Reciprocating compressors have been extensively used in different HVAC, refrigeration and industrial applications. These compressors have been the focus of machinery industry for better performance and reliability. The emphasis has been on the valve design, pulsation control, capacity control and performance monitoring. One important task has been the development of better pulsation control techniques using sophisticated mathematical and computer modeling. In addition the capacity control and the issue of speed variation and optimum capacity control combinations have been important topics for modern reciprocating compressors.
Traditional reciprocating compressor packages were based on constant-speed low-speed compressor trains. Previously flexible unloading schemes, combinations of suction valve unloaders, automatic clearance volume pockets and others, were used with constant-speed drivers. Even currently many engineers prefer such traditional low-speed contrast-speed reciprocating compressors to other modem designs.
In the last decades, high-speed reciprocating compressors have become common for many applications. Nowadays they are specified and used in HVAC, refrigeration and industrial services offering economic and reliable gas compression units. Proper compressor control systems, auxiliaries, package design, pulsation control, and torsional design are required for modern reciprocating compressors using high-speed, variable-speed technologies.
High pulsations, high vibration, cylinder valve issues, and unloader problems are common occurrences in reciprocating compressors. Cylinder valves and unlodaers have been vulnerable parts in reciprocating compressors. Pulsation could affect whole the piping and mechanical equipment in the upstream and downstream of a reciprocating compressor; it can generate high vibrations. Vibration produces alternating stresses in the materials. Alternating stress levels beyond the endurance limit of the material causes fatigue failure. Failures of this type can be catastrophic. Pulsation can also produce noise. Consequently, measures should be taken to avoid high pulsation. Pulsation and vibrations can usually be attributed to the mechanical and acoustical design of a reciprocating compressor package.
Compressor Efficiency
Many different variables affect the performance and efficiency of a reciprocating compressor. In broad terms, efficiency is the ratio of work output versus work input. For reciprocating compressors, efficiency is a measure of the amount of work applied to the gas versus the total amount of work applied to the compressor. Work is applied to gas in the form of pressure and temperature increases. While imparting this work, certain mechanical, thermodynamic, and other losses occur in the gas compressor. Common losses include compressor valve loss, passage friction losses, pulsation losses, etc. These losses can range from about 12% to 38% in extreme cases.
Pulsation Control
Reciprocating compressors emit pulsations. Pulsations travelling away from and to the compressor cylinders will set up standing wave patterns that result in unbalanced pressure forces in the piping and facilities at upstream and downstream.
An optimum design for reciprocating compressor applications poses several challenges for the pulsation study and mechanical design such as:
• Designing pulsation and vibration control for all operating conditions.
• Minimising pressure drop to achieve acceptable compressor efficiency.
• Ensuring that performance requirements (particularly capacity) are met while observing all limits such as driver power limits.
Accurate pulsation studies are required for all reciprocating compressors. Even low power reciprocating compressors should receive attention and extensive studies for pulsation and vibration control. Serious vibration, resulting from extreme pulsation levels, can still be a major concern on any reciprocating compressor sizes; small, medium and large machines. Sufficiently sized pulsation bottles are always recommended for any reciprocating compressor. One pulsation bottle for each cylinder suction or discharge is the best option to battle the pulsation problems.
Expanding the piping directly attached to the compressor cylinders (larger size piping) has been used by some vendors as an alternative to pulsation bottles, argueing that these large size pipes can provide volumes of gas equivalent to pulsation bottles. The expansion of the piping attached to cylinders is not an affective pulsation control practice and properly sized and designed pulsation bottles are always a better option. The piping line expansion (using larger piping sizes to act as the gas volume for the pulsation reduction) will behave like an ineffective long bottle which is inefficient in the pulsation control. Even worse, the piping line expansion sometimes increases the pulsation level. There were many cases in which the piping line expansion had not reduced pulsation levels, but actually increased pulsation levels. A properly sized pulsation bottle, with sufficient volume and correct length-to-diameter, should always be considered.
For pulsation bottles, a lower length-to-diameter (L/D) ratio can result in more effective pulsation reduction. An L/D ratio between “2” and “4” is recommended. In other words, L/D ratios lower than “4” are always preferred. Practically an L/D ratio more than “5”, a long-bottle with diameter less than 20% of its length, would result in an inefficient pulsation bottle.
Often the suction pulsation bottle is bigger than the discharge one because the gas volume in suction is more than the discharge one; it depends on the compressor ratio as well. Also pulsation bottle sizes are a function of gas molecular weight.
As a very rough indication, the following is noted for the pulsation bottle initial sizing check. For low and medium pressure compressors (say 5-25 Barg), the pulsation bottle volumes could be around 5-14 times the cylinder swept volume. The above-mentioned are just noted as a very approximate factors and even a reasonable estimate should consider many parameters such as gas composition, operation pressures, pressure ratio, temperature, etc.
Properly designed and sized pulsation bottles, with correct length-to-diameter ratio, can reduce the pulsation levels to less than 5% of original pulsation values. Another area of concern is the piping between pulsation bottles and other mechanical equipment, such as coolers. For example, in many compressor packages, high pulsation and high vibration were reported for discharge piping between the discharge pulsation bottle and the cooler. First and second orders of compressor speed have been reported as main excitation frequencies for many compressor packages. Often, higher orders could also make problems. In some reciprocating compressors, the predominant frequency of vibration was the fourth order of compressor run speed. Too often, the high order vibration was caused by high pulsation-induced unbalanced forces in the piping system.
All connections to the piping or facilities related to a reciprocating compressor, particularly those relatively close to the compressor which could be affected by the pulsation, should be designed to minimise overhung mass and should also be reinforced to avoid breakage due to vibration. Particular attention is required for small size connections. The above-mentioned reinforcement should preferably be designed and fabricated by the use of integrally-reinforced forged nozzles or by bracing back to the main pipe in at least two planes.
Guest blog by Amin Almasi, rotating equipment engineer.
