3 Steps to Size a Deep Well Pump Control Valve

3 Steps to size a Deep Well Pump Control Valve

Sizing a deep well pump control valve (DWCV) correctly is essential to ensuring smooth system operation and long-term reliability. This article is a follow-up to our Deep Well Pump Control Valves 101 article, where we covered how these valves function and their key components.

In this installment, we’ll dive into sizing principles, walk through a three-step methodology, and address common complicating factors that can arise in real-world applications. Always remember that every control valve application is unique, and the valve should be selected for the specific application.

1. Deep Well Pump Control Valve Sizing Principles

Sizing a DWCV can be a tricky application challenge. They serve an important function in a well pump station, but to do so they must be sized in coordination with the pump and other valves in the wellhouse. Below are two sizing principles and how they relate to the desired valve functionality.

Principle 1: Not too Small

The first function of a DWCV is to open before the pump turns on to exhaust initial flows to waste. To prevent that flow from going into the system a check valve downstream of the pump and DWCV needs to be held closed by system pressure. If the initial pumping flows generate too much pressure, the check valve may open sending sandy water and air into the system. Therefore, pumping pressure through the DWCV needs to be lower than system static pressure.

My general rule of thumb is the desired pumping pressure through the DWCV (and the entire waste line) should be approximately 70% of system static pressure. Cla-Val’s 61-02 engineering data sheet says at least 10 psi less. The principle is the same—we want a buffer to make sure system pressure holds the system check valve closed.

Principle 2: Not Too Big

The second function of a DWCV is to slowly open and close, which will rise and lower pumping pressure to open and close the system check valve. If the DWCV raises and lowers pressure too quickly, that can cause the check valve to open and close too quickly, leading to surges in the system. Speed controls on the DWCV allow for some control of the valve’s speed, but if the DWCV is too large by the time it starts to build pumping pressure it will be nearly closed and the valve stroke to valve flow coefficient (Cv) is very high. The goal is to have a smaller valve that generates high pumping pressure that opens the check valve in the middle of the valve’s stroke. At that point the valve percentage to Cv is lower and allows for much smoother transfer of flow. Therefore, we want to size the DWCV as small as possible for smoother control.

In addition, if the DWCV is too large, the pump to waste line may not generate enough back pressure for the pump to safely operate. The 70% static goal allows for smooth transfer and usually keeps the pump on its curve. If the pump needs more back pressure when pumping to waste, alternative solutions are needed. (We'll expand on that more in Complicating Factors.)

2. Three Steps to Size a Deep Well Pump Control Valve Just Right:

With the above principles in mind, below are three steps for sizing your DWCV:

Note: This basic methodology assumes the pump is single speed (Not VFD), there is a basic system check valve to the system (not a control valve) and that the system static pressures are not too high (below 80 psi).

Step One: Multiply the System Static Pressure by 0.7 to get desired DWCV line target backpressure (TargetBP)
Step Two: Select a DWCV with Full Open Head loss at the pump’s expected flowrate (Flow rate may vary based on backpressure) as close to the TargetBP as possible.
Step Three: Calculate the total head loss of the DWCV exhaust line. The total head loss should not exceed TargetBP (not by much at least). If it does, adjust to better match TargetBP.

Example: Well pump example station:

Pumping rate of 600 gpm
System Static Pressure of 50 psi
Waste piping out of building provides 5 psi of head loss at 600 gpm.

Step One: Take 50 psi of system static pressure times 0.7 to get 35 psi TargetBP.
Step Two: Using the 100-02 Hytrol valve head loss chart find 3” globe pattern 100-02 creates 28 psi of head loss at 600 gpm.
Step Three: Take 3” globe valve head loss of 28 psi with 5 psi of pipeline losses to find 33 psi of total back pressure, only 2 psi from target.

Solution: 3” Globe Pattern Valve

Note:

4” globe valve has 9 psi of head loss at 600 gpm, so Too Big
2 ½” gl has 50 psi of head loss at 600 gpm, so Too Small

3. Complicating Factors:

The above formula correctly sizes a DWCV for a simple well pumping station. However, below are a few complicating factors that can affect DWCV sizing:

When available sizes do not match the desired backpressure:
There are only so many options when sizing a DWCV. No one makes a 7” valve, or a 3.14159” sized valve, so sometimes the desired back pressure is in between sizes. In those cases, one option is to create more backpressure elsewhere in the exhaust piping. Smaller piping, a longer run, or a partially closed valve are all options for creating more backpressure when the DWCV options aren’t right.
When pumping and static pressures are too high:
Using a DWCV with a system check valve runs into physical limitations when pressures get too high. For example, if static pressure is 120 psi, we would want at least 84 psi of back pressure from the DWCV line. But that much back pressure is not feasible—either flow would become choked out, or that much back pressure would result in excessive velocities that could wear out the DWCV. For short periods of time, it’s acceptable for a DWCV to flow at high velocities (20-35 ft/sec), but when flows near up to 40-45 ft/sec excessive wear can negatively impact the life of the valve.

In the case of high pumping and static pressures, I usually recommend the use of a control valve to the system instead of simple check. That way the DWCV can be sized larger to keep velocities within reason, and the system control valve will control for smooth introduction of flows. In addition, sometimes a pressure sustaining feature is needed on the DWCV to provide adequate back pressure on the pump.
What about cavitation?
Cavitation is a natural phenomenon that occurs when a valve flows with high pressure differentials, especially when flowing to atmospheric discharge. Those conditions perfectly match the operating conditions of deep well pump control valves, so should we be concerned about cavitation? In short, not usually.

While deep well pump control valves often operate in cavitation damaging conditions, the valves are generally only operating for short periods of time—at most a couple times a day—and are predominately fully open. Cavitation damage may occur in the valve, but under those usage conditions it takes decades of service for cavitation damage to be a concern. Generally, in those conditions the cost of replacing the valve after thirty or forty years of service is more agreeable than trying to protect the valve from all cavitation damage.

It is possible that in severe applications cavitation damage could impact the service life of the valve. Particularly if the valve is operating frequently, flowing for long periods of time, has very high velocities (35 ft/sec+), or has a modulating pilot system. In those cases, steps should be taken to address or reduce the cavitation conditions.

Conclusion:

The steps outlined above are a good starting point for sizing a deep well pump control valve in standard applications. Always keep in mind that real-world systems often present unique challenges—such as high static pressures, non-standard valve sizes, or other complicating factors. Selecting the right solution requires careful consideration. If you have a specific application or need guidance in optimizing your system, we’re here to help—feel free to reach out to discuss your project.