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Most control valve manufacturers offer control valve sizing software which includes model specific Cv data and relevant sizing constants for their currently offered valve products. Some manufacturers’ software tools also include actuator sizing, specification sheet generation, physical constants for common fluids, prediction curves for installed conditions as well as noise or cavitation phenomenon. Some sizing software also features analysis with the use of upstream or downstream resistors (pressure differential producers or restrictors) to take up some of the available pressure drop that would otherwise occur at the control valve alone.1
One instrumentation sizing software suite includes control valve sizing data for multiple control valve manufacturers, allowing comparison between different valve models or sizes, as well as options for sizing analysis, which may include the use of multiple pressure reducing resistors in conjunction with the control valve.2 Regardless of complexity or capability, all of these tools require accurate process data in order to produce appropriate results.
Determining accurate process information for control valve sizing can be a common challenge for instrumentation engineers and maintenance personnel specifying control valves and/or engaging in redesign of applications.
Modern sizing tools are typically very intuitive to use so that even someone inexperienced in the calculations can be easily trained in how to input the correct data and generate results. However, regardless of the utility of the chosen software, input data for sizing can sometimes be questionable or incomplete. A review of historical client requests reveals some common concerns:
- The pressure (P2) just downstream of the control valve isn’t provided at all. Many times, there is misunderstanding of how to calculate or even estimate P2 in a piping system.
- A static pressure condition is given for the control valve inlet pressure (P1) without considerations for elevation head (liquids) or friction losses upstream of the control valve to arrive at a reasonable figure for P1.
- Both the provided inlet and outlet pressure values are suspect. This occurs more frequently in vacuum applications.
- It is unclear whether pressure values provided are in vacuum or absolute engineering units.
- Gas flow rates are given in actual volumetric engineering units without reference to specific flowing pressure and temperature values or standard conditions.
- One flow condition is given without an understanding of whether a normal operating point or maximum. Sizing with a maximum and minimum set of operating conditions is recommended.
- The inlet and outlet pressure conditions don’t change even at multiple flow rates. In liquid transfer systems utilizing a centrifugal pump and/or in a system with long piping runs with higher than recommended fluid velocities, this can especially be of concern.
- Physical constants such as density, specific heat ratio, viscosity and vapor pressure for uncommon process media are not provided, which can lead to the use of default assumptions, affecting sizing results.
- There may be a lack of understanding of how precise engineering data needs to be in order to arrive at a reasonable sizing/selection evaluation. Too little care can lead to errors, but high levels of precision may be unnecessarily cumbersome to determine, leading to wasted time and effort.
The reasons for these shortcomings in given process information is that, in many cases, those requesting sizing assistance are not informed as to what is required and/or how to obtain the needed information. Some might suggest that an instrument engineer should work with a competent process engineer to arrive at reasonable control valve sizing information for inquiry submittals, even performing their own sizing calculations and subsequently asking the vendor/supplier for assistance only with model selection or final checks of vendor specific data.
Additionally, there are many reputable software packages which engineers can utilize to perform detailed flow engineering calculations from a broader system perspective. Such software accounts for piping friction losses, pump curves and implications for control valve sizing at various flow cases.
Even so, process industry plants may lack the staff to perform this analysis or have funding constraints for such engineering services. Plant operations, project engineering or maintenance personnel, working in complement with a knowledgeable technology or sales consultant, may be able to recognize when given process details for sizing appear questionable and then assist in estimation of reasonable sizing input data. Some real-world examples are detailed below.
ABC Refinery asks for control valve sizing assistance and provides the following process data:
- 6” SCH10S piping on inlet and outlet of control valve
- Inlet Pressure (P1) = 20.5 iwc vacuum
- Outlet Pressure (P2) = 12.0 iwc vacuum
- Max Flow Rate = 40,000 scfh
- Process Media is air @ 120F
- Gas specific gravity = 1
- Ratio of Specific Heats = 1.4
Implications: The error here is related to the fact that pressure on the inlet and outlet of the control valve is expressed in inches of water vacuum. It is important to remember that pressures expressed in this fashion use atmospheric pressure as a starting point or reference, whereas absolute pressure measurements are referenced to absolute zero or complete vacuum. The given engineering units are stated as “iwc vacuum” so the pressure value is referenced to 0 gauge or atmospheric pressure. In and of itself, this isn’t a concern. However, the pressures are expressed such that the inlet pressure is lower than the outlet pressure, which means the flow would be backwards through the control valve. The “backwards flow” is more evident if one changes the given inlet and outlet pressures to absolute units (using 407 iwca as atmospheric pressure at sea level). Converting, P1 would equal 386.7 iwca and P2 would equal 395 iwca. This could be a simple transpose error of given inlet and outlet pressures but could also signal a greater problem with care for accuracy in the submitted data or a misunderstanding of how pressures can be expressed in the vacuum range.
ACME Paper asked for help with an existing temperature control valve that is failing prematurely on a desuperheating system. See the process diagram below for reference. The control valve (TCV-1) throttles boiler feedwater to a desuperheating spray nozzle inserted into a steam line. Put simply, as the temperature rises the controller (TIC-1) signals TCV-1 to open some, increasing the addition of atomized spray water through the nozzle to reduce the steam temperature which is measured at TE/TT-1, downstream of the nozzle. As the temperature falls below the target set point, the controller signals the valve to move more closed to bring the temperature back up to set point. Mill maintenance personnel note the globe control valve shows severe trim erosion and sometimes even washing of the body within 10 weeks of service. Plant personnel have inquired about a replacement control valve with erosion resistant materials. Other system components with similar materials of construction and subject to the same process media, pressure and temperature have acceptable service life.
Solution: Before proceeding with re-design of the materials of construction, the throttling conditions should be evaluated. Given this, one might be quick to request sizing data at various flow conditions. This can be difficult information to obtain. Pressure indicating instruments may not be present or practical to install, and flow measurement for the desuperheating water is not typically available in such installations. In this actual case, a trend of the steam flow was shown as holding relatively steady and the feedwater supply pressure appeared to be without appreciable variation. Thermodynamic calculations could be used to find the required desuperheating water demand at various steam flow rates for the temperature set point.
However, a more obvious evaluation path is available. The controller (TIC) output, or implied valve position, can be trended to observe at what percent open TCV-1 commonly operates, especially after the control valve is replaced. Given the steady steam flow rate and feedwater supply pressure to TCV-1, the desuperheating water demand shouldn’t fluctuate widely and consequently, there should be little need for regular, major travel changes at TCV-1. If the valve positioner is operating correctly, then the percent controller output should be approximate to the percent valve opening. A review of this trend revealed that the controller output was running from 6 to 12% open right after replacement and from 0 to 7% open before a then degraded control valve is changed. This low operating travel indicates the valve is oversized with subsequent high velocity flow impingement – likely causing the erosion.
Rather than collecting detailed sizing data, one could simply evaluate the Cv curve or tabular values for the existing control valve. Since the maximum implied valve position is 12% right after replacement, the Cv of the installed control valve can be found by researching the Cv table provided by the manufacturer for this specific valve/trim size and characteristic at this percent opening. Using that Cv value, one could evaluate the Cv tables for a trim reduction change or replacement control valve that has the same Cv value but at around 75% valve opening.
Certainly, there could be other process issues or even problems with the control valve application such as cavitation or even flashing. However, it is fairly typical to find an unlikely maximum flow case used for system sizing. In other words, TCV-1 was sized considering a maximum steam flow case and therefore, commensurate maximum desuperheating water demand, with little consideration for the normal or minimal operating cases. Engineers may err toward over-sizing rather than under-sizing process equipment. A more detailed analysis of this particular problem would obviously include existing sizing conditions at both minimal and normal operating conditions. In some cases, a solution can be parallel staged or split ranged control with one desuperheating water valve handling the normal operating conditions and another valve that opens to provide additional water flow when normal conditions are exceeded. Further details for such a solution are beyond the scope of this document.
Consider the simple system shown in the process diagram below:
A recirculation line with segmented ball control valve is shown off the discharge of a centrifugal pump in a dilution water application. This is a common arrangement, sometimes utilizing discharge pressure measurement in order to keep the pump in an acceptable operating range and to ultimately prevent dead-heading if the downstream flow demand falls to an unacceptable level.
Due to the relatively high pressure drop, it is also common for the recirculation control valve to cavitate.
A client asked for assistance in resolving the short service life of the cavitating control valve in this application.
An existing system is being evaluated in this example. Had this been an engineering design evaluation ahead of actual construction, a detailed hydraulic analysis of the system would be in order, where pump and control valve are concurrently specified. Our purpose here is to demonstrate how minimal data gathered in the field can be used to provide a reasonable estimate of control valve sizing data, ultimately leading to a solution.
Observations: Audible indication of cavitation at the control valve is evident when near the system (sounds like gravel being pumped instead of water). A field survey of the system reveals that the recirculation line is discharging into the top of the atmospheric tank. Therefore, the pressure at the recirculation discharge is known to be 0 psig. The pressure transmitter on the discharge of the pump shows the pressure varying from 66 to 70 psig. The lower pressure at the higher flow rate is expected due to the pump curve even though downstream friction losses will add more back pressure at higher flow rates. The controller output is trended to show a properly calibrated control valve to be operating between 40% and 70% opening and this is confirmed by visual observance of valve travel. There is reason to believe that standard engineering practice was used to size the piping for flow velocities between 3 to 10 ft/sec. There is no flow instrumentation on the recirculation or downstream line but given the expected design velocity range and evaluation of the pump curve data, the min and max flow rate shown is a good estimate.
Estimating P2 (control valve outlet pressure): Since the pressure of 0 psig is known at the discharge into the tank, the P2 pressure at the outlet of the control valve can be closely estimated by working backward from this known pressure. The difference in the known pressure and the control valve outlet pressure is the elevation head difference plus friction losses in the pipe. The elevation difference is approximately 10 ft so there is at least (10 ft of water / 2.31 = 4.3 psig) of pressure due to elevation head alone assuming a full pipe and some siphoning effect.
Using Crane Technical Paper No. 410, page B-11, one can read a pressure loss of 2.2 psid at the 900 gpm flow rate but for 100 feet of 6” SCH40 pipe. The pressure loss is only 0.233 psid at the low flow condition of 300 gpm. There is 14ft of pipe on the outlet of the control valve plus one 90-degree EL. (2.2 psid per 100 ft)(14 ft)= 0.31 psid at the high flow condition and negligible loss at the low flow condition. A worst-case pressure loss for the elbow, again using calculation methods shown in Crane 410, reveals a 0.65 ft or 0.28 psi loss at the high flow condition.
Therefore, the P2 is estimated as follows:
P2 = 0 psig @ discharge into tank – 4.3 psi elevation head + 0.31 psi pipe friction loss + 0.28 psi loss in the elbow = -3.7 psig
Note: If the flow was actually known to be 900 gpm at the maximum observed control valve opening, control valve sizing calculations using the Cv at this opening could be used to back calculate P1.
However, P1 can be estimated by starting with the known pressure of 66 psig near the pump discharge (high flow case) and then subtracting head losses due to elevation and friction loss.3 Thirty feet of elevation head (ambient water) is 13 psi. The friction related pressure loss for 34 ft of piping from the pump discharge to the control valve inlet is obtained from Crane TP-410: (2.2 psid per 100 ft)(34ft) = 0.75 psi. The friction loss through the converging pipe tee if all flow is recirculated could be as high as 2.8 ft or 1.2 psi. Therefore, in the high flow case the control valve inlet pressure is estimated as follows:
P1 = 66 psig – 13 psig – 0.75 – 1.2 = 51 psig
Armed with the inlet and outlet pressures and estimated flow rate, control valve sizing software can be used to determine whether the observed percent opening of the existing valve is near what is expected and whether cavitation phenomenon is predicted.
A solution would typically involve the use of a control valve with cavitation attenuating trim, such as Neles Q-RE or similar, or even adding a restriction or flow resistor just downstream of the control valve thereby increasing the pressure at the control valve outlet and reducing control valve pressure drop. This could be an orifice plate, baffle plate or even a manually adjustable V-port valve. In some cases, both flow division and pressure drop staging with the Q-RE trim and a downstream restrictor may be required to reduce cavitation to an acceptable level.
1) When evaluating relatively short piping runs which are sized for generally recommended liquid velocity ranges, friction losses due to piping, fully open block valves and fittings may be inconsequential for control valve sizing estimates, as 1-3 psi of inlet or outlet pressure difference may not appreciably affect control valve sizing results. This is especially the case for rotary control valves with higher rangeability. If minimal upstream and downstream pressure differences do impact results in a “what-if” analysis, a more detailed hydraulic assessment may be justified.
2) Elevation changes typical in plant liquid piping systems can significantly affect inlet and outlet pressures at the control valve.
3) The pressure drop “caused” by the control valve isn’t considered in this analysis, since the final control element in the system will take up whatever pressure drop is available.