Optimizing seal oil system safety and reliability

by Michael S and William E Forsthoffer

Check your plant machinery asset list and see how many compressor seal oil systems there are.  Then, compare the availability or MTBF of these systems to your compressors with dry gas seal systems.  Before you decide on conversions, consider the cost, potential critical speed vibration and stability issues and the possibility that the budget will be approved!  The purpose of this memo is to present the key items to monitor and cost effectively modify these systems if required to optimize your seal oil system safety and reliability. The items are arranged in descending order of exposure to potential revenue loss.

• Operating Procedure: Seal oil System on before gas is introduced into the compressor.The # 1 cause of compressor oil seal failures we have experienced are due to debris entering the seal chamber when the operating procedure does not require the seal oil system to be on before gas (N2 or process gas) is introduced into the compressor case.  All seal oil systems are designed to prevent excessive oil entering the compressor case during start-up.  Introduce gas immediately after the seal oil systems are in operation.

• Utilize dry, clean sweet buffer gas whenever possible in applications that can foul.Using a dry clean buffer gas source between the compressor shaft labyrinths and the oil seal chamber will prevent debris and sour gas from entering the oil system. The buffer gas should be introduced just after the oil system is put in service but prior to gas is introduced into the compressor case.  The buffer gas needs to be compatible with the Process.
• Modify existing internal type seal oil drainers to external type if drainers have either stopped draining or hung open (Gas was transferred to oil reservoir).If a source of clean, dry and sweet buffer gas is not available or the compressor is not designed with a buffer gas labyrinth, modifying your drainers to drain pots with external level control valves, will eliminate any drainer malfunction.
• Modify the drainer vent system if oil ingested into the process system has caused process rate losses.If issues have arisen with seal oil contaminating the process system, installing a drainer system vent control system(to be able to vary the flow of the gas being vented from the drainers as sour oil leakage varies over the life of the seal) will assure that seal oil leakage will be sent only to the drainers and not to the process system throughout the duration of the seal run time.
• For High Suction Pressure Service (> 1000 psig) that starts up with low pressure (@ less than 100 psig),consider a False Reference signal to provide adequate flow through the Seal Oil Atmospheric Bushing during start-ups.

• Implement the following seal oil system condition monitoring tasks: Trend all control valve positions – to trend the oil flow thru the atmospheric bushing of the oil seals Perform Oil drainer flow checks when required – to trend the oil flow thru the process side seal Trend oil consumption – to determine the condition of the shaft end labyrinth seals and the drainer vent system Trend oil filter differential pressure – to determine the amount of debris in the oil system and the necessity of an oil flush at the next Turnaround. Trend oil cooler differential temperature – to determine the condition of the oil coolers and the need for cleaning Perform quarterly oil analysis including flash point. To assure that the oil seals and system are effective and preventing process gas from entering the plant. There should also be a second sample point downstream of the filters to check particle count and assure filters are not bypassing.(The article has been written by Michael S and William E Forsthoffer of FORSTHOFFER ASSOCIATES INC. Email: bill@forsthofferassociates.com)