Figure 3: While not intuitively obvious, it is possible to determine inlet and outlet pressures, as well as flow rates, from knowledge of pump curves, head loss, and process data. π www. valve-world. net Valve World November 2026 37
Finding Big 5 data
The fluid type is usually straight forward( air, nitrogen, natural gas, steam, water, etc.). However, it can get much more complicated in facilities that are processing mixed streams of organics and / or hydrocarbons where physical constant data is more difficult to obtain. Many standard fluids are programmed into the sizing software, so in that case, one can simply pick a value from the database list, and the calculation automatically determines the physical constants based on the inlet pressure and temperature. If the fluid is a mixture, then the user will likely need to consult their process engineers or experts to estimate the specific data required. One note of caution, users must be aware when the fluid might be multiphase or contain slurries as this can radically alter the sizing and selection process. Inlet pressure and temperature are usually well known. Often there is instrumentation to directly indicate what these values are during normal operation, or they can be estimated from pump curves and head calculations( Figure 3). The difficulty with these parameters is choosing normal operating parameters versus min / max design conditions. That topic will be further discussed in the next section.
The outlet pressure can be more difficult to determine as it is not typically measured in many applications, but it is critical for valve sizing as it determines the pressure drop across the valve, which can induce cavitation, flashing, and / or choking in the valve, further restricting flow. If a direct measurement is not available, the outlet pressure can be back calculated from downstream vessel pressures, head calculations, and flow calculations. The complication here is that the downstream pressure is often dependent on the flow through the valve since that imparts pressure drop throughout the piping network. These pressure drops can lower the inlet pressure and significantly raise the outlet pressure at the valve, particularly if the downstream piping is lengthy. Flow can be one of the more confusing parameters to obtain. Ideally there is a flow meter somewhere in the line which provides a direct measurement, or the process design should provide a typical flow that is expected during normal operation. Flow can also be inferred from pump curves or level changes in upstream or downstream tanks. For compressible fluids, the volumetric flowrate needs to be in standard or normal units. For
CONTROL VALVES
liquids, actual volumetric units should be used.
The art of valve sizing
If valve sizing were easy, anyone could do it. However, there are a lot of design decisions and process knowledge that determine what data should be used, much of which can radically alter the valve selection process. As a result, the user is strongly encouraged to take an active part in valve sizing to make sure as much process knowledge as possible has been considered. As a start, it is important to understand the difference between design data and operating process data. Design data is typically the maximum and minimum pressures, temperatures, and flows the valve could see. For instance, a valve might normally operate at 600 PSI but could see 750 PSI during an overpressure event. The 750 PSI value is important to ensure the valve body and components can withstand that pressure, but the sizing data will typically use the 600 PSI value. Temperature is much the same as design temperatures may be used for body and packing material selection, but the normal process temperature is used for sizing.
Figure 3: While not intuitively obvious, it is possible to determine inlet and outlet pressures, as well as flow rates, from knowledge of pump curves, head loss, and process data. π www. valve-world. net Valve World November 2026 37