Operation and Maintenance of TurboCompressors – Issue 3 – October 2016 – Fred Geitner

“Operation and Maintenance of Turbocompressors”

Centrifugal compressors or turbocompressors are well known and they are widely used because they are reliable due to their robust and relative simple design. Their main representative is the inline type, often also the “pipeliner”, typically designed to industry standard such as API 617 “Centrifugal Compressors for Petroleum, Chemical, and Gas Services Industries” .

A prominent, but dated source cites 13% of all failures of turbocompressors as being due to errors or omissions in condition monitoring and maintenance. With the advance in monitoring technology and modern operating and maintenance practices one would assume that this general number might not be near as high today. What then are good monitoring and maintenance practices around turbocompressors?

Compressor condition monitoring has the following components:

Proper response to supervisory instrumentation such as alarms and trips.
Periodic observation and evaluation of operating parameters such as the compressor physical condition and its performance efficiency. This would include measuring and judging the rate of deterioration of mechanical and performance conditions for input into maintenance plans. Vibration analysis and aerodynamic performance calculations come to mind. Daily compressor operator rounds should be structured following the principles of Operator Driven Reliability (ODR).
Evaluation of operating trends. This should include auxiliary systems such as lubrication and seal oil consoles, compressor on-line washing facilities and dry gas seal support systems.
Periodic testing of lubrication and seal oils. Six basic analysis are required: Appearance test, testing for dissolved water, flash point test, viscosity test, the determination of the Total Acid Number (TAN), and the determination of the additive content.
Periodic testing of emergency safety and shutdown devices (ESD) and other fail-to-danger components, such as exercising the compressor’s surge control valve loop and the trip and throttle (T&T) valve on steam turbine driven compressor trains.
Data logging and automated record keeping such as the number of unplanned trips per train per year as a basic indication of compressor reliability.
Diagnosis of problems, appraising their severity and deciding what action to take.
Remedial action and execution planning.
Corrective measures should be preferably applied on-stream to reduce the impact on compressor availability. On line washing would be a good example.

Generally, turbocompressors have maintenance inspections, overhauls and repairs (MIO&R), also referred to as turnarounds, in intervals ranging from two to ten years depending upon the type of service. Maintenance intervals in clean services in the hydrocarbon industries of six to ten years are not uncommon.

The extent of MIO&R efforts ranges from simple bearing inspections to opening the compressor and replacing the rotor with a spare rotor drawn from specialized spare parts storage. Used rotors are examined for rubs at labyrinth seal locations and for fissures and cracks around impeller eyes on radial compressors. On axial compressors moving and stationary blades receive thorough attention. In all cases non-destructive test (NDT) procedures are being applied.

As the opportunity for a compressor turnaround approaches we should review the machine’s operating and maintenance history and if there are any defects noted it would be well to ask the following questions:

Are any of the defects repeaters?
If so, can they be expected at this turnaround?
What steps can be taken to eliminate them?
What action should be taken at this time?

A thorough pre-turnaround review should be undertaken in order to plan the work required. It should consist of:

An assessment of the compressor’s mechanical condition
A performance check
Review of the machine’s past history

Bearings Each turnaround but not more frequently than every 18 months.
Control System Inspection based on current instrumentation practice & pre-turnaround response check.
Couplings Each turnaround but not more frequently than every 12 months.
Internal Inspection Whenever cover is removed according to the following schedule:
1st Run…………3 years
2nd Run…………6 years
3rd and subsequent runs…10 years
Lube & Oil System Clean & refill reservoir, check coolers for leaks & replace filter cartridges at each turnaround provided not done within last 3 years.
Non-return Valves Examine undamped swing checks & Mission Duo checks at same frequency as internal inspection.
Seal Eductor Systems Inspect piping every turnaround
Seals Every turnaround on mechanical oil seals operating above 15500rpm
Dry Gas Seals: Leave well enough alone
Trip Systems Check all trips at each shut-down