Why Mechanical Seals Fail

Sept. 30, 2009

Larry Bachus (a.k.a. “Pump Guy”)Officially, I am retired, although I can’t totally give up working. So I dabble as a pump consultant and mentor in my waning years. I

Larry Bachus
(a.k.a. “Pump Guy”)

Officially, I am retired, although I can’t totally give up working. So I dabble as a pump consultant and mentor in my waning years. I give a few pump lectures each year. As I begin each lecture, I poll the classroom. What do you want to learn in this lecture? What issue regarding pumps brings you to this class?

Invariably, more than 10 percent of the responses deal with mechanical seals. When six people speak up in a class of 50 students, it really means that more people are concerned with the issue. But the others are too timid to express their thoughts aloud in the classroom on the first day.

In 2009, we can calculate with precision the solar eclipses in the next century, perform brain surgery, send man to the moon, and harness atomic energy for consumption as household electricity. But we still can’t get satisfactory service from a standard industrial pump seal. Illogical, but true!

Can you believe the mechanical seal was first patented over a century ago, in 1903? The internal combustion engine was patented that same year. Harley Davidson Motorcycles and the Ford "Model A" automobile went into production in 1903, and the Wright Brothers initiated powered aviation in 1903.

More than 100 years later, internal combustion engines are reliable and mostly perfected even as they evolve, and most people would agree that modern aviation is mastered and reliable (technically speaking, of course). But a century later, too many production mechanical seals are unreliable and fail mysteriously and prematurely in industrial pumps. And this is sad.

The automobile industry makes fuel-efficient cars for environmentalists. The industry makes off-road vehicles for campers, ranchers and farmers. It makes big cars for big families. The industry makes giant SUVs for short, bald guys with ego problems. And it makes high-maintenance cars for high-maintenance people. You’d think that the mechanical seal industry would make reliable seals for the needs of the process industry. What does the process industry need in a seal?

We need a seal for cavitating pumps. We need seals for deadheaded pumps and dry-running pumps. We need seals designed for pumps that are forced to operate on the extreme end of the performance curve for extended periods. We need a seal for starved pumps. We need mechanical seals designed to handle production upsets. These are the real reasons that too many pumps suffer premature seal and bearing failure. Why doesn’t the seal industry recognize this?

Imagine a cruise ship crossing the ocean … and mysteriously, the rudder falls off. This is the image in my mind as I ponder the existing mechanical seal industry, lacking direction and leadership.

We don’t really need another seal that can be installed by an apprentice mechanic in five minutes … every four months. The seal manufacturers continue to crank out ads promoting "easy installation," "interchangeable spare parts" and "off-the-shelf availability." How about LONGER SERVICE LIFE in a process pump? Well, DUHH!

As a rule, pumps generally don’t break – they leak. I mean the impeller never splits into seven pieces. The shaft sleeve doesn’t fracture into three pieces. The pump can overheat and continue pumping. The process engineer will give the order to continue running the pump even if it is vibrating wildly. This is called "Running-to-Failure" (a concept that deserves its own article). Eventually, the heat, vibrations, or operational abuse will take out the mechanical seal. Then the pump goes into the shop for repair, with a spewing seal.

The "failed seal" sends the pump into the shop. So I’ll offer a few thoughts on preventing leaks and improving mechanical seal reliability.

Mechanical Seal Reliability
I used to work with a wisecracking Canadian engineer, Dann. Dann and I spent a few months back in the 1980s wrestling with hydraulic cylinders and actuators. One day Dann said, "I could seal a million pounds of pressure if I could control the extrusion gap at the piston and rod seals."

A creative mechanic fashioned a trough to channel the mechanical seal leakage from this pump toward a nearby floor drain.

Dann’s words were a variation on a statement made by the ancient thinker Archimedes in Sicily before Christianity. Archimedes said something like, "Give me an adequate fulcrum and an adequate lever, and I can lift the weight of the world." Strangely, Archimedes also developed the "Archimedes Screw," the world’s first rotary, constant flow, positive-displacement, sealless pump.

Mechanical seals leak either at the seal faces or through the secondary elastomer component, normally an o-ring. I promise you the liquid is not extruding through the seal’s stainless steel body, or through the setscrews or springs. The drip doesn’t find a microscopic pathway through the non-porous ceramic ring. All leakage is through the union at the faces or through the o-rings or gaskets.

I mean, if you have a drop of oil on the ground under the engine where you park your car every night, that drop of oil leaked through a rubber or cork seal on the joint at the oil pan. Or did you think it splashed from the dipstick spout?
This is the "extrusion gap" referred to by my friend Dann. The seal companies and pump companies must jointly direct their brains and efforts toward the seal-face union and the secondary elastomers.

Previously, this was impossible. Forty years ago, no seal manufacturer was intimately associated with a pump manufacturer. But now, with corporate mergers and "team-playing" mega-conglomerates, it is very possible. There are a couple of conglomerates with both a pump company and a seal company in the portfolio.

Evidence suggests the seal manufacturers don’t care, because we haven’t seen any great leaps in mechanical seal MTBF (I hate buzzword acronyms.) in recent years, as the sealless pumps gain market share. What is available in mechanical seal technology?

High-tech polymer/elastomer technology:
Per-fluoroelastomer rubber for o-rings and other secondary seals was developed over 30 years ago. This permitted seals that could withstand temperatures up to about 600 F (315 C), and greatly improved the range of chemicals and fluids that could be contained. There have been variations on this same product, but to date there is no successor. Where is the 800 F o-ring? Or the 1,000 F o-ring?

Rebuilt seals, at one-half the price of a new mechanical seal:
Some seal manufacturers call them "Golden Handcuffs." These seals still don’t address the need for improved service life. Is there a real advantage in paying half price for an inadequate product that’s already passed through the goose at least once?

Disposable seals:
Is this good service? "Service" is the word used at a stud farm for racehorses. Most maintenance engineers are tired of being "serviced" by their seal suppliers.

Cartridge seals:
They were definitely a significant improvement. They’ve been around for over 40 years. It’s time for something new.

Double cartridge mechanical seals:
These have also been around for a while, but they have never come into their own. I believe the seal sales reps and purchasing agents both consider these seals too expensive. I know this: If I were to return to work full-time and step into a hornet’s nest of premature pump and seal failure, I’d look to double cartridge seals. I’d convert all single seals into double cartridge seals. They can be pressure and vacuum tested in the shop, meaning you know the seal works as designed. They can be configured for absolute zero leakage. They offer extended service life with adequate environmental controls. They can survive "dry running," cavitation, air entrainment and vibrations that cause failure with single seals. In my opinion, all process and reliability engineers need additional training to explore double cartridge seals.

Dry gas seals:
These seals will figure prominently in the coming years, when all pumps and systems have adequate instrumentation. The equipment operators need leadership from the engineers to control their pumps and negotiate system upsets. Dry gas seals have an uphill battle as long as pumps are allowed to run indiscriminately all over their curves. It makes no sense to install a mechanical seal where axial tolerance required at the faces is stricter than the permissible axial play in the shaft bearings. Right now, these seals are a distraction (spin control) to draw attention away from the sealless pumps.

Sealless pumps:
This is actually a misnomer. Sealless pumps have seals, but they don’t have mechanical seals. We should call them "mechanical sealless pumps." Mechanical sealless pumps are available in a wide range of designs, covering PD and centrifugal types. Mechanical sealless pumps are thought to be new. Actually, they are the oldest of designs. Archimedes developed the first one about 2,200 years ago.

Peristaltic and diaphragm pumps are among the modern PD mechanical sealless pumps. These pumps enjoy a good reputation. Amazingly, when these pumps leak – or fail to generate pressure – it is normally through a failed elastomer seal (o-ring, diaphragm, or tube). But they don’t have mechanical seals.

And, there are centrifugal mechanical sealless pumps. Again, one of the earliest designs of centrifugal pump was the small vertical sump pump. The motor is above the sump, and the shaft extends vertically toward the volute and impeller, which are submerged in the fluid. It has no mechanical shaft seal. It uses the submerged depth, or static head above the impeller eye, as the sealing mechanism. I think this design concept could be explored further.

Mag drives and canned motor pumps are in the family of centrifugal mechanical seal-less pumps. Again, these pumps have seals, but not mechanical seals. These pumps may be the future of centrifugal pumps if the seal companies don’t act. Some new generation HVAC units feature a mag drive or canned motor compressor. Next-generation clothes washing machines and automobiles will feature mag drive or canned motor water pumps if we don’t see something impressive soon from the seal companies.

As you read this article in mid-September 2009, I will attend a two-day pump conference – not as the lecturer, but as a student. Don’t ever let your education stop. Register now for my Oct. 13-15 Pump Guy Seminar in Pittsburgh. See the promo box below for more details.

Larry Bachus, founder of pump services firm Bachus Company Inc., is a regular contributor to Flow Control magazine. He is a pump consultant, lecturer, and inventor based in Nashville, Tenn. Mr. Bachus is a retired member of ASME and lectures in both English and Spanish. He can be reached at [email protected] or 615 361-7295.

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