4. Size control valves to absorb about rd of the total system pressure drop.
5. Control valves should not be less than half the pipe size. Normally the valves exclusively envisaged for shutoff service shall be line size.
Alternatively, they could be sized as control valves.
6. In the case of lines with a diameter of up to 1" the valve size shall normally equal that of the line. In the case of lines with a diameter larger than 1" the valve size shall not be less than 1".
7. Valves shall generally have flanged connections as per rating envisaged in the line specification with the exception of valves with a nominal diameter smaller than or equal to 1½" which shall have a minimum rating of 300 ANSI.
8. If you are
dealing with a corrosive fluid, choose the valve body and trim material to match the pump casing and impeller.
9. Velocity is the key to handling abrasive materials. Keep line velocity of about 10 ft/s for clean liquid. If you
have a fluid that is abrasive, keep the
velocity as low as possible, without having the particles dropout of suspension. When dealing with high pressure drop situations try always to keep the velocities below 0.3
mach on the inlet pipe,
valve body, and outlet pipe.
10. Always sense pressure where you want to control it. Many control valves and pressure regulators do not function properly simply because they are sensing pressure atone point and being asked to control it somewhere else.
11. If you use
a transducer in a control loop, specify a positioner on the valve. Otherwise the transducer will rob the
actuator of available thrust, and the valve will leak when it is supposed to shutoff. In cavitating fluids be sure to allow a straight run of downstream pipe after the valve - even when the control valve has cavitation trim in it. If there is a pipe "Tor
elbow immediately downstream, the flow will choke out and backup into the valve.
13. Remember that control valves only do what you tell them to. Many control valve problems turnout to be a problem somewhere else.
