1. Gas Compression and Flow Dynamics NGT 150
Reciprocating Gas Compressors
Chapter 5
Operation
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2. Compressor Unit Capacity
Factors that affect compressor unit capacity and developed horsepower include:
Compressor cylinder clearance
Cylinder displacement
Suction pressure
Suction temperature
Discharge pressure (compression ration)
Ratio of specific heats (K value)
Speed

3. 1. Compressor cylinder clearance (p. 76)
The flow capacity of a cylinder is proportional to the percent clearance (CP) per volumetric efficiency equation.
Therefore since percent clearance (CP)is determine by dividing clearance volume (VC) by piston displacement (D).
The flow capacity of a specific cylinder (with fixed pressure and temperature) is then a direct function of the amount of clearance (VC) in the cylinder in cubic inches

4. See Regulation devices (p. 42)
Regulation devices are used to vary the amount of gas being delivered to the cylinder.
They are located on the cylinder
Two basic methods are used:
Hold the intake valve open with a plunger which causes the gas to be pushed rather than compressed
Change the amount of the fixed clearance in the cylinder. Special valves open or close to increase or decrease the cylinder volume or capacity (manual & automatic)
As the flow capacity is reduced, horsepower is reduced. The compressor is unloaded.

5. Vary flow capacity and horsepower.

6. Varying compressor cylinder clearance
Permanent clearance
Remove a small portion of the end(s) of the compressor piston.
Add a spacer ring between the cylinder head and cylinder body
Shortening the projection of the cylinder head into the cylinder. (problem if head is water cooled)
Temporary
Installation of head-end fixed volume clearance pocket (fig. 48)
Insertion of a side passage plug (fig. 50)
Attach an external clearance bottle opened by hand wheel
Attach a valve cap clearance pocket opened by a hand wheel

7. Clearance Pocket

8. Varying compressor cylinder clearance
Excessive amount of additional clearance can sometimes cause slamming of the discharge valve.
When clearance is added to a cylinder the discharge takes place later in the stroke.
If enough clearance is added no gas will be discharged at all.
This condition is called “shutoff” or “standoff”.

9. Compressor cylinder clearance equipment summary
Head-end fixed or variable clearance pockets
Side-clearance passages
External clearance bottles
Crank-end or head-end valve cap clearance pockets
Suction valve lifters or deactivators
Inlet valve unloader
Inlet port unloader

10. 2. Cylinder displacement (see slide 2)
Almost all compressor cylinders are double acting
Flow capacity can be significantly reduced by unloading one end
Best method – deactivate or lift suction valves on one end
This can be done manually when compressor is shut down
Or by use of a suction valve unloader or lifter when in operation (Fig. 53)
This prevents one end of a double acting cylinder for compressing gas.

11. Suction and Discharge Valve

12. “Raising” a Valve Lifter
Open – not seated
Closed – seated

13. 3. Suction pressure
As suction pressure increases, the cylinder flow capacity increases, but the horsepower per million cubic feet of gas required for compression will decrease.

14. 3. Suction pressure
As suction pressure increases, the compression ratio decreases
The decrease in compression ratio reduces the horsepower required
But the flow capacity has increased (due to higher suction pressure) at a faster rate resulting in an overall increase of horsepower required (paradox)
Clearance must be added to avoid overloading the prime mover or driver

15. 4. Suction Temperature
Cylinder flow capacity is inversely proportional to suction temperature.
As suction temperature increases gas expands
As the volume is fixed - less gas will fill the cylinder
Required horsepower is not affected as temperature varies
Thus a reduction of suction temperature will increase compressor flow capacity – without requiring more horsepower
So precooling of gas can optimize cylinder capacity (Valerus – cooling between stages)

16. 5. Discharge Pressure
In general changes in discharge pressure will have little effect on the cylinder flow capacity
Flow capacity will decrease when discharge pressure is increased
However horsepower will increase significantly
Because horsepower is almost directly proportional to compression ratio

17. 6. K Values
K values are ratios of specific heats
Specific heat – raise 1 gram of substance 1 degree C in temperature
K values: 1.15 for propane, 1.25 for natural gas, and 1.40 for air
A higher K value will result in higher volumetric efficiency for a compressor cylinder
Thus a cylinder will have a higher flow capacity compressing natural gas than propane.
However, gas composition cannot always be controlled.

18. 7. Speed
Cylinder capacities are directly proportional to engine speed and developed horsepower
If compressor speed is reduced the flow capacity of the compressor will be reduced (and vice versa)
Likewise developed horsepower will be reduced
Adjusting engine speeds to maintain suction pressures at a desired level is a common practice

19. Efficient use of horsepower
Changes in field operations can affect existing gathering system pressures
Appropriate adjustments in should be made in compressor cylinder operation to fully utilize available horsepower
These include:
Controlling suction pressure
Introducing gas interstage
Fuel economics
Dual service compressors
Controlling discharge pressure

20. Controlling suction pressure
If field gas production is increased then suction pressure can be increased while maintaining existing horsepower.
As suction pressure increases cylinder flow capacity increases
Field methods include:
Careful scheduling of production days (p. 82)
Looping existing gather systems
Increasing field separator pressure (p. 82)

21. Introducing gas interstage
A high pressure gathering system that utilizes high pressure separators may be more economical than adding another compressor
Fuel economics
Operating a smaller number of compressors at full load rather than a larger number at partial load
Dual service compressors
Utilizing a spare compressor to operate at other suction or discharge pressures to support the remaining compressors

22. Controlling Discharge Pressure
Though rather insignificant – reducing pressure drops on the discharge side of a compressor does save horsepower.
Reducing unnecessary pressure drops caused by control valves, pulsation dampening devices, and small piping should be considered

23. Operating cautions
Care should be exercised when making changes in operating conditions or modifying compressor cylinders.
Make sure the new operating conditions do not exceed design limits of the compressor
Rod loads and horsepower requirements should be calculated to determine if the cylinder can be operated under the new conditions