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Aug-2014

Optimise control systems with preventive maintenance

Ensuring equipment integrity by conducting thorough maintenance provides multiple benefits for your operation, from maximising machine uptime and minimising process shutdowns, to prolonged equipment lifespan and safety.

Pier Parisi and Mikael Gustafson
Compressor Controls Corporation
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Article Summary
This is true and well understood in the industry when it comes to mechanical equipment. Yet, when it comes to controls and monitoring systems, opportunities to improve abound.

Maintenance approach
A plant’s equipment maintenance approach typically falls into one of two categories: reactive maintenance and preventive maintenance.

Reactive maintenance refers to repairs made upon equipment failure. Reactive maintenance takes place when a business chooses to wait for a pipe to burst or a part to fail before taking action. It is waiting for a compressor’s performance to degrade so far that it’s no longer able to support its downstream companions before studying the cause of the degradation. Thus, reactive maintenance doesn’t prevent subsequent safety and monetary consequences — it’s simply waiting for a negative consequence be- fore taking action.

Reactive maintenance comes with a number of negative consequences — scrambling to find a replacement from spare stock, or ordering a replacement from a supplier, which can result in long lead times or exorbitant expediting fees, all headaches compounded by the fact that as you wait, losses are stacking up quickly due to lack of production. Reactive maintenance really isn’t maintenance at all; rather, it is constantly living on the verge of a potential catastrophe.

Preventive maintenance (PM) goes above and beyond reactive maintenance by involving intelligent monitoring of process variables and mechanical conditions to identify potential causes of failure before they become actual causes of failure. Preventive maintenance involves using past failure data to put into place measures that ensure ongoing product performance versus awaiting potential failures.

The link between maintenance and system optimisation

An important step in preventive maintenance is monitoring ongoing machine operation and performance for irregularities. This includes monitoring the mechanical condition of rotating equipment and performing scheduled inspections of pipework, valves, pressure vessels, and other components. A major element of preventive maintenance is empowering personnel with the knowledge and expertise necessary to intelligently interpret this gathered data so they are capable of recommending the proper course of action based on the reports.

Too often, control systems known for their reliability become victim of the “set it and forget” syndrome. While they perform as originally designed, processes around them change, creating the real yet unrecognised need for life cycle optimisation. Regular audits by qualified personnel of suction and discharge pressures and temperatures, process gas flow rates, machine vibration, and steam flow, motor power, or turbine power, can reveal significant details about a machine’s health long before the overall process suffers. Correctly applied, a preventive maintenance plan means that a drop in compressor performance caused by seal and bearing degradation is properly identified before it causes a process upset or shutdown.

Preventive maintenance means valves are regularly stroked to ensure behaviour remains fast, accurate, and smooth. This is especially critical for recycle or blow-off valves performing antisurge functions, as these control elements may remain fully closed for months at a time, but must be able to open immediately in order to protect their machine

Example 1
Compressor Controls Corporation (CCC) was recently asked to assist in troubleshooting the cause of several trips and a subsequent inability to restart a turbo-expander re-compressor unit at a gas production facility in Northern Africa. As CCC’s representative inspected the process layout, piping, control elements, and fast trend data taken during the trip events for the turbo-expander re-compressor, it became clear that during a time when the turbo-expander re-compressor was operating at more than 20,000 rpm, a trip in the downstream export compressors caused an upset in the process. The turbo-expander re-compressor’s control system recognised the upset and immediately called for the antisurge valve to open, but the valve did not respond for more than two seconds. During those two seconds, the re-compressor experienced a surge event, suffered a hard landing against its magnetic bearings, and tripped. After this happened for the third time within three weeks, the unit had experienced such significant damage that once restarted, it would not come up to speed at all.

The resulting damage required installation of an entirely new rotor within the turbo-expander — a costly replacement for the facility in terms of parts and downtime. After determining the root cause of the failure, the CCC representative assisted in retuning the antisurge valve to ensure proper response of the valve to its control signal.

Had the facility performed regular performance tests of their control elements, a poorly-functioning valve would have been identified before its behaviour caused the damage it did, and the valve could have been retuned during a convenient shutdown or turnaround.

When preventive maintenance is in place, upon discovering a point of concern or irregularity, a facility has time on its side and is able to contact vendors for materials and support well in advance of a significant equipment degradation or costly adverse consequence. Once preparations have been made, the repairs can be completed at a convenient time, with a much smaller impact on production — and the facility’s revenue stream.

Balancing Controls with Current Process Parameters
Another important aspect of preventive maintenance that goes beyond preventing equipment failure is ensuring controls are operating based on current process parameters. If the process or equipment have undergone significant (or even rather insignificant) modifications since commissioning, the controls may not react properly, resulting in process instability, wasted energy, and reduced operator faith in the controls themselves.

If the general throughput of a plant has come down over time, it’s possible that an antisurge controller will demand recycling for the safety of the machine, where there previously was none. For instance, at the time of commissioning, because the actual flow rate through a machine was predicted to be high, it may have been decided not to perform testing to determine the actual location of the machine’s surge line, choosing instead to abide by predicted surge curves. But, because plant conditions now require constant recycling, taking the time to perform actual surge testing may allow for minimising of surge control margins and appropriate tuning of the control response to maximise the allowed envelope of operation, and reducing unnecessary recycling.

When changes have been made to a machine’s throughput capabilities by means of alterations to piping, rebundling the machine, process gas composition changes, or driver upgrades, the control strategy and response should be closely evaluated to ensure that the control configuration is still applicable to the machine’s operation, and that no energy is being wasted due to improper parameters.
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