There are several ways to prevent the contents from exiting a piece of equipment at the point a rotating shaft enters it. You can slip an O-ring on the shaft, you can use any of a variety of lip seals or packings or, for a few dollars more, fit a mechanical seal or bearing isolator to the interface. Each approach has good and bad points. Even if you, the plant professional, aren’t being paid to know which is better for a given application, it wouldn’t hurt to learn something about mechanical seals so you’ll recognize a snow job when you see it.
That’s why I’m inviting you to take another dive into that expanding morass we call the Web in search of practical, zero-cost, noncommercial, registration-free Web resources that will help you understand the economics of spending the money that mechanical seals command. Remember, we search the Web so you don't have to.
Your tax money at work
Part of the rationale for installing mechanical seals on rotating equipment is to minimize or eliminate shaft leaks. But there are leaks and then there are leaks with a capital “L.” Those are leaks that release wet nasties to the environment we’re supposed to be protecting from the ravages of mankind and the industrialized world. Because we’ve already demonstrated that, collectively, we’re responsible corporate citizens, the U.S. EPA weighs in on the leak issue and mandates that we do something about them. The working concept here is called leak detection and repair (LDAR). A search of the EPA Web site turns up many pages about the topic. A typical example, “Polymers and Resins IV Inspection Tool,” is found at www.epa.gov/ttn/atw/pr4/privinspect.html. If you get there, I’d direct your attention to Chapter 9, which has 21 pages that provide a list of eligible piping system components, the frequency with which they should be monitored, the method of inspection, and the test results that constitute a leak. It also discusses compressors, vent systems, exempt components and those having reduced monitoring requirements.
Finally, it tells you about recordkeeping and reporting requirements. If you fish around at the EPA well of abundance, you’ll probably locate an inspection tool that closely matches your own operations.
As Method 21 seems to be the EPA’s preferred test method for the purposes of leak detection with VOCs, you might as well learn how to perform it properly. Send your leaky mouse over to the EPA Emissions Measurement Center at www.epa.gov/ttn/emc and access the drop-down menu for Promulgated Methods. Scroll down to “21 - VOC Leaks” and click the go button. This gives you the option of getting it as either a WordPerfect file or as a PDF.
From those who know
The industrial revolution introduced the idea of interchangeable parts, which made mass production possible. Then, we imposed dimensional standards on those parts, which made it possible to purchase interchangeable parts from a variety of vendors. The differences among such standardized devices are generally related to quality, pricing and performance under fire. Getting that performance is the central idea behind the Texas A&M Turbolab’s International Pump Users Symposium, a biannual vendor-neutral event that has a heavy emphasis on mechanical seals. Mike Huebner from Flowserve Corp., Deer Park, Texas, is one of the regulars you can meet at the event. In his presentation in 2004 at the 21st symposium, “Overview of API 682 and ISO 21049,” Huebner discussed the standards that apply to seals.
The variables he addressed included seal categories, types, arrangements and configurations as well as flush plans, accessories and testing.
In his presentation at the 22nd symposium, “Material selection for mechanical seals,” Huebner discussed the options you have when specifying seal faces and the tribological implications of using different mating materials. Then, he addressed secondary seals and the choices you have among the many available elastomeric materials. Finally, he tells you about the options you have for the metallic components. These two presentations provide a concentrated dose of what you should know when selecting a seal to enhance plant reliability. Read the details when you pay a visit to http://turbolab.tamu.edu/pubs/papers.html and click on the appropriate icon that’s over on the right side. When the page loads, scroll down to access Huebner’s presentations.
William McNally, owner of the McNally Institute in Clearwater, Fla., is another big name in the business. He has been involved in some aspect of the pump and seal industry for nearly 50 years. During that time, he’s published numerous papers and given countless training classes. Although now retired, he posted much of his knowledge base to your favorite Internet for all to access. His site is truly a treasure trove. The topics he addresses are too many to enumerate, so you’ll need to explore them yourself. There are at least two ways to see this treasure. You can pop over to www.mcnallyinstitute.com to explore the site, or you can make your way to www.mcnallyinstitute.com/home-html for some material I couldn’t access via the first approach. For a starter, I’d recommend his technical papers. Then, you might want to investigate his published articles. Those and much more, await the patient browser.
What would Sherlock say?
If you install them correctly when the stars are lined up in your favor, your mechanical seal should operate leak-free for a long, long time. If yours fail and start leaking while there’s still functional thickness remaining on the mating faces, you’re doing something wrong. If it happens twice, perhaps you ought to consider doing a bit of sleuthing to identify the root cause of this reliability-killing problem. Granted, troubleshooting might be as much art as science. Whatever the case, though, effective analysis can find the root cause and thereby cut you a little slack to attend to other more pressing matters.