I have usually called this an external seal. The flexible element (which could be single spring, multiple spring or bellows) is outside of the stuffing box. This places the pumped product on the inside diameter of the seal. This creates a couple of potential problems. If there are solids in the pumped product, they can pack up the inside of the seal as a result of centrifugal force. For many pusher seals, this would require that the dynamic o-ring moves toward the process as the seal wears. This can result in hang-up of the seal as the o-ring tries to move onto a dirty surface. If the seal rotary head is truly external, it may need to be protected from environmental damage, corrosion or contaminants. The seal faces will have to be designed for a pressure balance with ID pressure. But, this should not be a problem as long as the seal manufacturer knows how you are planning to install the seal. Our plant has about 1600 pumps with mechanical seals. I can't think of a single one that has this arrangement. But, I can't say that it is a bad idea for all possible applications.
External spring designs are a very good design and not necessarily pressurised internally. Slurry seals use external springs, some API seals are external design with standard OD Balance. They are used on high viscous or abrasive application. As the seal slides the dynamic oring slides on a clean surface of the balance diameter. I have used these seals before and are a very reliable design. Johnny is correct in what he mentions and those arrangements are rarely used today. So you need to understand which design you have and what your application is. Perhaps you can send a cross section together with your application for better advice.
Thank you, Flexibox. I do not have a particular cross-section of a seal in mind.
I was thinking more in terms of what would be the disadvantage of having the multi-springs outside the product on a single seal that is not outside mounted...yes such as a slurry seal.
It occurred to me that there could be a disadvantage if the service is one that produces highly reactive fumes as these pass through the faces and dynamic o-ring and mix with air thus, attacking the small springs somehow. I just want to check my line of thought..
