Using Control Valve Installed Gain Calculations

Reference 4 does not include the ability to graph what is happening to P1 and P2, and thus the pressure differential available to the valve. However, the reference 4 worksheet has a tabulation of P1 and P2, so constructing a graph of P1 and P2 versus relative valve travel was simple. In the installed pressure level graph, as valve relative travel approaches 0.8 (80% valve travel), the pressure drop available to the valve decreases rapidly.
 

Different pump, different valves

The user found a pump with a slightly higher and flatter head curve. A new analysis of the upstream system gave the revised values of P1 and DELTA P shown in red in Figure 4. Putting these new values of P1 and DELTA P into the user’s valve-sizing program and into the reference 4 worksheet gives the graphs of Figure 4. The valve is now operating between 25% and 75% travel. The maximum design flow is now at slightly less than 80% of the fully open flow, giving ample safety factor at the high end of the range. The installed gain graph is much flatter and well within the suggested limits.

Figure 5 is based on an application where the system designer recommended a 10-inch segment ball valve after examining the installed flow and gain graphs and determining the segment valve was a good choice. The purchasing agent commented that a 10-inch high-performance butterfly valve would cost approximately one-third less than the segment ball valve. The system designer agreed to investigate the applicability of a high-performance butterfly valve, knowing the two valve styles have quite different inherent flow characteristics. Segment ball valves tend to have a nearly perfect equal percentage characteristic. High-performance butterfly valves tend to have an inherent flow characteristic between linear and equal percentage.

Which pressure drop?

A question arose regarding the pressure drop to use when sizing a control valve. Assuming a system that has already been designed, the sizing pressure cannot be arbitrarily assigned, but the values of P1 and P2 need to be obtained by an analysis of the frictional pressure losses and static pressure changes in the system both upstream and downstream of the control valve. The ideal situation is where the person selecting the control valve has a say in determining what the control valve pressure drop will be, most often by specifying the pump that will be used. Using an installed gain analysis of various pumps that might be suitable can be helpful.

To demonstrate how this can be done, three possible pumps for the system shown in Figure 6 will be considered, and the one that allows satisfactory controllability while minimizing energy consumption will be selected. Curves of P1, the pressure just upstream of the valve, are shown for each of the three pumps, along with the power required by each at a normal flow rate of 400 gpm. These curves slope downward in proportion to the flow squared from the 100 gpm pump head (45, 60, and 75 psig, respectively, for pumps A, B, and C) to a pressure 10 psi lower due to the combined effect of the 5 psi pressure loss in the upstream piping and the 5 psi decrease in pump head from 100 gpm to 600 gpm stated in the figure. The curve for P2, the pressure at the control valve outlet, starts with the 10 psig static head of the tank at very low flows and increases in proportion to the flow squared to 30 psig as the downstream piping and heat exchanger pressure losses increase to their 600 gpm values.

The control valve pressure drops (the difference between P1 and P2) are indicated in the figure by the arrows at the left side of the figure for 100 gpm and at the right side of the figure for 600 gpm. The analysis is performed based on using a segment ball valve. The graph in the lower left of Figure 6 shows the calculated installed flow characteristics. Keep in mind the installed flow graphs generated by the worksheet of the reference 4 graph is relative flow, so 1.0 is 100% of the fully open flow, which is different for each of the three cases. What is interesting is the installed gain graphs.