What are the most common methods for sizing control valves?

1 Basic principles

Before diving into the methods, you need to understand some basic principles of control valve sizing. The first one is the flow coefficient (Cv), which is a measure of how much fluid can pass through a valve at a given pressure drop. The higher the Cv, the larger the valve opening and the lower the resistance to flow. The second one is the valve authority (N), which is the ratio of the pressure drop across the valve to the pressure drop across the entire system. The higher the N, the more control the valve has over the flow. The third one is the valve characteristic, which is the relationship between the valve opening and the flow rate. The most common types are linear, equal percentage, and quick opening.

3 Simpler method

The simpler method for sizing control valves is based on the rule of thumb that the valve should have a Cv value equal to or greater than 25% of the system flow rate in gallons per minute (GPM). For example, if the system flow rate is 100 GPM, the valve should have a Cv of at least 25. This method involves determining the system flow rate in GPM and then multiplying it by 0.25 to get the minimum Cv value. After selecting a valve size that meets or exceeds this minimum Cv value, you should check the valve performance and adjust the size if necessary. This simpler method is easy and quick to use since it does not require any complex calculations or tables; however, it has some drawbacks, such as not considering pressure drop or valve authority which may affect control quality and efficiency, potentially resulting in oversized or undersized valves which can cause poor regulation, excessive noise, or damage to the valve or system. Additionally, it may not be accurate or reliable for some applications or conditions that require more precise or specific sizing.

4 Software method

The software method for sizing control valves is based on computer programs or applications that can perform calculations and simulations. This method involves inputting data and parameters, such as flow rate, pressure, temperature, density, viscosity, and composition; selecting a suitable valve type and characteristic; running the software to calculate the Cv and N values; choosing a valve size that matches the desired Cv and N values; and checking the valve behavior. This approach is convenient and flexible because it can handle different types of fluids and valves, as well as incorporate various factors and scenarios. However, this method also has some challenges, such as needing access to reliable software and data sources, depending on the quality of input data, and potentially requiring field testing for verification.

5 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?