 above: (a) An orifice plate that is removed from the piping so that the relative size of the hole (orifice) can be viewed. (b) An orifice plate mounted in the piping so that it can create a pressure drop that can be used to calculate flow. |
One of the simplest methods to determine the amount of flow is by placing an obstruction in the flow such as an orifice plate to create a pressure drop. These types of sensors are categorized as obtrusive-flow sensors. Since they are mounted directly in the fluid flow, they tend to cause small amounts of disturbance to the flow. In most cases the amount of disturbance is not important, but in some fluids the disturbance will create problems that will affect the quality of the product. If this occurs, an alternate method of measuring flow may be needed. The diagram on the left shows the orifice plate that is mounted in a pipe. When fluid begins to flow in the pipe, a pressure is created on the side identified as P1. When the fluid flows through the orifice, a lower pressure is created on the opposite side of the orifice plate identified as P2. The simplified formula for flow using the pressure drop is: Q = k√(P1 - P2) Q = flow in gallons/minute (gpm) |
Exercise:
If the pressure P1 is 4.5 psi and pressure P2 is 4.1 psi across an orifice plate and the value for k is 10.6, what is the total flow in gpm?
Solution:
Using the formula:
Q = k√(P1 - P2)
Q = 10.6 x √(0.4)
Q = 6.7 hpm