- 06 January 2006 -

Seal selection for rotary plant

The choice of what is the best seal for the job often comes down to personal preference as much as objective decisions. For many the choice is between a packed gland or mechanical seal – soft and conformable or hard and rigid. Here Andrew Goulding of James Walker & Co Ltd presents an overview of sealing options for centrifugal pumps.

The predominant role of fluid seals in rotary plant such as centrifugal pumps and mixers is to retain pressurised fluid media within a process. Unless a vacuum is involved, the seal rarely needs to prevent the ingress of external contaminants, as the positive pressure differential will achieve that naturally. Two types of rotary sealing device effectively contend for this form of dynamic duty, namely, the simple packed gland (typically containing five rings of pliant material) and the somewhat more complex mechanical seal. Another favoured sealing device for rotary shafts, the elastomeric lip seal, will efficiently protect bearings, but rarely holds a pressure greater than 4bar or works at under-lip temperatures above 200°C.

Although compression packing is a traditional sealing device that predates the industrial revolution, constant advances in materials technology, lubricant systems and manufacturing methods over the past 100 years have kept it at the forefront for both OEM and maintenance applications.

Some 20 years ago, a sharp decline in the use of asbestos-based materials, from which most packings were then made, combined with the increased popularity of mechanical seals to force traditional gland products out of favour for a time. This inspired high levels of research to produce low-friction compression packings from materials such as exfoliated graphite, graphite yarn, PTFE, and new synthetic fibres.

The aim was to provide better chemical compatibility, abrasion resistance, temperature characteristics and service life than obtained from asbestos packings, at a comparable price. Within just a few years, and a decade before the UK government's ban on white asbestos came into force in November 1999, new packings of advanced design were exceeding the performance of top-of-the-range asbestos types.

Before discussing in detail the many aspects relating to seal selection, an overview of pros and cons of both compression packing and mechanical seals is needed.

From this table it can be seen that both packings and mechanical seals have their own specific benefits on rotary duties. The mechanical seal is usually considered a fit-and-forget device, until its maintenance period is due. A packed gland, however, needs occasional adjustment to take up wear and compaction whenever the drip rate of fluid becomes excessive (ie, more than one drop every few seconds).

Trained personnel are needed for the adjustment procedure: the machinery will be running, and it is easy to over-tighten a gland and lose the fluid film that is needed for lubrication. When gland follower adjustment fails to reduce the leakage to the required level, the time has come to repack the gland. At this stage it is usually possible to run with slightly excessive leakage until it is convenient to take the plant off-stream for maintenance attention.

With mechanical seals, the on-site assembly and installation can be a lengthy, complex task demanding clean working conditions and precision adjustment. Moreover, a mechanical seal gives little or no warning of a premature failure. The result can be serious in terms of fluid loss, pump damage, plant downtime and the cost of replacement or refurbishment. Periodic maintenance usually involves the removal of the complete unit to replace rotating faces and/or seats, as well as any elastomeric or polymeric components such as ‘O’ rings or bellows. Full refurbishment often comes to 50 per cent of the original purchase price of the seal - and as refurbishment is usually performed off-site, a replacement seal is needed in the short-term to keep a process running.

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