A Scintillating Seal Story

Dr. Robert M. Gresham, Contributing Editor | TLT Lubrication Fundamentals October 2014

Industry and government form a partnership to reduce methane emissions.
 

Methane contributes 28 percent of the potential warming that CO2 contributes.
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KEY CONCEPTS
• To some controlled degree, leakage is necessary to lubricate a seal.
• As natural gas usage increases, there’s growing concern methane pollution will increase as well.
• EPA has formed an industry consortium for those working with all phases of natural gas.

I MUST ADMIT THAT MY KNOWLEDGE OF SEALS is just sufficient to be dangerous. That said, off we go.

For many years, STLE’s Seals Technical Committee was one of the society’s most prominent groups. Unfortunately, the seals industry went through a period of consolidation where companies acquired each other, and the sense I got was that, at least for the time being, sealing technology was pretty much adequate for our needs.

Remember, depending on seal design and fluids to be sealed, there is always a finite amount of leakage. In many cases, this leakage is necessary to lubricate the seal. Thus, if you are willing to spend the money on a relatively sophisticated seal, install it properly and maintain it in use, you can do a pretty good job of sealing almost anything to a very low level of leakage.

Previously, I have written articles in TLT about the emergence of natural gas as a source of relatively clean energy for electric power and for cleaner burning power for short-haul fleet trucks. Engineering research efforts are directed toward future applications for expanding its use. Also, we know that recent advances in oil shale extraction have resulted in vast quantities of natural gas becoming available. Thus, we are now pumping, storing and transporting far more quantities of natural gas—and we anticipate much more in the future. Dovetailing with all this, I recently had an interesting conversation with an individual from the Environmental Defense Fund (www.edf.org).

What I learned is that there are currently about 50,000 reciprocating compressors operating in the U.S. natural gas industry, each with an average of four cylinders, representing 200,000 rod packing systems (see the accompanying diagram.)


Figure 1.

These systems contribute about 72.4 Bcf of methane to the atmosphere annually. The leakage comes primarily through the seals making up the stuffing box in the diagram. As it happens, methane gas is a far more potent greenhouse gas than CO2. While there is currently 200 times more CO2 in the atmosphere, methane contributes 28 percent of the potential warming that CO2 contributes. Historically, methane pollution has steadily increased since 1900 to 1999. At that time, it began to level off. However, with the anticipated increase of natural gas usage, there is concern by the EPA that methane pollution will again increase.

The EPA has established the Natural Gas STAR Partner program with those involved in all phases of handling natural gas. Working as a group, the partners determined that with current technology, a new packing system (similar to that shown in Figure 2), properly aligned and fitted, may lose 11 to 12 standard cubic feet per hour (SCFH). As the system ages, leak rates will increase as a result of wear of the packing rings and rods. One partner has reported measuring emissions of 900 SCFH on one worn rod system.


Figure 2.

The initial approach by the partners for reducing emissions, using company-specific financial objectives and monitoring data, has identified the specific level of emissions at which it is cost effective to replace the rings and rods. The benefits of calculating and utilizing the “economic replacement threshold” include both methane emission reductions and cost savings. The savings come from less wasted gas, longer rod life and, of course, optimized maintenance costs. One partner reported saving $233,000 annually at 2006 gas prices. Another partner, Consumers Energy, replaced worn compressor rods packing rings on 15 compressor units with total estimated savings from reduced leakage of $49,000 (based on leakage reductions of 7,000 Mcf per year and assuming a price of $7 per Mcf). The cost of replacing all the packing rings, including materials and labor, was $23,000, resulting in a payback period of six months at 2006 costs. That’s a pretty good return on investment! (You can Google Reducing Methane Emissions from Compressor Rod Packing Systems for an excellent EPA article on the subject.)

However, these examples were a little simplistic. In reality, there are many variables cited as affecting potential savings such as cylinder pressure, fit and alignment of packing parts and the amount of wear on the ring and rod shaft, as well as company-internal decision criteria. However, the partners all agree that this is a practical method for reducing costs and emissions.

For the future, consideration of new materials can improve the life and performance of the equipment with the concomitant reduction in emissions and increase in savings. For example, carbon-impregnated Teflon is replacing the use of bronze rings as the Teflon rings can last as much as a year longer. New piston rods coated with tungsten carbide or chrome has been shown to increase rod life. While these kinds of coatings have been around for a while, they are apparently new to this industry, or at least this application. By using the established best practices for determining when to replace, the financial benefits of the coatings are established.

These are all pretty good examples of a win-win collaboration of industry and government working well together optimizing current technology and best practices. These days, we don’t get to hear many of these kinds of stories. And that is surely a scintillating seal story!


Bob Gresham is STLE’s director of professional development. You can reach him at rgresham@stle.org.