Mechanical Seal

Honestly I do not have the faintest idea of what you are asking and want to know.

Er, do you mean tandem seal arrangement? After rummaging about in a skip at the back of Saxone I found this: <cough>

http://en.wikipedia.org/wiki/End_face_mechanical_seal

If you could attach a sketch it might help. You might give SKF a call. They have seen everthing.

www.skf.com.

Buffer fluid pressure is zero intentionally so your pumping product that leaks past the primary seal leaks into the buffer fluid and not out to the environment. All mechanical seals leak intentionally to cool the faces. The leak rate is about 50 ml per day for a fluid that almost vaporizes after the seal and for viscous fluid 100 cP or more it is about 30 ml / hour. This buffer fluid catches the leak and will not allow vapors or the pumped hazardous fluid to exit to the atmosphere and cause a fire or a health issue. It is like a fluid that accepts the contamination intentionally and must be kept at a lower pressure than the primary seal chamber so the leak is from the pump into the buffer. The pump shaft has a pumping ring to circulate the buffer fluid to the pot and cooling system in the pot. The buffer fluid must be changed and the pots cleaned from time to time. The pumped product touches the primary seal faces. This is a Plan 52 system and I hate them. These are back to back seals, not tandem.

Tandem seals face the same way are are two seals, one is primary and the other is in case the first one fails. The second one is normally a dry running backup seals that runs touching but unloaded (so it doesn't get hot). Then if the first one leaks, the pressure builds up between the seals and pushes the second one closed under the leak pressure. Some backup seals are not contact types.

Barrier systems, Plan 53 A, B or C or Plan 54 are back to back seals and in between the seals is a barrier fluid kept at 4-6 barg above the seal chamber pressure. This way the barrier fluid leaks into the pump and out to atmosphere intentionally. the pump product does not touch the primary seal faces so in barrier systems we don't want the pumped product to touch, and harm, the seal faces. Such as dirty service, or highly viscous service that destroys seals. On Plan 53 systems the seal pot is pressurised by various means, normally nitrogen. The Plan 54 is a seal oil system with a separate pump, tank, heater, filter similar to a lube oils system. The barrier fluid circulates into and out of the seal chamber back to the seal system tank. You must top up the barrier fluid each week in the tank (normally 250 liter tank). The top up might be a few liters a week. The seal people can tell you the leak rates of the 2 back to back seals.

Your post is not off topic. There is a very complete reference guide that a good seal man let me see years ago when dealing with a sealing issue on a hot oil pump. Hopefully Petro Power can provide the name and source, (maybe API?). There are probably about 100 various seal arrangements shown and described. If you have the interest and need for a wide variety of pump sealing problems it can be a very valuable resource. With the short review I had, I learned that steam can be a very effective cooling fluid for hot oil pumps. A good saleman helped extend our seal life from 3 months to 12 months.

Just to understand the terms first, and I think you do already; Buffer fluid is a sealing fluid at a pressure (usually atmospheric) that is lower than the pressure in the seal chamber. Barrier fluid is a sealing fluid that is at a pressure higher than the seal chamber pressure.

Tandem, back to back and face to face are all double seal arrangements. Due to the different arrangements they are used for very different purposes. If you let me know your application I can be specific. (Your seal supplier if he is any good, should be able to help you, but I have met some very bad ones).

Tandem seals are basically two single seals in tandem with a low pressure buffer fluid between them. The product side seal runs on the pumped product and the outboard seal runs on the buffer fluid. This results in small leakage of the product into the barrier fluid. If the inboard seal fails the outboard seal takes over and runs on the product. In the back to back arrangement the barrier fluid leaks to atmosphere and into the pumped product.

Now to answer the actual question. Because the inboard seal is balanced, it will only function with the higher pressure on the outer periphery of the face (assuming rotating seal). If you increase the pressure on the buffer system the inboard seal will see the higher pressure on the ID of the seal and this will tend to force the faces open. The outboard seal will be fine. It is possible to design a tandem seal arrangement that will accommodate a barrier fluid, but then there has to be built in reverse pressure capability, usually a moving O ring to shift the balance line.

Also remember that the seal systems that you use for barrier and buffer arrangements are substantially different.

Dear Albert Caspers:

Petro Power and The Prof discussions are precise.

For more understanding refer these sketchs...

many many thx ths was the real reason tendom seal arrangement can not use in place of back to back arrangement by by keeping prr. higher in API 52 PLAN

IF WE LOOK ON THIS ARRANGEMENT THEN WE WILL FIND THAT INBOARD SEAL INSIDE FACE WILL OPEN IF WE INCREASE PRR. OF BUFFER FLUID

To idea to remember buffer (Plan 52) and barrier (Plan 53 and 54) is simple.

The English word 'buffer' means to lessen the affect of something, like the headache pain medication named "Bufferin". Open this link. Visually remember this blue box when discussing buffer systems in this way "Give me your headache and I will lesson the affect", like Bufferin headache pills.

The buffer fluid takes on the 'headache' . . . . the fluid that you do not want to drip outside the primary seal. Maybe the drips will cause an explosion or maybe ruin the environment, but the idea is to collect them and 'buffer' them.

ALL full contact mechanical seals leak intentionally. I call them leaks, not seals. So if you are pumping a fluid that you do NOT want to leak or drip out to the environment, such as gasoline, the the gasoline leaks into the buffer fluid that is trapped between two seals, dual seals, and circulates up to the seal pot (chamber) with a small little pumping ring that spins on the pump shaft. That is why the buffer fluid is at low pressure. The vapors in the pot are vented to a safe place. These systems are high maintenance as you need to keep them clean. The buffer fluid leaks a tiny bit past the second seal, so you need to top it up.

For Plan 53 and 54 are Barrier systems: The English word barrier means to prevent entry. So this fluid is trapped between two back to back dual seals and we intentionally want the barrier fluid to leak into the pumped fluid. The barrier fluid is higher pressure than the seal chamber making a 'barrier to entry' for the pumped fluid. We do this when we do not want he fluid to touch the seal faces fearing damage to the seal faces by the pumped liquid, or, for the reasons of the Plan 52 above. Plan 53 is more expensive than Plan 52, so it is mostly used to prevent pumped fluid to touch seals, not for environmental or hazard reasons. The barrier fluid must be compatible with the pumped fluid as barrier fluid leaks into the pump. The barrier fluid also leaks out to atmosphere in the seal drain piping. So the barrier fluid must be environmentally safe as well. Plan 53 using seal pots and pressure system for those pots (hydraulic or a gas blanket like Nitrogen). The pressure must be 'topped up' from time to time and the barrier fluid must be topped up also, without loosing pressure in the barrier fluid while ding so. I don't like Pan 53 at all.

Plan 54 is a barrier fluid system but uses and external seal oil system for pressure and flow, very expensive but if done correctly you can get 10 years life from a set of dual seals.

THX PETRO

Thanks for saying thanks ! Not too many people come back and thank all contributors. I want to thank all you as well. Good learning for me also.