pump run dry

[rika kose]

We use a centrifugal pump to unload acetic acid from tankers.
Discharge side of the pump is vertical line to the pipe rack 10m high. from the loading pump to the storage tank is around 100m.
differential head of the pump is 30m @ rated flow.
The suction side has no sensor. Operators normally set a timer. quite often, tanker is already empty, but pump is still running.

I want to install a protection to the pump like flow switch or level switch to protect the pump from dry run.
Operators say if the pump run dry, it can transfer all the acid to the tank.
I don't think so, because if there is only air (N2) in the pump, the pump can not deliver so much pressure to push the liquid to 10m high and 100 m away.
There must be some acid remaining in the pump.

 

[TiCl4]

What is your question? Are you asking if the centrifugal pump can push air through the line to clear it of acid? The answer is generally NO. Unless the centrifugal pump is of the self-priming variety, it cannot push any significant amount of air through the lines. This is due to the lower viscosity and compressible nature of air - almost all of the air pushed out along the impeller vanes will recirculate internally and not generate flow. You would need a positive displacement pump to draw/push air through the lines. Even with a PD pump, at some point you will reach annular flow regimes where the remaining liquid phase in the outer annulus is thin enough that there is essentially no flow. At this point, the remaining air being pumped through the line will only serve to remove additional material through evaporation.

The pump and loading lines will have some remaining acid in them, yes. The amount greatly depends on the piping configuration.

As a side note, running the pump dry will increase wear on your sealing mechanism. Generally, packing or double mechanical seals with a barrier fluid will handle dry running better than a single mechanical seal. Single mechanical seals can be ruined in less than a minute if run dry.

 

[EdStainless]

Most pumps rely on the pumped fluid to cool the seals and maybe the bearings also.
The outlet pressure should drop significantly when the pump starts 'sucking air' so a simple pressure sensor may do the job.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[Latexman]

I want to install a protection to the pump like flow switch or level switch to protect the pump from dry run.

Is there a vertical section of the suction you can put a dP cell on/across? When the pump is just running dry, that should go to ~ 0 mm of liquid height. You want the pump to empty the suction side as best it can. There will be acetic acid in the pump and discharge line.

Good Luck,
Latexman

 

[LittleInch]

So what's your question?

It's one of those things that shouldn't be done but in reality right at the end of the tanker unloading you will be pumping a mixture of liquid and n2.

The vast majority will make it up the 10m and then move along the pipework.

Your pump might not last many years though.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Compositepro]

The pump will not be running "dry" with 100 m of discharge pipe and 10 m of head. It will be acting as a self-priming pump trying to prime.

 

[MJCronin]

There is no question here ... If you want to use these fora, you must explicitly ask a question.

If your question is "How do we design tanker truck offloading piping systems ?" then let me offer the following:


1) Your truck piping tank configuration is very typical. Pumped liquid must go from a truck up to a rack and then to the tank.

2) Your 100m piping distance to the tank is NOT typical. Most chemical process facilities group storage tanks and have a truck offloading pumping facility nearby. You cannot locate any plant component anywhere on a large plant site and expect things to operate well

3) I agree with TiCl4, there is acid left in the piping. Centrifugal pumps are not compressors and will not "pump air". The question is will the remaining acid cause you any problems and, aside from representing some stranded money, how bad is it ? Some centrifugal "offloading" pumps are specifically designed to run dry and have associated features to immediately stop the pump.

https://www.northridgepumps.com/article-263_tanker-unloading-pump-kit-for-ethanol

4) Some plants have an operator use low-pressure compressed air (10-25 psi) to push the remaining heel of acid into the storage tank. The pump must be valved off. and this must be carefully done with extreme caution. The storage tank must have a large emergency vent installed. This can be very risky business ...

Please provide information such as PIDs, pictures of the pump and top of the tank for us to help you

What is the material of construction for the pump, piping and valves..

What is your fundamental question ?



MJCronin
Sr. Process Engineer

 

[pierreick]

Hi,
What is the material ? Diluted acid or glacial acetic acid?
Where is located your facility ? Risk of freezing ? Etc .
Consider to talk to the supplier of this material , he may be able to provide support (other technology or mean to transfer material).
My 2 cents
Pierre

 

[rika kose]

Thanks all for the valuable information.
Sorry for not making the post complete. My question is " when suction side is empty, will the pump and discharge side completely dry?"

 

[vt2012]

As others have said - no. Acetic acid will remain in the suction piping, the pump itself, and the discharge piping.

If you're looking for a quick way to detect when the pump stops pumping, an ammeter can sometimes be a quick and dirty way (without having to tap in to the pipe).

 

[LittleInch]

If the volume and flow of air or nitrogen is large enough and goes on for long enough then yes, it might blow all the acid out the pump and pipe work.

But the pump on its own acting as a gas blower and not a pump is unlikely to clear the pump and pipe completely. But it might if you left it long enough to clear 90%.

A liquid centrifugal pump is a poor blower/ compressor, but it will create a small gas pressure.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.