Sizing a Valve
Selecting the Right Control Valve
Sizing a control valve may seem like a no-brainer (how tough can it be?), but it’s actually quite the opposite. The selection process goes way beyond just looking at the size of the pipes. The gas or fluid (and its viscosity), pressure and pressure differential, as well as the flow rate and required flow characteristic all must be factored into the decision…and that’s just to determine the size.
The last thing anyone wants is for a valve to fail prematurely or unexpectedly. Some of the most common causes of failure are damage from:
One way to reduce the likelihood of failure is to be sure to select the right size, style or type, and body and trim material up front.
Consequences of Using the Wrong Size Control Valve
A valve that’s incorrectly sized can lead to problems:
- If it’s too small
- It cannot handle the flow that it needs to.
- If it’s too large
- It cannot be adjusted to the required flow rate with any relative precision. Changing the valve’s position just slightly will create big, non-proportional changes in the fluid’s flow…it becomes overly sensitive.
- The valve’s precision will be further reduced by any friction in the system (which will cause stickiness).
- To top it off, purchasing a larger valve will cost more than it needs to.
Tips for Determining the Right Size Valve
According to Control Valve Primer – A User’s Guide, some recommendations to help guide you to the right control valve are:
- An equal percentage valve works well for a system with a lot of pipe.
- Linear valves work better for systems with a small amount of pipe.
- Standard practice is to use a control valve that’s no larger than the line.
- Never install a valve on a pipe that’s more than twice the diameter of the valve.
- Globe valves are commonly one size smaller than the line. If you get a different result, you may wish to double check your information.
- You’ll achieve the best control with a valve that’s sized to operate at:
- 60 to 80 percent open at the maximum required flow and
- around 20 percent open at the minimum required flow.
Other Factors Affecting Valve Size Selection
Flashing and cavitation
Any time there is risk of flashing and cavitation it can and should be factored into the sizing calculations and selection criteria. Cavitation creates noise and vibration and can cause a lot of damage to the internal parts of a valve and the pipes downstream.
In rotary valves predicting the level of cavitation that will cause damage requires more than a simple calculation. Localized areas of pressure drop followed by recovery may cause cavitation way below the threshold that causes the flow to be fully choked.
With flashing and cavitation comes the added problem of choking. Flashing, or the formation of vapor bubbles just past the vena contracta, limits or chokes the flow through the valve. If the downstream pressure drops past the point of flashing, the flow will become choked.
The basic liquid valve sizing equation must be modified based on this pressure differential and using a valve recovery coefficient.
Separate calculations are needed to determine the pressure at which cavitation will occur and choked flow will begin.
When dealing with the added friction of a highly viscous liquid, the valve sizing coefficient needs to be increased. Viscosity is only an issue when it exceeds 40 centistokes or when the valves are to be very small with a valve sizing coefficient of less than 0.1.
When either of these situations exists, additional factors and calculations have to be figured into the equation.
High noise levels cause vibration which may damage pipes as well as other equipment. Noise levels are even more likely to exceed standards in steam and gas service (even when the pressure drop isn’t extreme), especially as the valve and pipe size increases.
Noise should be considered in the calculations for selecting and sizing control valves, particularly for oil and gas service.
The flow characteristic, or the relationship between the valve stem travel and the flow through the valve, typically should be linear (when plotted on a chart). The inherent flow characteristic (which comes from the manufacturer) usually changes once put into operation. Things such as drops in pressure choked flow (flashing and cavitation), and location of the valve within the system will have an impact on the flow characteristic.
The actual flow once installed, referred to as the installed flow characteristic, will be hard to control if it has strayed too far from a linear flow.
Computer Analysis to Determine Control Valve Size
All this being said, it is important and useful to understand the how and why of valve selection, but to fully cover the sizing of control valves requires a small textbook or at the least, a large user manual.
For those of us who love performing calculations, we can run through pages upon pages to work out the best solution.
For everyone else (and even for the former to verify our conclusions), a software program will do the heavy lifting for you and is recommended.
Avoid potentially costly mistakes by doing your homework (on a computer) the first time around.
Just in case you really wanted some math…
The following formula applies to about 80 percent of liquid sizing applications. Gas or steam valve sizing requires a different formula.
q = Cv √ΔP/Gf
Where: Cv = valve sizing coefficient
q = the flow rate in U.S. gallons per minute
ΔP = change in pressure – inlet minus outlet pressure in psi
Gf = specific gravity of a liquid at a given temperature
Remember: the calculation will need to be adjusted when more viscous fluids are processed or when cavitation or choking is expected.
Selecting the Type of Control Valve
There’s less calculating needed to determine the type of control valve to use, but the decision still requires some thought. The same criteria for valve selection that we’ve talked about in the past apply to control valves: the liquid or medium, pressure and pressure differential, operating temperature, environmental conditions, cavitation risk, and flow characteristic.
CPV Manufacturing’s control valves are available in a range of options for all of these criteria including temperature, pressure, and flow characteristic. O-SEAL®, G-Series®, Mark VIII®, and FloMaster® Valves are all available to fill your control valve requirements. The tapered plug in our needle valves provides a large flow area and superior flow control. Our leak-proof designs are trusted worldwide and always exceed quality standards.
Take the time, in the beginning, to ensure you install the right valve for the job. For help or questions regarding any of CPV’s products, contact us.