Lars Josefsson | Flow velocity in control valves

Flow velocity in control valves

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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 1/2 - 2 7.5 25 90 300
65 - 100 2 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 1/2 - 2 12 40 105 350
65 - 100 2 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.

Control Valve Installation
Correct sizing and selection procedures, proper installation techniques. and periodic preventive maintenance are all factors that can lengthen control valve service life. Most valve manufacturers furnish detailed installation and operation instructions with each valve. These instruction sheets normally outline specific installation and maintenance procedures which apply to the particular valve described. Naturally, the specific instructions should be read by the purchaser prior to valve installation and closely followed during installation and operation. The suggestions furnished below are general in nature and should not take precedence over the valve manufacturer's detailed instructions for a particular valve.

Be Sure the Pipeline is Clean
Foreign material in the pipeline could damage the seating surface of the valve, or even obstruct the movement of the valve plug or disc so that the valve could not shut off properly. To help reduce the possibility of a dangerous situation occurring, all pipelines should be blown out with air prior to valve installation. Make sure pipe scale, metal chips, welding slag, and other foreign materials are removed. Also, if the valve has screwed end connections, a good grade of pipe sealant compound should be applied to the male pipeline threads only. Do not use sealant on the female threads in the valve body because excess compound on the female threads would be forced into the valve body. This could cause sticking of the valve plug or accumulation of dirt which would prevent good valve shutoff.