Reciprocating Compressor ROD Drop?

[10815L]

Hi,
We got ROD drop alarm and compressor was changed over with stand by compressor, when compressor was unloaded to zero load, lot of fumes was observed by field operator and on opening of inspection window we found cross head shoes are broken.
My question is, if ROD drop alarm is there then piston ring must go and 2nd question is what is the relation with ROD drop and cross head shoe damage.
Please explain about this condition.
Thanks
108/15L

 

[MintJulep]

One condition that can cause a compressor to break a connecting rod is trying to compress liquid.

When that happens lots of stuff gets damaged.

 

[DLite30]

The cross head supports the piston rod. A decrease in the thickness of the crosshead bearing surface will "drop" the piston rod the same amount, and ends up with the rod moving in a motion like a saw instead of straight on.

 

[CaracasEC]

One purpose of the rod drop monitoring is to know the wear of the wear band. If the thickness of the wear band already reach the setting of the rod drop (proximity sensor) then an alarm will be triggered. There will be a lot of causes for the cross head shoe damage and troubleshooting this would require the system to be analyzed. Since we are dealing with a positive displacement compressor, we have a variable head, constant flow and is not self limiting. So head will be developed corresponding to the available driver power. Thus a pressure relief valve is installed at the discharge, and is this working/calibrated. How about the lubrication system? Is the flow and pressure sufficient to dissipate the heat? Was the crosshead shoe in proper place. Usually, the crosshead had its "C" Shoe and an "O" Shoe.

 

[10815L]

Hi,
Before we changed the compressor with standby, Lube oil DP start increasing and went upto 2.1barg in 40 minute, after 30 minute and then ROD drop alarm appread and once we changed the compressor lot of fumes generated in the cross head compratment.
One thing to be remember in this problem is, lube oil supply pressure remained constant even DP across the filter went hi 2.1 barg, normal DP is 0.35barg.
Hope you can explain more about this problem.
Thanks
10815L

 

[CaracasEC]

I consider your compressor to be a double distance piece design as per API 618. The fumes could be on the distance piece near the piston rod packing. Fumes could be the result of a leak in the piston rod packing that mixed with the oil. For the lube oil pressure to remain constant, im sure that your lube oil pump is a positive displacement pump which could be a rotary type that is shaft driven because if it is a centrifugal type, for a higher delta P the flow based on the characteristic curve will turn to the left of the BEP as the head go up. The high delta P of the filter should alarm the DCS to switch on to other filter as this is a duplex type im sure of. Once the filter was not switch to the other, though you have enough pressure as a characteris of a positive displacement pump having variable head and having a constant flow up to the filter, then the amount of flow from the filter affected by a high delta P would mean that the amount of lubrication oil to the system is not sufficient enough. Normal flow of the lubrication if using the crankshaft driven pump would be a suction filter form the crankshaft casing to the pump then to the main bearings,connecting rod bearings then to the crossheads thru the connecting rod. If you are using the standby pump the normal flow would be pump to the discharge filter, to the crankshaft main bearings, connecting rod bearings and then to the crossheads thru the connecting rod. The root cause of this problem could be due to insufficient amount of lubrication oil to the crosshead shoe. The rod drop alarms could be because of the thin film that is equivalent to the amount of sufficient lubrication oil is beyond the design limits which affect the position of the shaft and the setting limit of the rod drop sensor (proximity probe).

 

[10815L]

Hi,
Main oil pump is crankcase driven and auxiliary is centrifugal type.Therefore, we does not change the pump.
Filter was changed over but second filter DP was also increased within 10 minute of change over.
Thanks
10815L

 

[CaracasEC]

This means that your main oil pump that is crankshaft driven is a rotary type probably a gear pump and your auxiliary pump is a centrifugal pump driven by motor. You are using dual filter and as soon as the delta P becomes high you switch to the other filter and after 10 minute change over DP increased then a rod drop alarm was triggered. Upon unloading, fumes was observed at the inspection window and upon opening at compressor shutdown the crosshead shoe is damage. The fumes is a result of the piston rod packing leakage. The piston rod packing leaks because your rod drop / piston rod runout has exceeeded its design/maximum limit.
What is your maintenance record of the piston rod run-out on your last overhauling? How about your piston rod deflection? Are they within limits? How about your crosshead shoe clearances? Are they within design limits?

 

[10815L]

Hi,
Please correct the Lube oil pumps, our auxiliary lube oil pump also gear pump.sorry about my typing mistake.
Rod Drop alarm limit is -0.8mm.And fumes were generated at cross head side not in piston side.
Thanks
10815L

 

[CaracasEC]

Please confirm if your compressor, is a double distance piece design as per API 618. Also, are the limits during your last overhauling within design limits 1. Crosshead shoe clearance 2. Piston Rod Run-out 3. Piston Deflection 4.Wear Band Clearance

 

[DLite30]

CaracasEC:

3. Piston Deflection

That's a new one to me.

What exactly are you talking about?

 

[CaracasEC]

Hi Dlite, its a piston-rod deflection, not piston deflection...refer to API 618 for a detailed explanation..thanks

 

[10815L]

Hi,
I'm operation guy and data you asked not in my access but I will try to get it from reliability department.
thanks
108/15l

 

[DLite30]

ah...you're talking about rod sag.