Hydraulics and pneumatics
Reciprocating compressors are the most common compressor worldwide. The advancements of reciprocating compressor technology have made it possible to achieve high pressures and high capacities in compact and efficient compressor packages using high-speed, variable-speed reciprocating compressors.
Modern high-speed reciprocating compressors are manufactured in the 800 to 1600 rpm speed range; they can also offer a superior operational flexibilities using variable-speed operation usually in a wide-speed range. However, more sophisticated designs and techniques are required for these modern machines.
For example, some of high-speed reciprocating compressors operate less efficiently than traditional low-speed compressors due to higher dynamic effects and higher f low-rates that lead to higher associated f low related losses through cylinder valves and pulsation control devices.
Also, with higher power reciprocating compressors operating over a relatively large high speed range, cylinder valve operation could be more complicated, pulsation amplitudes tend to be significantly higher, all these leading to higher dynamic losses and high risk vibrations on the cylinders, cylinder valves and surrounding piping and facilities.
Advanced techniques such as modern pulsation control techniques are needed to accommodate the increase in compressor power and the range of running speeds as well as variations in operations.
The f low resistances play a critical role in any reciprocating compressor package. The direct effect of the f low resistance in the suction and discharge gas passages is the increase of discharge and suction pressure ratio inside the compressor cylinder.
The flow resistance has some effects on the suction and discharge gas temperature variations; such effects should properly be modelled and evaluated carefully.
A modern development in the reciprocating compressor industry is the step-less capacity control systems. They are advanced and expensive systems that are superior options for some applications – where they can offer considerable operating cost reduction and a better energy management.
This section focuses on a step-less capacity regulation system for reciprocating compressors in the capacity range between 20 percent and 100 percent.
This system is based on the so called “reverse flow regulation principle”. The principle works by reversing part of the total gas that has been taken into the cylinder by conveying it back to the suction chamber by holding the suction valve open for a controlled and variable proportion of the compression stroke.
Therefore, the discharge capacity and power reduction are virtually proportional. Such a step-less capacity control system uses a very sophisticated hydraulic system to unload the suction valve for a controlled proportion of the compression stroke. There is a relationship between the variation of the capacity and the opening time of the suction valve in a compression stroke.
This operation needs complicated and high performance control and hydraulic system. A new capacity regulation system usually consists of a hydraulic unit, a distributor, an unloader system, and a complex control system.
Such a step-less capacity control system is recommended for a relatively reciprocating compressor (say above 1.5MW) that should be operated extended periods of part-load operation.
On the other hand, a step-less capacity control system is a complex and expansive system which require special operation and maintenance. The commissioning, repair, maintenance and overhaul of such a system are costly and special which can only be done by direct involvement of step-less capacity control sub-vendor specialist(s) which associated with significant costs and complexity. A step-less capacity control system is only recommended for large compressors with operation profiles that required such a wide range and flexible capacity control. This is not recommended for small and medium size reciprocating compressors in ordinary services that most of the time will work at full-load or defined simple part-load steps. A step-less capacity control system is a modern, complicated and expensive capacity control device that should only be used in right applications.
There is a wide range of gas turbines available on the market for mechanical drive and power generation applications. The speed variation capabilities are critical requirements for the operational flexibility and the energy management of these machines. Some famous single-gas turbine drivers such as “Frame 6”, “Frame 7”, and “Frame 9” can only offer very limited speed variations. These gas turbines need the help of large variable-speed electric motor for the start-up. Split-shaft gas turbines might be heavy-duty machines (such as “Frame 5” gas turbines) or aero-derivative machines (such as LM2500 series). Some gas turbines are multi-spool machines, but with no free-power turbine, that still exhibit a large speed range, such as the LM6000.
The generated power of a typical gas turbine decreases approximately 0.7 percent for every one degree centigrade increase in the ambient temperature. The ambient temperature change could impact both the gas turbine and the driven equipment such as a compressor or a generator. A compressor train or a generator train, which uses a gas turbine driver, should be able to manage the power in different ambient temperatures.
The move in modern plant towards aeroderivatives gas turbine drivers is mainly about the efficiency, operational flexibility and a better energy management. As an indication, aero-derivatives gas turbines can offer efficiencies around 41–46 percent compared to 30–39 percent efficiencies for heavy-duty machines.
Variable frequency drives (VFD) can offer high starting torque which makes them the ideal solution to start large compressor trains in case of single-shaft gas turbine drivers. The VFD driven electric motor can function not only as a starter, but also as a helper to integrate the
gas turbine output in hot ambient temperatures or as a generator producing electric power when the gas turbine output is higher than that required by the compressor. Major plant operators are interested in electric motors driven compressors and pumps. Many clients prefer this solution as opposed to gas turbine driven trains. The main reasons are:
Higher availability. Reduced maintenance costs and downtimes.
Higher operational flexibility particularly by using variable frequency drives (VFD). This solution provides excellent start-up capabilities as well as the compressor/pump speed variation.
Decoupling of the plant production and the ambient temperature. Reduced plant emissions. The electric motor cost and delivery period are competitive compared to ones for a gas turbine.
A centralized power generation system can reduce greenhouse gas emissions and significantly improve thermal efficiency when compared to individual gas turbine driven trains. Electric motor driven compressor (or pump) availability can be very high (more than 99 percent), definitely higher than the 95-96 percent typically cited for large (conventional) gas turbine driven trains. Electric motors can be kept operational for six years or more without any shutdown.
However, the electrical power supply has a great commercial impact (particularly on the initial plant cost). All different scenarios and conditions should be considered. For example, an electrical motor driven train can be the best option from both capital cost and operation cost points of view, when cheap electrical power is available and can be imported to the plant. Another example, when there are electrical power demands existed nearby and the electrical energy from a bigger sized modern, combined-cycle power plant can be sold.
Modern, large combined cycle power plants can offer efficiency more than 61 percent. In this way, using electrical motor driven plants can result in a higher thermal efficiency. The losses in electrical transmissions, and electric motor drives are against this higher thermal efficiency. Typically, the increase of thermal efficiency is estimated by about 15-20 percent more than gas turbine driven trains and loses are calculated about 3-4 percent. The overall efficiency increase of around 11-17 percent can be achieved using a modern power plant and large electric motor driven compressors and pumps in a plant. Adding to this, the higher reliability, availability and operational flexibility, the operational cost reductions can usually justify the additional capital costs.