1. Natural Gas Production Chapter 7 Compressors and Prime Movers
PTRT 2323
Natural Gas Production
Chapter 7
Compressors and Prime Movers

2. Compressors
Most common item of equipment in handling and transporting natural gas
Vary in size
Belt-driven 50 HP
15,000 HP and higher
Most common – gas-engine-driven reciprocating unit
Gas-turbine-driven centrifugal units are coming into use
Electric-motor and steam-turbine driven for special circumstances rarely found in field operations

3. Reciprocating Compressors
Driver (Prime-mover) – internal combustion gas engine
Thermal efficiencies approaching 40%
Steam turbine only about 33%
Auto engine only 25% efficient
Numerous types of engines listed on Page 87 of the text.

4. Belt-driven Gas Engine Unit
Engine crankshaft connected to compressor crankshaft with a belt
Usually operate at high speeds

5. Direct-drive Reciprocating Unit
Engine crankshaft connected to compressor crankshaft with a coupling or clutch
Usually operate at high speeds

6. Integrally-connected
Compressor cylinders mounted on the engine frame
Usually operate at low to intermediate speeds

7. Prime Movers
All types of gas-engine are basically reciprocating machines with rotating crankshafts
Fundamental difference lies in the combustion cycle
Two-cycle or four-cycle
No clear advantage of one to the other

8. Two-stroke engine
Two piston strokes for each single rotation of the crankshaft in each complete cycle
Exhaust ports open by being uncovered by the cylinder during the stroke
Scavenger piston preps the new charge of air to support combustion

9. Typical two-stroke design

10. Four-stroke engine
Four piston strokes and two rotations of the crankshaft in each complete cycle
Cycle begins with intake stroke with piston at top and intake valve open. As piston moves down intake valve is opened and fuel/air mixture is drawn into cylinder

11. Four-stroke engine and compressor

12. General Comments
Fuel consumption of slow-speed integral units varies between 9,000 and 7,000 BTU/bhp/h depending on type and make
Lower end requires turbocharging (supercharging)
Maintenance costs are approximately the same for two-stroke or four-stroke units
As engine size increases unit costs ($/bhp/yr) generally decrease.

13. Horsepower Ratings
Depend on amount of air that can be supplied to power cylinders.
Definite fuel/air ratio is require to support combustion
Naturally aspirated
Supercharging
Additional air-scavenging cylinders (two-stroke only)
Larger air scavenging cylinders (two-stroke only)
Belt-driven air blowers
Electrically-driven blower
Centrifugal compressor driven by exhaust gases (turbocharger)

14. Supercharging

15. Compressors Majority are reciprocating piston type Ringed piston
Cylinder
Cylinder head
Suction and discharge valves
Mechanical linkage to convert rotary to linear motion
Connecting rod
Crosshead
Wrist pin
Piston rod

16. Typical parts of reciprocating compressor

17. Compressor cylinders Materials dependent on rated capacity Solid-body
Cast iron (good to 1500 psi)
Nodular iron (ductile iron)
Cast steel (good to 2500 – 3000 psi)
Forged steel (good for higher pressures)
Solid-body
Cylinder changes require boring
Limited changes allowable
Liner-type
more flexible since liner/piston combination can be inserted

18. Theoretical Considerations
Ideal Gas Law PV = ZnRT
Work = Force x distance (no physical NET movement means no work has been performed)
Force = action that produces the motion
Energy = potential for doing work
Potential
Kinetic
Power = rate of doing work (ft-lb/min)
1 HP = 33,000 ft-lb/min
Velocity = speed of body in motion (ft/sec). Gas molecules are always in motion except at absolute zero temperature. Pressure is determined by the average velocity of the molecules of gas

19. Thermodynamics of compression
Isothermal compression – compression of gas under conditions that keep the temperature constant
Adiabatic compression – compression of gas under conditions that prevent heat transfer between the gas and surroundings (i.e rapidly compressed)
Compressors operate under neither condition perfectly, however these are useful approximations that make predictions possible

20. Specific Heat
Expressed in BTU – amount of heat required to raise 1 lb of gas by 1 ⁰F
Constant volume (Cv)
Constant pressure (Cp)
Specific heat varies with temperature (except for monatomic gases)
Ratio of specific heats called N-value or K-value of the gas
For air N = Cp / Cv = / = 1.406
Dry natural gas N = 1.265
Wet casinghead gas N = 1.1
Use engineering curves to find the correct value
Power required to compress a gas is very dependent on the value of N

21. Compression Ratio
Defined as discharge pressure divided by suction pressure
Example: intake pressure at 0 psig (14.7 psia) and discharge pressure at 40 psig (54.7 psia) has a compression ratio of 54.7/14.7 = 3.72
Compression ratio is generally limited to less than 5.5 usually between 2.5 and 5.0
When compression larger than these values is required, two or more stages of compression are used
If you need to compress 500 psig up to 4000 psig that requires /514.7 = 7.8 compression ratio
Must use two stages each with about 2.79 compression ratio to accomplish this

22. Delivered Volume Affected by clearance of the unit.
Clearance is the space reserved for mechanical tolerances plus valves, etc. “unused” volume to participating in compression
Percent clearance is defined as
For small diameter cylinders this difference in volume can be quite large

23. Volumetric Efficiency
Piston will not usually deliver the geometric volume available
Wire drawing (throttling effect of the valve stems)
Heating of the gas during suction
Leakage past valves and rings
Re-expansion of residual gas trapped in clearance pocket spaces (by far this has the greatest effect)
Given by one of two simple expressions involving several thermodynamic terms

24. clearance
AROD
AHE
ACE

25. Piston Displacement Single-acting cylinder Double-acting cylinder
A and S are sq in and in respectively converts cu in to cu ft

26. Cylinder Capacity
Cylinder capacity does NOT equal the piston displacement: Why? =1
Converts cfm to Mcf/d

27. Rod Load Also called pin load, frame load Manufacturer defines a limit
Force = pressure x area
Rod load is the force difference between the two ends of the cylinder
Will usually be different in compression that in tension
Usually lower for single-acting than double acting compressors
Note: area of crank end equals area of head end minus the cross-section of the rod

28. Discharge Temperature
Difficult to calculate except for adiabatic case
Ratio of specific heats
Compression ratio

29. Efficient Utilization of Horsepower
Efficient use of compressor involves running the engine at design speed
Capacity is dictated by a variety of factors:
Cylinder clearance
Suction pressure
Suction temperature
Discharge pressure
Speed
Properties change from day to day so a certain flexibility is designed into the compressor but horsepower is generally fixed

30. Altering compressor size
Change cylinders (time consuming and expensive)
Change cylinder capacity by adjusting the volumetric efficiency using changes in the clearance
Remove portion of the cylinder end
Add spacer ring between the cylinder head and the body
Shorten the projection of the cylinder head into the cylinder
Raise the suction and discharge valves by installing spacer rings
Installation of clearance pockets (unloaders) is a very common solution (adjustable or fixed)

31. Head-end Fixed-volume Clearance Pocket
Variable-volume Clearance Pocket

32. Compressor Selection COST Specific HP requirements
Matching existing units
Unbalanced loads
Cylinder selection and arrangement
Portability
Maintanence
Project life

33. Safety Considerations
ANSI Code for Pressure Piping
B31.1 Petroleum Refinery Piping
B31.8 Gas Transmission and Distribution Piping Systems
Special provisions for venting to avoid excess pressure (through manifolds) on low-pressure equipment
Starting and stopping instructions posted
Auto shut-down devices
Low lube oil pressure
Overspeed
High jacket water temperature
Two shut down methods
Ground the magneto
Block and vent the fuel gas lines
Scrubber upstream of each compressor stage w/ auto drain and high level shut down – WHY??? Liquids cannot be compressed equipment breaks instead

34. Safety Considerations
High discharge temp shutdown
Low suction pressure shutdown
Incorporate a speed controller to maintain approximately constant pressure OR cycling regulator to send discharge gas back to the suction side of compressor BOTH waste horsepower but are often required to maintain satisfactory operation
To prevent excess suction pressure a pressure-reducing inlet regulator or a back pressure regulator to flare can be used
Wells must auto shut in if flare is not used or gathering system MUST be designed to handle the MAX wellhead pressure
Manual shut down stations physically removed from compressor area (usually two on perimeter of site)

35. Review Compressor Start up document
Review Chapter 7A slides
Simtronics Reciprocating Compressor Simulation