CONTROL VALVES
Big 5 parameters for sizing control valves
Here, Mark Nord from Emerson talks about the Big 5 parameters you need to size a control valve and explains where to find that information. He also shows how to choose the correct process data to generate an acceptable control valve solution for a given application.
By Mark Nord, Emerson
The idea for this article started as an informal quiz, which I posed to numerous clients and colleagues over a couple of years. The question was:“ Name the five pieces of information you need to size a control valve.” The seemingly simple question stumped a surprisingly large number of people. Just for fun, stop reading now, pull out a piece of paper and write down your answer before continuing! The fact is that control valve sizing is becoming a dying art. Many people think one can plug some data into a computer application and generate the right answer. However, one must know what data to enter and more importantly, how to choose the right data to get an accurate control valve sizing solution versus a wildly overpriced, oversized, or undersized model.
The Big 5 parameters
To run an initial valve sizing calculation, five main items of information are required( Figure 1). More complicated control valve applications may require more information than is listed below, but every valve will need at least this information.
FLUID TYPE INLET PRESSURE( P1) OUTLET PRESSURE( P2) INLET TEMPERATURE FLOW
Figure 1: These Big 5 parameters are required to initially size any control valve. More complicated applications may require more information.
• Inlet Temperature: What is the fluid temperature as it enters the valve?
• Flow: What is the flow through the valve? Note this parameter is usually more than one value, as one needs to know the minimum controlled flow, the normal flow, and the maximum flow that must pass through the valve.
These parameters are the minimum necessary to even start a control valve sizing calculation. Unfortunately, there is a lot more to sizing a valve than just plugging in these Big 5 parameters, but it is important to realise that one cannot even get started without at least this information.
Valve sizing calculations
Control valve sizing calculations are not for the faint of heart. The process involves complex mathematics to determine fluid flow and velocity, pressure changes, cavitation, sonic velocity and noise, actuator forces and counter forces, and other factors( Figure 2). Fortunately, valve sizing software is typically very good at math, so it can certainly help crunch the numbers once the proper data has been entered. If the data entered into the valve sizing program is incorrect, the answer will also be incorrect. There is also a fair amount of art in choosing what data to enter into the program. For instance, does one enter the maximum inlet pressure and temperature the valve will ever encounter, or the normal operating pressure and temperature? Flows through the valve typically vary, so which values does one use?
The Big 5 parameters are:
• Fluid Type: What is the fluid and is it a liquid, gas, or steam? If multiple fluids are involved, what are they and what are their percentages? Note that knowledge of the fluid allows the user to obtain additional details required by the sizing calculations, including density, viscosity, vapour pressure, and other physical constants.
• Inlet Pressure( P1): What is the pressure of the fluid before it enters the control valve?
• Outlet Pressure( P2): What is the desired pressure of the fluid after it leaves the control valve?
Figure 2: Control valve sizing calculations can be quite involved. Fortunately, valve sizing software can handle the math, but it is utterly dependent upon the user to enter the correct information.
36 Valve World November 2026 www. valve-world. net