Why is my mechanical seal continuously failing? 1

[DJKeng]

Hey guys,

I'm Running white latex paint for abrasive wear test.
Mechanical seals are failing early, and at low pressure.
Why?

<50psi
silicon carbide is the stationary seat.
hardened steel is the rotating face.

I've only been able to find one ref. online saying latex balls up between seal faces. can anyone point me to a good ref.?

 

[cponnu]

Face combination seems to be okay; what type of seal you are using ? What about the drive elements, the loading mechanism and the secondary seals ? What is the type of failure

 

[Sealman]

your problem is common when handling latex based media. The comment you found is probably a good enough explanation of the failure mode. The latex is shear sensitive and since, in a single seal application, the media forms the lubricating film between stationary and rotary faces the latex agglomerates and forms a non contiguous layer of hardened latex on the faces. This build up is huge in relation to the normal &quot;sealing gap&quot; of about 1 micron and allows high levels of leakage. There are two answers. One is to use a pressurised double seal where the sealant media is itself the lubricant film between the faces. The downside of that is that a loss of sealant pressure can allow latex into the barrier system and screws up the whole system. Also an unpopular solution with process people in case the inner seal fails and allows dilution/contamination of the media.
The other solution is to go for a knife edged seal. These are usually formed metal bellows seals with tungsten faces one of which is extremely narrow. The principle is not to allow the formation of a fluid film and use very wear resistant faces. The heat generated by high, effectively dry, contact pressure is dissipated by a low pressure water quench on the atmospheric side of the seal (usually less than 0,3 Bar) at a flow rate of around 1/L.Min. You should normally get up to 2 years life with this approach.
If you would like more info please em me.

 

[tothepoint]

I agree with sealman, however another approach is to use a multi lip seal instead of face seals. Filled PTFE lips ride against a hardened shaft in this design. There are no contacting faces, so you eliminate the sealing layer and latex build-up. This type of seal is commonly used in adhesive or syrupy services. All major seal manufacturers make this style seal (Garlock PS2, Crane MLS, etc.), and they can help you with any environmental controls you may also need if your latex tends to set up.