Mind your size: What is valve sizing and why is it important?

By "Apollo" Valves
February 20, 2018

Control valves come in many different styles, shapes, and sizes. As we’ve written about before, choosing the right valve type for a specific application can be a daunting task for a person without experience in this field. Additionally, it can even be more difficult to find the right valve size for your application.

Valve sizing refers to selecting the correct valve sizes for specified flow applications. Valve sizing is absolutely critical in any system because it not only affects the amount of flow through a valve, but it also determines the flow coefficient and general performance of the valve.

General rule in sizing valves

The general rule of thumb when sizing valves is to make sure the selected valve operates anywhere between 60 percent and 80 percent open at the maximum required flow rate. The valve also should not operate below 20 percent open at the minimum required flow rate. This ensures that a wider range of the valve’s operation scope is utilized while sustaining a practical safety factor.

If your valve sizing is wrong, you will typically end up with an oversized or undersized valve, which will create problems.

Undersized valves

Undersized valves create inadequate flow. The system then is forced to compensate for this by raising the pump’s head pressure. As a result, there is a high differential pressure within the valve.

As the liquid leaves the valve, there will be a sudden drop in pressure, causing cavitation. Cavitation is responsible for high vibration and noise levels within valves, and it also can lead to the rapid destruction of the valve’s seals and seats. This significantly reduces the life of the valve. Furthermore, cavitation also can weaken your system’s piping and lead to leakages.

Oversized valves

Oversized valves may not cause as much physical damage to the system as undersized valves, but they are equally as problematic because of their controllability issues. Oversized valves tend to be very sensitive, meaning that any small changes in the valve’s position results in excessive changes in flow. This makes it quite difficult or even impossible to adjust the valve to the required flow.

As you can see, choosing the right valve size for your application is imperative. The perfect valve size will operate more quietly, control flow more accurately, prevent leakages, perform at optimal levels, save you money in the long term with minimal repairs, and definitely last longer. If you are not sure what size valve you need, always consult a professional valve engineer for help.

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