Capacity Control – Conventional vs. Step-less
One of new technologies presented for reciprocating compressors is the step-less capacity control system. A step-less capacity control system uses the finger type unloader and unloads the suction valve for only a portion of each compression cycle to achieve an adjusted capacity. This is a hydraulically actuated system with a very complex mechanism and a very sophisticated control which is offered by very limited manufacturers. The selected step-less capacity control device should only be used with a valve from the same manufacturer.
The finger type unloaders have potential for damaging valve sealing elements and require more care for maintenance. Valves and unloaders cause around 44% of unscheduled reciprocating compressor shutdown.
The selection of capacity control method (the unloader system, valve types/details, and others) can affect the reciprocating compressor reliability and maintenance. In addition, the unloader selection has strong effect on performance, operational flexibility, start-up and shut-down of a reciprocating compressor. The “plug type” or the “port type” unloaders can offer better reliability and performance compared to the “finger type” unloaders. However, “plug type” unloaders (or “port type” unloaders) are not available for small sizes.
Great care should be taken for the unloader selection of a small reciprocating compressor. The “finger type” unloaders are only available option for some small reciprocating compressors. For some tiny machines (say below 100 kW), even a 100% spillback (the recycle loop) may seem an acceptable capacity control solution, because the wasted power is low. However, the “operational flexibility” generally is considered more important than around 10% added reliability and the “finger type” unloaders are provided (usually in addition of a 100% spillback loop) for small machines in critical applications such as refinery, petrochemical or gas processing plants.
For medium size machines (from 300kW to 1.4 MW), the best capacity control configuration is the selection of part-load steps based on plug/port unloader, and if necessary the clearance pocket. Clearance pockets should also be dealt with care since they could offer some reliability and operational issues.
New technologies (such as step-less capacity control systems) are not suitable for all applications. Step-less capacity control devices are only recommended for large machines (say above 1.5 MW) with great durations of part-load operation.
The step-less capacity control system is a fast-acting, accurately controlled arrangement for the energy-saving operation and the rapid control of reciprocating compressors. The step-less capacity control system allows an operator to compress only the required amount of gas in a very dynamic fashion. However, this system uses special instruments and actuators and nearly always brings a long list of deviations (to compressor manufacturers and project specifications) and special requirements in design, installation and operation. Of course proper manufacturer guarantees can be offered by its manufacturer and satisfactory assistances in all levels could be received. Overall, this is a modern and special-purpose system that should only be employed when really necessary. Only for large machines with long durations of part-loads and requirements for fast follow-up, this special system is recommended.
Theoretically, when using the step-less capacity control system, the bypass loop could be eliminated. However, this is just theory and the best recommendation is to provide a 100% recycle loop (a 100% bypass loop) for the operational flexibility and continued operation if the automatic step-less capacity control system shows a problem. A modern step-less system is only expected in critical applications and a 100% recycle system could be justified for such critical services. Modern step-less capacity control systems are relatively reliable devices and their record of reliability is not worse than conventional unloader systems. On the other hand, these complex systems include various mechanical, hydraulic, electrical and control sub-systems and their reliability could not be higher than a certain limit. An important issue could be this system cannot hold its position in case of a problem. In other words, the system should be set to the “100% load” or the “100% unload” in case of a failure or a problem (such as a hardware/software issue, an actuator problem, a hydraulic system issue, an instrument failure, or another operational problem). The full-load option (the “100%-load” in case of a failure) plus a 100% bypass can offer a good operational flexibility up to the shutdown of machine in the first possible opportunity.
Modern Condition Monitoring
Condition monitoring systems should be particularly cost effective; at the same time they should include all necessary items to identify malfunctions at an early stage. The result of an optimum condition monitoring system should be a relatively low maintenance cost and the lowest risk. An advanced vibration monitoring system includes:
- The continuous vibration monitoring of the compressor and the driver. Velocity-transducers are preferred over accelerometers because of a better signal to noise ratio. For relatively large machines (>0.7MW), both (velocity-transducers and accelerometers) should be employed. An advanced configuration is the vibration monitoring at each end of the crankcase about halfway up from the base-plate in line with main bearings.
- The accelerometer at each cross-head.