Viewpoint

Emissions-. alternative perspect ives by John Plumridge,

Technical Services Manager, Eastern Hemisphere Operations, EG&G Sealol.

It was perhaps inevitable that the opening salvoes in these Viewpoint articles would focus on mechanical seals; however, manufacturers must sometimes look back with some nostalgia to the days before 'fugitive emissions' and 'API 682'.

These matters have, of course, recently dominated not only these pages, but also conferences, work- shops and a large slice of company energy. There is absolutely no doubt that these new demands from regula- tions and standards on operating performance, reliability and life will be to the long term benefit of equipment specifiers, seal makers and plant operators alike.

In my experience, the debate on relative performance of mechanical seals took off in the early 1980s. There was a prolific output of articles and papers. Operators expressed concern at the general performance level of seals, notably through statis- tical comparison with bearings, and provided much detail on the resulting cost to operators for hardware repla- cement and downtime. Very few offered solutions, although two nota- ble exceptions were from UK opera- tors, the f'trst of w h o m addressed the problem by analysing good perfor- mers rather than the more normal practice of looking at the failures, and another which concluded from his particular plant:

"The examination of plant data showed that seal failures are now no longer the predominant reason for pump failures. On the contrary, seals tend to outlive pumps and while it is true that seals fail unpredictably, the base reason for seal failure is external

events. There are the occasional upsets and they happen at random. The mean time be tween failures, due to careful operation and maintenance, has been shown to be high and is no longer the prime concern for the single-stream pump installation."

Seal manufacturers working in a very competitive market wrote com- petitive papers and made competitive presentations. Things came to a head as far as the UK was concerned through two seminars at the Institu- tion of Mechanical Engineers, where interested parties at first closed ranks. In summary, the operators demanded better performance, the OEMs and engineering contractors demanded lowest cost and the seal makers advised that having both was difficult and seal failure was anyhow due to external events.

A wider debate suddenly opened with the formation of a Working Group at the Institution resulting in both the publication of their guide to 'Mechanical seal practice for im- proved performance', and also much greater user-led activity towards in- creased reliability.

This has continued through other national multi-interest working groups, most notably that which produced the recent American Pet- roleum Institute API 682 Standard 'Shaft Sealing Systems for Centrifugal and Rotary Pumps'.

This document should be widely welcomed in its scope to improve the state-of-the-art; its objectives are no- ble and in most respects the bul l of the document is in line with its mission. But it is bulky, and I have some reservations regarding some of

the qualification and product assur- ance testing requirements, and more particularly the level of detail in seal types and selection. This is based on selecting only those products which have wide usage experience and this, in a small number of instances, results in non-preference of newer technol- ogy. However, these are minor mis givings on an otherwise excellent standard

in latter years, the seal companies themselves have been collectively active in both the USA (Fluid Sealing Association) and more recently in Europe (European ,Sealing Associa- tion), most particularly to look in detail at actual seal performance in plant and steps which need to be taken on the newer legislation on the environment (particularly in the USA and Germany). In particular, wide debate has been aroused on the selection principles of sealed or seal- less equipment, on whether emission regulations should be based on flow or concentration levels and on how best to measure these emissions. All of these efforts are for the common good of both maker and user, Addi- tionally, over this last decade there has been much more general colla- boration between seal maker and user, at tirst mainly for new projects for the specifying process, but more recently this has extended to full partnership agreements to improve performance levels and rationalise inventor3" at existing plants.

During all of these changes, the seal companies themselves have been improving existing products and de- veloping new ones at a rate which I believe has been more rapid than in most other sectors of mechanical engineering. The sceptic might say that this has been forced upon us by the events that l have described, but most makers would point to the fact that many of the seeds for improve- ment had already been sown and that we, as an industry, have been quick to take up newer design techniques and materials

Existing products have had their operating limitations extended, but perhaps more importantly have be- come more tolerant to process upsets, which in the past gave an impression of unreliability', and this in turn has led to much improved life expectancy. Many new products have been intro-

14 Sealing TechnologF No. 9

Viewpoint

duced over this period; in some instances, for example the dry gas seal for high performance turbine driven compressors, new sealing technology has almost revolutionised an industry.

A little down the scale, there have been a number of developments on primary seal products operating at near-vapour conditions and their complimentary secondary back-up seals and supporting systems. A pre- vious contr ibutor to these columns related much of this to the new fugitive emission regulations based upon a global clean-up and concen- trated on the excel lent measured performance generally of single seals. However, this area of activity origi- nated with two very different driving forces from the present effort based on environmental protect ion. These were: • to p roduce an improved back-up

seal which would retain the low cost and s implici ty of a single mechanical seal and which would mee t the life requi rements of a primary seal whilst running at the prevailing a tmospher ic (or more normally leakage recovery) condi- tions. This would retain leakage w he n the primary seal deter iorated or failed and permit the use of a much simpler external system than the c o n v e n t i o n a l l iquid sys tem which r e p r e s e n t e d a substantial additional purchase cost together with high operating and mainte- nance costs.

• To use this 'new' technology to improve the safety on less hazar- dous duties where traditionally the pumps incorporated a single seal and thrott le bushing.

I relate these original requirements for three reasons: • I believe that the focus on environ-

menta l r e q u i r e m e n t s is en t i re ly appropriate, but this has pushed debate on safety into the back- ground. I offer two areas of support to this view. Firstly, search through published articles or con fe rence papers over, say, the last five years and you will f ind tha t v a p o u r l eakage is d o m i n a t e d by EPA, CAAA, TA LUFT. Secondly, in my experience, most duties which have incorporated the 'new' technology using dry sliding back-up seals have been based on environmental rather than safety requirements.

. Safety has general ly b e e n main- tained or improved by the contin- ued or ex tended use of traditional systems on new plant. However, permanent ly contact ing dry run- ning back-up seals have been devel- oped and field proven to the point where their use as a safety device should be considered. Technology has ensured that they can provide the combined benefits of long term normal operating emission control and shorter term 'safe' containment in the event of primary seal dete- rioration or even full failure.

• By the same token, it appears to me that operators have been slow to introduce these technologies into existing pumps which presently use single seals. The rate of seal conver- sion using these simple tandem seals is slower than the seal indus- try anticipated during the original deve lopment effort. There is no doubt that if this plant were to be uprated, it would be because emis- sion levels e x c e e d e d 1000 p p m rather than for absolute safety.

FinaLly, I turn to emissions of an altogether different nature. Debate on the emission of global warming and green-house gases on the interna- tional scale in the environmental and political arena is front-page news and presently puts fugitive emissions of volatile hydrocarbons relatively into the small print. This has not followed through to the level of seal perfor- mance as yet, so this is an at tempt to redress the balance. Activity in the refrigeration industry in general, and for instance the automobile industry in particular, is quite huge in response to the need to contain leakage in existing plant and to prepare for the phasing out of the existing and even the newer gases in very short periods of time through the effects of the Montreal Protocol.

Considering seals for the com- pressor industry, the requirements for performance are less well known than some of the areas I have previously discussed. The following gives an appreciation of the industry expecta- tion and what is achievable from highly developed single seals with no external containment, operating in fairly consistent and controlled con- ditions with efficient oil-based lubri- cation.

Oil Leakage Rate: acceptable 0.05 - 0.25 ml /hour achieved 0.01 - 0.05 ml /hour

Gas Leakage Rate: achieved 5-25 grams/year

Typical Seal Life: acceptable 3 years achieved 5 + years

It should perhaps also be remembered that the sealed produc t in this industry is quite often very aggressive to all but the most highly developed elastomeric materials.

A comparative reference could be made with the most recent studies made in the USA where the new federal requi rement of 1000 ppm (EPA Method 21) has been judged to represent a leakage flow of around 0.56 grams/hour (or 5000 grams/ year).

In this case the 'partnership' is very much be tween the seal maker and the compressor maker (rather than the end-user), but the end result is still a step change in the perspec- tive of sealing capability.

It also strongly re-enforces the argument from previous contributors that given close co-operation be- tween all parties involved, the seal maker can meet the challenges of today and the future.

The editor w o u l d w e l c o m e any correspondence on this or any other subject.

Contact: Parry Ray, Elsevier Ad- vanced Technology , PO Box 150, K i d l i n g t o n , O x f o r d OX5 1AS, UK. Tel: + 44 865 843848; Fax: + 44 865 843 971.

Sealing Technology No. 9 15