PCI20401

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Note: The source of the technical material in this volume is the Professional

Engineering Development Program (PEDP) of Engineering Services.

Warning: The material contained in this document was developed for Saudi

Aramco and is intended for the exclusive use of Saudi Aramco’s

employees. Any material contained in this document which is notalready in the public domain may not be copied, reproduced, sold, given,

or disclosed to third parties, or otherwise used in whole, or in part,

without the written permission of the Vice President, Engineering

Services, Saudi Aramco.

Chapter : Instrumentation For additional information on this subject, contact

File Reference: PCI20401 E. W. Reah on 875-0426

Engineering EncyclopediaSaudi Aramco DeskTop Standards

Severe Service Control

Valve Specification Issues

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Engineering Encyclopedia Instrumentation

Severe Service Control Valve Specification Issues

Saudi Aramco DeskTop Standards

CONTENTS PAGES

CONTROL VALVE SERVICE CATEGORIES .............................................................................1

Introduction .......................................................................................................................1

General Service..................................................................................................................1

General Service Applications ...............................................................................1

General Service Control Valves ...........................................................................1

Severe Service....................................................................................................................2

Severe Service Applications.................................................................................2

Severe Service Control Valves ............................................................................2

Super-Severe Service .........................................................................................................3

Super-Severe Service Applications ......................................................................3

Super-Severe Service Control Valves...................................................................3

Course Focus .....................................................................................................................4

SEVERE SERVICE BOUNDARY CRITERIA ..............................................................................5

Introduction .......................................................................................................................5

Boundary Criteria As “Flags” ............................................................................................5

Service Conditions ...............................................................................................5

Fluid Characteristics.............................................................................................6

Specific Applications ...........................................................................................7

Valve Constructions .............................................................................................8

THE IMPORTANCE OF PROPER CONTROL VALVE SPECIFICATION .................................9

Review Of The Saudi Aramco Selection And Procurement Process ..................................9

Specification.......................................................................................................10

Procurement .......................................................................................................10

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Saudi Aramco DeskTop Standards

Impact Of Errors In The Selection And Procurement Process .........................................11

Opportunities For Errors ....................................................................................11

Courses Of Action To Compensate For Selection Errors ...................................11

Preventing Valve Performance Problems ...........................................................13

Critical Steps In Valve Selection And Sizing...................................................................14

Introduction........................................................................................................14

Verification Of Process Data ..............................................................................15

Initial Valve Selection........................................................................................15

Initial Valve Sizing.............................................................................................15

Final Selection And Sizing.................................................................................15

Specification On The ISS ...................................................................................15

GLOSSARY..................................................................................................................................16

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Saudi Aramco DeskTop Standards 1

CONTROL VALVE SERVICE CATEGORIES

Introduction

The objective of differentiating between general, severe, and super severe services is to ensure that every valvethat is specified receives the appropriate amount of engineering attention. By providing engineering attention

that is commensurate with the severity of the application, the specifier may prevent operational problems and

costly valve and process failures.

General Service

General Service Applications

General services are those in which the fluids are reasonably clean, inlet pressures and pressure drops are low,

and operating temperatures are less than 400 degrees F. Refer to Figure 1.

General Service Control Valves

General service control valves are those with standard trim, standard materials of construction, and standard

ANSI class ratings below ANSI Class 600. General service valves are “off the shelf” assemblies; i.e., they are

listed as standard products in manufacturer’s catalogs. Refer to Figure 1. As shown in Figure 4, approximately

70 percent of the control valves in Saudi Aramco operations are classified as general service control valves.

General Service Applications General Service Control ValvesGeneral Service Conditions General Service Construction

Moderate Temperatures and Pressures

Clean Fluids

Moderate Pressure Drops

Standard Constructions

Standard Trim

Standard Materials

General Service Applications General Service ValvePlant Utility Applications

Fuel gas control

Low pressure water, air, and steam

Use Word 6.0c or later to

view Macintosh picture.

Typical General Service Valve

Figure 1

General Service Applications And General Service Control Valves

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Severe Service

Severe Service Applications

Severe service applications are those in which the fluid is corrosive, erosive, or at an elevated temperature. Inaddition, applications with pressure drops that are large enough to cause cavitation, flashing, and the generation

of unacceptable levels of noise are categorized as severe service applications.

Severe Service Control Valves

Severe service control valves include “standard options” for trim and materials of construction that have been

made available to ensure good valve performance and long service life in specific severe service applications.

The term “standard option” refers to an option that is readily available from a valve manufacturer without

special engineering. Refer to Figure 2. As shown in Figure 4, approximately 28 percent of all control valves in

Saudi Aramco operations are classified as severe service control valves.

Severe Service Applications Severe Service Control ValvesSevere Service Conditions Severe Service Construction

Corrosive fluids

Erosive fluids

High temperature

Flashing

Cavitating

Excessive noise

Special valve constructions

Special trim designs

Non-standard materials

Non-standard accessories

Typical Severe Applications Example Severe Service Valve

Separator dump

Water injection

Amine letdown

Boiler feedwater Glycol stripping

Vents

Wellhead choke

Pump recycle

Surge control



Typical Anti-Cavitation Valve Trim

Figure 2

Severe Service Applications And Severe Service Control Valves

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Super-Severe Service

Super-Severe Service Applications

Super-severe services are those in which the fluid characteristics are very demanding, the service conditions areextreme, or when multiple severe conditions are present. For example, if an application involves a fluid that is

highly corrosive, highly erosive, and cavitating, the application is super-severe.

Super-Severe Service Control Valves

Super-severe service control valves are custom constructions - often one-of-a-kind - for which the body design,

trim design, and/or construction materials have been custom-engineered by the valve manufacturer to satisfy the

requirements of a specific, highly demanding application. Refer to Figure 3. As shown in Figure 4, only 2

percent of all control valves in Saudi Aramco operations are classified as super-severe service control valves.

Super-Severe Service Applications Super-Severe Service Control ValvesSuper-Severe Service Conditions Super-Severe Valve Construction

Intense conditions

Combinations of severe conditions

(See Figure 2)

Unique designs

Considerable engineering input

Typical Super-Severe Service Applications Example Super-Severe Service Valve

Water Iijection (seawater/brine)

Wellhead chokeUse Word 6.0c or later to


Special Valve For Extreme Cavitation

Figure 3

Super-Severe Service Applications And Super-Severe Service Control Valves

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Figure 4

Percentage Of Control Valves In Each Service Category Within Saudi Aramco

Course Focus

This Course will focus on severe-service applications. The reasoning for this focus is explained as follows:1. Most super-severe applications provide highly visible clues that signal the need for careful engineering

attention; accordingly, super severe-applications do typically receive adequate engineering attention.

2. Most general service applications, by definition, do not present significant problems in valve selection

and sizing.

3. Most specification errors occur because of a failure to distinguish between general and severe service

applications.

Because of the common failure to distinguish between general and severe services, this Course will focus

severe services and related issues.

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SEVERE SERVICE BOUNDARY CRITERIA

Introduction

Because of the importance of identifying severe service applications during control valve selection, it is usefulto identify some of the criteria that define the boundaries between general and severe service applications.

Boundary Criteria As “Flags”

The boundary criteria can be viewed as "flags" to indicate that additional engineering attention and

documentation may be required during the valve specification process. The severe service flags that will be

discussed in this section fall into one of the following categories:

• Service Conditions

• Fluid Characteristics

• Specific Applications

• Valve Constructions

Service Conditions

Differential Pressure Limitations - Large pressure drops can signal the potential for flashing,

cavitation, excessive noise, and erosion from high velocity flows. One subjective method for

estimating the pressure drop at which valve problems may occur is expressed as follows:

Maximum P psidD

∆ ,,

≥1000

(where D = the nominal valve size in inches)

Fluid Temperature Above 400 Degrees F Or Below -20 Degrees F - This is a firm guideline that

signals the need for special materials of construction.

Services Where Cavitation Index Ksa Is > 90% Of Kc - This is a guideline that signals that the potential for cavitation damage is significant. The terms Ksa and Kc will be defined later in this

Course.

Services Where Cavitation Index Ksa > .80 - This is a guideline that signals a high potential for

cavitation.

Dry Gas And Superheated Steam At Velocities > .25 Mach - Fluid velocities in this range are often

associated with erosion.

Wet Vapor And Saturated Steam At Velocities >.15 Mach - As compared to dry gas and

superheated steam, wet vapors can cause erosion damage at much lower velocities. The increased

potential for damage is caused by the presence of vapor particles that can impinge on critical valve

components.

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P1/P2 > 5 For Dry Gas And Superheated Steam Services - P1/P2 ratios in this range can signal the

potential for high-velocity and erosion damage as well as high levels of aerodynamic noise.

P1/P2 > 3 For Wet Vapor, Saturated Steam, And Gas Or Vapor With Entrained Particulates -

Vapors, steam, and fluids with entrained particles are typically controlled at reduced velocities tominimize the potential for erosion damage.

Control Valve Inlet Velocities In Excess Of 0.3 Meters/Second - Valve inlet velocities in this

approximate range can lead to the generation of excessive noise and erosion damage.

Cavitating Fluids - Fluids that are known to be cavitating present the potential for rapid and total

valve failure of standard control valve constructions.

SPL Limitations (Noise) - Valve generated noise levels above 110 dBA will cause valve damage and

noise levels above 90 dBA will cause hearing loss in workers who are exposed to the noise for

extended periods.

Fluid Characteristics

Corrosive Fluids - Many fluids cause corrosion damage to “standard” control valve materials such as

316 and 416 stainless steels and WCB body castings. The need for special materials signals a severe

service consideration.

Erosive Fluids - Fluids with entrained particles, dirt, sand, or other debris always present the potential

for erosion damage to critical control valve components.

Flashing Services With Downstream Vapor Content > 5% By Weight - Liquids that vaporize as

they pass through the valve increase the potential for erosion damage because the vapor droplets can

impinge at high velocity on critical valve surfaces.

Two-Phase Flow Services With Downstream Vapor Content > 5% By Weight - The concern for

two phase (liquid/gas) flow is the erosion that can result from the high-velocity impingement of

particles on critical surfaces.

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Specific Applications

Valves Used For Anti-Surge Control - Control valves in compressor surge control applications are

required to respond very quickly to changes in the control signal, and to provide very short stop-to-

stop stroking times. Special instrumentation is often required to achieve the required performance.

Valves Used For Recycle - Control valves in pump recycle applications are required to respond

quickly to operating transients in a hard (liquid) system. Special instrumentation is often required to

achieve the required performance.

Well-Head Choke Valves - While special choke valves are typically selected for this application,

control valves are occasionally specified. High inlet pressures, high pressure drops, corrosion, erosion,

and combinations of cavitation, flashing, and outgassing combine to form a particularly challenging

application.

Water Injection Services - Water injection is a challenging application because large pressure drops

increase the potential for cavitation, sour water and/or brine solutions are highly corrosive, and

entrained sand and dirt can cause extreme erosion.

Boiler Feedwater Service - Boiler feedwater valves control water at high pressure drops and low flow

rates. Severe cavitation is nearly always expected. In addition, additives such as amines that are used to

treat the water can cause rapid corrosion of components unless the appropriate materials are selected.

Condensate Drain Service - Depending on the application, condensate drain valves may be subjected

to erosion (caused by entrained particles ranging from rocks and sand to vapor particles), corrosion (by

treated fluids or sour fluids), and flashing and cavitation (which result from large pressure drops).

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Valve Constructions

ANSI Class Over 600 - An application that requires a control valve body rating above ANSI 600 is

one which is operating at high pressure, high temperature, or both.

Body Size Over 16-Inch - Because the effects of fluid forces do not always scale in a linear fashion

from small to large valve sizes, large valves can present some unusual problems. For example, local

areas of high and low pressure around the valve plug can cause valve plug instability in large valves

that is not generally observed in smaller valves. Non-linear scaling effects have also been documented

for cavitating fluids.

Valve Body Materials Other Than Carbon Steel - A valve specification that includes a non-standard

body material can be interpreted as a signal that corrosion, erosion, flashing, cavitation, or high-

temperatures have been identified as a concern.

Special Actuator Requirements (Short Or Long Response Time) - Many applications have specific

requirements for control valve stroking times. The specified time may be short, as in recycle and surge

control applications, or it may be long, which is often the case when the objective is to prevent pipelinesurge and water hammer.

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THE IMPORTANCE OF PROPER CONTROL VALVE SPECIFICATION

Review Of The Saudi Aramco Selection And Procurement Process

Because most control valve problems can be traced to an error or omission in the selection and procurement process, it will be helpful to review the selection and procurement processes as implemented at Saudi Armaco.

Refer to Figure 5 and the discussion that follows.

Use Word 6.0c or later t


Figure 5

Overview Of The Saudi Aramco Valve Selection And Procurement Processes

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Specification

Acquisition Of Process Data - In order to select and size a control valve, the specifier must first obtain

all pertinent process data; i.e., the fluid characteristics, the fluid physical properties, and the service

conditions (operating pressures, pressure drops, flow rates, and temperatures).

Initial Valve Selection From Fluid Characteristics - The initial selection of a particular valve type

and various options is typically based on an analysis of the fluid characteristics; whether or not the

fluid is corrosive, erosive, or at high temperature.

Initial Valve Sizing - The initial sizing calculations are performed to determine the valve size and to

allow the specifier to ensure that the preferred valve style is available in the needed size. The

calculations should also reveal the presence of choked flow, flashing, cavitation, and whether or not

excessive levels of noise will be generated.

Final Selection And Sizing - After the specifier is satisfied that the initially selected valve type and

size are satisfactory for the application, minor corrections and modifications may be made to account

for valve availability, material requirements, installation details, actuator and accessory selection, andother specification issues.

Specification On The Saudi Aramco ISS - To develop a formal specification, the specifier finally

enters all specification data on the Saudi Aramco ISS (Instrument Specification Sheet).

Procurement

Request For Quote - The control valve specification is forwarded to the Saudi Aramco procurement

division where it is distributed to control valve vendors as a request for quote (RFQ). The result to

Saudi Armaco may be a low first cost; however, the installed cost (product life cost) may be very high

because of poor control valve performance and/or premature valve failure.

Technical Review - When the quotes are received from the valve vendors, they are reviewed to ensure

that the quoted control valves conform to the specifications on the ISS. Vendors who submit quotes

will be influenced by two competing objectives: (1) winning the bid award, and (2) providing the best

control valve for the application. Vendor’s commercial interests are very strong and it is not

uncommon for a vendor to quote a low-cost, general service valve into a severe service application for

the purpose of winning the bid award. During the technical review, it may be difficult to defend the

selection of a superior control valve if there are many bids for lower cost control valves.

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Commercial Review - Vendors and manufacturers must also pass a commercial review to ensure that

they are in compliance with Saudi Aramco standards that address manufacturing standards, quality

control, documentation requirements, and various other business issues.

Final Selection: Low Bid - The objective of Saudi Aramco’s procurement division is to acquire theneeded products and services at the lowest possible cost to the company. Accordingly, the contract or

purchase order is awarded to the vendor who has submitted the lowest bid.

Impact Of Errors In The Selection And Procurement Process

Opportunities For Errors

There are many steps in the selection and procurement process. As a result, there are many opportunities for

error.

Specification Errors - Errors can occur in the specification process as a result of failing to identify

severe service conditions, failing to select an appropriate valve, or failing to accurately document the

specification on the ISS.

Procurement Errors - Errors in the procurement process include failure to interpret the requirements

on the ISS and placing an undue emphasis on the criterion of low cost.

Courses Of Action To Compensate For Selection Errors

Any error in the selection and procurement process will become most visible in the performance of the valve

after it is installed. The valve may provide poor performance, it may provide a short service life, or it may

totally fail upon installation. As shown in Figure 6, there are several courses of action that can be taken when a

valve provides less than acceptable performance.

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Figure 6

Courses Of Action To Compensate For Selection Errors

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Enduring Poor Performance - The simplest course of action is to do nothing. This option is possible

only if the installed valve provides some measure of control. However, enduring poor performance can

have significant impact on the quality of process control, the quality of the product that is being

produced, downstream processes, and the overall economy and efficiency of the process.

Retrofit/Upgrade - Retrofits and upgrades such as replacing trim, replacing actuators, and other

“fixes” require additional expense, additional engineering resources, and often require costly process

downtime.

Scrap And Replace - If the installed valve proves to be entirely unsatisfactory, it must be replaced.

Replacement results in a duplication of all efforts in selection, procurement, and installation and typical

requires significant process downtime.

Preventing Valve Performance Problems

Because of the high costs of dealing with a control valve failure, it is clearly preferable to take the steps that are

necessary to ensure that an appropriate control valve will be selected. The specifier’s last opportunity to prevent

the selection of a less than adequate control valve is during the preparation of the control valve specification.

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Critical Steps In Valve Selection And Sizing

Introduction

To ensure that the valve specification is accurate and complete, additional engineering attention must be appliedat several critical points in the valve selection and sizing process. Refer to Figure 7 and the discussion that

begins on the following page.



Figure 7

Critical Steps In Control Valve Selection And Sizing

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Verification Of Process Data

Documentation Vs. Actual Operating Conditions - In many instances, the valve specifier is

presented with process data that describes the fluid, fluid properties, and service conditions. There are

many sources of such data; e.g., original piping diagrams and design documents. However, theavailable information may not accurately describe the as-operated conditions. For example,

undocumented changes in system parameters may have been made, upstream or downstream pressures

and flow rates may have changed due to a change in piping or pipeline equipment, or the system may

be operating at reduced capacity or increased capacity.

Information Sources - Accurate and up-to-date process and fluid data may be obtained from updated

process diagrams and/or by contacting operations personnel, maintenance personnel, SA process

personnel, or CSD consultants.

Initial Valve Selection

The best time to identify corrosive applications, erosive applications, or high temperature applications is during

the initial valve selection step. These severe service conditions can be identified from a description of the fluid,the fluid characteristics, and the service conditions.

Initial Valve Sizing

During this step, the specifier is examining the fluid properties and service conditions (flow, velocity, and

pressure conditions) for the purpose of sizing the valve. Because the severe service conditions of cavitation,

flashing, and excessive noise can also be determined from the fluid properties and the service conditions, this is

the appropriate time to apply additional engineering attention to determine if these severe service conditions are

present.

Final Selection And Sizing

During the final selection and sizing step, it is incumbent on the specifier to integrate all of the normal

requirements of capacity, flow characteristics, etc. plus the severe service requirements into the valve

specification.

Specification On The ISS

After a satisfactory valve construction has been identified, it must be accurately specified (clearly documented)

on the ISS. If the control valve specification is complete and precise, there is little opportunity for error in the

downstream processes; i.e., vendors will be forced to submit quotes for acceptable valves , and quotes for

unacceptable valves will be identified during the technical review.

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S di A D kT St d d 16

GLOSSARY

∆∆P The pressure drop across the valve; P 1-P2 in psid.

cavitation The potentially damaging formation and collapse of vapor bubbles that

occurs when the pressure at the valve vena contracta (Pvc) falls below the

fluid vapor pressure (Pv) and pressure P2 is greater than the fluid vapor

pressure.

corrosion The removal of material through various chemical attack mechanisms.

erosion The removal of material through various physical attack mechanisms.

flashing The flowstream phenomenon of fluid vaporization and increased fluid

velocity that occurs when the pressure at the vena contracta (Pvc) falls

below the fluid vapor pressure (Pv) and the downstream pressure (P2) is

greater than the fluid vapor pressure.

general service A service that is categorized by clean fluids, moderate pressures,moderate pressure drops, and fluid temperatures below 400 degrees F.

high-temperature

application

Any application that operates at a temperature that is greater than 400

degrees F.

ISS Instrument Specification Sheet; the Saudi Aramco engineering

document that is used to prepare a control valve specification.

K sa An index that describes, on a scale from 0 to 100, the relative potential

for cavitation in a given system; calculated as the ratio of the pressure

drop across the valve (P1-P2) to the value of P1-Pv.

severe service A service in which the fluid is corrosive, erosive, flashing, cavitating at

high temperature, and/or one at sufficient velocity to generate excessivenoise.

SPL Sound pressure level.

super-severe service A service that is characterized by extremely difficult fluids, extreme

pressures, pressure drops, or fluid temperature, or combinations of

severe conditions.

two-phase flow A flow that consists of fluids in two different states; e.g., a liquid and its

vapor or a liquid and a gas.

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