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Catching VOCs (Compression Methane Reduction)

jon360i

The oil & gas industry has been scrutinized for many years for it's products and their perceived impact on the environment.  What people are not aware of is the effort this industry has made towards reducing emissions to meet Federal and local government regulations, which seem to be a never-ending revolving door that changes with each new administration.  The government’s current objective is methane reduction.


Catching VOCs will provide information on M-Squared’s efforts to reduce methane emissions from current reciprocating compressor projects using our patented PLDs® (Packing Leak Detectors).


The California Air Resource Board (CARB) is a state council that regulates air emissions.  Currently, CARB has identified one of the larger methane emissions leak sources that comes from many natural gas reciprocating compressors cylinders packing glands.  CARB has set acceptable leakage limits at 2.0 Scfm of leakage per cylinder on an aggregate basis.  Therefore, if a six-throw compressor is running six cylinders, the acceptable aggregate operating leakage would be 12.0 Scfm.  12.0 Scfm aggregate leakage equates to 6.30MMscfd per year of acceptable leakage for this unit.


Recently we installed an aggregate PLD system on a gas storage plant in northern California.  The plant consisted of five (5) electric driven Ariel KBBV reciprocating compressor packages.  All packages were two-stage, six-throw compressors with six operating cylinders on a single service.  The units are housed in an insulated building with the heat exchangers situated external to the building.


After installation of the PLD aggregate system was completed on all five compressors, minor adjustments were made to each units packing system to establish a real-time “Zero-Leak” baseline.  These adjustments can range from adjusting lubrication rates, emery cloth abrasion of the piston rods, changing out improperly installed packing, correcting packing material used, and, in some cases, replacement of piston rings that may cause rod drop.  Once these adjustments were completed, the packages were placed into service.

Each unit was connected to the client’s PLC via modbus registers, where they were able to set leakage rate limits, alarms and shutdowns, while continuously monitoring the packing gland health 24 hours per day, 365 days a year.


During normal operations, the PLD continuously monitored the aggregate flow of all six cylinders per compressor package at rates well-below the aggregate limit of 12.0 Scfm.  However, when the compressors were shut down “loaded” (current practice for pipeline gas transportation and storage units), the PLD-monitored, leakage-rates were well above the accepted aggregate limit of 12.0 Scfm.  The client called M-Squared for an explanation of this issue, concerned there might be an issue with the compressors or the PLD system.


Response:  Dynamic vs static seal system?


The reciprocating compressor packing gland system is a “Dynamic Seal System,” which means they are functional when the compressor is running.  The packing gland seals are designed to seal against the piston rod with an injected film of oil while the piston rod oscillates back and forth inside the packing gland material.  When compressors are shutdown “loaded,” the compressed gas still trapped in the cylinder bores, pulsation bottles, and piping no longer have the resistance of the packing gland seal system now in static state.  This static state allows the gas to travel down the piston rod, through the packing gland assembly with minimal resistance, and, thereby, leaking out of the packing at rates exceeding acceptable limits.


The PLD is the only current API device designed to handle the arduous oil and gas environment, while continuously monitoring, measuring, and provide leakage data 24 hours per day, 365 days a year, whether in dynamic or steady state.


Resolution:

The PLD provided our client the necessary information to address their leakage problem when shutting their compressor units down “loaded.”

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