Condition monitoring of Recip Air Compressors 4

[NeilSF]

I have been tasked to come up with a plan for the maintenance our compressed air system. I was wondering if anyone is aware of a good condition monitoring regime for recip air compressors. I'm interested in the state of the cylinders and of the pistons. Two ideas are listed below.

-max flowrate at set pressure.
-max pressure reached at zero flow.

In your experience is there any value in condition based maintenance of said machinery or is it best to simply stick to the manufacturer's time based recommendations.

We have 4 Bellis and Morcom recips each capable of compressing 1200 normal m3/hr to 9 bar.

Thanks for any thought given to this.

Regards,
Neil. Scotland.

 

[quark]

I do check flow rates periodically by receiver fill method and it will be rather easy if you have flow meters installed. However, it is always better if you have retrospective data with you.

I warn you about max pressure reached at zero flow as there is no significance for it with PD machines and can be disastrous. PD machines develop pressure with a blocked discharge till they get damaged.

Regards,

 

[pmover]

recommend monitoring operating data (i.e. pressures, temps, etc.) if available. additional preventative or condition based measures would be to sample lubricating oil periodically and perhaps vibration monitoring data. any deviation from baseline is an indicator of an anomaly that should be investigated.

good luck!
-pmover

 

[Montemayor]

NeilForster:

I've operated a lot of recips in the Americas and in Europe, but I'm not familiar with Bellis or Morcom. No matter, I'm confident that I could disassemble both without any instructions and put them back in original state. The reciprocating compressor is probably one of the only mechanical devices that has never changed much from country to country or culture to culture. By "condition based maintenance" I assume you mean: "Don't do any maintenance until something starts going wrong or fails".

I've had to operate recips 330 days/year, 24 hrs/day and I've also had to do the following regarding maintenance:

1) Under an austere operating budget and a penny-smart, dollar foolish General Manager I've had to wait until something went wrong with the machine and a shut down was inevitable;
2) When I had control of the operations budget I always planned a strict and rigidly timed interval when each recip underwent a complete inspection and overhaul (if needed); I kept rigourous logs on the mechanical condition of each of the critical components that I know of by experience - depending on the type, stages, fluids, and other characteristics of each machine. This program was in line with what I learned through the years working with such machines as Ingersoll-Rand, Clark Bros., Cooper-Bessemers, Chicago Pneumatics, Norwalks, Worthington, and many more. I think my program was more strict than the manufacturers - mainly because I had more to lose than the manufacturer in the event of an unscheduled shutdown due to mechanical failure.

I always monitored my recips whenever and whereever I could. The monitoring I did involved those critical components that are unique to each machine depending - as I stated above - on the type, stages, fluids, and other characteristics of each machine. Some of the critical monitoring critical to any recip are the following:

1) The discharge temperature (taken at the immediate valve discharge valve outlet within the discharge chamber) on each discharge valve. This is probably the most important item for a recip and my favorite diagnostic indicator as to the running condition of a recip. This item indicates not only the condition of a discharge valve, but also is an excellent indicator of how the piston rings on a double-acting cylinder are sealing. When ring wear starts to become a factor, the discharge temperature starts to climb due to internal by-passing of the rings and subsequent re-compression of hot gas withing the cylinder. I always painted my compressors with paint on the discharge valve covers that would char or burn at temperatures exceeding 300 oF. Since I have religiously always specified maximum discharge temperatures of 275 oF for all recip stages, I could spot a faulty, over-heated valve from across the room.

2) The suction valve temperature; this indicates the leakage of the seats on the valve. Sometimes I would opt for not monitoring this (because of expense) but, instead, depended on the operators to place their hand palms on the valve covers on a routine basis each time they made their rounds. This method would immediately alert an operator of a leaking suction valve. Today, this is almost impossible with DCS, everything centralized and operations being so down-sized in personnel. With all these managerial "economies", the trade-off is more thermocouples placed on the compressor valves. Sorry, but nothing comes free when monitoring your expensive investment.

3) Compressor frame vibration; this is an attempt to spot any bearing wear and knocking. Depending on your design, you still may have to contend with babbitted bearings and crosshead tolerances that are subject to normal mechanical wear and tear. One test I always did on my running recips was to balance an "Indian Head" Nickel coin on edge on the top of the leading cylinder on the compressor. If the Nickel would remain on edge and not move, the machine was balanced and the bearing were running well. This test has never failed me.

The above items - valves, rings, bearings - have always been the main items of concern for me in any recip design or application. There are other items that must be considered, of course, such as oil lubrication but those are the main ones for me. Strangely enough, in more than 15 years of exposure to recips in the field I never suffered more than 2 shut down events due to mechanical failure - one was a shattered discharge valve and the other was an electric motor drive burn-out. Love, care, and attention (as in any marriage) are items that a reciprocating compressor thrives under. I could continue to write about reciprocating compressors -complete with photos of all the compressors I operated, but a forum is certainly not the place conducive for such a novel.

I hope the above experience is of some guidance for you.

Art Montemayor
Spring, TX

 

[zdas04]

There's not much to add to Art's list. The only additional thing that I do is compare actual conditions to the manufacturer's sizing program. Input actual suction temperature, discharge pressure, clearance, and rpm and then compare the program's interstage pressure and temperature, and discharge temperature to your measurements. If you end up way off, there is something wrong or going wrong with the compressor.

Air service is a bit easier than field natural-gas service since you tend to have an almost constant inlet pressure in air service. In gas service (for the community's general fund of knowledge), a significant reduction in suction pressure (say 10% of design absolute pressure) will starve the first stage cylinders, increase the actual compression ratios, and increase rod loading. You'll see this quickly on interstage temperature. The same thing happens in air service when the suction path gets fouled.

David Simpson, PE
MuleShoe Engineering

http://www.muleshoe-eng.com

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[NeilSF]

All,

thankyou very much for your compreshensive replies.

I shall let you know how I get on.

Best regards,
Neil.

 

[quadtracker]

In addition, take a monthly oil sample for analysis, the lab will check for unusual elements in the oil and alert you to the likely cause. Some labs will graph you samples, and over time you will notice changes before they become critical. Take a daily log of readings, oil temps, intake and discharge valve temps, coolant water in/out temps, oil pressure, oil level (consumption?), check valve unloaders for proper function, etc. Alot of companies are using a PRIDE gun record equipment readings, how it works is , the handheld gun has a keypad in which you enter your data from each piece of equipment that you scan daily. You program the gun to ask the user all the questions you want to monitor and record for each indiviual piece of plant equipment. Each piece of plant equipment has its own bar code in which the user scans daily on his/her route. After the route is completed, the info is downloaded in a computer, where it is stored and graphed to show mechanical changes over time in which you may not notice the small changes in equipment performance. The PRIDE gun is battery operated,uses a lazer scanner for the bar code, is intrinsicaly safe, rated in class 1 div. 1 group c&d areas, (Explosive proof). The program can red flag (Alarm) any reading taken in which you set limits, on temps,levels, pressures etc. The system is used worldwide by BP, but there must be others in use, by other suppliers.

 

[NeilSF]

Thankyou all for your replies regarding the condtion monitoring of recip compressors. Now can I venture another querie?

I am trying to establish the best way to determine the running condition of each of our 4 compressors. We could carry out a full test involving flow rate measurement, power use, etc. We do not have that equipment offshore however, so I was wondering if it is possible to tell tell the condition of the compressor from suction and discharge valve temperatures? I understand that once you have the baseline performance of your healthy machine then these parameters are useful in determining current condition, but when you've no baseline? Can you tell to what extent the machine churning just air back and forth inside if you can't take air flow rates?

I am going off to take all measurements from our compressed air system now and will try to make sense of it on my return.

best regards,
Neil.

 

[zdas04]

Neil,
You probably should have started a new thread with this.

Like I said above, your "baseline" can be the manufacturer's sizing program. I looked at a 3-stage compressor last week that had first-stage temperatures that were 50F higher than the program said they should be. This is an indication that the valves are too stiff and the cylinders were partially filling. Partial filling of the cylinders results in the actual pressure being well below the pressure you read on the panel, which results in higher compression ratios and higher discharge temperatures. The fix in this case is to reduce the spring stiffness.

Using the program as a baseline has worked very well for me.

David Simpson, PE
MuleShoe Engineering

http://www.muleshoe-eng.com

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

The Plural of "anecdote" is not "data"