4 replies to this topic

[John000]

Hello Everybody
Feels great to be a part of great team having enthusiastic and intellectual contributors. This is my first topic and need your valubale inputs on the same.

I m working on project proposal for compressor revamp. Details are furnished for yr ref -
Exisitng Configuration - 3 compressors in parallel, Model – Ariel JGT/4 Single stage, Rod diameter – 2”, Cylinder bore – 6.75”, Stroke - 4 ½” (114.2 MM),RPM – Max 1500, No of Throws – 4
Power – 1940 kW (2600 Hp)
Gas stream – Natural gas with water/or heavy hydrocarbons (Methane < 90%, SG> 0.7, Propane > 8%)

Driver Details
Waukesha Gas Engine, Model – L 7042 GSI, Governing speed -1200 rpm, Power – 1458 Hp/ 1054 kW

Designed operating conditions
Suction pressure – 31 Kg/cm2, Discharge pressure – 79 Kg/cm2, suction temp 60 deg c
Capacity – 0.66 mmscmd, Temp – 35 deg C

Proposed configuration - Suction pressure – 10 Kg/cm2, Discharge pressure – 75 Kg/cm2, Flow compromise is acceptable.
I need to design either Single compressor or two compressors in Series?

Will u please advise on how to go about it and your recommendations for performance / concerns / modifications required in system?
Thanks in advance for gr8 help

John -

[Zauberberg]

I'm not an expert on the subject (recipe machines) but here are a few practical tips:

- Accounting for 35C inlet temperature, the proposed pressure ratio will result in >200C (or >400F) compressor discharge temperature (single-stage machine). For majority applications, I believe 135C can be considered as practical maximum. 175C is the highest discharge temperature I have seen so far.
- Mechanical reliability issues such are rod loading and packing life will probably also call for lower compression ratios and lower discharge temperatures.
- Higher compression rations result in lower volumetric efficiency of the machine, and therefore bigger compressor (larger cylinder) will be required for the same unit capacity. In your particular case, it means that you will loose some of the capacity on existing machine.

It would be good if you can explore API 11P "Specification for Packaged Reciprocating Compressors for Oil & Gas Production Services" while we are waiting for Art Montemayor - he's the expert on this subject.

[S.AHMAD]

Hi John

The choice of the number of compressor in series with inter cooler is determined by discharge temperature and pressure ratio limitations.
The maximum allowable discharge temperature is 150C for most hydrocarbon system except for hydrogen services where it is limited to 135C.
The maximum compression ratio is normally limited to 5 for low suction pressure and to 2.5 for high suction pressure (>70 bar).The pressure ratio is limited by the stress imposed on the piston rod. Your new conditions give a pressure ratio of 7 which is definitely requires 2 stages compression system. Existing system pressure ratio is about 2.5 which is acceptable.
I suggest you conduct an optimization study to determine optimum compression ratio for each stage. Normally the 1st stage is set at higher ratio than the 2nd stage.

Hope the above gives you some idea where to begin.

[Art Montemayor]

John:

You should furnish all the basic data involved, such as:

• Is each Ariel compressor driven by an individual Waukesha engine?
• What is the rated maximum horsepower of each Waukesha?
• What is the horsepower input of each Waukesha on each compressor at design operating condition?
• What is the design operating rpm of each Waukesha?
• You state that design operating , suction temperature is 60 ^oC; then you say that the capacity is 0.66 mmscmd at a temperature of 35 ^oC. What does this mean? Is your suction 60 or 35 ^oC? I don’t recommend a suction at 60 ^oC. Why don’t you cool it down?
• You say you will accept using a 2-stage configuration. But you state what capacity reduction you will accept with a 2-stage setup. What capacity do you propose?
• Your compression ratio proposed is much too high for a single stage cycle. You need to do 2-stages and this will reduce your capacity.

Go to the Ariel website and download their compressor rating program. Use this program to rate your compressors at the different available and recommended speeds. Consult with Ariel as to what design limits you have on your machines regarding rod loading, speeds, cylinder MAWP, valve velocities, discharge temperatures, and BHP. Using this information, you can start to identify what maximum performance you can expect from your machines and driver(s).

You basically have a high-speed, field natural gas machine that is suited for the service and the duty expected for a certain amount of time. I do not consider this type of Ariel as a long-term, heavy-duty reciprocating machine. It is basically a field machine that is designed for a specific duty and project service time. Beyond that, it must be totally revised and some parts replaced or rebuilt. If it was me and I required long-term service, I would run it at a maximum piston speed of 700 ft per min. Most of the Ariel JGTs I have seen operate from 900 to 1,200 ft per min. This is OK for short-term life expectancy on the gas project, but it depends on what you want and expect from the machine and the application. That is your choice.

If you go to a 2-stage setup, then you will require an intercooler and a liquid-vapor separator in between stages. You should design based on equal work in each stage, which means that the ratio of compression of each stage will be the square root of the TOTAL ratio of compression over both stages. This will indicate to you what the interstage pressure should be – and the 1st stage discharge temperature.

Do not revamp or reconfigure these machines without consulting with Ariel and obtaining their approval on what you propose to modify or change.

[akslzf]

First of all, state your case clearly. Provide a process flow diagram of the existing & proposed conditions. And use the right symbols. "Temp – 35 deg C" that you've mentioned can also be construed to be "minus 35°C". Why at all do you want to run the compressors in series?

Anyhow, one important aspect to bear in mind is this.

Discharge pressure of recip compressors, like all positive displacement compressors, floats with the discharge header. Therefore, despite any drop in suction pressure, the discharge pressure will continue to match the discharge header pressure.
This causes an increase in dP across the compressor, whenever the suction pressure drops, as in the case you've stated here.
From the machine design point of view, an important term is "Rod load reversal", which ensures that a film of oil remains between the cross head & the connecting rod, thereby preventing metal to metal contact. Before increasing the dP across the compressor, check this value (Refer API 618) to ensure that the oil film is not squeezed out, as otherwise, it could kill the compressor.