November 2010 Vol. 237 No. 11
Features
Exploring The Cost Of Lost Natural Gas
Volatility in natural gas prices for the past decade has not only impacted consumers, but also has changed corporate thinking. This volatility in the commercial, industrial, transportation and electric power sectors, in concert with global greenhouse gas (GHG) emissions concerns, has created a paradigm shift in corporate attitude.
The EPA’s GHG reporting rule, Subpart W, requires monitoring and collecting data on GHG emissions for the gas industry, beginning Jan. 1, 2011. The gas industry is now scrambling to find effective and cost efficient methods to reduce emissions.
Previous industry practice when gas prices were at the $2-3/Mcf levels found little incentive to reduce, much less monitor, GHG emissions. Gas was viewed as plentiful, inexpensive and largely unregulated. Blowing down compressor cylinders and flaring vent gas were commonplace practices at many compressor facilities.
However, corporations now face a complex energy landscape requiring more environmental stewardship. This change is becoming most evident with the current administration, with clear signals that President Obama intends to regulate GHG emissions. According to Michael Gerrard, Director of the Center for Climate Change Law at Columbia University in New York City, there is little question that this is heading in the direction of federal regulation of carbon dioxide. Subpart W is that regulation, and it will apply to some 3,000 sites in the United States in just months.
Due to the significant dollars involved, and the concerns regarding global warming, natural gas gathering and transmission companies now routinely initiate comprehensive monitoring programs to minimize GHG emissions. Maintenance and operations seek cost-effective, non-intrusive solutions to reduce lost gas. Regulations are currently in place for the exploration and production sectors, and many gas processing, transmission, storage and distribution facilities must begin monitoring and collecting data on GHG emissions on Jan. 1, 2011.
To address and capture a significant portion of GHG emissions from reciprocating compressor packing cases, Compressor Engineering Corporation (CECO) developed Low Emission Packing (LEP). CECO LEP® is a proven technology to reduce vent line leakage by restricting the amount of gas flow before it exits the packing case vent. Unlike some other costly packing retrofits utilizing purge gas, CECO LEP can easily be installed in existing packing cases, often with no modifications to the packing case.
CECO initially implemented LEP® for Gas Transmission reciprocating compressor applications more than seven years ago to solve leakage problems at a mainline compressor station. LEP® is still installed in this unit – it is not commonplace in the industry for packing to continue to perform at near zero leakage after this amount of time. In subsequent years, LEP® has resolved many critical leakage situations.
Rod packing has historically been an overlooked component of reciprocating compressors. Careful attention to the engine primarily, and compressor cylinders secondarily, was commonplace among operators. Unless gas blew significantly from the vent line of the packing case or distance piece, monitoring and replacing rod packing was not a major consideration for many companies.
Rod packing was often made of bronze or cast iron for lubricated service and upon startup took much longer to seal against the piston rod compared with non-metallic seals. With the advent of Teflon® (PTFE) materials, pressure packing could conform much easier to the piston rod and seal better than metallic rings.
However, even with improved materials, packing leakage was generally ignored and seals were not normally replaced unless a serious leak occurred. In addition, the condition of the packing case (consisting of a series of metallic cups) that contains the packing rings was often not considered.
Conventional rod packing consists of a pressure breaker ring in the cup closest to the pressure to reduce gas flow and allow subsequent seals to break the pressure down to atmosphere at the end of the case. This type of packing is dynamic as it requires the gas pressure to exert force on the packing to seal against the rod.
Depending on a number of factors, “standard” leak rates average in the 1-3 scfm range and have been recorded as high as 15 scfm. At this rate, a four-throw pipeline reciprocating compressor would leak 86,400 cubic feet into the atmosphere in one day. Since pipeline stations contain multiple compressors, this figure would be even higher for the entire station.
LEP® has revolutionized leak rates by drastically reducing gas loss commonly to within a range of .2-.5 scfm, and in many cases even lower numbers have been reported. Unlike conventional packing, which uses only a dynamic seal, LEP® utilizes both dynamic and static seals to prevent gas from escaping the packing case. This unique ring design does not require gas pressure to activate and seal against the piston rod. Instead, it uses a side-loaded design to restrict gas from flowing past the seal, thereby greatly reducing emissions to the vent line.
The benefits are tangible, both in terms of lost gas reduced as well as significant cost savings during operation. Customers for LEP® include gas transmission, gas processing, CNG, petrochemical, refining and the air separation markets. CECO continues to expand the use of LEP® based on customers’ growing need to reduce emissions.
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