1. Gas Compression and Flow Dynamics NGT 150
Reciprocating Gas Compressors
Chapter 3
Design of Components
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2. Quality of Cylinder and Valve Design Determines compression efficiency
Determines continuity of operation
Factors in choice of design:
Choice of material
Size of cylinder
Valving
Cooling system
Regulation devices
Piston and piston rod seals
Cylinder heads
Access for inspection
Available area for additional connections

3. Choice of material
Operating pressures
Resistance to thermal or mechanical shock
Nodular iron (p. 2)
Handling of extremely corrosive gases
Special metals or alloys
Wear compatibility of rubbing parts
Piston contacts cylinder bore
Rod contacts sealing rings
Depends whether cylinder is lubricated
If not - thermoplastic rings or rider bands (prevent piston contact with wall)
That is - made of nylon, acrylic, or Teflon

4. 2. Size of cylinder
Influences mode of construction
Large cylinders are cast
Small cylinders or forged
Large cylinders with many valves
Are limited to low pressures
Thus made by casting
Smaller cylinders for higher pressures
Very high pressure cylinders made by forging

5. 3. Valving
Excessive pressure drop through the valves wastes horsepower.
Therefore valve velocities should be kept low
Gas passages to and from the cylinder must be adequately sized to minimize flow losses
Valve selection considerations include pressure level, compressor rpm and compression ratio

6. 4. Cooling system
Low ratio compression does not require liquid cooling.
Some cylinders with sub-zero intake gas temperatures are static cooled – that is, coolant is not forced around the bore
Intermediate temperatures use natural convection (thermosyphon – natural flow due to temperature differential around a closed loop)
High temperatures require force circulation with pumps and coolers.

7. 5. Regulation devices (p. 42)
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
As the capacity is reduced, horsepower is reduced

8. 5. Regulation devices (p. 42) DUPLICATE
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
As the capacity is reduced, horsepower is reduced

9. 5. Regulation devices (p. 42)
Single-acting cylinders compress gas in only one direction of piston travel. They can be head end or crank end.
Double-acting cylinders compress gas in both directions of travel.

10. 6. Piston and rod seals
Reduce the pressure loss around the piston and the piston rod
Thermoplastic materials are used for piston rings and rod packing to prevent metal to metal contact
7. Cylinder heads
Final closure to cylinder. Provides access to the cylinder bore
8. Access for inspection and maintenance
Covers and ports for valving, packing, etc.
9. Available area for additional connections
For lubrication, flushing and gauges

11. Valve-in-Body Design
Used in moderate to low compression ratio applications
Position of the valves allow for higher clearances than cylinders with valves in the head. (See “clearance volume” p. 101)
Valves can be larger and thus provide lower valve velocities for more efficient compression
Cylinders have larger gas passages and thus produce less resistance to gas flow
Some designs us a dry liner. A dry liner does not come into direct contact with coolant. This eliminates joints between water and gas. Where as wet liners do. (dry liner instal. p. 44) (cylinder liners – see page 2)

12. Dry liner characteristics
Installation performed at the factory by heating the cylinder body in an oven
No direct contact with cylinder coolant
Absence of joints that seal both water and gas with a common gasket.
Little stress due to gas load

13. Wet liner characteristics
Removal performed through the disassembly of studs and nuts
Gas load fully on the liner, thus becoming a structural consideration
Advantage in ease of replacement

14. Valve-in-Head Design
Used for low pressure or vacuum pump service
Because of low pressure the density of the gas is low. Consequently efficient operation is maintained.
May be supplied with wet or dry liners
Valve head design presents maintenance problems. Due to stud placement all valve assemblies must be removed requiring extra time.
Double-acting cylinders compress gas in both directions of piston travel.

17. Piston Design (p. 45)
Design depends on type of cylinder pressure and service in which the piston is used
Conical pistons are associated with working pressures below 250 psi
Cylindrical pistons are used for both high and low pressure service
Low pressure, large pistons are usually hollow
Hollow pistons provide proper unit balance and piston wear
Solid pistons are used when strength is a requirement

18. Piston Design
Cast iron, nodular iron, aluminum, and steel are used in the construction of pistons
Physical weight and strength is a factor in material selection
Low weight reduces bearing pressure thus minimizing wear
Cast iron and nodular pistons are low friction material
Aluminum and steel are higher friction material and are often fitted with thermoplastic wear band material

19. Pulsation (Society of Petroleum Engineers)
The flow of gas through a reciprocating compressor inherently produces pulsation because the suction and discharge valves are not open for the entire compression stroke.
Pulsation damping is needed to create a more uniform flow through the compressor to assure uniform loading and to reduce piping vibration levels.

20. Pulsation control
Intake and discharge of a reciprocating compressor are of a pulsating nature
If the pulsations are not taken into consideration in the design – problems can arise
Pressure waves (or pulsations) amplified by resonance may build up and cause destructive vibrations
Problems from these excessive pulsations include low compressor capacity, valve failures, excessive engine load and/or destructive pipe vibration

21. Pulsation control
Troublesome pulsations can be avoided by:
Proper use of adequate intake and discharge piping and
The installation of proper dampening or pulsation bottles to the compressor inlet and discharge flanges of the cylinders
A single bottle may serve one or more adjacent cylinders
The capacity or cubic content of a pulsation bottle depends on the number of cylinders connected to it, gas delivered by each stroke, and the timing of the impulses.

22. Pulsation control (SPE)
Adequate piping - long straight runs of piping of the same diameter as the compressor cylinder line connection, and the stage power is less than 150 hp.
The addition of orifices at key locations in the piping can also reduce piping pulsations.
For most applications, volume bottles or pulsation vessels with internal baffles and/or choke tubes should be located as close to the cylinder as possible for optimum valve reliability.

23. Pulsation control (SPE)
Suction scrubber (p. 24) - Ingestion of liquids into the compressor through the inlet gas stream can cause damage to the compressor internals. For this reason, an adequately sized suction scrubber with provisions for draining is required.
The scrubber may be part of the pulsation control when properly planned.

24. Valve Design
A compressor valve is a device used to permit relatively unrestricted flow of gas in one direction, but to block all flow in the opposite direction.
Each operating end of a cylinder must have two valves or sets of valves – one to admit gas for compression and the other to discharge gas after compression.
Compressor valves are used over a wide range of gas conditions – dry & clean, wet & dirt, or corrosive.

25. Valve Design
There are five kinds of valves –
Channel
Poppet
Feather
Plate
Ring
Channel and plate are the most common
Two basic objectives in valve design
A valve that has the least possible resistance to flow
A valve that will have a very long lifetime

28. Valve Design
All valves today are automatic in operation
They open and close solely through differential gas pressure between the inside of the cylinder and the cylinder gas passage.
Closing is assisted by a spring.
Every time a valve opens there is an impact against the stop plate, and every time it closes there is an impact against the seat.
The majority of valve failures are due to impact