Using clean buffer gas in oil seals ensures plant safety

October 24 2014 - William E. Forsthoffer

Use clean, sweet buffer gas whenever process gas is sour and/or can contain debris to optimize oil seal MTBF (in excess of 120 months).

The entrance of sour gas and/or gas with debris into the oil seal system will expose the seal oil system to the following issues that can reduce seal MTBF, cause unscheduled shutdowns and pose a safety hazard (gas release to the plant environment):

  • Mechanical or iso carbon inner seal hang up and high seal oil leakage
  • Blockage of internal seal oil drainer orifices causing ingestion of seal oil into the compressor
  • Contamination of overhead seal oil tanks with oil sludge, eventually blocking internal seal oil ports and exposing the seals to low or zero seal oil to gas differential and possible gas release to the plant

The use of a sweet and clean buffer gas will positively eliminate the above problems to optimize the life of the seals and the reliability of the seal system and corresponding unit (99.7% +).

The function of any buffer fluid system is to continuously supply clean buffer fluid to each specified point at the required differential pressure, temperature and flow rate. There are different types of fluid buffer systems:

  1. Compressor buffer systems
  2. Steam buffer systems
  3. Liquid buffer systems or pump flush systems

Compressor buffer gas system

There are various types of compressor buffer gas systems for different applications. The types and functions are:

  • Dead ended or seal chamber reference system

          Prevent contamination of seal oil by process gas

  • Dead ended type

          Warm suction end of compressor

  • Vented trap system to flare or atmosphere (where acceptable environmentally and safely)

          Prevent contamination of seal oil by process gas

          Prevent seal oil from entering the compressor

  • Vented trap system returned to compressor
  • Prevent contamination of seal oil by process gas
  • Vented trap system

          Prevent seal oil in compressor

          Warm suction end of refrigeration compressor

  • Flow ratio controlled vented trap system

          Prevent seal oil in compressor

          Positively prevent contamination of seal oil by process gas while ensuring seal oil does not enter compressor

Note: Systems 1-5 are differential pressure (buffer gas to process gas) controlled

System operation

The reliability of the external source will directly influence the reliability of a typical buffer gas system. The gas flow rate is differential pressure controlled by the control valve. In a single valve design, the reference pressure closest to the balance drum must be used, since this pressure will be slightly higher than the suction pressure as a result of the balance line pressure drop.

Referencing in this manner ensures that the differential of buffer gas on both ends will be at least the minimum amount set. The suction end buffer gas differential pressure will therefore be slightly higher.

It is important to note that differential pressure control is historically used for most buffer gas systems. However, when one considers the concept of an equivalent orifice, it can be seen that this valve is actually referencing flow in order to provide a constant flow across the buffer gas system. The flow then enters both ends of the compressor downstream of the buffer gas valve.

Buffer gas flow then encounters two equivalent orifices on each end of the compressor. The wear of either orifice will result in increased flow through that orifice. Wear can be the result of vibration, liquid entertainment, abrasive material or other causes.

The flow on each end is then distributed back to the compressor and towards the seal. This action ensures that the seal interface is always in contact with buffer gas, which is a controlled gas that meets unit seal cleanliness requirements. Depending upon the oil trap configuration, buffer gas may flow back to the unit. In this case it will flow to the compressor.

It is important to note that this system only ensures that the process gas will not come in contact with the seal. Greater flow towards the compressor than towards the sour oil traps could result in seal oil ingestion into the machine. If this is not acceptable, a modification to this system must be made.

Typical buffer gas velocities towards the compressor end seal are 6-9 m/sec (20-30 ft/sec) but should take into account maximum (worn) labyrinth clearances.

Sources of buffer gas

The sources of buffer gas can vary with the application. Buffer gas can be obtained directly from the compressor if the gas is compatible with the seal oil, or from any external source that meets buffer gas specifications. When buffer gas is provided from an external source, one must consider the reliability of the source in assessing total buffer gas system reliability.

Note that when buffer gas is taken from the compressor, it should be superheated to a sufficient value to prevent condensation across the labyrinth. Consideration should also be given to applications that are using water and/or solvent injection to prevent fouling.

Preventive maintenance

Typical preventive maintenance of buffer gas systems should include the following:

  • Periodic valve movement
  • Switch calibration
  • Flow meter measurement and calibration
  • Buffer gas valve stability

Failure to design and use a sweet and clean buffer gas in oil seal systems has resulted in low seal MTBFs (less than 12 months) and has caused gas releases in plants that have caused fires and personal harm.

I have known of a case of loss of life that was caused by an internal valve oil drainer remaining open in sour gas service, which released H2S gas to the plant while an operator was checking seal oil leakage from this drainer.

This best practice has been used and recommended for new projects and plant modifications, since the above incident occurred in the late 1970s, to ensure plant safety and optimize seal oil system reliability.