Gain is defined as: Now, at our three flowrates: Qmin = 25 gpm Qop = 110 gpm Qdes = 150 gpm we have corresponding Cv values of 6.5, 28, and 39. The corresponding stroke percentages are 35%, 73%, and 85% respectively. Now we construct the following table: Flow (gpm) Stroke (%) Change in flow (gpm) Change in Stroke (%) 25 35 110-25 = 85 73-35 = 38 110 73 150 85 150-110 = 40 85-73 = 12 Gain #1 = 85/38 = 2.2 Gain #2 = 40/12 = 3.3 The difference between these values should be less than 50% of the higher value. 0.5 (3.3) = 1.65 and 3.3 - 2.2 = 1.10. Since 1.10 is less than 1.65, there should be no problem in controlling the valve. Also note that the gain should never be less than 0.50. So for our case, I believe our selected valve will do nicely! OTHER NOTES: Another valve characteristic that can be examined is called the choked flow. The relation uses the FL value found on the valve chart. I recommend checking the choked flow for vastly different maximum and minimum flowrates. For example if the difference between the maximum and minimum flows is above 90% of the maximum flow, you may want to check the choked flow. Usually, the rule of thumb for determining the maximum pressure drop across the valve also helps to avoid choking flow.

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