How much, if at all, mechanical seals do, must or must not leak is a controversial subject for which thereare few clear answers. This document is intended to clarify what to expect on mechanical seals found inImo Pump products.

　　A mechanical seal, figure 1, consists of a stationary member (seat) and a rotating member (head). The headis statically (or nearly so) sealed to and rotates with the pump shaft. The seat is statically sealed to a non-moving cover. The head has a highly polished, very flat face that is spring, and frequently pressure, loadedagainst the seat face, also a highly polished, very flat surface. Primary sealing takes place where the rotatingand stationary faces are in near contact.

　　The seal faces cannot be permitted to run dry (in hard contact) as the frictional heat build-up will very quicklycause severe seal face damage and extensive leakage. Because the faces cannot run dry, they aredesigned to allow a very thin film of liquid to exist between and migrate across the seal faces. In a properlyoperating mechanical seal, this thin liquid film, of necessity, leaks to the low pressure side of the seal, usuallyoutside the pump. In a single type mechanical seal pump, this liquid is normally the fluid being handled bythe pump. There are more complex designs with double, tandem or other arrangements which place abarrier fluid between seals such that one of the seals is exposed only to the barrier fluid while the inner seal(s)are exposed to the liquid being pumped. There are also seal arrangements that use pressurized gas at theseal faces to prevent liquid leaking out and also to prevent actual seal face contact.

　　Many pumped liquids are sufficiently volatile that the fluid frictional heat gained while traversing the sealface, causes the leakage to vaporize. Such fluids may display no apparent or visible leakage. Leakage is,however, still taking place. Other liquids will not so readily vaporize and thus may show an accumulationof leaked liquid outside the pump. Liquid to be sealed, sealing pressure, face loading, speed, size, sealing

　　parameters all impact how much parent leakage will be incurred. Some seal manufacturers design sealsfor high face loads that minimize leakage but also shortens the seal life. Others use lightly loaded faces forlong life but these seals are more prone to greater leakage, figure 2.

　　The main point to be understood is that ALL MECHANICAL SEALS MUST LEAK to operate properly.MECHANICAL SEALS ARE CONTROLLED LEAKAGE DEVICES, NOT ZERO LEAKAGE DEVICES.This is not what anyone wants to hear but that is the current state-of-the-art in single mechanical seals. Theamount and cosmetic visibility of the leakage will vary from pump to pump. A leakage rate of 10 drops perhour or less per seal is considered an acceptable leak rate for mechanical seal pumps manufactured in ourNorth American or European facilities. Many if not most pumps will operate well below these limits but upto these limits, we will not consider the seal operation improper. Note that seals will sometimes “run in” andleakage will gradually diminish over several hours operation with new seals. Disturbing (separating) sealfaces may cause it to leak slightly more than before being disturbed since the faces will not be “tracking”in exactly the same manner. These, too, may wear back in and cease or nearly cease apparent leakage.Seal systems with liquid or gas barriers can minimize detectable leakage at significant increase in expense.Beyond that solution, there are sealless pumps such as canned or magnetically driven designs which canalso be quite expensive but totally leak free. If the leakage is not an immediate health hazard, some systems,such as lubricating oil packages, can use vertically mounted pumps located inside the oil reservoir. Properlydesigned, these pump arrangements need not have shaft seals and flow that would normally cool andlubricate the seal is simply allowed to exit the pump and run directly back to the reservoir.

　　Startups can be particularly hard on mechanical seals. Insure that the pump seal chamber is filled withappropriate liquid so the seal does not start dry. Otherwise, initial dry start damage will progress to a severeleak. Dirt and other fines will also deteriorate the surface finish of the mechanical seal faces resulting inpremature leakage. The most common mechanical seal face material combination is carbon against anickel-iron (Ni-Resist). Very hard face materials such as tungsten carbide, silicon carbide and others arealso available to better resist abrasive wear albeit at higher initial cost. Hard-on-hard face combinations,such as silicon carbide on tungsten carbide, are very susceptible to heat checking (formation of surfacecracks) if allowed to run dry for even a few seconds. They should not be applied where the liquid side ofthe seal is exposed to lower pressure than its atmospheric (air) side, usually less than 1 atmosphere (14.7PSIA). u

More details for Mechanical Seal, pls click www.othchina.com, or email to oth01@othchina.com