First in a Series on how mechanical seals work
Mechanical seals touch nearly every aspect of industrialized society. Wherever a rotating shaft moves fluid, mechanical seals play a key role in sealing process fluids in, keeping contaminants out, or both.
A few basic components and principles in mechanical seal design contribute to a working seal at the interface of the rotating shaft and stationary pump/mixer/seal-chamber housing. Mechanical seals are usually end-face seals or rotating-face seals, but in some designs they can be circumferential or even a hybrid of lip-type seals. In either case, the following components are common to all mechanical seals:
- Rotating primary sealing element: fixed to/driven by the shaft and seals against the stationary primary sealing element
- Stationary primary sealing element: fixed to the stationary housing of the pump, mixer or other equipment through which the rotating shaft passes and seals against the rotating primary sealing element
- Closing force: biases the primary sealing elements in contact to initiate sealing
- Static and/or dynamic secondary seals: seal between the mechanical seal components and the equipment shaft and housing
Rotating & Stationary Primary Sealing Elements
The more common end- or rotating-face mechanical seal designs feature mating faces as the primary sealing elements. Rings of ceramic, carbide, carbon or composites of these materials are lapped flat in the range of less than 1 micron on an axial end face. These lapped faces run against each other, one rotating with the shaft and the other stationary with the equipment housing.
The sealed fluid migrates between the flat faces and forms a stable fluid film at this interface. During shaft rotation, the face materials heat up, wear and degrade quickly without a lubricating fluid film between them. The sealed fluid creates this thin lubricating film.
In a lip-seal-type mechanical seal, a thin film of sealed fluid also lubricates the sealing interface. Rather than two flat rings, the sealing interface is a polymer material deflected against a hard material. This material could be a hardened, coated or plated metal, ceramic, or carbide face or sleeve. One of these elements rotates with the shaft while the other is stationary with the equipment housing.
Leakage is a function of the mathematical cube of the film thickness, so to minimize leakage, the gap at the sealing interface must be kept at a functional minimum. Closing forces are used to optimize this design parameter throughout the operating range of the mechanical seal.
The initial closing force ensures that the seal will function properly from startup. In end- or rotating-face mechanical seal designs, the initial closing force is provided by a spring component, which can be a single coil spring, multiple coil springs, a deflected bellows unit (elastomer or metal), or formed or flat springs. Initial biasing forces also can be created by magnets, compressed elastomers or any other means of applying a closing force between sealing elements. In a lip-type mechanical seal, the initial closing force is typically from the deflected polymer of the lip-type seal or a garter spring for less resilient materials.
Static/Dynamic Secondary Seals
The sealing elements must be secured to the rotating shaft and stationary housing of the equipment being sealed. O-rings, gaskets and other elastomer seals stop leakage at these interfaces.
A static secondary seal stops leakage between components that do not move relative to each other. One example is the interface between a sleeve and a shaft, where both rotate but do not move relative to each other. A dynamic secondary seal, on the other hand, stops leakage between components that move relative to each other. An example is a spring-mounted seal face, where the face is free to move as the spring deflection allows, and the secondary seal will stop leakage between the seal face and the component to which it is resiliently mounted.
A lip-type mechanical seal may only require static secondary seals because the deflection of the lip-type seal accommodates equipment operating motion. All effective end- or rotary-face mechanical seals require at least one dynamic secondary seal. This is because the mating faces of the sealing interface are rigid materials that cannot comply with any equipment shaft/housing misalignments, thermal growth and shaft end-play. The dynamic secondary seal will accommodate the relative motion between at least one of the seal faces and the component to which it is mounted.
Factors that increase seal life include seal design and material selection, process and environmental controls, and equipment optimization.
Mechanical seals are used with many process fluids. Each fluid has different lubrication qualities, but a thin, lubricating film at the sealing interface is always needed. A film that is too thick will increase leakage and may allow particulate between the faces, which will increase wear from abrasion. A film that is too thin will generate heat and cause materials to degrade. Keeping the sealing interface cool and clean will promote longer seal life.