Understand the technology behind reciprocating compressor capacity control

The reciprocating compressors (known also as “piston-type” compressors) are the best compressors for any service that the capacity is relatively low and the differential pressure is relatively high.

Maintenance costs of reciprocating compressors are approximately 2-3 times greater than those for centrifugal compressors.

This shows the importance of proper condition monitoring and automation for the reciprocating compressors.

Cylinder valves are the most critical components of reciprocating compressors and strongly influence reliability, operation, performance and efficiency of these machines.

Cylinder valve defects are obviously responsible for most of unscheduled maintenance events of reciprocating compressors.

Valve components should be operated (opened and closed) several billions of times during their operating life without being affected by fatigue or other degradation mechanisms.

The correct material selection and proper component design are keys to achieve a successful valve operation. Reciprocating compressor valves should be supplied from a reputable valve manufacturer with proper references and a long-term successful production history.

Some compressor manufacturers are also active in the cylinder valve business. Usually, cylinder valves manufactured by compressor vendors should be dealt with great care.

The valve design and manufacturing should be considered as very delicate tasks. This is only performed successfully by a few professional manufacturers. Sometimes, for special applications, there are only three qualified cylinder valve suppliers with satisfactory references.

Advanced polymers have excellent mechanical properties and are capable of working at hostile cylinder valve conditions which also include extreme mechanical stresses and relatively high temperatures.

In most cases, modern, sophisticated, low mass polymer sealing elements can vastly increase the valve life (could reduce energy consumptions and maintenance costs).

The main advantages of modern polymers over old-fashioned metallic valve parts are: improved heat resistance, high fatigue life, high tolerance to dirt and corrosive traces (liquids or chemicals) particularly those in dirty gases, reduced wear and improved sealing capabilities.

Main valve types are:

  • Ring type valve
  • Plate type valve
  • Poppet valve

For large compressors (generally low speeds and high pressure ratios) and small machines (relatively high speeds) the “ring type valve” and the “plate type valve” are the best option, respectively.

Figure 2 Examples of reciprocating compressor valves (plate type valves for medium size compressors).

The best valve size should be selected with respect to efficiency, reliability and performance requirements considering many operational and machine design factors such as the minimum clearance volume.

The valve “lift” is the distance travelled by valve moving elements. The higher the lift, the higher the valve flow area, the lower the valve pressure drop, the less consumed power, the higher moving elements impact velocities and the lower valve durability.

An optimum valve size and an acceptable valve lift should be found for each application. The optimum valve spring stiffness is also important.

Too stiff springs can lead to the valve flutter (more compressor power and considerable wear rate) or early closing of valve (reduce the capacity). Too light springs cause valve late closing and the reverse flow (higher velocity, less reliability and reduced capacity).

An example of a horizontal-type reciprocating compressor.

An example of a horizontal-type reciprocating compressor.

Traditionally, “poppet valves” were popular many years ago. Previous generations of poppet valves have left service because of poor performance, low reliability and operational problems. Today, some compressor manufacturers offer new versions of specially-designed poppet valves with great promises.

The performance and long-term references of these valves should be considered carefully. Sophisticated evaluations are required to highlighting clearly advantages and disadvantages compared to commonly-used valves (such as the “ring type valve”) for each application.

Capacity control: conventional vs. stepless

One of new technologies presented for reciprocating compressors is the stepless capacity control system. A stepless 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 stepless 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 stepless 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.

Examples of reciprocating compressor valves (plate type valves for medium size compressors).

Examples of reciprocating compressor valves (plate type valves for medium size compressors).

The stepless capacity control system is a fast-acting, accurately controlled arrangement for the energy-saving operation and the rapid control of reciprocating compressors. The stepless 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 stepless 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 stepless capacity control system shows a problem.

A modern stepless system is only expected in critical applications and a 100% recycle system could be justified for such critical services. Modern stepless 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.

An advanced monitoring should include:
1- The gas discharge temperature, pressure and flow for each cylinder.
2- The pressure packing case – piston rod temperature.
3- The crosshead pin temperature.
4- Driver strategic temperatures, particularly the driver bearing temperature.
5- The valve temperature.
6- The oil temperature, flow and pressure.
7- The jacket water temperature for each cylinder.

Proximity probes should be located under the piston rods and used to measure the rod position (the rod run-out) and determine malfunctions such as wear of piston, rider band problems, a crack in the piston rod (or a crack in any piston rod attachment), a broken crosshead shoe, or even the liquid carryover to a cylinder.

The latest recommendation is to use rod run-out measurements just for monitoring and alarm (not for trip). Recommended limits for the cold run-out and the normal operation (hot) run-out are 60 microns and 170 microns (peak to peak), respectively.

All shutdown functions should be 2 out of 3 voting to avoid unnecessary trip. Generally minimum numbers of shutdowns should be assigned for a reciprocating compressor in critical services such as hydrogen units in a refinery, gas processing crucial roles, important refrigeration modules and so on.

The low pressure trip of the lubrication oil system is considered an essential shutdown case. Operators always encourage a very high vibration level for a shutdown (even sometimes 6-8 times than normal). There are always discussions about the high discharge temperature shutdowns.

Many experienced operators argue that they prefer to tolerate relatively high discharge temperatures (and high temperatures of the cylinder valves, which could result in the valve and all wearing parts life reduction), compared to an unscheduled trip of a machine that can result in a critical refinery/process unit shutdown with production losses of an around 0.5 million dollar per day.

Of course safety risks should always be assessed in these situations. The author’s recommendation is to consider a high discharge temperature trip (since this is a code mandatory requirement and constantly insisted by safety teams).

However, the trip level should be set properly high (based on accurate simulations and realistic thermal/safety evaluations) to avoid unnecessary shutdown.

Advanced passive vibration control

Usually, the preferred design of reciprocating compressor for small and medium sizes is a two-cylinder machine. For large machines, four-cylinders and six-cylinders are commonly used.

Sometimes, odd number of cylinders is unavoidable. In these cases, a dummy crosshead should be used to reduce the operating vibration. The state-of-art spring-mass-spring systems can be studied for the passive vibration control (more reduction in the vibration).

This is a new technology. In this innovative system, the dummy crosshead on the one hand is attached to a movable piston assembly by a flexible member and on the other hand to the stationary compressor casing using auxiliary mechanical springs.

Masses, dimensions and stiffness are optimally calculated to offer the minimum operating vibration.

Irregularity setting

For all reciprocating compressors, the flywheel is mandatory to regulate variable reciprocating torques. The irregularity degree for the mechanical component reliable operation is around “2%”.

This value can be considered as the minimum requirement for all reciprocating compressors. Generally in accordance with specific requirements of driver (especially the current pulsation of electric motors), torsional vibration considerations, and other operational issues, a lower irregularity value is specified.

Reliability studies have indicated an irregularity value between 1-1.5%. It is strongly recommended to obtain 1% irregularity (if practically possible) for special-purpose reciprocating units for a smooth and trouble free operation.

For very large machines (>6 MW), there are sometimes manufacturing limits, for example, extremely large flywheels could not be supplied or integrated.

Amin Almasi[Amin Almasi is a rotating machine consultant in Australia. He specialises in rotating machines including centrifugal, screw and reciprocating compressors, gas turbines, steam turbines, engines, pumps, subsea, offshore rotating machines, LNG units, condition monitoring and reliability. Almasi is an active member of Engineers Australia, IMechE, ASME, Vibration Institute, SPE, IEEE, and IDGTE.]