Microdrive Series Instruction Manual
4201-109 Rev I
104
DESCRIPTION OF OPERATION
This system is similar to that described in Appendix 4C except
that use is made of additional features to overcome the following
problem common in pressure control systems.
Problem
Under conditions of very low or no draw off, centrifugal pumps
still maintain speed to maintain pressure. Due to this speed there
may be high losses in the pump, even to the extent of boiling the
fluid.
The normal solution to this problem is the addition of a non-return
valve. This maintains pressure, however the pump may
continue to run.
As a further solution to this problem the above system using
feedback relays is implemented to automatically stop and start
the pump. Key points are as follows:
i)
A minimum speed is set which guarantees to
provide a degree of over-pressure under the
conditions that the pump is supposed to stop
(i.e., under zero flow conditions).
ii)
A feedback hysteresis level sets the over-
pressure level (the amount above the set point)
at which the hysteresis relay opens. Under
zero flow conditions, the minimum speed setting
must cause the pressure to exceed this level
for this system to work.
iii)
The feedback hysteresis relay opens (the stop/
start circuit) under the above condition, and
stops the drive. The non return valve maintains
the system pressure.
iv)
As flow resumes, the pressure drops. When
the pressure drops below the set point minus
hysteresis level, the feedback hysteresis relay
closes, starting the drive again.
v)
Apart from this automatic stop/start mechanism,
the system operates as a normal pressure
control system.
Содержание D3-2.5
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