rotary screw compressor 2nd stage damage

[Esoliman]

what will happen if a rotary screw compressor discharge air flow restricted,
we have a rotary screw compressor 10 bar 20 nm3/min, when the compressor started it tripped by inter-stage pressure high. the trip was 16 seconds from start-up
when the compressor checked after trip,the 2nd stage found defected and the drive shaft of the 2nd stage found broken.the mechanical maintenance suspecting that the compressor started on closed discharge valve, but even if that happened the compressor is equipped with pressure relief valve which suppose to open and protect the compressor.
Assuming discharge air flow restricted, would the compressor in 16 seconds increase air pressure to tremendous values that would break the 2nd stage.
attached is a photo of 2nd stage broken shaft
Thanks

 

[georgeverghese]

Though no photo is attached, it should be obvious the PSV wont help if it is located downstream of the 2nd stage discharge valve(s), and all the discharge valve(s) are installed wrong.
By the way, another protection mechanism for high shaft torque in this case would be the motor overcurrent relay..

 

[Esoliman]

Georgeverghese, Pressure relief is installed correctly upstream compressor Discharge valve, "attached drawing"
As mentioned earlier, the compressor started for only 16 seconds and its motor is star/delta starting.
the problem is why the protection didn't operate before stage damage?

 

[rotw]

Not sure what you mean with "closed discharge valve". Compressor line on final discharge has check valve, cooler and separator with PSV in between. If the final outlet is blocked or obstructed, PSV is there to relief the pressure.

BTW, you also depict a pressure measurement on discharge, hopefully this is connected to some PLC or DCS, why don't you look at the historian / trends for process data (pressure levels) before failure?

 

[georgeverghese]

Okay, we see that interstage press trip means stage 1D high pressure trip.

In my previous post, I meant the internal discharge valves within the stage 2 compressor casing, not the external isolation piping valve. Wrong installation of these internal valves can also result in stage 1D PSHH.

By the way, a common cause of shaft breakage in these PD machines is moisture or liquids entrained in the feed, in my experience - this may not be the cause in your case here ? - we see you have a primitive moisture separator for the feed to stage 2.

 

[Esoliman]

rotw, Thanks, it looks like that you have investigated the drawing very well.
What i am going to add is that we have a manual valve down stream the moisture separator which is not shown in the drawing and it is used to isolate the compressor for maintenance.
We have four compressors in parallel, one is duty and the remaining three are stand by.
This compressor was out for suction regulator maintenance and after they have finished the operation tested the compressor and as soon as the compressor started the problem happened, the compressor ran for only 16 seconds from starting to trip.
Unfortunately, the 2nd stage discharge pressure reading is not recorded in historical events and it is used in compressor loading and unloading setting,if it is recorded it would help in the investigation.
The maintenance accusing the operation of starting the compressor with discharge valve closed which led to 2nd stage damage while the operation denied any wrongdoing.
What i am thinking about, even if operation left the valve closed, the damage should not had been occurred as the compressor is equipped with pressure relief valve and ran for only 16 seconds besides it is star/delta started.
Could any one gave and explanation for what really happened.

 

[rotw]

You did mention maintenance on pressure regulator on suction. Is this routine/planned maintenance or has this been triggered by some malfunction, known upfront? what do you know about the history of this machine?

Looks like the pressure regulator acts across the 2nd stage, possibly to maintain stable in/out pressure or keep the operating pressures within a predefined envelop to cope with the process, including (I know medium is air) parameters variations of the 1st stage. Possibly also that the envelop I refer to here is connected to power/torque limits which are not to exceed.
Corollary of this is that pressure regulator acts as safeguard for the 2nd stage, including for the absorbed power such that it does not exceed predefined limits (note here that one motor shares its power between two stages...). If something goes wrong with the regulation itself then power/torque could increase above the torque capability of the shaft, of which torque could have been made available by the motor, all in all creating a undesired combination where shaft breakage occur.
What does the operating manual says? do you have certain forbidden zone(s) of operation with regard to torque limits?

Especially because this was a test condition, I deem there is no guarantee that the regulator fulfilled the function it was designed for. Possibly this should have been functionally tested apart first, before it is fitted on/tested with the machine.

 

[zdas04]

When you say "rotary screw compressor" is it oil flooded or dry?

In either case (the big difference is that in a dry screw, the timing chain will always break before the drive shaft), if the compressor case and the discharge piping were liquid filled (against a shut discharge valve) you would reach torque numbers adequate to break a drive shaft before the PSV could operate. If there were any gas volume in the process then the PSV would have time to protect the second stage from mechanical damage. The only scenario that makes sense is you started a PD machine full of liquid with no place for the liquid to go. I've done that. Pressure increased so fast that you couldn't see the needle on the pressure gauge move, it just seemed to jump from zero to full travel in an instant. Luckily our drive-coupling failed before anything expensive broke, but I see a lot of PD compressors with couplings that are stronger than the shaft.

[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

 

[Esoliman]

zdas04: The compressor is oil free "dry"
Could you explain why the timing gear should damage before the drive shaft, could you please provide supporting documents.

 

[zdas04]

Esolinman,
Sorry, do I work for you? For free you get statements that amount to opinions, hire me and I'll provide documentation. You seem to have very high expectations of free consultants.

My experience has been that I've dealt with 4 broken timing devices (chains, belts or gears depending on the vintage and manufacturer) and zero broken shafts or couplings on dry screws.

[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