Splitting the cost

The application of a tried-and-tested technology is claimed to offer all process industries improved reliability, increased uptime and reduced maintenance costs. Mark Venables explains.

Mechanical seals are an integral part of power transmission systems which need to prevent leakage between a rotating shaft and its housing under conditions of extreme pressure, shaft speed and temperature.

They can be single or double acting. Single seals have one sealing gap. The lubrication film required by the sliding seal faces is provided by the medium to be sealed.

By contrast, the lubrication film required by the faces in double seals is provided by a higher-pressure buffer medium (sealant liquid) that is compatible with the pumped product. The sealant liquid is at a higher pressure, so any leakage across the faces will be of the sealant liquid into the pumped product. This buffer serves to separate the product and the atmosphere.

Common applications for mechanical seals include pumps, agitators or mixers; marine stern tubes (propeller shafts); gas seals (spiral groove seals); and cryogenic seals. The seal can be internally or externally mounted.

When searching for mechanical seals, important shaft size and service limits to consider include nominal diameter, speed, alternate or rubbing speed, operating pressure, and operating temperature. Common features for mechanical seals include balanced or unbalanced construction, whether it is dependent or independent on direction of rotation, whether it can handle slurries, and whether or not the spring element is encased.

The direction of rotation — the direction as seen from the drive — is also an important factor. Seals that are dependent on rotational direction are those that transmit torque using a conical spring, or ones equipped with a pumping screw. The direction can be clockwise or anti-clockwise.

A recent analysis by sealing systems supplier John Crane on a variety of products for pumps and rotating equipment confirmed that split seals offer significant, measurable advantages for process industries — specifically improved reliability, increased uptime and reduced maintenance costs.

A proven technology on Royal Navy ships and submarine propeller shafts for more than four decades, split seals have also become a preferred solution for large industrial equipment, such as mixers and agitators, because the technology provides effective solutions for horizontal and vertical pumps in a variety of duties.

Since the seal’s components are split in half and can be installed directly to the shaft or packing sleeve, split seals were originally selected for difficult-to-disassemble areas. but today’s models are a universal solution with environmental benefits, as well as a means to increase production and cutting costs.

‘John Crane’s advanced designs require minimal installation time — as few as 30 minutes as compared to two hours or more with packing,’ said John Crane’s marketing manager Tom Evans. ‘The latest cartridge type of split seal comes with many of the components already sub-assembled, eliminating the need to measure or centre the seal to the shaft, and providing direct flush connections for cooling fluid and removing debris.’

Advanced materials and design enhancements also contribute to easy installation. While old designs used ceramic face materials susceptible to fractures from thermal shock, modern ones use a rigid mating ring assembly.

The ring’s halves, made from carbon-impregnated silicon with a rough surface at the split, nest together securely with no chance of misalignment. A retaining ring holds the halves together while the installer secures the assembly. The result is a rigid mating ring assembly that keeps the seal faces flat during assembly and operation. The secure clamping gives this seal its vacuum capability.

In addition to easy installation, today’s split seals can operate maintenance-free for extended periods of time — averaging three years compared to an average of six months with packing technology. And since they are hydraulically balanced, the seals are not prone to failure during system upsets. Clogging problems associated with small coil springs are eliminated and bearing failure is reduced with the elimination of excessive packing leakage.

So the advantages of split seals in pumps and other rotating equipment include no leakage, high reliability and low maintenance. This in turn translates into greater productivity and cost savings for all process industries.

The MSS seal from Flowserve is designed for use on mixers, reactors and other containment vessels, but is particularly suited for use on older equipment where there is severe shaft wobble and run-out eccentricities, or where compression packing is being replaced by a mechanical shaft seal. It provides advanced performance for wet or dry-running and is specified on mixer drives used for the blending of pharmaceuticals, food supplements, polymers, solvents, acids and other chemical products. Under dry-running conditions the seal provides the practical alternative to packings and liquid running seals on top entry mixers. It is also a good choice for side entry reactors or other conditions where wet face running is anticipated.

The design ensures that installation can usually be completed in less than an hour, even within tight spaces and without the need for special tools. Cumulative shaft run-out, eccentricity and deflections of up to 0.150in TIR can be accommodated. There is no need for time-consuming removal of gearboxes or motor drives, and the external seal assembly ensures simple installation and ease of inspection and maintenance.

The design and construction of the seal ensures that only non-metallic parts are in contact with the process fluids or gasses, eliminating the possibility of adverse chemical reactions. Seal faces are also self-lubricating with good heat dissipation properties.

Because these seals can also withstand pressure reversals, the problem of product contamination is also avoided when vessels are emptied or drawn down. Most sizes, from 25-305mm dia, can be provided with non-standard imperial and metric shaft dia sizes also available.

Operating pressures range from full vacuum up to five bar for dry-running, and up to seven bar for wet-running, with speeds ranging from 350 rev/min (dry) up to 1,750 (wet), at operating temperatures up to 121oC.