Lars Josefsson | Flow velocity in control valves

Flow velocity in control valves

It is recommended that the maximum inlet flow velocities applied to control valves should be as shown in the tables below.
Globe Valves Size Liquid Steam or Gas
mm inch m/s ft/s m/s ft/s
15 - 25 1/2 - 1 9 30 120 400
40 - 50 1/2 - 2 7.5 25 90 300
65 - 100 1/2 - 4 6 20 75 250
150 - 200 6 - 8 6 20 70 225
250 - 400 10 - 16 4.5 15 55 175
Angle Valves Size Liquid Steam or Gas
mm inch m/s ft/s m/s ft/s
15 - 25 1/2 - 1 13.5 45 135 450
40 - 50 1/2 - 2 12 40 105 350
65 - 100 1/2 - 4 10.5 35 90 300
150 - 200 6 - 8 9 30 85 275
250 - 400 10 - 16 7.5 25 70 225
Not only the tables maximum inlet flow velocities applied to a control valve is important. A control valves' opening characteristic is also very important for the smooth function of the entire control loop.

LINEAR (Flow is directly proportional to the plug lift.) Use this trim characteristic if the difference pressures over the control valve differ less than 20%.
EQUAL PERCENTAGE (With unit change in plug lift, the flow changes by a constant percentage of its instantaneous value.) Use this trim characteristic if the difference pressures over the control valve differ more than 20%. The equal percentage trim characteristic is also the best when the sizing data is a little uncertain and the valve may be oversized.
QUICK OPENING (Capacity increases rapidly in linear relationship with plug lift, reaching maximum value with small opening travel.) A plug-lift equal to a quarter of the plug diameter gives full passage.

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