In this article we will quickly and easily as we can size a control valve in such a way that fits our needs and costs.

Assume we have a control valve regulating the outflow of fluid in a tank where you want to control the level to 25 feet. The inflow varies from 0 to 125 gallons per minute:

Figure 14 Dimensions of tank valve for simple

The maximum outflow Q, must be equal to the maximum inflow ie, 125 gpm. Since 1 foot water develops a pressure of 0.433 psi, with a level of 25 feet in the tank pressure through the valve will be:

P = 25 x 0.433 = 10.8 psi

Applying the formula of the preceding article paragraph (Cv of a valve) we know G and Ap.

For example:

G is the specific gravity of water, ie 1.0

Ap is the pressure drop, because the air discharge tank, will that causes the liquid to the inlet valve, ie 10.8 psi

Cv = Q / [(Ap / G) ^ 1 / 2] = 120 / [(10.8 / 1) ^ 1 / 2] = 36.5

Figure 15 shows a valve manufacturer's table, detailing the diameter of the valve Cv versus its corresponding to a percentage of valve opening.

Figure 15 - Globe Valve Cv equal percentage

Returning to our example, we now find the smallest valve capable of delivering a Cv of 36.5. For this we refer to Figure 15 and found that this is a 2-inch equal percentage valve. No delivery valve smaller this value of Cv. It is imperative to make the valve Cv occurs between 10% and 90% of the race, in our case to 100% open Cv 56.2 provides more than enough for this case.

If now instead of downloading the valve to the atmosphere, it does assume that a second tank 15 feet as noted below:

Figure 16 Control in two tanks

In this case, being the input and output flows the same as for the previous case, the pressure drop through the valve will be:

AP = (25 ft - 15 ft) (0.433) = 4.33 feet

Cv = Q / [(Ap / G) ^ 1 / 2] = 120 / [4.33 / 1] ^ 1 / 2 = 57.7

Again we seek the smallest valve capable of delivering a Cv of 57.7. In Figure 15 we find that this is a 2-inch valve and a half. Note that the capacity of the valve depends on both the size of the pressure drop in the system.

This is the end, as you can realize a valve sizing is not very difficult. As a recommendation to verify that the viscosity is not high, ie Raynols your number is not greater than 2000. If you review the table of the previous article and apply the correction factor.