9.1.1 Introduction to High Pressure Drops

Flow moves through a valve due to a difference between the upstream and downstream pressures, which is called the pressure drop ( ? P) or the pressure differential. If the piping size is identical both upstream and downstream from the valve and the velocity is consistent, the valve must reduce the fluid pressure to create flow by way of frictional losses. A portion of the valve s frictional losses can be attributed to friction between the fluid and the valve wall. However, this friction is minimal and is not sufficient to create enough pressure drop for an adequate flow. A more effective way to create a significant frictional loss in the valve is through a restriction within the body. Because many valves are designed to allow a portion of the valve to be more narrow than the piping, they can easily provide this restriction in the fluid stream. Because of the laws of conservation, as the fluid approaches the valve, its velocity increases in order for the full flow to pass through the valve, inversely producing a corresponding decrease in pressure (Fig. 9.1). The inverse relationship between pressure and velocity is shown by Bernoulli s equation, which is

Figure 9.1: Location of vena contracta from point of orifice restriction and pressure and velocity curves. ( Courtesy of Fisher Controls International, Inc.)

where ? = density units<i class="emphasis">        V</i><sub1</sub> = upstream...