Question about maximum possible discharge pressure (considering blocked outlet scenario)

[Acapulko]

Hi to everybody!

Last few days im thinking about one question (and i will appreciate if someone will help me to find right answer).

I need to determine max. possible discharge pressure of liquid ring compressor considering blocked outlet scenario. I need this to check if we can exceed the design condition of pipelines or not (do we need PSV, or not).
Basically the porblem that I faced is: I took into consideration highest suction pressure, and back pressure. Is it also neccesary to disregard the rated motor power? I mean if i consider motor rated power, we will never face the situation where we will exceed design pressure of pipeline (because of the motor protection), but of course if I wont consider motor rated power and assume that the motor without any over current protection, than the pressure will continiously rise and exceed desighn pressure of pipelines.

All answers will be highly appreciated!

 

[LittleInch]

What does the compressor curve or data sheet say?

I don't know a lot about this type of compressor so not sure if it will stall like a centrifugal or just keep pumping like a piston or screw compressor.

At no flow power is often quite low but torque is high and hence pressure can be high.

Any model of compressor you'd like to share?

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

 

[Acapulko]

LittleInch,
First of all, thanks for fast reply.

1). Compression ratio remains the same. So, higher inlet- higher discharge, but of course torgue is high.
2). This compressor will keep working.
3). Model - NASH 2bq1.

I also dont know a lot about this type of compressor, but the main point for me is not the compressor, i know that it will continue to pump at no flow. My main doubt - how to find max. possible pressure with blocked outlet. From one side i have rated shaft power of the motor, so in case of blocked outlet the motor protection will shut down system and protect from exceed of design pipeline pressure. From the ohter side i also think that probably i should not take into consideration motor (rated power etc.), but in this case i feel like im doing some kind of over-design.

 

[Compositepro]

" i know that it will continue to pump at no flow." That statement is nonsense.

Liquid ring compressors will simply stop pumping when the discharge pressure gets beyond their capability, just like a centrifugal pump. If I am reading the curve correctly it looks like your compressor's max pressure is slightly over 180 psig.

http://www.gdnash.com/liquid_ring_compressors/2bq_compressor/

 

[zdas04]

A dynamic machine will stall because the increasing pressure comes from the properties of the fluid under flowing conditions, when flow stops the conditions that create the increasing pressure stop.

A positive displacement machine keeps increasing the downstream pressure by trying to stuff fluid into the downstream piping until the driver stalls or something breaks.

Liquid ring compressors are positive displacement until they aren't. When pressure reaches some maximum the machine blows all its seal liquid out and it starts just spinning in the chamber.

But in answer to your basic question--of course you have to consider driver capacity in a credible scenario analysis. It is not credible to assign a volume flow rate/pressure pair that exceeds the horsepower available. With an electric motor, there will be an excess-current trip. With an engine there will be a governor and/or a high manifold pressure trip or the engine will stall. If you think that it is credible that the outlet valve can be shut at the same time the over-current trip fails than you need to protect against it. Personally I think that would fit the "double jeopardy" definition.

[bold]David Simpson, PE[/bold]
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

 

[LittleInch]

Acapulko,

You need to watch this which might explain what ZDAS04 is talking about. This is from NASH pumps

https://www.youtube.com/watch?v=Y99ulE85e8Q

Q - My main doubt - how to find max. possible pressure with blocked outlet. A - Ask the vendor

From one side i have rated shaft power of the motor, so in case of blocked outlet the motor protection will shut down system and protect from exceed of design pipeline pressure.

A- You can't possibly state this with any certainty. At no flow the pressure might go up, but overall if the compressor keeps spinning and the shaft power is lower than power at max flow, then the motor will not cut out. Most motors are oversized compared to shaft duty as they go in steps of power and most protection circuits look at motor max power, not pump power.

"but the main point for me is not the compressor" - That's your error here. Of course it is the main point. Find out if you have a problem by looking at the compressor - forget about the motor until you know what the compressor does on no flow.

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

 

[LittleInch]

BTW, the NASH 2BQ1 appears to have a max discharge pressure of 3.75 bar abs....

http://pdf.directindustry.com/pdf/nash/nash-new-series-compressors-2bq1/11710-88454.html

Also every liquid ring performance curve I can find has the power at zero flow lower than at operating point, therefore your cut out on the motor won't work.

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

 

[Compositepro]

littleinch, look at the link I posted. 3.75 bara is the max operating pressure, not max deadhead pressure.

 

[LittleInch]

I did look at it, but the link seemed to cover a wide range of compressors.

The specific 2BQ1 data sheet states 3.75 bara, and has a pretty straight line on the flow vs head curve.

Sure it might suddenly go higher at no flow, but seems kind of unlikely to me. But like I keep saying to the OP - ask the vendor.

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

 

[Compositepro]

The operating envelope of the 2BQ1 is a small rectangle on the shaded performance curve. But I agree that the manufacturer should answer the question.

 

[georgeverghese]

In many compressors, the following control and protection mechanisms are all provided:
a) Discharge overpressure control loop
b) Discharge high pressure trip loop
c) Driver side overload trip loop

In addition, a blocked discharge PSV is provided. In some cases, the design case relief load at this PSV may be reduced by taking credit for the action of one of these trip loops. But you cannot altogether eliminate the requirement for the PSV. This is standard good practice.
Check if you have all of these in place, else you have to show that this compressor simply cannot develop a discharge pressure (without taking credit for any of these control or protection devices coincident with max possible operating suction pressure) that will exceed pipeline MAWP (or the limiting design pressure of any other component connected to the pipeline) at all flows.

Besides, when this compressor is operating at deadhead, the motor may most likely not be drawing high current.

 

[zdas04]

Liquid ring machines are unique in that the compression elements are liquid. When you get much above 5 compression ratios the heat of compression begins boiling the liquid off. In fairly short order the liquid is gone and the impeller (very different shape then a centrifugal compressor impeller, and a very different function) just spins in an empty chamber. They tend to not have any of the protection mechanisms that georgeverghese listed.

[bold]David Simpson, PE[/bold]
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist