Establishing Proper Pressure Drop for Feedwater Flow Control Valves

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Establishing Proper Pressure Drop for Feedwater Flow Control Valves2/01/2014 | S. Zaheer Akhtar, PE

eedwater control valves play a critical role in boiler operation. One important parameter of their design is the pressure drop at the rated condition as well as off-design conditions.

owever, conventional methods used for establishing control valve pressure drop cannot be used at face value without reviewing all plant operating scenarios.

(http://cdn.powermag.com/wp-

ontent/uploads/2014/02/PWR_020114_IC_FWCV_splash.jpg)

n power plants with drum-type boilers and constant-speed main boiler feed pumps, the feedwater control valve (also referred to as the drum level

ontrol valve) provides the means for controlling flow to the boiler. On the other hand, in power plants equipped with variable-speed turbine-driven

main boiler feed pumps, the feedwater control valves are usually eliminated from the main circuit but may still be used on the startup circuit with thmaller motor-driven startup feed pump.

n either application, the feedwater control valve is in critical and severe service. As such, it must be sized and designed to provide adequate drum level control and cope

arying drum pressures expected over the range of plant operating conditions. In this regard, one of the important parameters to be evaluated is the control valve press

rop at the rated condition, as well as during off-design conditions.

he control valve pressure drop needs to be established carefully, as it is a performance debit resulting from increase in horsepower associated with the pressure head o

oiler feed pump. Use of variable-speed drives or turbine drives on the boiler feed pump can avoid this debit by eliminating the control valve altogether. Boilers designed

iding pressure operation generally utilize variable-speed drives or turbine drives not only to eliminate the control valve penalty but also to take advantage of minimized

erformance debits at part loads due to lower pump head. The part-load advantage is not available with fixed pressure operation, where boiler pressure remains consta

he pump pressure head must remain high, even at part loads.

his article highlights the various plant operating scenarios that must be considered while evaluating the control valve pressure drop. It also points to the fact that theonventional methods used in industry for establishing control valve pressure drop cannot be used in power plants without reviewing all plant operating scenarios.

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ote that the difference between the boiler feed pump head-flow curve and the system resistance curve (Figure 1) provides the basis for the pressure drop available for t

rum level control valve. During startup and low-load operation, when drum pressures are low, the valve may experience severe service due to high pressure drop. These

onditions could lead to valve cavitation and subsequent destruction of the valve trim along with pipe hammer, which could lead to piping and piping support damage. It

herefore, essential that the sizing and design of the drum level control valve be such that these problems are avoided. For this purpose, the entire range of service condi

hould be provided on the valve data sheet, as this will enable the valve supplier to make the correct valve/trim selection.

(http://cdn.powermag.com/wp-

ontent/uploads/2014/02/PWR_020114_IC_FWCV_figure1.jpg)

Establishing drum level control valve pressure drop. Courtesy: Bechtel Power Corp.

Conventional Methods for Establishing Control Valve Pressure Drop

n general industrial applications, control valve pressure drop has commonly been established by one of the three methods discussed below. However, note that these

methods may not be adequate for feedwater control valve applications, which require additional evaluation taking into consideration the high static pressure head involv

umping feedwater to the boiler.

raditional Method. This method traces back to the ISA Handbook of Control Valves by J.W. Hutchison, which provides guidance for control valves in a pumped circuit.

ccording to this method, the pressure drop should be 33% of the dynamic loss in the system at rated flow, or 15 psi, whichever is greater. In this context, the dynamic lo

he system is expected to include the pressure drop of the 100% open control valve. Other references allow the control valve pressure drop to be 25% to 50% of the dynam

oss in the system, exclusive of the control valve pressure drop.

onnell Method. In this method, the minimum pressure drop assigned to the control valve is based on pump discharge pressure, increased frictional pressure drop due

maximum flow rate, and base pressure drop to account for the fact that even in the wide open position the control valve generates some pressure drop. The Connell

orrelation is expressed in the equation below:

(http://cdn.powermag.com/wp-content/uploads/2014/02/PWR_020114_IC_FWCV_equation.jpg)

where:

P is differential pressure,

s is the pressure at the beginning of the system (pump discharge),

d is the design flow rate in the line,

n is the normal flow rate in the line,

is the differential pressure (dP) across the process equipment at normal flow,

is the dP across only the piping and valves at normal flow, and

is the base dP for the control valve.

http://cdn.powermag.com/wp-content/uploads/2014/02/PWR_020114_IC_FWCV_equation.jpghttp://cdn.powermag.com/wp-content/uploads/2014/02/PWR_020114_IC_FWCV_figure1.jpg


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Minimum Control Valve Pressure Drop Method for Pumped Application. In this method, attributed to F.C. Yu, the control valve in a pumped application is assigned a

minimum pressure drop at maximum design flow rate and maximum 80% control valve open position (assuming control valve regulating range is 20% to 80% valve open

osition). The control valve opening is then checked at normal flow to make sure that the opening is not below the minimum 20% open limit. The minimum pressure dro

o 15 psi greater than the pressure drop at valve full-open conditions.

Method Application Examples

elow is the computation used when applying the various methods for determining the assigned control valve pressure drop for the drum level control valve. Note that t

alues are not exactly comparative because each method uses parameters that are not common to all three methods.

raditional Method:

Dynamic losses = 75 psi at normal flow

Control valve dP = 33% of dynamic losses, including control valve dP

Therefore, dP / (75 + dP) = 0.33, where dP = 38 psi

onnell Method:

Pump discharge pressure = 3,000 psi.

Base dP for control valve = 11 psi.

Dynamic losses at design flow = 1.2 times normal flow.

Control valve dP = (0.05 x 3,000) + 1.1 x [(1.2)2 – 1] x 75 +11 = 197 psi.

Minimum Control Valve Pressure Drop Method for Pumped Application:

Obtain valve characteristic curve showing the coefficient of flow (Cv) versus % valve opening and check Cv at 80% open (for example: Cv = 150 at

80% open).

Using maximum design flowrate (Q) of 2,300 gallons per minute (gpm) and Cv value at 80% open, calculate the control valve dP as follows: Cont

valve dP = (Q / Cv) 2 x specific gravity = (2,300 / 150) 2 x 0.9 = 212 psi. Ideally, this is the pressure difference, which should be available between t

pump’s head-flow curve and the system resistance curve (exclusive of the control valve pressure drop) at the maximum design flowrate of 2,30

gpm.

Now, using the same valve characteristic curve, select the Cv at minimum controllable 20% open (for example: Cv = 45 at 20% open).

Assume that the minimum operational flowrate (for example, 800 gpm) will be handled by the valve at minimum 20% open position, calculate th

valve dP as follows: Control valve dP = (Q / Cv) 2 x specific gravity = (800 / 45) 2 x 0.9 = 284 psi. Ideally, this is the pressure difference, which shou

be available between the pump’s head-flow curve and the system resistance curve (exclusive of the control valve pressure drop) at the minimum

operational flowrate of 800 gpm.

ote that due to high static head of a boiler feed pump in a power plant application, the Traditional Method for establishing control valve pressure drop provides a low

ressure drop, which is inadequate for this application. The remaining two methods (Connell Method and Yu’s Method, or Minimum Control Valve Pressure Drop Method

umped Application) consider this high static head in the calculation and result in a more reasonable value. These methods can be used as the first iteration but should n

sed without additional checks. The additional checks should be done to ensure that the selected valve sizing (Cv and dP) is able to deal with all operating scenarios with

pening remaining within the acceptable controlling range of the valve. For this purpose, it is essential to recognize the various operating scenarios and functional

onsiderations at the power plant, which impact the control valve dP.

unctional Considerations for Drum Level Control Valves

he basic functional consideration for a drum level control valve serving a power boiler is that it should be capable of covering a wide range of operating parameters.

ormal Flow Versus Minimum/Maximum Flow. In a power boiler fitted with a constant speed motor-driven pump, the drum level control valve should be capable of

andling the required flow during normal plant load as well as that required at reduced load (including startup).

n addition, the drum level control valve should be capable of handling maximum flow requirements, which could arise due to a combination of feedwater flows in additio

he rated flow, such as boiler blowdown, sootblowing, or spray water usage.

startup or low-load operation results in too severe of a range for one valve, a second startup control valve may be required to split the service conditions.

bility to Restore Drum Level. The maximum flow requirement established should be utilized and checked against the flow requirement when trying to restore drum le

om low level to normal level. In case the drum level drops to around the low-level mark and the boiler is operating under full-load conditions, it may not be possible to

estore drum level without reducing load, unless the feedwater system has been designed for the extra flow capability.


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onsider a boiler drum with a 3-minute storage between the normal level and the low-low level in a plant that is operating at full load. If the drum level dropped to the low

evel due to transient conditions, in order to restore drum level in a short period of time (for example, 15 minutes) without decreasing load, the feedwater pump and the

ssociated control valve will be expected to handle 20% flow above normal load operation (since 3 minutes x 100% = 15 minutes x 20%). On the same basis, if the drum lev

o be restored over a longer period (for example, 30 minutes), then the feedwater pump and the associated control valve would need to handle only 10% flow above norm

oad operation. This additional surge capability is built into the control valve normally operating at 80% of full travel and the pump’s design point being above the 100%

perating point.

oiler Makeup During High Drum Pressure Condition (Pressure Safety Valve Discharging). The ASME Boiler & Pressure Vessel Code, section I, paragraph PG-61.1 req

hat the source of feeding shall be capable of supplying water to the boiler at a pressure of 3% higher than the highest setting of any safety valve. Under the conditions

mentioned in the code, the increase in drum pressure reduces the pressure drop available to the drum level control valve. This reduces the feed flow to the drum, and the

ontrol valve is required to open further to compensate, if possible.

the control valve is already fully open and unable to compensate for the flow reduction, then care should be taken that the reduced flow rate does not decrease to a po

where it falls below the pump’s minimum flow recirculation point; otherwise, all flow will be diverted to recirculation, and no flow will reach the boiler drum. Such a condi

unacceptable, as it would violate the ASME code requirement.

A Graphic Example

hese design conditions and the control valve pressure drop variation can be well represented on a graph similar to the generic one shown in Figure 1, which includes th

oiler feed pump (constant speed) head-flow curve, system resistance curve, and the control valve pressure drop (dPcv).

s flow increases, the available pressure drop across the control valve decreases. As a result, the control valve opening increases. The increase in control valve opening is

owever, restricted to around 80% to 85% (due to controllability considerations), and the corresponding Cv establishes the maximum flow capability of the control valve.

gure 1 also shows that as the pressure head increases due to high drum pressure (highest set pressure of pressure safety valve plus 3%), the system resistance line mopward and cuts back on the available pressure drop across the control valve. At this point, the control valve opening increases, reducing the valve dP to compensate for

ncrease in pressure head.

he result is that the operating point moves to the left side of the head-flow curve. At this point, it is important that the control valve opening remains within its operating

ange; otherwise, the drum level may not be controllable. It is also critical that this operating point falls on the right hand side of the minimum recirculation flow line;

therwise, the entire flow through the feedwater pump will go toward recirculation, and no flow will reach the drum.

pecific Design Considerations

ome of the specific operating cases and functionalities of the drum level control valve that also need to be considered are discussed below.

ombined Cycle (2 x 2 x 1) Plant Operating in Single Train (1 x 1 x 1). In the case of a 2 x 2 x 1 plant—assuming one high-pressure/intermediate-pressure (HP/IP) boile

eedwater pump per heat recovery steam generator (HRSG)—the HP pressure could operate at 135 bar. However, this HP pressure could decrease significantly to 85 baruring 1 x 1 x 1 operation (50% steam turbine load). Under these conditions, the pump continues to operate along the pump characteristic curve, but due to lower pressu

he drum, the control valve closes and has to take a high-pressure drop.

he control valve pressure drop could be as high as five to six times the normal pressure drop. At this point, the question to be addressed is whether the control valve rem

within the regulating range when it closes and experiences the high dP during 1 x 1 x 1 operating conditions. If at this high dP the control valve is out of regulating range,

different base dP for the control valve must be selected.

ombined Cycle Bypass Spray Operation. On a steam turbine trip, the HRSG steam is bypassed (usually 70% to 100%) and the bypass sprays are placed in service. The H

old reheat (CRH) bypass valve spray water is often taken from the IP section of the HP/IP pump. However, in some cases, the IP section spray water pressure may not be

nough, and the spray water is taken from the HP discharge section of the pump. In either case, the spray water requirements enhance the flow capacity of the HP or IP

ection of the pump, depending on the location of the takeoff spray water line.

onsider the case of added capacity when spray water is taken from the HP discharge section and assume that the HP section flow has to be upgraded by 20% to include

ypass spray water flow. This 20% additional capacity can be considered as spare during normal operation and can be utilized, if required, for drum level makeup (from lo

evel to normal level), for example, in a 15-minute time period.

herefore, the drum level control valve should be suitable for handling the 20% additional flow with a significantly reduced pressure drop as projected by the difference

etween the pump curve and the system resistance curve. In other words, under these conditions the control valve will open wide but must remain within the regulating

ange of the control valve.

ombined Cycle Part-Load Operation at 75% Load. In the case of part-load operation with combustion turbine generators at 75%, the HP feedwater flow can decrease

round 60% of normal flow while the system head could decrease by around 75% to 80% (due to lower HP steam outlet pressure). With a constant speed feed pump, the

eedwater flow along with the drop in system head requires the control valve to absorb the additional pressure drop and, as a result, the control valve tends to close. The

xtent of valve closure should be verified to ensure that the control valve remains within the controlling range under these conditions.


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Establishing Proper Pressure Drop for Feedwater Flow Control Valves