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[Firoz Raja]

Quite often I came across with comments/queries from process consultants and/or end user (operator) stating that why is it necessary to blow down or flare the inventory for a typical compressor system before starting the machine at settle out condition. So in best of my experience of all the compression packages that I’ve been part of its design, it is always recommended that the compressor is never started at a higher pressure (settle-out pressure) than it’s design suction pressure. There are following reasons for not starting up the compressor at higher pressures.

• The compressor driver end (electric motor or a gas driven engine) is designed for a certain starting torque. At settle-out pressure, the starting torque would be much higher for the driving end and hence it can lead to serious damage to the driving end (gas engine or even electric motor).
• Suction pressure at settle-out pressure would be higher than the designed suction pressure and this disturbs the compression ratio of the compressor. This problem is more severe for a reciprocating machine where it can lead to higher discharge temperature, rod loading of the piston and even bearing damage. This usually occurs very fast before any other compressor inherent trip kicks in to safeguard the machine.
• When the inventory (gas/vapour) is held up for longer duration, then there are chances of condensation. And any amount of hydrocarbon or aqueous liquid is absolutely detrimental to the compressor.
• For smaller reciprocating and even centrifugal compressors (mechanical) seals are not designed to hold the pressurized gas in standby condition. The seal would start leaking and eventually would be damaged too if the pressurized gas (even at settle-out condition) is contained for longer duration.

 

Further it is recommended that the operator attends the process upset and makes sure that the root cause is identified and properly mitigated so that it is not cascaded to any other process facility.

 

The usual practice is NOT to contain the pressurized gas for longer duration. The inventory is completely blown down or partially depending upon the operator’s preference. A partial blow down till the system pressure is near to the design suction pressure of the compressor is useful as it helps not to flare/vent the hydrocarbon.