6 replies to this topic

[nadavran]

Hi everyone,

 

The control engineer in my company uses the following technique to detect conditions of low feed tank level or dry runing:

He is using the pressure transmitter installed at the pump dischrage and trips the pump once there is a sharp drop in the discharge pressure caused by air entering the pump (loss of priming).

 

 

I would like to know if this routine is safe to use.

 

Regards,

Ran.

[samayaraj]

Hi Nadavran,

 

Its good practice to use the discharge pressure for tripping the pump. But in some cases, if the discharge valve kept closed, the above said condition will not safeguard the pump. It better to take motor current as an input in addition to the discharge pressure for trip.

 

If the pump is running dry/ at shut off pressure, the current drawn by the motor will be less. If the motor takes very less current (Minimum shaft input), sudden trip to be actuated. Because the pump is running dry. If the motor takes current more than minimum shaft power but less than ordinary condition, you can give little time to operate the discharge valve. If the time exceeds valve opening time, pump need to be tripped to avoid from overheating.

 

Hope this is clear to you.

 

#Samayaraj

Edited by samayaraj, 17 November 2014 - 01:37 PM.

[breizh]

http://www.pumpsands...prove-pump-0512

 

A lot of information are available on website .Take a look . Google " centrifugal pump Running dry prevention "

 

Note : I used liquiphant (fork) from E&H  with good success !

 

revised

 

Breizh

Edited by breizh, 20 November 2014 - 01:10 AM.

[nadavran]

Thanks breizh,

Can you please send that hyperlink again? for some reason it sends me to google search result.

 

Regards,

Ran.

[fallah]

nadavran,

 

It can't absolutely be a safe method for pump protection against dry running, because trip the pump after air entering inside; on the other hand it might before that happening, pump NPSH had been fallen below required one... Then use tripping the pump on low low liquid level...

[farid.k]

I do agree with fallah.normally trip when low low level. The protection of the pump should be before the pump cavitation happen and to avoid this,do trip the pump when tank level transmitter trigger low low.

I also have and experience regarding to low current input will trip the pump.this method you should deal with pump vendor since this will not link to other external iniator input as per previous where by the trip will depend on tank level transmitter signal. They also shoul have pump motor overload trip.

[senthil13]

I have seen couple of different things to protect the dry runs

1. Power trip

2. Temperature transmitter on the pump casing

3. One of our plants in Germany uses Level transmitter on all pump suctions and route the pipe as such to measure the level in the pipe.