How To Avoid Oil Dilution With High Dew Point Temperature Gas

By Glenn Schuster

“Oil-flooded screw compressors are great for low suction pressure compressor applications!”  This statement is very much true and screw compressors have found many applications within the Oil and Gas Industry.  However, their advantage over other forms of compression can become a detriment in some low pressure applications.

Why are screw compressors so good for low pressure applications?  During compression, oil is introduced into the gas stream.  The oil is introduced for many purposes: to clean and lubricate the bearings; to clean, lubricate and help seal the rotors.   Beyond these reasons it is also injected to absorb the heat of compression.  This permits many ratios of compression to be achieved in a single stage.  A typical design for screw compressor in natural gas compression will be for 5 to 15 ratios with a design discharge temperature of between 200°F and 220°F (93°C and 105°C).  In these applications, a single stage of compression with a screw compressor can replace 2 or 3 stages of compression with a reciprocating compressor.

When should there be concern over using a screw compressor?  The benefits derived from introducing oil into the gas stream can, in some instances cause operational difficulties.  For example, having the compressor lube oil exposed to the process gas exposes the lube oil to being contaminated by the process stream.  Particulate contamination can easily be filtered out.  However liquid contamination cannot. Liquid contamination can become a concern if the dew point of the gas at compressor discharge pressure is within 40°F (22°C) of compressor discharge temperature. So caution should be exercised if any one of the following conditions are present.

     1) The gas is ‘Hot’.  Tin > 90°F (32°C)
     Hot inlet gas from a source that will be water saturated, ie a well-head, treater, battery,
     tank, etc, will carry a larger percentage of water than it would at a lower temperature,
     thereby increasing the discharge dew point temperature of the gas.

     2) The gas is ‘Heavy’.  SG > 0.90
     As the mole weight of a hydrocarbon gas increases, the discharge hydrocarbon dew
     point temperature increases.  Gases with a high percentage of C3+ should be looked at.

     3) The gas is ‘Sour’ H2S > 10 ppm
     H2S on its own is not a concern for condensation.  However, with the presence of water
     in the gas stream, any concentration H2S creates concerns around condensation and
     the formation of acids.  

Example 1, Effect of temperature on water solubility
Referring to the water solubility graph below from GPSA we can see that at 14.7 psia and 60 °F a saturated gas will carry ~800 lbs water / mmscf of gas.  Keeping the pressure constant and increasing the temperature to 90 °F the same gas will hold ~2000 lbs water / mmscf of gas.

Figure 1
Gas Processors Suppliers Association, GPSA Engineering Data Book, 12th Edition, 2004, Water Content of Hydrocarbon Gas

Example 2, Typical P-T compression curve
In Figure 2, below, we see a representative dew point curve of a heavy mole weight gas. The redline represents a typical pressure/temperature curve of an oil flooded screw compressor. In this example the gas starts out 100% water saturated at about 105 °F and 7 psig at suction. It is then compressed to 150 psig with a compressor discharge temperature of 195 °F. At this point the gas has only 13 °F of super heat which is well below the 40 °F recommended superheat. Running at these conditions water would likely condense on the walls of the oil separator and associated piping systems, contaminating the oil.

Figure 2
Ariel Corporation. Phase Envelope with mark-up.

How can you manage dew point concerns?  To manage discharge dew point concerns proper oil selection becomes critical and control needs to be exerted over the discharge temperature.  So in order of application the means of dew point control are:
  • Select the proper oil for the application.  A dilution check needs to be performed by the oil vendor.  If they cannot do so their oil should not be used in any situation.
  • Control lube oil temperature supplied to the compressor with a 3-way temperature control valve.  As well it is beneficial in colder climates to prevent excessive oil by cooling the oil through an indirect mean (IE.: indirect glycol cooling circuit).
  • Increase the discharge temperature by decreasing oil injection flow.
  • Maintain the compressor in a warm ambient condition.  This means housing the compressor package in an enclosure c/w heating as required for cooler climates.
  • Provide direct heat into the compressor lube oil by means of an immersion heater or external heat source.
  • Prevent heat loss by insulating the compressor discharge oil piping, separator and lube oil piping as required.

In addition to the above measures, further care and attention need to be paid at start-up and shutdown.  A compressor that may be operating without issue at steady-state condition can run into trouble during these times.  Measures that can be employed to manage dew point at these times, in no specific order, are:
  • Blowing down of the oil separator to as low a pressure as possible, either suction pressure or atmospheric pressure at shutdown.
  • Introducing a sweet/dry purge gas at either or both shutdown and start-up.

In closing, enough emphasis cannot be placed on managing dew point concerns.  In applications where dew point is an issue and it is not properly addressed a screw compressor can have a catastrophic failure in a matter of hours.  However, in those same conditions when screw compressor package is properly designed and the high discharge dew point issue is appropriately addresses and controlled, a screw compressor package can have many years of trouble-free operation.

So if you have an application that you believe may be of concern with regards to a high dew point gas, ensure that you are talking to an individual with experience in the field and whom can properly address the concern.

Photo - Compass Compression
1478HP Low Pressure Pipeline Screw Compressor Package

1 comment:

  1. Your work is extremely smart and that i appreciate you and hopping for a few additional informative posts.many thanks for sharing nice info .


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