This article is based on a real-world situation related to to my recent article (Sept. 4, 2020) on the value of machinery-quality assessments (MQAs) in selecting best-available technologies for plant applications. Here, I discuss how a former manager’s feedback provided the incentive for a U.S. oil refinery to re-examine seal selection and flush practices in the facility’s coker unit. The unit had also experienced unremarkable “borderline” seal life, and the search for an optimized flush application merged into a quest for both better mechanical seals and flush-plan alternatives.
A bit of research confirmed that for temperatures much above 205 degrees C (400 degrees F) one would normally favor metal bellows seals. Below 205 degrees C pusher seals (which incorporate O-rings) would be quite acceptable.
A generic recommendation for selecting mechanical seals was made to to the refinery. It explained how, through a series of steps and questions appropriate to the task, i.e., taking an MQA approach, we would be able to determine if a vendor had relevant expertise. For this particular situation, a typical, but effective, MQA would would proceed as follows:
01. Establish formal communication with the four largest mechanical seal manufacturing companies (in 2020 and listed in alphabetical order, these four were: AESSEAL Inc., John Crane, Eagle-Burgmann, and Flowserve).
02. Provide e-mails to the local representatives of each, with invitation to reply by a deadline date (typically one week to 10 days).
03. Provide API data sheet(s) to each of the four potential manufacturers or vendors.
04. Request experience data, wherein each manufacturer discloses proposed seal type and estimated or known average seal life attained.
05. Make (04), above, a training experience whereby (highly undesirable) experimentation is disclosed as such. Accordingly, request feedback from the vendor who floated the idea of gas-lubricated and/or upstream pumping seals for the application at issue here.
06. Request comments on any sealing-system design elements, e.g., throat bushings that stabilize the shaft, “excluder” components that keep away solids, possible seal-chamber modifications) required for the application.
07. Provide a proposal drawing with a detailed bill of materials showing the basic seal design and sealing system requirements.
a. Request each manufacturer’s comment on minimum required seal-cavity dimensions.
b. If warranted, consider involving seal manufacturers in your overall pump-upgrade strategies or plans. In case a seal manufacturer is unresponsive or limits its interest to selling seals, consider involving a CPRS (competent pump-repair shop) with applicable experience in pump upgrading.
c. Emphasize that you seek feedback on possible stuffing-box modifications.
08. Compare seal leakage with each vendor’s proposed flush plan against buffer fluid lost into the pumpage (possibly a contractor/consultant calculation) to determine incremental savings.
09. Request API Flush Plan Number and/or modification recommendations to an existing flush plan. Be sure to become familiar with API Plans 53a/53b/53c and Plan 54
10. Ascertain the heat removal rate (Kcal or BTU/hr) if a cooler/heat exchanger is needed in high- temperature services. Note that some pumping ring designs are considerably more efficient than others; establish facts and vendor experience, but realize that Plan 54 is superior to Plan 53.
11. Compare vendor-supplied data and look for deviations from standard, or customary, answers.
12. Follow up on deviations and resolve differences between the answers submitted by different vendors.
The above recommendations and sequences have been validated by many decades of field experience. Be prepared to be ignored by some seal vendors, but you will find quality if you look for it. In the meantime, my next article for The RAM Review will provide some best-practice details on seal-flush plans.TRR
ABOUT THE AUTHOR
Heinz Bloch’s long professional career included assignments as Exxon Chemical’s Regional Machinery Specialist for the United States. A recognized subject-matter-expert on plant equipment and failure avoidance, he is the author of numerous books and articles, and continues to present at technical conferences around the world. Bloch holds B.S. and M.S. degrees in Mechanical Engineering and is an ASME Life Fellow. These days, he’s based near Houston, TX. Email him at firstname.lastname@example.org.
Tags: reliability, availability, maintenance, RAM, machine-quality assessments, mechanical seals, seal-flush plans