Dry Running Protection

[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?


Regards

UkGrad

 

[LionelHutz]

I wouldn't think it's a very good method. You could use current or power to detect no flow.

 

[OperaHouse]

Thermal detection probably involves heating the sensor. When flow slows or stops the fluid can not carry the heat away and the sensor heats up.

 

[ScottyUK]

If you can afford it a flowmeter with a trip amp or DCS alarm threshold is the best way to detect flow. Less chance of a false indication than from a switch, and not trying to imply what the fluid is doing by measuring power into the motor on the other side of the coupling.

 

[1gibson]

Can you not just trip on low motor amps?

 

[PersonalProfile]

http://www.eng-tips.com/viewthread.cfm?qid=321456

Regards,
Lyle

 

[ScottyUK]

1gibson,

The relationship between load power and current isn't linear. Motors draw a fair bit of magnetising current even when they are running open shaft, so it isn't a good measurement to use to detect low flow. A power meter takes account of the phase displacement between voltage and current to calculate active power while ignoring the reactive power component, rather than just mutiplying current and voltage which gives the apparent power.