Protective Systems

Systems providing alarm only action should be subject to periodic testing, however delays to such tests, or known calibration errors, do not cause a direct safety risk and can be managed. Trending of the data prior to an alarm coming in can give valuable pre-warning; intelligent use of combined trends can indicate developing fault conditions.

The compressor must operate with closely defined parameters, the protection provided will be designed to suit a particular installation, a typical installation may include :-

x Low suction pressure x High suction pressure x High interstage pressures x High discharge pressure x High differential temperature x High suction temperature

x High discharge and interstage temperatures x Gland leakage detection

x Vibration levels for each cylinder x Motion works lubrication oil pressure x Lubrication flow to gland and cylinder x Bearing temperature protection

These are measured and protective action automatically initiated when the machine exceeds defined levels. The safety protective systems require tests and, must have a checked return to operation after test. Trip test procedures must be validated and adhered to. Trips may be disabled during machine start - ideally the measured values are recorded at high sample rates during a start, for examination should there be a problem. Also the enabling process must be

validated - particularly on a new machine or after software / instrument modifications. Failure of a single trip should not lead to an unsafe condition - a problem is likely to be detected in several different ways, and the machine tripped before a dangerous condition is reached, though there may be some internal damage e.g. to a bearing.

Process Isolations

As referred to in Section 4.4.3.1, upstream and downstream process isolation valves should be tripped closed on a machine trip. In addition, there should be a self-acting non-return valve. These valves prevent reverse spinning, and minimise the extent of a process gas release from a damaged machine. Similarly if there is a process system trip, this may require the compressor to be tripped. If the trip is not safety-related, it may be delayed while vent / purge actions are completed.

Use of Software Trip Systems

While it is recognised that the reliability, cost-effectiveness and sophistication of modern digital control systems mandates their use for the management control of complex plant equipment, the risk of common systems failure must be accepted. Key trips (e.g. seal pressure, oil pressure) may justify duplicate hard-wired trips. Software trip sequences require rigorous change control procedures. This is true even of "vendor standard" upgrades, particularly on process systems which have probably been "tailored".

The three most critical protective systems on reciprocating compressors are :-

Low Suction Pressure Trip. This can prevent excessive differential pressures, which can overload the piston rods, also unintended sub-atmospheric operation can draw air in through vents or valve glands, potentially creating an explosive or corrosive mixture.

Discharge Relief Valve(s). Positive displacement compressors can create dangerously high discharge pressures, particularly if the suction pressure is allowed to rise higher than normal (e.g. blocked discharge, low flow, gas source continues to supply, suction pressure rises, discharge pressure rises to dangerous levels). The compressor should have a discharge pressure trip which acts first, but the relief valve(s) is the ultimate protection. There should be a system to alert the operator when the valve lifts.

Low Lubricating Oil Pressure. Lack of oil to the motion works will lead to major bearing damage. One or more oil pressure switches, located remote from the pump, should bring in standby oil pump(s), finally trip the compressor. Loss of oil pressure may be the first indication of major compressor damage.

4.4.7.6 Bearings

The bearings in gas compressors are dependent on a continuous supply of oil for lubrication and, more importantly, cooling. Generally, oil must be pressure-fed prior to machine start, and continue to flow until the machine has fully stopped. Reciprocating compressors stop very quickly, but it is good practice, on a normal stop, to leave the oil pump running for a few minutes to cool the bearings. Hence possibly a selection of oil pumps, driven by AC power, machine shaft. Oil quality in terms of grade, cleanliness, low moisture content, temperature and adequate flow are vital. Oil grades, even "equivalents", should not be mixed without draining the complete system.

Radial bearings are large plain white metal type, to suit low speed and high loads. Temperature sensors should measure bearing metal temperature, not the oil.

The “little end” bearing, linking the connecting rod to the crosshead, is special as it has a reciprocating loading with very little rotation. If the loading does not reverse with every stroke, the oil film will fail and the bearing will wear rapidly. This reversal will be established as part of the design process, unloading systems must work correctly as designed or the reversal effect can be lost.

For details of Bearings and related hazards see Section 5.9 – Ancillary Systems & Equipment.

4.4.7.7 Cooling

The main obvious cooling requirement on a gas compressor is the lubricating oil; this can be cooled against air or water. Loss of cooling effect normally results in a controlled trip with no safety implications. A cooler tube failure can be more subtle - air cooled tube failure can spray oil mist causing a fire risk, water tube failure can contaminate the oil causing rapid bearing failure. The cooler design should permit tube inspection / testing, corrosion resistant materials must be used, and gasketed water / oil joints avoided.

Cylinder walls and piston rod packings are cooled to remove frictional heat. This cooling should be established just before start-up, but not left on when the compressor is stopped, as it can cause condensation.

The process gas coolers (inter, after, recycle) provide a cooling load dependent on process duty requirements. The cooling load may be massive (of the order of the compressor drive power) thus taking a considerable part of the plant's available cooling capability. Any limitation in cooling will give higher than intended gas temperatures, reduced compressor efficiency. Provided that gas cooler inlet and outlet temperatures and pressures are available, loss in cooling performance can easily be detected and creates no hazard. Carrying out an energy balance across a cooler is notoriously difficult, cooling water flow measurement is seldom anything like accurate enough. For details of Cooling Systems and related hazards see Section 5.11 – Ancillary Systems & Equipment.

4.4.7.8 Sealing