Home > How to get the most out of piston type compressors (Part 3)

How to get the most out of piston type compressors (Part 3)

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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.

 

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