pressure reversal in dual mechamical seals 1

[behar]

Hi guys!
Can anyone explain to me what is pressure reversal in dual mechanical seals and what are the effects with specific reference to an unpressurised buffer service?
I have been searching the net for info, but there is hardly anything.
Regards
Behar

 

[JJPellin]

In general, mechanical seals are designed for pressure in one direction. For most seals, that is pressure on the OD. It is possible to reverse balance a seal so it can take some pressure in the opposite direction (pressure on the ID, for my example). However, the seal cannot take as much pressure in reverse as it can take in the normal direction. If the seal sees exessive reverse pressure, the most likely affect would be that the seal faces would open up and the seal would leak catestrophically. For an unpressurized double seal, this could happen in a number of ways. Pressure could build up in the seal pot. This could be pressure from the flare or vent system backing up in to the pot. It could be an unvented pot that pressurizes from heat on a hot sunny day. The other other way reverse pressure could be seen is if very low pressure or vacuum were to develop inside the pump. This possibility depends on the particular process. For most seals in this configuration, reverse pressure across the primary seal would cause the seal to "burp" some or all of the barrier fluid into the pump. If normal pressure is restored, the seal may not have any damage and could be refilled with fresh barrier fluid and run. As long as the barrier fluid was fairly clean so it did not contaminate the seal faces with solids, etc. you may not have to reseal the pump. It is more common at our site to have reverse pressure problems for pressurized double seals (Plan 53) if the pressure in the seal pot is lost. In that case, product is "burped" into the pot which contaminates the barrier fluid and often causes a failure of both seals.

 

[longeron]

JJPellin is exactly right.

There are some ways to counter act this with balance ratio.

Another issue is how your mating ring, usually the stationary seal face, is mounted. Most often seals are designed without a method to retain this face in reverse pressure- O-ring friction and an anti-rotation pin are most often used. But in a reverse pressure situation, it might be possible to unseat this seal face. Many seal mfgs offer mating rings that are balanced in much the same way that the rotating faces or primary rings are... Ask your representative about this.

 

[behar]

Thanks guys,
The information was extremely useful in justifing the installation of pressure gauges on the barrier tanks of seals piped up with a combination of plans 41 & 52 without vent of tank to flare. As of now there is no method of telling what the pressure is inside the tank on ; for example a sunny day as JPellin mentioned.
Regards
Behar