Control valves are critical elements in industrial process control. They are used for controlling various types of fluid. However as fluid passes across a control valve, the pressure regimes across the valve especially pressure drop changes with flow. A basic understanding of the pressure regimes across a control valve will help in the valve sizing process.
To accurately size a control valve, we must correctly predict the pressure drop across the valve from minimum flow to normal to maximum flow.
Typically as flow increases across a valve, the pressure drop across the valve reduces until it gets to the minimum allowable pressure drop across the valve at maximum fluid flow. As flow decreases, the pressure drop across the valve increases commensurately. These variations are illustrated in the diagram below:
As shown in the diagram above:
1. As flow increases across the valve , upstream pressure P1 drops
2. The pressure drop across the valve, ∆P, decreases as flow increases
3. At zero flow, ∆P is maximum and the downstream pressure P2 = 0
4. As flow increases, downstream pressure, P2 increases.
5. At maximum flow across the valve, ∆P is minimum
