Large multistage, multi-cylinder, double-acting crosshead designed reciprocating compressors are most commonly used to move natural gas within the natural gas production industry, generate low pressure plant air, and supply high-pressure gas for oil well drilling. While low speed reciprocating compressors can be used for a variety of other processes, they are most prevalent in industrial plant air requirements and gas transmission pipeline applications.
Types of Reciprocating Compressors
When is comes to reciprocating positive-displacement machines, there are two general types:
- Low speed reciprocating compressors (integral)
- High speed reciprocating compressors (separable)
For the purpose of our article today, we will focus on integral (low-speed reciprocating compressors).
Integral Low Speed Reciprocating Compressors
The word “integral” is generally used in association with this system. This is because of the compressor’s main drivers, the power cylinders, which are mounted integrally within the frame that contains compression cylinders. Integral units are known for their lower speeds – averaging between 200 rpm and 600 rpm. These speeds are generally preferred for use in pipeline service and gas plants where long life and fuel efficiency are of critical importance. Integral, low speed reciprocating compressors may also be supplied with anywhere from 2 to 10 compression cylinders and powers ranging from 140 hp to 12,000 hp.
Integral compressing units require less maintenance than separable units and offer a favorably high efficiency over an array of operating conditions. However, integral units typically require higher degrees of pulsation and vibration suppression, heavy foundation, and must be field erected. This initial cost is higher than separable units.
Major Components
Low speed reciprocating compressors are available in a collection of arrangements and designs; however, within these options the major components stay the same:
Frame
A reciprocating compressor unit’s rugged and heavy housing is comprised of the crosshead guide’s and cylinder’s rotating parts. Frames are rated by manufacturers in regards to frame load and maximum continuous horsepower. Engine-power cylinders and the compressor are generally mounted on one side of the frame and driven by the same crankshank (not balanced-opposed like separable).
Cylinder
A cylinder is a type of pressure vessel that holds gas during the system’s compression cycle. Cylinders that are single-acting compress gas in a specific piston travel direction. This can be located on either the crank or head end. On the other hand, cylinders that are double-acting work to compress gas at both ends of the piston travel. The majority of reciprocating compressors utilize double-acting cylinders.
A cylinder material choice will be determined by pressure of operating. Cast iron is the general selection for pressures up to 1,000 psi, nodular iron for pressures ranging from 1,000 psi to 1,500 psi, and cast steel for pressures ranging between 1,500 psi and 2,500 psi. Running part wear compatibility is also a consideration for material selection (seal rings and piston rod, cylinder bore and piston rings, etc.). Rider bands and thermoplastic rings are the most prevalent in reciprocating compressors in order to reduce wear.
Distance Piece
This component provides a separation between the compressor frame and cylinder. Distance pieces can either hold a double or single-compartment arrangement. Within the single design, the space between the diaphragm and cylinder packing is lengthened so that no aspect of the rod enters both the cylinder stuffing box and crankcase. Oil is migrated between these two components. When contamination is a concern, an oil slinger will halt the packing oil from infiltrating the compressor frame. In the presence of a toxic service, a two-compartment design is implemented.
Piston
The piston, set on the end of the piston rod, acts as a movable barrier within the compressor cylinder. Material selection is based on compatibility, weight, and strength of the gas that is being compressed. This component is generally constructed from a lighter material such as steel or cast iron with a hollow core or aluminium. Rider or thermoplastic wear are fitted to minimize the risk of piston-to-cylinder contact and increase ring life, in addition to distributing piston weight.
Bearings
All throughout the axial positioning and compressor frame are bearings. Main bearings are fitted within the frame in order to position properly into the crankshaft. Located at the bottom and top of each crosshead are crosshead bearings, while the center of the connecting rod and crankshaft houses crank pins. Most of the bearings utilized by reciprocator compressors are lubricated, hydrodynamic bearings. Oil and gas that has been pressurized is supplied to each bearing grooves located on the bearing surfaces. These grooves are specifically sized to prevent overheating and to ensure adequate oil and gas flow.
Compressor Valves
The primary functions of compressor valves are to block the flow of gas in undesired directions, while permitting gas flow in the desired directions. Each one of the compressor cylinder’s operating ends must hold two sets of valves. The of suction valves (inlet) work to allow gas to enter the cylinder. A set of discharge valves are used to eliminate compressed gas previously located in the cylinder. The manufacturer typically makes a specification of valve size and type.
Plate Valves
One of the most common valve types, plate valve, is fashioned from rings that are connected by webs into a singular plate. Depending on the plate material, the valves have the ability to handle temperatures up to 500°F, speeds up to 2,000 rpm, differential pressure to 10,000 psi, and pressures as high as 15,000 psi. The inherent advantages and benefits of ring valves include:
- Low repair cost
- Moderate part cost
- Strong ability to handle liquids
Poppet-Style Valves
Poppet-style valves provide performances that are superior to concentric ring and plate valves. The poppet style utilizes round, separate poppets to sit against holes located with in the valve seat offering a low-pressure and high life drop, along with better efficiency of fuel. These valves are most commonly used in natural gas pipelines, conditioning of natural gas, and natural gas processing facilities. Poppets crafted from metallic work well at:
- Temperatures of 500°F
- Speed up to 450 rpm
- Differential pressures up to 1,400 psi
- Pressures up to 3,000 psi
Thermoplastics Poppets
Thermoplastic poppets on the other hand can be utilized in applications with:
- Temperatures of 400°F
- Speeds up to 720 rpm
- Differential pressures up to 1,500 psi
- Pressures up to 3,000 psi
Why Are Reciprocating Compressors Important?
Reciprocating compressors and the stations in which they are housed are crucial to the natural gas transportation system. It’s components and the quality of their installation enables natural gas to travel through this network of pipelines to destinations around the world. This is vital not only to the regular flow of gas through the network, but also in rerouting gas to storage areas during periods of lower demand. Above all, low speed reciprocating compressors allow natural gas to be transported safely and efficiently throughout all stages of the gas transmission pipeline application.