Pipe strain check for reciprocating compressors

[svkd]

Guys,

While connecting the discharge and suction piping to the cylinders of reciprocating compressors,Which is right location to put dial indicator to see the strain coming from piping to the compressor? Some one told me to put dial indicator on the crank shaft coupling hub to see the piping strain coming to the compressor but makes no sense to me I don't think the forces due to suction/discharge piping strain is strong enough to move the crank case so that we can see deflection in crank shaft hub?
What do you think guys? Where to check the pipe strain effect Is it really on crank shaft hub?

 

[JJPellin]

I would not recommend the use of a dial indicator to evaluate pipe strain on a reciprocating compressor. I would focus on piping alignment.

Johnny Pellin

 

[DubMac]

If you have measurable pipe strain on the crankshaft hub, you have a piping alignment problem. The nozzles are not pipe-hangers. You should be able to freely move the bolts in and out of flanges. Don't let lazy pipefitters ruin your equipment.

 

[lilliput1]

Use flexible connections and support piping independent of the compressor. Piping should not impose force on the compressor.

 

[svkd]

Guys, My question is How to check the pipe strain on reciprocating compressor ,For example in pumps we mount the indicator on pump coupling hub while bolting up the suction/discharge piping to see the pipe strain so for reciprocating compressor which is right location to put dial indicator to see the strain coming from piping to the compressor?

 

[DubMac]

It's your equipment, do what you want with it. However, the only way you can assure no pipe strain to the compressor is to make sure there are NO forces or moments applied to the nozzles themselves (or at least below the allowable F&M values given by manufacturer). Unbolt the piping; if the pipe moves, you need to realign pipe to the free nozzles.
Measuring at the coupling hub may mask strains in the case which, over time, can cause damage other than just to the rotating element. Such as case cracking.

 

[JJPellin]

I would not use a dial indicator at all. There is no good location to mount the indicator.

Johnny Pellin

 

[svkd]

Guys , Just found that API 686 ,recommends as below for pipe strain measurement for reciprocating compressor

Clause 4.8 Pipe Strain Measurement
4.8.7 Reciprocating compressor piston rod run out shall be measured before and after connection of process gas
piping to the compressor cylinders and/or pulsation vessels and compared to the compressor manufacturer’s
allowable run outs or API 618, as applicable.

I think it is good idea to check piston run out before and after connection of process piping , if there is strain coming to the cylinder then it will show up in piston run out ,
I believe that checking at the crank shaft coupling hub may not give the correct reading since the shaft rests in sleeve bearings & had clearances so it will mask the shaft deflection(if any due to pipe strain) while bolting up the piping to the cylinder.

What do you think Guys?

 

[JJPellin]

Taking rod run-out before and after hooking up the process piping is a good practice. But, I would still not consider this as definitive proof that the pipe strain is acceptable. Verification of piping alignment when the lines are made up will verify that the strain is low. We have specifications regarding gap and angularity when the flange is hanging free. And, as noted above, you should be able to slip the studs through the flange holes without the need for a crane or come-along to pull the pipe into place.

Johnny Pellin

 

[svkd]

Johnny,

After achieving the pipe alignment within specification ,when inserting and torquing the bolts do we still need to put some kind of dial indicator on the crank shaft to see the shaft deflection due to pipe strain?

 

[bingopin]

As stated, it is unlikely that pipe strain would show up at the crank coupling. If it did, I believe the pipe misalignment would certainly be easily visible before being made up. Take the advice and focus on piping alignment. For small vent and drain connections a visual check will do. For suction and discharge connections, measure gap and angularity of the flanges. My memory is that we used the angularity spec of 1/64" per foot of measuring diameter as max allowable but I would defer to Johnny's specification.

Rod runout readings are important but out of spec readings can be caused by any number of other things such as x-head alignment or foundation issues.

 

[bhaskar5150]

Dear SVKD !!

I do not know why you are going through a dial indicator to know pipe strain.

I can comment on pulsation study though :-

There are parties who carries out pulsation study of recip machines by considering gas effects and vibrations.
For a reciprocating compressor, it is always a good practice to carry out pulsation study of the complete system after commissioning in the max load scenario and/or after any modifications done to the machine (For eg. changing to stepless capacity control system etc).

Piping modifications (if required) are easily identified and necessary corrections can be made well in advance.

Thanks,
Bhaskar5150