Dry Running Protection 1

[ukgraduate]

Hello,

We are looking at purchasing some helical rotor pumps. The vendor has offered an RTD installed into the casing of the pump which will monitor the stator temperature. We can of course take this into our PLC or motor protection unit at trip at a desired setpoint.

We have also previously used a flowswitch after the pump. When the pump runs dry there will be no flow and we can trip the motor. We prefer not to use the paddle type as unless the flow is a steady stream it can lead to spurious trips. We have used thermal dispersion switches. This also has the added benefit of alerting the user that the pipe may be blocked down the line and may not be reaching the target.

Is an RTD an acceptable and reliable method of DRY RUN PROTECTION?

UkGrad

 

[bimr]

It is a difficult to respond without knowing what you are pumping and the pressures.

 

[itdepends]

Flow switches/flow interlocks are useful to prevent operating with the pump at low/zero flow conditions but they need to be disabled during pump startup (to allow the pump to establish the flow). This can result in damage due to dry running during the few seconds you allow in your control logic for the flow to establish.

Flow related interlocks would be more robust than the RTD the vendor is proposing.

If you have sufficient suction head available then a pressure switch on the suction of the pump could be used to provide dry run protection by only allowing the pump to run after the switch detected sufficient suction pressure.

More details on your application would help.

As a chem eng/metallurgist the first part of any answer I give starts with "It Depends"

 

[georgeverghese]

The flow switch is backup instrumented shutdown protection to the pump discharge overpressure protection - low flow trip before PSV blows and rattles all the piping around it to bits. Check that the FSL is located after the PSV and not before it.

 

[rotw]

could you put an interlock / protection based on motor current set point ?
Just a thought...

"If you want to acquire a knowledge or skill, read a book and practice the skill".

 

[ScottyUK]

Hi rotw,

Current is a poor indication of what the pump is doing because the line current has two components: magnetising current which is pretty much constant regardless of load, and active current which does useful work. It isn't uncommon for the open shaft current to be more than half of the full-load current.

A power-monitoring relay which disregards the magnetising current is an option, but they still only measure power into the motor and there are credbile failure modes which they won't detect. Ultimately the best way to detect low flow is to measure flow, and the best way to detect high temperature is to measure temperature - attempting to infer either from the input terminals of a motor will at best only gives a vague idea of process conditions, regardless of how accurate the electrical measurement.