Mechanical Seals Failures

Mechanical seals have been increasingly used to seal rotating shafts for more than seven decades. Currently, they are the most common types of seals found on items such as centrifugal pumps and compressors used in the oil and gas industry. Over the years, the failure of mechanical seals has become an important issue. For example, a study conducted in a petroleum company reported that 60% of plant breakdowns, directly or indirectly, were due to mechanical seal failures [3]. 

A study of mechanical seal failures in a company reported the following causes for the failure of mechanical seals [3]  

Wrong seal spring compression Shaft and seal face plane misaligned Metal particles embedded in the carbon External system or component failure (e.g., bearings) 

Highly worn carbon (i.e., greater than 4 mm of wear) 

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By design, mechanical seals are the weakest link in a machine train. If there is any misalignment or eccentric shaft rotation, the probability of a mechanical seal failure is extremely high. Most seal tolerances are limited to no more than 0.002 inches of total shaft deflection or misalignment. Any deviation outside of this limited range will cause catastrophic seal failure. 

The replacement of elastomer seals with welded metal bellows shaft seals in the refining industry is discussed with reference to tests which claim to show large maintenance cost savings. Reasons why mechanical seals fail and methods of reducing or eliminating the major causes of mechanical seal failure are described. The cost of conversion and estimated cost recovery time is also outlined. 

Additional information on mechanical seal failure modes is available in... 

Skewis, W.H., Mechanical Seal Failure Modes, Sup>port Systems Technology Corporation, Gaithersburg, Maryland, retrieved on May 28,2015 from website http // docslide.us/documents/mechanical-seal-failure-modeslitml. 

Severe mctrculation will cause hydraulic loss and reduce pump efficiency. It will also cause damages at impeller eye and vanes, impeller fiaictiire, mechanical seal failure, and bearing failure. Therefore severe recirculation in pump needs to be avoided. 

The most common maintenance problem with centrifugal pumps is with the seals. Mechanical seal problems account for most of the pump repairs in a chemical plant, with bearing failures a distant second. The absence of an external motor (on canned pumps) and a seal is appeahng to those experienced with mechanical seal pumps. 

Not in-kind Ensure that the replacement satisfies the require-replacements (e.g., ments of duty gaskets, rupture, Implement management of change review disks, packing, mechanical seals) resulting in failure. Possibility of hazardous release. CCPS G-22 CCPS G-27 CCPS Y-28 OSHA 1910.119... 

First, one pump running in a parallel system tends to suffer from cavitation because operation to the right of the BF.P indicates that the NPSHr of the pump rises drastically. To survive this condition, you should use dual mechanical seals on these pumps. Dual or double mechanical seals can withstand cavitation better than a single seal. There is a discussion on this in the mechanical seal chapter of this book. Many engineers perceive that parallel pumps are problematic becau.se they appear to suffer a lot of premature seal failure. Parallel pumps deserve double seals even if it s only a cold water system. 

It has always seemed strange to us that the mechanics, or the manufacturer, are blamed when a mechanical seal fails after 3 months of service. If the seal fails on start-up, maybe you could point to the mechanic or the seal, but not after 3 weeks or 4 months of operation. This would most likely be an operational failure (a failure in operations), or a design failure (a failure in the system s design). And what is really amazing is that this statement and these words have never been recognized or said before.)... 

This report briefly describes a group of reactor coolant pump (RCP) seal failures that occurred at Arkansas Nuclear Unit 1, Calvert Cliffs Unit 1, and Indian Point Unit 3. Both mechanical and maintenance-induced RCP failure are discussed. For each event, the following information is provided the... 

A mechanical seal s performance depends on the operating condition of the equipment where it is installed. Therefore, inspection of the equipment before seal installation can potentially prevent seal failure and reduce overall maintenance expenses. 

There are defect limits that are associated with random failure modes. For example, if there is a leak from a mechanical seal on a pump, where do we decide that the leakage is excessive and requires immediate maintenance Vibration analysis severity levels are also typical examples of when do we have severe enough conditions to warrant equipment shutdown and overhaul. In such circumstances, the defect limit is dependent upon individual subjective judgment. 

Mechanisms Symptoms Carbon corrosion (air-air start) Gas impurities (e.g., CO, H2S, Sp2, ) Contaminants (e.g. some transition metal cations, anions) Catalyst instability (pt sintering, dissolution, re-crystallization) GDL loss of wet-proof (flooding) Seal failure (gross leaking) Membrane failure (pinholing, and tear)... 

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