Reciprocating Gas Compressor Systems Reports Example

Abstract

Reciprocating compressors are pumps that force gasses into a limited and confined space thereby converting electric or chemical energy into potential energy by raising pressures. Reciprocating compressors are some of the most important equipment in many industrial setups. Large and critical industries such as gas pipelines, petroleum refineries, and petrochemical industries heavily depend on compressors. Poor compressor performance or faults could lead to high losses if operations stall or slow down due to inefficient compressor operation. Therefore, compressor selection, operation and maintenance are very important aspects of plant design and operation.

Introduction

A gas compressor system adds potential energy to a fluid by reducing its volume. A pump is used to compress the gas into an air tight cylinder where it is stored under pressure pending its application. Modern compressors are equipped with automatic control devices such as pressure sensors and gauges so that they can independently service a pressurized fluid requirement. Compressors vary in type and power according to application and fluid pressure requirements. The mode of gas compression in compressors, either through rotary impellers or by reciprocating pistons defines the type of compressor and the power produced. This paper details the mechanisms and operation of reciprocating gas compressors.
Compressors are classified according to their pumping mechanisms. Likewise, pumps can be classified according to the way in which they displace air, i.e. positive and dynamic displacement. Under positive displacement, there are two sub categories, rotary and reciprocating pumps. Under dynamic displacement, the subcategories are centrifugal and axial displacement (Bloch & Hoefner, 1996, 17). For the purpose of this case study, focus shall be given to the reciprocating gas compressors.
Reciprocating compressors have two advantages over other types of compressors: they can achieve high pressure ratios with relatively low mass flow rates and they are also cheaper (American Petroleum Institute, 1989, 54).
Reciprocating compressors compress air through the action of pistons driven by a crankshaft coupled to a motor or an engine. Gas at ambient pressure enters the intake valve and is driven to the compression cylinder by the suction system. After compression, the pressurized gas is driven to storage cylinders where it is stored ready for use. Reciprocating compressors are widely used in oil refineries, chemical plants, pipelines and processing plants among other applications (American Society of Mechanical Engineers, 1975, 33).

Types of reciprocating compressors

Reciprocating Compressors can be classified according to their mode of operation; single acting, double acting, and diaphragm compressors. Single acting compressors have gas acting on one the upper side of the piston only and rely on spring loading to return it to its resting position (Reciprocating Compressors, 2008, 117). Conversely, double acting compressors have gas on both the upper and lower side of the piston head in the piston cylinder. Diaphragm compressors use the action of a flexible membrane to alter the pressure and volume of a fluid. The diaphragm is driven by a rod attached to a crank shaft. Diaphragm compressors are used for pumping toxic or explosive gases and the membrane is designed to handle the pressure loading and the fluid toxicity.
Also, the compressors can be grouped according to their running speeds; high speed or separable compressors and the low speed or the integral compressors. Separable compressors are detachable from their drivers, usually electric motors or an engine and require a gear box in the compression rain. Separable compressors operate at speeds of 900 to 1800 rpm (Bassetto, Neto, & De Souza, 2009, 144). They also possess several advantages over the integral compressors such as:
easy to install,
low initial cost,
portability which make them easy to move from one site to another
low initial cost involved in purchasing the system,

And are available in a range of sizes which makes selection of the right compressor size easy.

Nevertheless, separable compressors are expensive than integral compressors.
Integral compressors have cylinders attached to the compressor frame. These type of compressors run at speeds of 200rpm to 600rpm and are used in pipelines and gas plants where durability and efficiency are paramount. The systems normally have a power capacity of 140 to 12000hp and may be powered by 2 to 10 cylinders. The advantages of integral compressors over separable compressors are high efficiency and low maintenance. Their major drawbacks include high initial investment accompanied by a heavy erection foundation, immobility of the system, and the need for a vibration and pulsation suppression mechanism.

Components of reciprocating compressors

Compressors have many components which vary from one system type to another. The major system components of reciprocating compressors are listed below.

Frame

The frame holds all the rotating and stationary compressor parts in place and it is designed to handle the mechanical stresses due to the machine operation. For this reason, compressor frames are usually rugged and heavy. Separable compressors have their components arranged in a balanced-opposed configuration such that the action of one component on the system frame is cancelled out by an opposing component. For example, the pistons are out of phase by 180 degrees and are separated by the crank web only. Integral compressors have the driving engine or motor and the power cylinders mounted on the same frame. The whole system is powered by the same crankshaft and the power cylinders are arranged on one side of the system only. Therefore, an integral compressor system requires a more robust frame than a separable system (American Society of Mechanical Engineers, 1975, 56).

Piston Cylinders

The piston cylinders house the piston heads and confine the gas during compression. Single acting cylinders compress the gas against the cylinder top while double acting cylinders compress the gas by the piston head or the crank end. The material used in the piston cylinder design depends on the operating pressures. Cast iron is used in compressors working with an average pressure of 1000psi, Nodular iron is used in 1500psi compressors, while cast steel and forged steel are used in 2500psi compression systems.
Cylinders are designed with the maximum allowable working pressure (MAWP) set at 10% higher than the design discharge pressure in order to allow for the installation of a high-pressure safety (PSH) at a point higher than the discharge pressure and also for the relief valve above the PSH pressure rating (Bassetto, Neto, & De Souza, 2009, 79).
Wear and compatibility also play a part in the selection of a cylinder and piston head materials. The moving parts in the cylinder include the piston bore, piston rod, piston rings, and the seal rings which wear over time due to continued friction. Cylinders wear at contact surfaces with the piston rings. Rider bands and thermoplastic rings are used in the design of thermoplastic compressors to reduce wearing.

Distance piece

The distance piece separates the compressor cylinder and the frame. A distance piece can be housed in a single or double compartment setup. In the single compartment system, the area between the diaphragm and the cylinder packing is elongated such that the rod does not reach the cylinder box and the crankcase. The two compartment design is used in toxic environments in which case the rod does not enter the crankcase and the compartment next to the cylinder (Almasi, 2010, 143).

The crankshaft

The crankshaft is found only in reciprocating type of compressors and it rotates along the frame’s axis driving the connecting rods, piston rods, and thus the piston heads. The connecting rod joins the crankshaft to the cross head pin which converts the circular motion of the connecting rod to a linear oscillatory motion that drives the piston within the piston cylinder. The piston rod joins the crosshead and the piston (Almasi, 2010, 145).

The piston

The piston head compresses air raising its pressure due to the force applied by the piston rod. Pistons are made from materials which can bear the pressure loading and remain unaffected by the working fluid. They are normally fabricated from light metals such as aluminum or cast iron. Pistons are fitted with wear bands to reduce frictional wearing. Thermoplastic materials are used to increase ring life and eliminate the risk of piston-to-cylinder contact (Rastin, 2000, 117).

Compressor valves

Compressor valves in reciprocating air compressors direct gases in the required direction and restrict flow in undesired direction. The piston cylinder has two sets of valves, inlet and discharge valves for letting gases in the compressor from the suction side and letting out the compressed gas from the cylinders respectively. Special valves referred to as plate valves are normally used in compressors and comprise of rings connected by webs thereby forming a plate. These types of valves are capable of operating within pressure ranges of 15000psi to 10000psi, average temperatures of 500F, and speeds of 2000rpm (Rigola, 1996, 56).
Concentric rings valves are also used in compressors. According to Rigola (1996), These valves have two advantages over the plate valves; low purchase and repair costs and they can handle liquids better than plate valves. The third type of valves, referred to as the Poppet-style valves are superior to the other two valve types. Poppet-style valves have round poppets which cover the valve holes in the valve seat. This action gives the valves a high pressure resistance and low efficiency drops, which reduces power consumption by the compressor. Poppet-style valves are used in pipeline applications, gas conditioning systems, and other processing facilities. Metallic poppets operate best at given optimum conditions; pressures of 3000psi, speeds of 150rpm and temperatures of 500°F.

Operation of reciprocating gas compressors

Reciprocating gas compressors have a crankshaft which operates a set of pistons that compresses gases in a cylinder. The crankshaft is rotated by an electric motor or a diesel engine. A reserve tank holds the compressed gas from where it is fed to the power tools for application. The compressor cycles between on and off states of operation in order to maintain the gas in the storage tank at a preset pressure level.
A valve system that rests on the cylinder head holds the compressors inlet and discharge valves. These valves are made of two thin metal flaps, one flap underneath and the other on top of the valve plate. During the downwards stroke, the crankshaft withdraws the piston and a vacuum is formed in the cylinder. Gas at atmospheric pressure is pushed by the differential pressure through the inlet valve forcing it open and thereby filling the cylinder. When the crankshaft rotation pushes the piston upwards, the air is compressed thereby forcing shut the inlet valve and pushing open the outlet valve. The gas is then pushed into the storage tank. Air is continuously fed into the storage tank until the required pressure is attained at which point a pressure actuated switch stops the motor.
A regulator is provided to set the tank pressure at required operation levels. Gauges positioned before and after the regulator monitor tank and airline pressure. Also, the tank is equipped with a safety valve which actuates if the pressure switch fails. An unloader valve reduces tank pressure when the compressor is not running.
Most compressors have two or one piston cylinder. The two cylinder models have two strokes per revolution and operate in two stages; the first cylinder raises the air pressure from ambient while the second cylinder increases it further (Rastin, 2000, 116).
Figure 1: Schematic diagram of a single cylinder reciprocating compressor

Efficiency calculation in reciprocating compressors in a refrigerator

Compressor efficiency is an important aspect as it determines the power consumed per work done. Low efficiency will cause high energy consumption with low work output which can make an industrial process uneconomical. Compressor efficiency is evaluated trough the study of the coefficient of performance, which is the ratio of the cooling capacity to power consumption. The compressor efficiency is defined by the volumetric efficiency, mechanical efficiency, volumetric efficiency, and the motor efficiency.
For a compressor working at a given speed and displacement, the cooling capacity is given as a product of the enthalpy change due to evaporation and the mass flow rate.
Q=qmΔH

The mass flow rate is a product of the volume flow rate and the gas density

qm=qvρ

The relationship between suction gas volume and the volume flow rate is given by the equation:

qv=qsuctionf
The volumetric efficiency is the suction volume divided by the piston swept volume in one revolution when other operational factors are held constant:
ηv=VsuctionVswept

The empirical values obtained from the equation above are compared with theoretical results given by the formula:

ηv=1-[τ1/γ-1]ε
The clearance volume ratio is given by the clearance volume over the compressor displacement. The experimental volumetric efficiency is always lower than the theoretical efficiency due to several factors such as:

Valves delay leading to back flow due to the action of pressurized gas in the tank.

Valves leakage
Lubrication oil mixing with the working fluid
Blow-by between the cylinder bore and the piston
Throttling and the fluttering of valves
Incorporating the pressure ratio in the empirical equation above gives:
ηv=1-0.05τ
The induced volume parameter determines the compressor’s capacity in handling air flow and is used as compressor selection criteria. The parameter is measured in standard conditions of sea level and atmospheric pressure. Despite the differences between the induced and delivered gas volumes, the mass per cycle in the two cases must be the same to ensure continuity of operation.
Figure 2: The compression cycle in a refrigerator
The expansion and compression is polytropic, and the value of n is equal to γ. The work done by the Compressor in one cycle is given as a sum of the gross work in the four stages
Work per cycle=(P2V2-P1V1)/(n-1) + P2(V2-V3) +(P4V4-P3V3)/(n-1) + (P4V4-P3V3)/(n-1)
For polytropic compression, p1=p4 and p2=p3,
Also mass delivered = mass induced, therefore the above equation can be condensed as below:
Work per cycle = [nn-1]P1(V1-V4){rpn-1n-1}
Power consumed by the compressor per cycle = work per cycle x cycles per second
For a polytropic process, n= γ =1.4 and rp= p2/p1, therefore:
Work per cycle =1.41.4-1100x1031-0.28001001.4-11.4-1=227205.252joulescycle
The refrigerator compressor above does work equivalent to 227.20kJ per cycle.

Conclusion

Compressors are a type of a pump that pumps gases such as air only. The compressed gas, such as nitrogen or air, is stored in pressure vessel awaiting usage. The pressurized gas from the storage tank is delivered to the application regions through a system of pipes and valves. The use of compressors to power a range of tools eliminates the need to have an elaborate system of chains and gears as in the case of the central motors. Tools such as nail guns and spray painters are powered from a central pressure tank coupled to an automated compressor.

References

Bloch, H. P., & Hoefner, J. J. (1996). Reciprocating compressors: operation & maintenance. Houston, Tex, Gulf Pub. Co.
American Petroleum Institute. (1989). API specification for packaged reciprocating compressors for oil and gas production services. Dallas, Tex. (1201 Main Street, Suite 2535, Dallas 75202-3904), The Institute.
American Society of Mechanical Engineers. (1975). PTFE seals in reciprocating compressors: manual of material selection, design, and operating practices. New York, American Society of Mechanical Engineers.
(2008). Reciprocating Compressors. 115-122.
Bassetto, I. F. F., Neto, A. H., & De Souza, G. F. M. (2009). Reliability Analysis in Reciprocating Compressors for Refrigeration Systems. HVAC & Amp; R Research. 15, 137-150.
Rigola Serrano, J. (1996). Parametric study of hermetic reciprocating compressors.Proceedings of the 1996 International Compressor Engineering Conference at Purdure. July 23- 26, 1996. Purdure University, West Lafayette, Indiana, USA.
Alexander S., Ashok K., & Matthew F. (2013). Compressors.
Rastin, T. (2000). Reciprocating Compressors Explored. Hydrocarbon Engineering. 5, 116- 119.
Almasi, A. (2010). A new study and model for the mechanism of process reciprocating compressors and pumps. Proceedings of the Institution of Mechanical Engineers, Part E: Journal of Process Mechanical Engineering. 224, 143-147.