Determining Pressure Drop for Control Valve Sizing Calculations
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The selection of pressure drop (Δp) for control valve sizing calculations is a critical aspect that is often misunderstood. It's essential to understand that Δp cannot be arbitrarily specified without considering the actual system in which the valve will be installed. Each component of the system, except for the control valve, has a fixed pressure loss at the required flow rate. This includes pipes, fittings, isolation valves, heat exchangers, and other process equipment. In contrast, the control valve, being variable and connected to an automatic control system, adjusts to achieve the specified flow, temperature, tank level, or other process conditions.
If you need to perform sizing calculations at more than one flow rate, such as at both maximum and minimum design flows, you must repeat the calculations of p1 and p2 for each flow rate. This is because the system pressure losses and pump head depend on the flow rate. Accurate determination of pressure drops at various flow rates ensures that the control valve is sized correctly for all operating conditions.
Accurately Determining Pressure Drop Across Control Valves: Steps to Follow
When sizing a control valve for a system that has been designed but not yet built, or for a running system where reliable pressure measurements near the control valve are unavailable, the following steps should be taken to determine the pressure drop across the control valve accurately:Step 1: Identify Upstream Pressure
Begin upstream of the valve at a point where the pressure is known. For example, you might start at a pump where the pressure can be determined from the pump's head curve. From this known pressure, subtract the pressure losses in each of the fixed elements in the system, such as pipes, fittings, and heat exchangers. This process continues until you reach the valve inlet, at which point you will have determined p1, the pressure immediately upstream of the valve.Step 2: Identify Downstream Pressure
Next, move to a point downstream of the control valve where the pressure is known, such as a tank where the head is known. From this point, work upstream toward the control valve, adding the pressure losses of each fixed element encountered along the way. It's important to add these pressure losses because you are working in the direction opposite to the flow. When you reach the valve outlet, you will have determined p2, the pressure immediately downstream of the valve.Step 3: Calculate Pressure Drop
The actual pressure drop across the control valve is the difference between the upstream and downstream pressures, calculated as Δp = p1 - p2. This pressure drop is a key parameter for accurately sizing the control valve.If you need to perform sizing calculations at more than one flow rate, such as at both maximum and minimum design flows, you must repeat the calculations of p1 and p2 for each flow rate. This is because the system pressure losses and pump head depend on the flow rate. Accurate determination of pressure drops at various flow rates ensures that the control valve is sized correctly for all operating conditions.