45
The Application Categories
5
Figure 12-2. Cv/Flow versus Travel.
3. Failure mode. Fail safe is the preferred failure mode and is usually in
place or last position. This means that all actuator types can be used;
pneumatic, electric, or hydraulic. There are always exceptions. One is in
the nuclear power industry where the normal feedwater regulators are
failed closed to eliminate an uncontrolled leak path, while the auxiliary
feedwater system takes control.
4. Stroke speed. Stroke speed is generally not a consideration because most
processes cannot change load quickly because of stored energy or productin the system. Thus, normal valve speeds are quick enough to respond to
the demands imposed.
5. Shutoff. This is usually not a consideration because these valves are
seldom shut. So, an ANSI/FCI-70-2 leakage class III or even less is
sufficient in most cases.
The most obvious example of a valve in this application would be a boiler
feedwater regulator valve where a constant-speed feed pump is used. The
pressure condition for the valve inlet (pump output) and valve outlet (system
pressure) are shown on Figure 12-1. A representative set of conditions for this
application would be a flow rate of 55,000 lb/h (metric 25 t/h) at a pressuredifferential of 735 psi (5.07 MPa) at start-up. At a full-load flow rate of 990,000
lb/h (450 t/h) the pressure differential is 30 psi (0.20 MPa). Thus the pressure
drop across the valve changes by 25 times as flow is varied by 20 times over
the load range. The resultant valve rangeability exceeds 100 in that the
minimum Cv is 4, and the maximum is 370. However, to provide control at the
maximum condition, a Cv of over 400 is required.
7/29/2019 Aplicaciones Valvulas de Control
12/45
Control Valve Applications
6
A range of conditions for the gas transmission metering valves would be a low
flow pressure differential of 900 psi (6.2 MPa) and a high flow pressure
differential of 100 psi (0.7 MPa). Another valve selection criteria in this
application that may be a consideration is valve-generated noise. With the high
compressible fluid pressure differentials, some valve designs could produce noise
levels in excess of 100 dBA.
Continuous Letdown
In this application category, the valve is located between two large reservoirs of
different pressures where the downstream reservoir could be the atmosphere. In
this case, the valve sees a constant pressure drop, so flow control is established bythe valve position. There is no need to provide anything other than a linear trim
characterization for this purpose unless extended duty at low feed rates is also
required by the process. An equal percent closure member will allow travel farther
off the seat for extended operation at low flows.
Reliability and ruggedness are the keys to this continuous-duty application, soselecting the proper valve design for the pressure drop condition is important.Table 12-1 provides a guideline for which type of valve to select for specific
pressure ratios.
Figure 12-3 shows a schematic of the situation covered by this application. The
most usual control condition is that of either upstream or downstream pressure or
level control. The feed rate is varied as necessary to maintain pressure or level. Anexample of upstream pressure control is the letdown from a process, such as a
chemical reactor vessel, a gas reservoir, or a reservoir level control. Probably more
numerous are the downstream pressure control requirements, such as for steam to
an auxiliary turbine or process reactor and for gas flow into a distribution system
such as for multiple burners. The control variable can also be flow instead of
pressure. This would be the case for burner control valves, spray or mixing valves
used for pressure or temperature control, and gas transmission pressure-reducing
valves.
A less obvious example of a valve in this application is that of a gas
transmission metering valve. The highest flows occurs at the lowest
differential pressure, and the lowest flow rates occur at the highest differential
pressures. This is caused by variable load resulting from changing weather
conditions. As noted by Reference 4,
When it is cold outside, consumers require large volumes of gas,
which results in great demand placed on the local distribution
company, LDC. The LDCs in turn require a greater amount of gas from
the pipeline system. This results in small differential pressure between
the pipeline and the LDC, with corresponding high volumes of gas.
This situation requires a valve with a high capacity.
In contrast, when the weather turns warmer, consumer demand
diminishes and the LDCs place very little demand on the pipeline. The
pipeline system pressure remains high because of the reduced
demand. Therefore, high differential pressures occur between the
pipeline and LDC with very little flow occurring. This situation requires
a valve that can handle low capacity.
7/29/2019 Aplicaciones Valvulas de Control
13/45
The Application Categories
7
Figure 12-3. Continuous and Intermittent Letdown Application.
In specifying conditions for these valve applications, the most common error is
omitting the off-load operating conditions. Usually, only one set of operating
conditions is provided, that of steady-state full load. The off-load conditions are
important because of the complexity of most plants start-up procedures, whichrequire holding at partial loads for extended periods. Thus, the need for good
rangeability is usually missed in these applications, resulting in poor valve
selection with the corresponding result of poor control of the process start-up or
part-load performance.
Typical names assigned to these valves are:
Attemperation
Blowdown Flow control Letdown Level control Pressure control
Pressure regulator Reducing Spray
Key attributes for the continuous letdown applications are the following:
1. Accuracy of control. As with the process control valve, this is the most
important function for this valve. Maintaining a near constant pressure,flow, or temperature condition is essential to the process product quality
and reliability of the equipment affected by the control. A continuously
varying output would have many short- and long-term damaging
influences.
This application is fairly routine in terms of its demand on the control
valve. Many different types of valve designs can handle the conditions
imposed without detrimental effects. There, of course, can be exceptions
where the attributes discussed here may assume more importance than
indicated here.
2. Rangeability. This is not generally an important issue because most valve
designs provide sufficient rangeability to meet the process needs. Areasonable need is for a fifteen-to-one rangeability. Since the pressure
drop across the valve is constant, the trim of the valve is linear, and for
most valves the travel position is a reasonable approximation of the load.
3. Failure mode. This depends upon the specific application, and all modes
of failures on loss of power to the actuator are used. If a generalization is
to be made, it would be to have this valve fail-in-place so that the system
is not disturbed by an abrupt change in pressure or flow conditions. Some
engineers may prefer to fail the valve closed so that the operator is alerted
to the valve failure.
7/29/2019 Aplicaciones Valvulas de Control
14/45
Control Valve Applications
8
4. Stroke speed. As with the process control/feed regulation application, this
is usually not a consideration.
5. Shutoff. This is not an important consideration because the valves are
rarely closed unless the process is off line and isolated by other system
valves.
Intermittent Letdown
As with continuous letdown applications, the valve reduces the pressure
between two large reservoirs, and frequently the downstream reservoir is the
atmosphere. Obviously, for fluids damaging to the environment and public health,
a downstream reservoir would capture the flow for further processing, such as
through a flare arrangement, where the fluid is burned before release.
The pressure drop could be constant or variable depending upon the specific
application. The variable case would occur on a blowdown situation, so the inlet
pressure would decrease with time. The valve stroke would be increased if a near
constant flow rate was desired to minimize blowdown time. Or, for the case of
upstream pressure control, the stroke would be varying depending upon the
process input and output of the upstream reservoir.Figure 12-3 is the same for continuous and intermittent letdown applications.
As implied by the name, the only difference in the two processes is the time of
operation or duty cycle on the valve. Even though the valve is used less frequently,
the conditions for the valve represent a tougher service. In this case, the control
valve must perform the dual functions of providing control and tight shutoff. The
latter usually means a Class V or VI leakage (ANSI/FCI-70-2), but in many cases
this is not sufficient. A block valve leakage requirement must be used to provide a
sufficient criterion for the permissible leakage. The reason a tight shutoff is
needed is that any leakage through this valve means a loss of process fluid that is
needed upstream to make the process product or output.
Valves in intermittent letdown service primarily perform a bypassing function.The valves are opened to bypass the entire process or parts of the process during a
start-up/shutdown function or a safety-relieving function. In some cases, the valve
can perform both functions of bypassing for control and safety relief. However,
local codes must be checked to see if a dual role is permitted.
Typical names assigned to various intermittent letdown valves are shown in the
following list. Frequently, the name of the equipment being bypassed or blown
down is used in combination with these labels:
Antisurge Dump Reject Auxiliary Extraction
Relief Blowdown Flare Start-up Bypass
Injection Vent(ing) Depressurizing
Letdown
The attributes for the intermittent letdown application are as follows:
7/29/2019 Aplicaciones Valvulas de Control
15/45
The Application Categories
9
1. Accuracy of control. For most valves in this application this is not an
important function because most control valves provide sufficient
resolution to meet the requirements. An exception frequently exists when
the valves are in a start-up bypass function where a fine, accurate control
is needed to maintain flow or upstream pressure conditions over extended
periods. The pressure drop across these valves is constant for most of the
operating conditions. As a result of this and the accuracy needs, a linearcharacterization of the trim is normally sufficient.
2. Rangeability. Normal design capability is sufficient.
3. Failure mode. Normally, this valve is in a fail-close configuration, its
normal status, so that if power is lost to the valve the process is not
disturbed unnecessarily. Occasionally, process system needs will require
the valve to fail in its last position.
When the valve is in a safety role, the valve configuration must be in the fail-safedirection, which is usually fail-open.
4. Stroke speed. The dual functions of these applications dictate the speed ofoperation. When the valve is used in a safety function, the speed needs to
be very fast, on the order of one-half to five seconds. The safety function
could be to protect personnel, equipment, or the process output. This
would only be necessary for the opening direction because, in this mode, it
is usually relieving pressure from an over-pressurized upstream reservoir.
When the valve is used in the process bypass mode, for start-up and
shutdown function, speed is generally not a priority consideration. Speeds
can be achieved without requiring special considerations because
commercial positioners usually have sufficient capacity for pneumatic
actuators. Electric drives, although they tend to be relatively slow, are fast
enough. The use of hydraulic actuators for speed purposes would be over
design except when the valve is used for safety relief.
5. Shutoff. This is a key and critical function of valves in this application,
regardless of whether the valve is performing a bypass or a safety role.
Any loss of fluid through this valve reduces the process efficiency. Either
pumps or compressors must work harder to overcome the leakage or the
maximum loads achievable are reduced.
The minimum shutoff requirement would be a Class V leakage for a control
valve. However, often a Class VI or even a MSS-SP61* block valve closure is
prudent for reliable long-term valve operation. For this reason, many control valve
* The MSS-SP61 is a standard issued by the Manufacturers Standardization Society of the Valve and Fittings
Industry, Inc. The SP61 standard is titled Pressure Testing of Steel Valves. It covers the requirements forthe shell and seat closure pressure testing of steel valves. The standard is not intended to be used for control
valves. However, as noted in this chapter, there are many control valve applications in which the valve mustperform the role of modulating control and then become a block valve when shut. For many valves that areused in safety relief and plant start-up or shutdown applications, these dual roles are frequent requirements.
If only a control valve leakage class is imposed then some leakage is permitted. The amount of leakage maynot be bothersome when the valve is new. However, since these valves are normally shut, fluid erosion ofthe closure members due to the leakage results in a complete failure of the valve to shut off. In the extreme
case, the ability to control is also lost. Manufacturers can produce control valve designs capable of meetingthe MSS-SP61 standard.
The standard calls for a leakage test pressure drop of 110% of the 100F (38C) ANSI class rating
pressure. If this high pressure may damage the seat, a possibility in control valve designs, then 110% of themaximum differential operating pressure may be used. The permissible leakage under this standard shall beless than 10 cc/h of liquid per inch of diameter of the nominal valve size.
7/29/2019 Aplicaciones Valvulas de Control
16/45
Control Valve Applications
10
designs use the fluid pressure to help the closure member maintain a tight seating
force. Soft seats are frequently used, but these may not be reliable in the high
pressure drop situations, over 1500 psi (10 MPa), that frequently occur in this
application category.
Recirculation
This application could be thought of as a subset of intermittent letdown in that
the duty cycle is intermittent and the function performed is generally a process
bypass situation. The bypass need is usually driven by the start-up, shutdown, orsystem-upset conditions. However, because the applications listed under this
category are usually the most severe within the process system, they need special
consideration to assure their reliable and long-term operability.
Thus, the recirculation application is reserved for the cases in which either a
pump or compressor is bypassed, as shown in Figure 12-4. In this case, the fluid
that has been pressurized or compressed is reduced in pressure and returned to the
pump or compressor inlet reservoir. The valves experience a wide range of
pressures and temperatures, however, within a specific application, rangeability
and control are seldom critical. The valve is usually closed, and its performance is
judged by how well it shuts off. If shutoff is not maintained, the fluid must be
repressurized for use by the process at increased energy expense. There have been
cases in which recirculation loss is so high that the total output of the system ismeasurably reduced.
A typical example of this application is the steam turbine bypass valve. This
valve is installed to bypass the steam around the turbine until pressure and
temperature are at appropriate turbine start-up conditions. The valves are
closed as the turbine is brought on line. These turbine bypass valves can also
be used for pressure relief valves per some code regulations, particularly in
Europe. For safety purposes, the American Society of Mechanical Engineers
Boiler and Pressure Vessel Code does not permit the use of this bypass control
valve as the pressure relief valve. For the turbine bypass function, the most
important valve attributes are usually shutoff, control, and opening speed.
Another example involves the turbine drive of a synthetic gas compressor in
an ammonia plant. The primary purpose here is not turbine protection butassuring there is high-pressure steam available to a downstream reformer if
the gas compressor is tripped. Steam is needed to avoid catalyst deterioration
resulting from carbon deposition and to provide time to achieve an orderly
shutdown of the reformer. Thus, the trip-open function is very important in this
application, requiring that the valves open in one to four seconds. Also,
because of the fairly high pressure drop of 1000 psi (6.9 MPa), noise control is
also a consideration in valve selection.
7/29/2019 Aplicaciones Valvulas de Control
17/45
The Application Categories
11
Because most fluidsare erosive, particularlyliquids, a small leak
through the valve willresult in the rapiddeterioration of theseating surfaces.
Figure 12-4. Recirculation/Recycle Application.
This valve will see the highest pressure drop in the entire plant when youbypass the main pump or compressor. The cost of fluid leakage through this valve
traditionally exceeds the purchase price of a new valve many times over. This loss
is seen in reduced plant efficiency, unavailable load and the increased pumping
power required to pressurize or compress the fluid stream. It is not an application
where the engineer should compromise on valve selection. The engineer must
select a valve that will meet the long-term seat integrity requirements to assure
leak-free operation. Frequently, this would utilize a design in which the fluid
pressure assists in assuring that there are good seating forces between the closure
member and the seat ring.
Valves in this application go by many names depending upon whether it is flow
through a pump or a compressor that is being bypassed. The most common names
are as follows:
Antisurge
Mini-flow Bypass
Recirculation
Dump
Recycle
Kickback
Return
Leak-off
Spillback
Letdown
Surge control
In addition to the comments about leakage and control attributes discussedunder the intermittent letdown application, there are two other significant
considerations concerning the selection of these valves. For liquid recirculation
applications, those considerations are cavitation and vibration, and for compressor
applications, they are noise and vibration. In both cases, the detrimental affect is
caused by the high pressure drop associated with these valves and the
accompanying low back pressure. The back pressure is usually near atmospheric
but could be a vacuum if the downstream reservoir is a condenser. These
applications frequently demand severe service valves that are specifically
designed to handle these conditions.
7/29/2019 Aplicaciones Valvulas de Control
18/45
Control Valve Applications
12
In summary, then, the key attributes for valves in this application, assuming
proper sizing, of course, are as follows:
Tight shutoff. Usually pressure assisted via piloted designs or unbalancedplug designs with large actuators.
Anti-cavitation/low noise trim
Pipe vibration elimination
Secondary considerations include the following:
Flow characteristics. Usually linear for pumps and characterized forcompressors
Failure mode. Normally open
Stroke speed. 2 to 5 seconds for compressible; up to 25 seconds forliquids
Frequently, the speed requirement for the compressor antisurge valve drives the
designer to question the capability of pneumatic actuators. High speeds can be
achieved with a pneumatic system as is demonstrated by the actual field test
results from a compressor recycle valve. As shown by Figure 12-5, that valve
7/29/2019 Aplicaciones Valvulas de Control
19/45
The Application Categories
13
opened to 95% of full stroke in only 1.7 seconds. The valve and actuator sizes and
conditions were as follows:
Figure 12-5. Valve Position versus Time Opening.
As this actual valve design demonstrates, a pneumatic actuator can achieve
very rapid position changes to provide good surge control of the compressor. The
air supply tubing to the actuator and the air source must be large enough to
provide the rapid air flow rate to the actuator. Otherwise, speed will be limited.
The air supply pressure to achieve this quick opening is dependent upon the valve
hydraulic and friction forces. Thus, the air supply pressure may decrease and still
allow the stroke time to be achieved, provided the pressure results in enough force
to move the valve closure member. Consult the valve manufacturer if you are
concerned that the air supply may be restricted.
Fluid Mixed Refrigerant
Actuator size 113 in2 730 mm2
Valve stroke 24 in 610 mm
Plug weight 1320 lb 600 kg
Inlet pressure 190 psi 1.31 MPa
Outlet pressure 45 psi 0.31 MPa
Flow rate 1,620,000 lb/hr 735 t/h
Air supply pressure 100 psi 0.69 MPa
7/29/2019 Aplicaciones Valvulas de Control
20/45
Control Valve Applications
14
Valves in Parallel
There are three primary reasons for having control valves in parallel. These are
illustrated in Figure 12-6. All of the situations illustrated in the figure are driven
by the process but are heavily influenced by the capability of the control valve
selected.
Figure 12-6. Parallel Valve Application.
The first reason for having control valves in parallel is to have a redundant
valve available in the event that problems with the primary valve develop. In this
case, it is very important that the process be kept on line and running. The usual
reason is the economics of the output product or the high cost of a shutdown. A
shutdown may require a lengthy start-up time, or in some cases an unsafe
condition could result from the lack of feed control. Certainly, the first
consideration by the engineer is to use a highly reliable valve in this situation, a
valve that is designed for the service conditions. An example in which these
conditions would exist is in coal gasification, where erosive fluids cause valves to
wear out quickly. To avoid repeated shutdown, parallel valves are used so that
repair can take place while the process remains up and running. Another exampleis the use of parallel or redundant valves in the nuclear industry, where decay heat
from the reactor core must be removed after a reactor is tripped. Thus, regardless
of the reliability of the valve, the consequences of even a remote possibility that
the primary valve will fail are offset by the availability of the redundant parallel
valve system.
A second reason for having two valves in parallel is to assure balance in the
process equipment. Balance may mean equalizing temperature or chemical
concentration, as in mixing situations, or splitting the flow stream for process
benefit. As shown in Figure 12-6, an example of thermal balancing would be a
7/29/2019 Aplicaciones Valvulas de Control
21/45
Valves in Parallel
15
heat exchanger that must have flow through both tube banks to assure that the
tubes do not overheat. In this situation, the most common problem is that small
signal changes to the valve result in too much flow change, a valve resolution
problem. Generally, the resolution problem is related to the valve selection. Too
much capacity has been installed, and/or the trim characterization does not
provide sufficient gain for good resolution. There are valves available today that
can provide extra rangeability and excellent characterization so that good controlis achievable. Valves that have an ability to achieve this capability but with
different degrees of success are illustrated by Figures 12-7 through 12-12. The
figures illustrate various design concepts used to expand the flexibility of the
valve to meet a wide range of flow conditions. In many cases, a single
manufacturer will supply more than one concept. It might be argued that the
rangeability and characterization needs could be achieved by the control system
feeding the correct signal to the valve. The problem arises in the valves ability to
respond to the small signal change with a correspondingly small flow change. The
trim design and characterization are key attributes that influence the flow response
to the change in control signal.
Figure 12-7. VRTTrim Valve. (Courtesy of Masoneilan)
7/29/2019 Aplicaciones Valvulas de Control
22/45
Control Valve Applications
16
Figure 12-8. V-Line Series Noise Attenuator Ball. (Courtesy of Fisher Controls International Inc.)
INSERT PHOTO HERE
7/29/2019 Aplicaciones Valvulas de Control
23/45
Valves in Parallel
17
Figure 12-9. Cascade Trim Valve. (Courtesy of Copes Vulcan, Inc.)
INSERT PHOTO HERE
7/29/2019 Aplicaciones Valvulas de Control
24/45
Control Valve Applications
18
Figure 12-10. Mark OneGlobe Valve. (Courtesy of Valtek International)
Figure 12-11. Q-BallValve Schematic. (Courtesy of Neles-Jamesbury Inc.)
7/29/2019 Aplicaciones Valvulas de Control
25/45
Valves in Parallel
19
Figure 12-12. DragTrim Valve. (Courtesy of Control Components Inc.)
INSERT PHOTO HERE
7/29/2019 Aplicaciones Valvulas de Control
26/45
Control Valve Applications
20
Another example of valves in parallel to maintain balance is illustrated by the
use of a three-way valve design to replace two valves in parallel, as illustrated in
Figure 12-13. Control of the temperature of the fluid leaving the heat exchanger
can be accomplished by bypassing a portion of the heating steam. To be
successful, experience has shown that the bypass flow must be less than 25% of
the total flow. As the bypass flow increases, the available pressure drop across the
heat exchanger decreases with the lower flow through the exchanger. Thisincreases the pressure drop across the exchanger steam control valve branch,
which now has to finely control the steam flow at a higher valve pressure drop in
order to provide accurate temperature control. The pressure drop across the bypass
valve branch has also increased with the lower exchanger flow. If the bypass valve
outlet pressure drops below the valve pressure ratio that causes choking, then the
ability to control temperature is lost since the bypass flow cannot be varied in the
choked condition.
That the mixing function of the three-way valve is an equivalent to the parallel
valves is also apparent from Figure 12-13. In this situation, the two fluids would
enter ports B and C and exit port A in proportion to the position of the valve
stroke and porting size used in the design.
Figure 12-13. Three-way Valve Substitute for Parallel Valves.
7/29/2019 Aplicaciones Valvulas de Control
27/45
Valves in Parallel
21
When everything has been done right to achieve good rangeability, it may still
be necessary to add a small valve in parallel with a larger capacity valve. This
represents the third example of valves in parallel as illustrated in Figure 12-6. The
smaller valve can either provide a fine control during the start-up or low load,
called sequencing, or it can provide a trimming function for small process changes
while operating at a higher load condition. The definition of the control logic will
be based upon which need is dictated for the valves. From a valve integrity andlong-term reliability standpoint, the control logic designer should work to make
sure the valves do not operate for extended periods with the valve closure element
very near the seating surface. Travel at a position that causes the pressure drop to
occur in the flow path between the closure member and the seating surface will
cause erosion of these surfaces. The result is degradation of control, instability,
and excessive leakage early in the valves life cycle.
When two valves in parallel are sequenced for rangeability, the valve design
and control logic selection are critical to a reliable system. The valve trim should
not be of an inherent linear characteristic. The reason for this is that there is a
discontinuity in the gain of the two valves when switching from one to the other.
This is best understood by an example for an application involving continuous
letdown and constant pressure drop.
The transition from one valve to the other is usually quite fast relative to the
process, which results in a bumpless transfer. The engineer should give
consideration to the transition point so these valves are not continually opening
and closing during normal operation. An operation that continually opens and
closes will experience a gradual degradation due to premature wear. This higher
wear rate can be avoided by performing proper Cv selection of the parallel valves.
Reference 5 includes additional information on parallel logic involving control
valves.
The total capacity of valves in parallel is obtained simply by adding the Cvvalues of each valve in parallel. That is,
Cv,o = Cv,1 + Cv,2 + + Cv,i (12-1)
Consider a small valve with a Cv of 10 that is being sequenced with a valve with
a maximum Cv of 100. Both valves have linear trim and actuators capable of
one percent resolution. At the transition from the small valve to the large valve,
the gain, or minimum change in Cv for the small valve is 1% of 10 or a 0.1 Cv
change. After transition, the large valve gain is now in effect, and its gain is 1%
of 100 or a Cv change of 1.0. Thus, the gain has increased ten times, creating a
strong potential for an unstable or an oscillatory control scheme.
When sequencing, the parallel valves should have a near equal percentage trim
characteristic and be of equal gain on each side of the transition. This is shown
in Figure 12-14 for the preceding example of two valves with a Cv of 10 and 100.
It is assumed that the large valve has a minimum controllable Cv of 1/30 of thefull Cv. For this case, as load is increased and the small valve reaches its
maximum Cv of 10, the larger valve is opened to a position of Cv equal to 10,
and the small valve is closed. The gains are equal at the transition, and thus
the process control resolution is not affected. Similarly, upon reducing load,
the large valve controls until the Cv is down to 3.3, after which it closes and the
small valve is opened to this capacity. The amount of overlap between valves is
dependent upon many factors, which includes stroke time, valve
characterization, and process response time.
7/29/2019 Aplicaciones Valvulas de Control
28/45
Control Valve Applications
22
where
Cv,o = overall Cv
Cv,i = Cv of the ith valve
Reference 6 discusses a fourth reason, controlling cavitation, for installing
valves in parallel. This arrangement takes advantage of the change in thecavitation index of a valve when it opens, that is, less cavitation occurs at small
openings. This reference provides measured cavitation index values for different
valve designs as a function of the valve opening. In this situation, small openings
refer to operation within 10 to 20% of the valve full-open position. If extended
operation below 10% is needed, then other problems of control develop in that the
valve trim parts may erode, vibrate, and create excessive noise. Another problem
is the increased cost of a second valve, unless redundancy is a requirement.
Reference 6 also discusses how cavitation could be controlled by placing two
valves in series, as discussed in the next section. Again, increased cost is a
disadvantage as is the potential that the upstream valve will adversely affect the
downstream valves performance.
Figure 12-14. Sequencing Two Parallel Valves of = % Trim.
7/29/2019 Aplicaciones Valvulas de Control
29/45
Valves in Series
23
Valves in Series
It is always desirable to
use one valve becauseof the controlinstabilities that arise inthese seriesconfigurations.
The use of control valves in series is usually driven by a desire to limit the
pressure drop across a single valve. This may have been necessary many years
ago. However, there are now valve designs that can absorb very high pressure drop
conditions in a single valve without the accompanying problems of cavitation,
erosion, vibration and noise. Table 12-1 provides pressure drop guidelines for
different valve designs that will aid in the decision regarding whether valves inseries is a possible configuration. The cost of the valves, associated controls,
installation, and calibrations would then be a second consideration.
Table 12-1. Valve Type Selection Guide.
Control instabilities are beyond the purpose of this discussion, but they canoccur when the fluid residence time between valves is much faster than the valve
response time. As a general guide, the time constant of the valve should be less
than the residence time of the fluid between the valves. For example, if the
residence time is one second, the valve time constant should be one second or less.
The time constant is defined as the time required to complete 63.2% of the total
rise or decay of a step change in the signal to the valve. This is illustrated in Figure
12-15. The residence time is calculated by dividing the distance between the two
valves by the average fluid velocity in the connecting piping. That is,
Residence Time = L/U = AL/w (12-2)
where
L = distance between valvesU = average fluid velocity
A = piping flow area
w = mass flow rate
= fluid density
Valve Type Flow Direction p1/p2 Limit* p/p1 Limit
Multi-stage, Multi-path ------ No limit 1.0
Multi-stage, Single-path ------ 5.5 .82
Single-seat Globe Open 3.3 .70
Close 2.3 .57
Double-seat Globe ------- 3.3 .70
Angle-body Cage Open 3.0 .67
Close 2.3 .57
Pinch ------ 2.9 .66
Butterfly, 60% Open ------ 1.5 .33
Reduced Ball, 80% ------ 1.2 .17
*p1/p2 = 1/(1 p/p1)
7/29/2019 Aplicaciones Valvulas de Control
30/45
Control Valve Applications
24
Figure 12-15. Single-Order Response Time.
Instability also results if the pressure drop across one of the valves, particularly
the downstream valve, becomes small, less than 20% of the overall pressure drop
assigned to the valves. To assure a stable operation from this standpoint, a good
rule is to have two-thirds of the overall pressure drop across the first valve and
impose 80% of the drop as a top limit.
Figure 12-16 shows a case in which control valves would be put in series, that
is, when multiple lines feed downstream parallel processes and the designer places
an inlet pressure reduction valve upstream of the distribution manifold. Such a
case would occur on a gas burner distribution system where the inlet pressure is
dropped and then individual valves control the gas flow to each burner.
Figure 12-16. Series Valve Applications.
7/29/2019 Aplicaciones Valvulas de Control
31/45
Frequent Application Problems
25
A gas transmission metering and reducing station is also an example of valves
in series. A primary control valve reduces the pressure, and a monitor valve
safeguards against the over-pressurization of the downstream line in case of the
failure of the primary control valve. The system is designed for the primary valve
to fail open and the monitor valve to fail closed.
The overall capacity of valves in series if needed can be determined from the
following equation:
(1/Cv,0)2 = (1/Cv,1)
2 + (1/Cv,2)2 + (1/Cv,i)
2 (12-3)
In these definitions, the overall pressure drop is used in the expression for Cv,0,
and the pressure drop across each of the components is used in the Cv,i definition.
This equation must also be used when a valve and another resistance component,
such as a diffuser or orifice, is installed in series. (The Cv of an orifice or a diffusercan be approximated by using thirty times the flow area, where the area is
expressed in square inches.) Failure to do so can result in an installed C v less than
that required by the process.
Frequent Application ProblemsIt is worthwhile to look at some of the common problems experienced when
using control valves. These problems tend to be independent of the applications
but can be aggravated by the unique needs of each installation. The root cause of
these common problems varies from a lack of understanding of valve design and
the selection of the wrong valve type (usually because of efforts to reduce initial
capital cost) to poor calibration and maintenance.
Controlling Pressure Drop
The first problem to be discussed arises from the purposes of a control valve,
which are to do the following:
1. Convert energy by reducing the fluid pressure,
2. Handle deviations from ideal operation,
3. Handle the influence of unavoidable process changes,
4. Permit the smooth transition from one load condition to another.
To be able to achieve these objectives, there must be some pressure loss at the
valve, and this results in an increase in fluid kinetic energy within the valve.
Guidelines that have developed through many years of successful applications
indicate that at least 10% of the system pressure drop should be available across
the control valve to provide some control. To achieve good control, a value of 30%
is desired. These guides are continually challenged, as in the case of the first
application category discussed in this chapter, process control/feed regulation. Inthis application the most efficient plant operation would be to control the full-load
condition without any energy absorption, an impractical goal. Thus, special
attention must be devoted to the sizing pressure drop conditions to assure that the
valve can modulate the feed flow and meet the control objectives.
Excessive Fluid Velocities
Throughout this discussion of control valve applications, we have referred to
the problems of erosion, vibration, noise, cavitation, and flow instabilities. All of
these problems can be eliminated by considering the control of fluid velocities
7/29/2019 Aplicaciones Valvulas de Control
32/45
Control Valve Applications
26
during the flow and the accompanying pressure drop through the valve. Excessive
fluid velocities cause widely varying local pressures because of the conversion of
static pressure head to kinetic energy. This results in extreme turbulence and
boundary layer separations as the fluid is forced to make directional changes in
the flow path through the valve. The high fluid velocities and locally varying
pressure differences cause the following:
Uneven forces on moving parts leading to vibrations that cause excessivewear, breaking parts, unscrewing bolts, and noise and flow rateoscillations.
Shock waves that lead to screech and high noise-level work environments.Even pipe breakage can occur, as noted in Reference 7, when the noise isin excess of 110 dBA.
Cavitation and the rapid erosive wear of metallic surfaces near thelocation of the bubble collapse.
Flashing of the fluid and the accompanying high-velocity liquid dropsthat erode metal surfaces.
Erosion of metallic parts when the fluid has entrained hard solids such assand, pipe scale, weld slag, and catalyst.
It should be apparent that good long-term control cannot be achieved when all
or even one of these problems are present. Guidelines on what velocities are
acceptable have been developed over many years of experience in a wide range of
applications. These are expressed as a limit to the fluid kinetic energy exiting from
the valve trim, as discussed in Reference 8.
For the kinetic energy evaluation, the location in the valve that is of greatest
concern is just downstream of where the fluid is throttled or controlled. At this
location, the flow area is the smallest, and the fluid velocity and kinetic energy are
the highest. The parts of the valve responsible for controlling and seating are often
located at this point and are therefore subjected to the highest energy fluid.Figure 12-17 shows the throttle area for various kinds of valve trim. For a top-
guided globe valve, the trim outlet flow area is the annulus area between the plug
and seat. In a cage-guided valve, the trim outlet flow area is the exposed area of
the windows in the cage. For a multi-path cage, the trim outlet flow area is the
total area of all the exposed flow paths. For multi-stage trims, the flow area from
stage to stage must not increase too rapidly or else the throttling will take place
across the first stages, and the later stages will be ineffective (see Figure 12-17e).
Butterfly and ball valves usually meet the presented criteria for kinetic energy.
The pressure drop across these types of valves is not large enough to accelerate the
flow to a high velocity level. Thus, a much lower value of energy is realized.
In a valve, the disk or plug moves to increase or decrease the area through
which flow can pass. For a given set of conditions, a fixed area of the trim is opento flow. Under any significant pressure drop conditions, this area will be
considerably less than the inlet or outlet area of the valve. As a result, the fluid
passing this point will have much higher velocities and kinetic energy levels than
in other valve locations. The only way to increase this flow area without
increasing the flow rate is to increase the resistance of the throttling flow path.
The flow conditions defines how far the valve is open, and the valves trim design
(flow path resistance) defines how much flow area exists at the trim outlet. Once
this area is defined, the continuity equation can be used to calculate the velocity of
the fluid at the outlet of the trim.
7/29/2019 Aplicaciones Valvulas de Control
33/45
Frequent Application Problems
27
Figure 12-17. Throttling Exit Area (Ao) Examples for Typical Valve Trim Design.
7/29/2019 Aplicaciones Valvulas de Control
34/45
Control Valve Applications
28
(12-4)
The fluid density and velocity are used to establish the fluids kinetic energy:
(12-5)
Values for the constants M1, and M2 are shown in Table 12-2.*
For gas or steam, the fluid velocity at the trim outlet may be sonic. If it is, the
density of the fluid at the trim outlet must be higher than the outlet density (o) inorder to pass the given mass flow rate, w. This higher density can be estimated
using Equation 12-4 by substituting the fluids sonic velocity, c, for the outlet
velocity, Vo, and solving for density. This density and sonic velocity are then used
in Equation 12-5 to find the kinetic energy.By including the effect of density in the criteria a single kinetic energy
guideline can be defined for all fluids. Thus, the velocity for high-density fluids,
such as liquids, would be much lower than for gases. Table 12-3 shows criteria for
a valve trims outlet kinetic energy. The valve trim should be selected to keep the
kinetic energy below these levels.
Table 12-2. Numerical Constants for Velocity and Kinetic Energy Equations.
For most conditions, an acceptance criterion of 70 psi (480 kPa) for the trim
outlet kinetic energy will lead to a trouble-free valve. In some applications, where
the service is intermittent (the valve is closed more than 95% of the time) and the
fluid is clean (no cavitation, flashing, or entrained solids), the acceptance criteria
can be increased but should never be higher than 150 psi (1030 kPa).
* From the general form of the energy equation, potential energy is expressed in terms of a column height ofthe fluid. Similarly, kinetic energy is expressed in this discussion as a head. The form that is commonlyreferred to is the velocity head. The units of pressure are traditionally used for the velocity head expression,which can also be converted to a height of the fluid to be consistent with the potential energy term.
For objects with a mass the kinetic energy is expressed as:
KE= (mass)(V2/2)
Similarly, the velocity head is an expression of the kinetic energy of the fluid, although in the form that
is relative to a unit volume of the flowing medium. Thus,
KE= (mass/volume)(V2/2) = V2/2
The gravitational constant is usually shown in the denominator of the velocity head expression. This is
required in order to convert from mass units to force units. The gravitational constant has been included inthe constant M2.
Constant Units Used in Equations
M w o Ao Vo KE
M1 25 lb/h lb/ft3 in2 ft/s -
1.0 kg/s kg/m3 m2 m/s -
M2 4636.8 - lb/ft3 - ft/s psi
1000 - kg/m3 - m/s kPa
Vow
M1oAo-------------------=
KE1
2---oVo
2
M2------------=
7/29/2019 Aplicaciones Valvulas de Control
35/45
Frequent Application Problems
29
Table 12-3. Trim Outlet Kinetic Energy Criteria.
In flashing service, liquid droplets are carried by their vapor at much higher
velocities than would be the case for a single-phase liquid flow. To eliminate the
risk of erosion in this situation, the acceptance criteria for flashing or potentially
cavitating service should be lowered to 40 psi (275 kPa). The same criteria exists
for liquids carrying entrained solids.
Special applications may require even more stringent kinetic energy criteria.
For example, pressure letdown valves used in pump test loops must be vibration-free so that a proper evaluation of the pump can be made. These valves are
designed with trims that reduce the kinetic energy to less than 11 psi (75 kPa). Gas
or steam valves with very low noise requirements may also result in extra-low trim
outlet kinetic energy requirements.
These kinetic energy criteria are additional sizing considerations that assure the
reliable long-term operation of the control valve. A decision to ignore these rules
may result in lower procurement cost but cause high operation and maintenance
costs. In some cases, the ability of the valve to perform its control function may be
jeopardized. References 9 and 10 present examples in which feedwater regulators
could not perform the intended control function. Control was achieved when a
trim was used that halved the fluid exit velocity of the originally installed cage
trim and local smoothing of the flow path was incorporated.
Remember to check thecalculated velocity tobe sure that it does notexceed the sonicvelocity of the fluid. Ifthe velocity is greater
than sonic it must beset equal to sonicvelocity.
The calculation of trim velocities and kinetic energy will require a knowledge
of the cross-sectional area for the flow channel of interest as well as the number of
ports. Approximate calculations can be made with some information about the
valve design, but it is best to work with the manufacturer to obtain more accurate
results. Another approach would be to use Equations 12-5 and 12-4 to calculate
the Vo andAo, respectively, by fixing the kinetic energy at the Table 12-3 values.
The area calculated is the area needed, at the valve travel, that passes the flow
input to Equation 12-4. Then Equation 12-5 is used to calculate the fluid density
so it can be used in Equation 12-4 with the velocity at sonic. This calculated area
can then be compared with the flow area for the valve being proposed by the valve
supplier. The actual area should always be greater than the calculated value in
order to meet the kinetic energy selection criteria. A larger area would also be
needed if cavitation or noise were major considerations in the application.
Calibrating It Right
A third problem involves achieving full-seat loading to maintain tight shutoff
when the valve is closed. A prevalent practice is to calibrate the valve, or the
bench set when a positioner is not used, so that the closure member (e.g., plug,
diaphragm, disk) is just positioned at the seat instead of also assuring that the
closure member is fully loaded against its seat.
Service ConditionsKinetic Energy
CriteriaEquivalent Water
Velocity
psi kPa ft/s m/s
Continuous Service, Single-
phase Fluids
70 480 100 30
Cavitating and Multi-phaseFluid Outlet
40 275 75 23
Vibration-sensitive System 11 75 40 12
7/29/2019 Aplicaciones Valvulas de Control
36/45
Control Valve Applications
30
Unfortunately, it is not obvious to the inexperienced engineer that there must be agood seat load between the two mating seating surfaces. If there is not a sufficientload, then fluid leakage will rapidly erode the surfaces, the erosion time depending
upon the valve pressure drop and fluid. If proper seat loads are not maintained, then it isimpossible to maintain design leakage rates.
The situation is analogous to the manual control of the home water faucet. If
the faucet is turned to just stop the water flow, it will not be long before it will startto drip. As a result of this experience, we subconsciously apply an extra torque to
load the faucet plug against its seat to assure there will be no distracting or
wasteful leakage.
Full loading of a push-down-to-close sliding stem valve is only assured when
the conditions within the actuator are as shown in Table 12-4. Single-acting
actuators are most frequently a diaphragm design. With this design, the spring is
either reducing the seating load or providing the entire seating force. The double-
acting actuator is typically a piston design. With the piston design, the supplypressure is not limited as with the diaphragm design, and full-supply pressure is
available to achieve higher seating forces. The higher pressures in the piston
design have the additional benefit of increased stiffness and better control
resolution.
Table 12-4. Actuator Seating Load Source.
As Baumann pointed out in his article on power signals (Reference 11), many
designers think of the 4-to-20-mA signal as an information signal instead of a
power signal. This signal, in the case of a control valve, is not only dictating the
required position of the closure member, it also drives the operating power to
position and seat the valve. When the valve is calibrated to be just closed at
exactly 4 mA, the extra power built into the valve design to assure seating load is
not applied. This seating load would only be applied when the control signal drops
below 4 mA, a signal that is usually not built into the control system. Thus, the
goal is to calibrate so that the valve positions and then seats with full loading
when it is closed.
CALIBRATION WITH POSITIONER
For small signal changes on double-acting actuators, positioners are typically
designed to maintain two-thirds to three-quarters of the supply pressure on the
side of the piston in the direction of stem movement. In single-acting actuators,
this bias pressure is replaced by a spring. In either case, the only way to ensure
that maximum actuator load is provided to the valve seat when the valve is closed
is through careful calibration.
Actuator Type Conditions for Full Seat Load
Single-acting, Spring to Open Supply Pressure on Top of Diaphragmor Piston Pushing toward the Seat
Spring Pushing away from Seat
Single-acting, Spring to Close Spring Pushing toward Seat
0 psig under Diaphragm or Piston
Double-acting Supply Pressure on Top of Piston
Pushing toward the Seat0 psig under Piston
7/29/2019 Aplicaciones Valvulas de Control
37/45
Frequent Application Problems
31
To ensure that the actuator fully loads the valve closure member against its seat,
the calibrator has to purposely create a sufficient error between the valve position
feedback to the positioner and the positioner input signal in order to cause the
positioner to try to correct its position in the closing direction.
To accomplish this, the positioner is typically calibrated to have the valve
closure member reach the seat within the signal ranges shown in Table 12-5. The
difference between these values and the true endpoint values of 3.0 psi, 15.0 psi(0.02 MPa, 0.10 MPa), 4.0 mA, or 20.0 mA is sufficient to create the error
required within the positioner to cause it to provide maximum load in the seating
direction.
Table 12-5. Calibration for Seat Load.
CALIBRATION WITHOUT POSITIONERS
The problem of sufficient seat loading is particularly acute in throttling control
valves that dont use a positioner. In these cases, the signal pressure to the actuator
is also the operating pressure for the actuator. Even with the older standard signal
ranges of 3 to 27 or 6 to 30 psi (0.02 to 0.19 or 0.04 to 0.21 MPa), resulting seat
loads are considerably less than those available using a positioner, where up to
full-supply pressure can be utilized to provide actuator thrust.
Valve throttling applications without positioner control utilize a spring to
oppose the operating pressure. They are generally of the spring and diaphragmstyle because of the low pressures involved and have large diaphragm areas
relative to piston actuators.
The user has very little flexibility in altering the calibration built into the
factory selection of the actuator and spring. The spring force can be adjusted to
increase the seating force, but if changed too much the valve may not fully open.
Also, adjusting the valve when the valve is not pressurized may result in incorrect
loads. This is because each valve design has its unique hydraulic and frictional
forces upon which the actuator was selected and of which the user may not be
aware.
The calibration of the actuator takes into consideration two factors. The first is
the actual signal pressure range. For example, if the designer counted on a 3-to-15
psi (0.02 to 0.10 MPa) signal to the actuator being able to actually range from 0 to18 psi (0 to 0.12 MPa), the additional 3 psi (0.02 MPa) at the closed end of the
stroke was assumed to provide seat load. If the valve is then calibrated in service
to have a minimum of 3 psi (0.02 MPa) and a maximum of 15 psi (0.10 MPa)
applied to the diaphragm, seat load will be reduced by 3 psi (0.02 MPa) times the
diaphragm area, which is commonly several hundred pounds (kilograms).
Reference 12 addresses this situation in the form of a standard that calls for an
extended signal range.
The second consideration in calibrating the actuator are the forces that the
actuator will have to work with and against. These forces are hydraulic unbalance,
Positioner SignalRange
Valve Action onIncreasing Signal
Positioner Signal when ClosureMember Just Contacts Seat
3.0 to 15.0 psi(0.02 to 0.1 MPa)
Opens >3.1 but 0.021 but 14.4 but 0.100 but 4.2 but 19.2 but
7/29/2019 Aplicaciones Valvulas de Control
38/45
Control Valve Applications
32
friction, and seat load requirements. For example, if an actuator is intended by the
user to be operated with a true 3-to-15 psi (0.02 to 0.10 MPa) signal, with no
under- or over-ranging, the designer will have to build these loads, including seat
load, into the actuator spring force or diaphragm force. This is commonly
accomplished by selecting hardware that satisfies a bench set pressure range
that may be quite different from the operating signal range of 3 to 15 psi (0.02 to
0.10 MPa). This bench set pressure range is intended to allow the actuator tocompensate for the forces just noted when the valve is in service so that it will
operate properly (e.g., provide the design seat load) within a 3-to-15 psi (0.02 to
0.10 MPa) signal. To compensate for the requirement that there be an infinite
number of springs to fit individual situations, spring adjusting screws are
normally incorporated into the actuator design to achieve accurate spring forces.
Thus, because of the relatively small available forces, when the technician sizes
and selects actuators for throttling applications without a positioner it is critical
that the technician calibrating the valve in the field know the operating pressure
assumptions that were used to select the actuator and to calibrate the actuator
accordingly. It should also be apparent that if the valve is moved to another
application or fluid pressure conditions change in the existing installation, the
technician will probably require new springs and a new calibration, or bench
set, to assure the highest seating load.
MORE CALIBRATION COMMENTS
The calibration methods described in the preceding section provide excellent
performance in almost every control system. In a control loop, the signal to the
valve is created from a measurement in the process. The measurement is
compared against a desired value, and a correction signal is then fed back to the
valve. The valve responds to this signal to correct by changing position. Thus, in
this basic function of the control valve there is no need for the travel stroke to
match the input signal. So the calibration described in the last section will assure
the design seat load when the valve is closed.
The calibration described in the last section will provide a sufficient indication
of valve position using the input signal to the valve. The position indication
seldom must be highly accurate because it only provides relative feedback on the
operation of the valve. If highly accurate position indication is required, a separate
position transmitter can be added to provide this information.
In the event that the calibration methods described in the last section are not
acceptable, there are electrical and pneumatic devices that can then be added to
the control schematic to cause a fully loaded seat interface when the signal
reaches the closed position. These devices could add to the initial cost of the
control valve, especially for small valves. However, this cost is much less than the
maintenance cost associated with a valve that seals poorly. An example would be
for a 3-to-15 psi (0.02 to 0.10 MPa) pneumatic signal, increasing signal to close,to add a high-gain relay so that at 15 psi (0.10 MPa) the power to the actuator
would be the maximum supply pressure. But even in this case it would be
advisable to set the relay at some signal level below 15 psi (0.10 MPa) to allow a
margin for calibration or signal error. The control schematic for this example is
shown in Figure 12-18 in which the full seating force is applied at 13.8 psi (0.095
MPa), 1.2 psi (0.008 MPa) below the full closed signal. Some manufacturers
current-to-pneumatic (I/P) converters have a snap-shut signal wherein the 3-to-15
psi (0.02 to 0.10 MPa) output signal drops to zero as the 4-to-20 mA command
drops below 4.2 mA.
7/29/2019 Aplicaciones Valvulas de Control
39/45
Frequent Application Problems
33
Figure 12-18. Control Schematic to Modulate between 10% and 100% Open.
The schematic shown in Figure 12-18 is for a 3-to-15 psi (0.02-0.10 MPa)
modulation signal with increasing signal to close the valve, using a double-acting
piston-type actuator. The actuator is fitted with a spring for fail-open on loss of thesignal. A regulator between the supply and snap-acting relay is set at 20 psi (0.138
MPa). The snap-acting relay is set at 13.8 psi (0.095 MPa), which represents the
signal for the 10% open position. When the signal is modulating between 3 and
13.8 psi (0.02 and 0.095 MPa), the high selector and snap-acting relays are in the
positions shown. Thus, the positioner receives the modulating signal and moves
the valve actuator in accordance with the demanded position. When the signal
exceeds 13.8 psi (0.095 MPa), the snap-acting relay changes position from
venting to straight-through. The 20 psi (0.138 MPa) signal from the regulator then
changes the high selector relay position and feeds this pressure to the positioner.
The 20 psi (0.138 MPa) signal into the positioner causes the valve to go fully
closed with the pressure above the piston at the supply pressure and the pressure
below the piston at atmospheric. Thus, the maximum actuator force is directed to
sealing the valve closure member.
Other schematics can be created to assure full seating load for decreasing signal
to close and for single-acting actuators.
Operating Close to the Seat
An application problem that is frequently encountered involves the extended
duration of operating the valve with its closure member close to the valve seat
(approximately 5% or less). Such operation causes seat deterioration. Under this
low-flow condition, the flow path is the small opening between the closure
7/29/2019 Aplicaciones Valvulas de Control
40/45
Control Valve Applications
34
member and seating surfaces. The fluid velocities can be extreme, resulting in
erosion, noise, vibration, and, for liquids, cavitation damage. In some cases,
erosion is so severe that the damage to the valve at low lift increases flow area, and
the valve cannot control a needed minimum flow. To eliminate this problem, the
minimum controllable flow must be determined from the process data and valve
designs selected that have the needed rangeability for the application.
The control schematic shown in Figure 12-18 can also be used to avoidoperation with the closure member close to the seat. When the valve stroke
reaches a preset position, the snap-acting relay changes positioner signal causingthe valve to fully close and apply the full seating force.
Poor Assembly
Another contributor to valve problems is poor assembly in the field. It is critical
that the manufacturers assembly instructions be followed to assure that trim parts
are aligned well and, as discussed earlier, calibrated properly. The mechanical
assembly of a valve may seem quite straightforward; however, the assembler may
not be aware of the special design features included to assure a good functional
control valve and its dependence upon proper assembly procedures.
The issue of the alignment of parts is particularly critical in many applications.If alignment is not maintained within tolerances then the seating line or contact
area between the closure member and the seat ring may not be as anticipated.
Significant leakage will occur with the long-term erosion of these sealing
surfaces. The erosion of parts also reduces the valves ability to provide good
control.
Another adverse effect of poor alignment is that the stem may be scratched and
galled because of the close tolerances within the spacers and guides used in the
packing box. If the stem becomes scored then the packing will have a dramatically
reduced life as packing material is removed from the packing box during
modulation. In general, the higher the fluid pressures, the tighter the spacer
tolerances. The tight tolerances are needed to minimize the extrusion of the
packing material from the box.Poor alignment results in high friction for the moving parts. This is not always
apparent at the time of assembly. Friction can become a major problem during theoperation of the valve as the fluid system reacts to the excessive movement of the
valve disk or plug in response to the higher friction. This excessive movement will
cause the valve to continually search for the position that corresponds to the
system control point.
Poor alignment generally results from improper torquing of the packing box
and the valve bonnet bolting. The latter is the body/bonnet closure joint that seals
the internal valve pressure from the ambient. Both of these bolted joints must be
tightened by bringing the mating components together while maintaining them as
close to their final and fitted relationship as possible. While parts will have tight-
fitting location registers, improper torquing will push the mating parts to theextreme of the tolerances, thus reducing design margins. Correct tightening of the
bolts is achieved by using a crisscross tightening pattern and working up to the
final torque value in small steps. If too large a torquing step is used, the parts
become skewed and remain so through the final torque sequence. The skewed
posture loads the internal mating parts non-uniformly and causes a misalignment.
Tightening the packing box is a critical operation in the assembly of most
valves. Regardless of the packing design supplied by the valve manufacturer, the
instructions should be rigidly followed to assure alignment and to avoid excessive
packing friction. Tightening is most critical in the high-temperature graphite
7/29/2019 Aplicaciones Valvulas de Control
41/45
Frequent Application Problems
35
packing designs. This procedure generally involves a sequence of torquing levels,
with the valve stem moved three or more times at each torque level until the
maximum sealability torque is achieved. The torque is then backed off to a
maximum operability torque level for optimum control. The torque levels
should then be verified periodically during the start-up and operation of the valve
to assure the optimum control performance. The maximum sealability and
operability levels will vary depending upon the pressure rating of the valves.
To assure that the finaltorque level results in aproperly bolted joint allmating parts includingthe studs and nuts,must be well lubricated.It is not uncommon tolose between 20 and40% of the torque valueto friction betweenthese parts.
Do not reuse old seals and gaskets when reassembling a control valve. The
seals and associated springs will take a permanent set in the application, whether
elastomeric or graphite. They may not have the strength to initiate a proper seal on
reuse. Elastomeric materials may be near the end of their lives and fail because of
brittleness or poor ductility. Small nicks in the seals may also contribute to leak
initiation and the subsequent erosion of the seal in the application. The
replacement seals should be of the original design because special compositions
or winding processes may have been specified by the manufacturer to assure
proper function and long life for the specific application. The small cost of using a
new and correct set of these soft goods is minor in comparison to the cost of
excessive leakage or poor control function. In some cases, the reuse of seals and
gaskets can even result in a shutdown of the process.
Depending upon the design, there are times when it is necessary to connect the
actuation system to the valve in a way that assures that an actuator stop does not
limit the seating load. In some cases, this could even hold the closure member off
of the sealing surface. The assembler must make sure that the seating load is not
limited. The manufacturer must provide instructions that will assure that this
attachment process can be done correctly.
Following the proper assembly procedure assures that the design features of the
valve are not compromised or negated. Thus, the reliability and performance of
the valve will fulfill expectations.
Actuator Supports
Occasionally it is necessary to provide added support for a valve actuator. Thismay be the case if the actuator is quite heavy, such as for many electrical and
electro-hydraulic designs, or when the valve is installed in a position in which theactuator is off the vertical centerline. In these cases, it is very critical that the
support be flexible enough to allow for the thermal growth of the piping system
and that the installation does not impose large lateral forces. Such forces could
cause higher friction on the valves moving parts.
Trying to stop pipelinevibration by holding theactuator is analogousto trying to stop a trainwith your car.
The ideal support system is simply a pulley with a dead-weight
counterbalancing the weight of the actuator, as shown in Figure 12-19. Other
support systems require enough design effort to be assured that the actuator is not
constrained or deflections imposed that add undesirable side loads on the actuator.
One criterion that should never be used when designing a support system is to
attempt to limit vibration and in particular to limit a pipeline vibration problem.
The former almost always results in the fatigue failure of a restrained component
and the latter will result in short-term destruction of the valve-actuator assembly.
The vibration issue must be resolved by using damping devices mounted directly
on the pipe or by eliminating the root cause of the vibration.
7/29/2019 Aplicaciones Valvulas de Control
42/45
Control Valve Applications
36
Figure 12-19. Supporting a Heavy Actuator.
System Debris and Flushing
During new construction or the major repair of existing equipment there is
always debris generated by the processes of cutting, machining, grinding,
welding, erection, and installation. This occurs in spite of efforts to make sure the
fluid side of the system is clear of all trash. Thus, it is critical that the system beflushed and the internals of valves and other critical equipment be protected by
removing debris or by providing upstream strainers during initial operation to
minimize damage caused by the debris. The effort to assure that a system is
cleaned by flushing it requires significant planning; however, the investment pays
itself back many times by extending the life of the many components in the
system. To assist in this planning process, the reader is referred to References 13
through 16. A measure of the significance of this issue is provided in Reference
15, which noted that nine tons of iron oxide were removed from the power plant
piping. The Installation and Location section of Chapter 18 ofControl Valves
includes other examples of debris problems.The influence of debris in a system can have a very detrimental impact on the
control valves. In the references cited in this chapter it is recommended that thetrim be removed before the flushing operations and that some sacrificial trim be
subs