Blowdown for compressor restart

[Dammi77]

Hello All ,
I have question regarding the reciprocating compressor restart upon shutdown .
Upto how much pressure should I blowdown the package so that the compressor can be re-started without any problem .
Blow down is preffered over settle out ........
Thx !

 

[georgeverghese]

The unit should be depressurised to below the PAH setting at stage 1 suction. If you dont have such a setting, it may be estimated from the suction pressure vs horsepower curve for this compressor - at some suction pressure, the compressor HP will approach the max continous driver limit.

 

[MortenA]

But you have to remember that the seals cannot tolerate being under high dP for a long time so you may have to have a time dependent BD of the casing

 

[BigInch]

US regulations require that the compressor and station piping to be blown down completely. If there is a problem with blowing down a compressor completely during a shutdown, the compressor most likely lacks sufficient isolation possibility from its suction and discharge piping. Bad design.

 

[georgeverghese]

Big Inch, Though I'm not familiar with US regulations, in other countries, partial blowdown on compressor stations is commonly done for all non emergency shutdowns, if it is possible. This saves considerable time in the subsequent restart operation, since an extended station purge sequence is avoided. Total blowdown is only required for cases where there is risk of loss of containment, for example some failure of the dry gas shaft seals or fire or gas detection from local detectors.

 

[BigInch]

I can't really see much sense leaving gas in a compressor during a partial blowdown. Surely a partial station blowdown would mostly be directed at the compressor itself, rather than suction, or discharge piping, etc. If the compressor was isolated with suction and discharge valves, blowing it down completely would not be a difficult thing to do, nor would it cost a lot of gas.

 

[georgeverghese]

Yes ,that is true, but what operators are after is to reduce the time taken to restart. For partial blowdown, we avoid some 10minutes or so for the purge sequence. I know of one case where Operations folks were up in arms because the design team hadnt built in the flexibility in the trip logics to enable compressor shutdown with partial blowdown.
In all cases, the entire train is blocked in for the shutdown and blowdown sequence, which always includes all inventories from the suction SDV right up to the final discharge SDV.

 

[LittleInch]

To me its a design and judgment call on what is the impact on pieces of plant of remaining pressurized whilst mot moving / no flow.

The key aspect is the seal arrangement and ensuring that all components are suitably rated for the pressure which could easily equalize between the inlet and outlet valves.

Each system will be different so the first point of call is the compressor vendor to find out what the impact is of certain pressures in the compressor when it is not moving.

After that it is a systematic examination of the system to see what is the potential impact on other bits of plant.

Legislative requirements and company procedures may not permit this, but they are location and client dependant.

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

 

[BigInch]

Why block it in, if you don't blow it down. Just to keep the bomb complete. Operations people know mostly how to operate things in the easiest manner possible; not always so safely.

 

[LittleInch]

Block it in because that's the safe way of stopping something and isolating it into discrete parcels.

It's not a bomb if it stays in the pipe and doesn't over pressure it.

Agree that operators sometimes find ways to defeat certain annoying features of a design, but equally venting gas each time you stop or trip is a waste of gas and delays re-start. That's the balance that good design brings.

The OP is asking a question no one can answer without the specific details, but is talking about a recip compressor so IMHO it will revolve around what the vendor is happy with for his particular machine.

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

 

[BigInch]

I get the isolation concept, that's why I suggested not having block valves would be a bad design.

The regulations requiring blowdown refer to station ESD, as opposed to momentary operational shut downs.

 

[dcasto]

No where does the term blowdown mean 100% evacuation of the compressor. Just like no where does it define the time to 100% depressurization.

 

[gcastan]

hey,
I'd contact your compressor manufacturer and provide them with the model and serial number.
They should be able to provide you with a better answer as they know how they fabricated your specific unit.

Regards

Gabriel Castaneda, P.E.

http://www.gabcheminc.com

 

[BigInch]

Title 49 §192.167 Compressor stations: Emergency shutdown.
(1) It <ESD system> must be able to block gas out of the station and blow down the station piping.

"Blow down" means 0 differential pressure remamining between station piping, equipment and atomospheric pressure.

I'd be happy to ask for confirmation of that definition from PHMSA, if you'd like me to.

 

[51stovebolt]

From the perspective of a compressor packager, I can say that the pressure in which a unit needs to be blown down to in order to allow restart varies based upon each different compressor package design. The limiting factor is generally the bypass/recycle line size, as they are commonly the smallest line in the process system when recirculating gas. Reason is that just as soon as the compressor begins to rotate, it begins to move gas. The amount of gas it moves is dependent on the pressure that is in the process system; the higher pressure, the higher the flow. As the gas begins to move through the process piping, friction of the gas causes a pressure drop. Another way of putting it is that discharge pressure builds, which in turn increases the power required to turn the compressor. If the power required exceeds what the driver can generate at the low starting speeds, the driver will stall and never achieve the minimum speed.

Bottom line is that figuring out the maximum allowable startup pressure is most commonly a trial and error process. Some calculations can be made to predict the pressure, but most of the time, either the engineering know how to do so, or a knowledge of the required detail of the package does not exist, thus one must resort to trial and error.

Will