| 1. If you are dealing with a corrosive fluid, choose the valve body
and trim material to match the pump casing and impeller.
2. Velocity is the key to handling abrasive materials. Normal city water
velocity is about 7 to 10 F.P.S. (clean liquid). If you have a fluid that
is abrasive, keep the velocity as low as possible - without having the
particles drop out of suspension.
3. 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 at one point and being asked to control it somewhere
else.
4. Velocity is the key to handling noise. Noise is energy. 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.
5. 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 shut off.
6. In cavitating fluids - even if the control valve has cavitation trim
in it - be sure to allow a straight run of downstream pipe after the valve.
If there is a pipe "T"or elbow immediately downstream, the flow will choke
out and back up into the valve.
7. If you use a control valve with a bellows seal in it, try to size
the valve so that its normal throttling position is near the bellows "at
rest" position. This will minimize wear on the bellows.
8. Don't use a valve below 10% of flow if at all possible. Even though
a valve may have good rangeability, if the valve is used in an abrasive
or erosive service (steam), it will not hold up unless it has hardened
trim.
9. If a PLC is being used to control the valves in a system, specify
the valves with a linear flow characteristic.
10. If a control valve is started up and fails to respond - or goes
to full open or full closed and stays there - check the controller and
reverse the controller's action. |
