A DP Endress and Hauser flow transmitter is always tripping
pump at low flow. set point is 74 cubic meter/hr. Thorough
impulse line flushing and zero calibration has been done,
yet problem persists. What do you think is the problem and
how do you rectify this fault?
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Answer Posted / carl ellis
The stated question assumes that the design criteria for a
low flow alarm SP at 74m^3/hr is valid. It is not clear
whether the problem is limited to the instrumentation
without a clear statement that the pump has been determined
to be operating properly. Since this is an instrumentation
exam, I'll assume the problem is in the instrumentation.
If the instrumentation is the problem, then the DP's
indicated DP/flow rate has to be lower than the alarm SP.
What can cause a DP transmitter to indicate a lower
DP/flowrate than it should?
If the equalizing valve is not tightly closed or leaks at
the valve seat, the DP will be lower than it should be as
the DP approaches zero, depending on the leak rate.
Damaged orifice plates read low, resulting in low indicated
DP readings.
Lower line pressure than the "design" line pressure will
reduce the indicated DP reading.
Someone changed out the orifice plate (OP)with an OP with
bigger bore or re-installed the correct OP backwards.
Since DP operation is "high side pressure minus low side
pressure equals indicated DP", a leak anywhere on the high
side will reduce the high side DP pressure and result in a
low indicated DP. That could be
- leak in the high side impulse line or its fittings
- manifold valve flange gasket leak on the high side or on
the process side
- vent leak
(however liquid leaks are usually visually evident)
In a mis-configured system, the PLC/DCS might have square
root extraction in addition to square root extraction on the
DP transmitter, which would cause a low flow rate indication
at the PLC/DCS.
Tell 'em Carl sent you.
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