Secondary seals are an integral part of any mechanical seal; this can be identified as o-rings, graphite rings and Teflon / PTFE wedges, etc. Under this document only o-rings as an elastomeric secondary sealing option are discussed.
We select o-rings according to the products that are pumped, and product information is captured in a centrifugal pump/product data sheet, see example below:
We then select the O-ring as secondary seals, according to a compatibility chart as published in the “Parker O-ring Handbook”. It is worthwhile to mention that this is not the only selection chart that can be used.
The selection criteria for this analysis consist of the following and are highlighted in the 2 columns above:
- Product / Liquid
- Pumping Temperature
- Compound Compatibility
The above criteria will assist with identifying whether or not you have the correct selection of secondary seals in the sealing arrangement selected for the application.
Normally we can easily identify premature seal failures on any of the seal components, in particular the elastomers (o-rings) that are discussed in this analysis.
Case History:
We had five mechanical seal failures on a MEK (Methyl Ethyl Ketone) application in the non-process, tank farm area. Each time we analysed the component seal parts we could not find any signs or obvious reason for the product leakage. After the fourth failure, an upgrade was done from a component seal into a full cartridge seal. The upgrade was done to eliminate the finger trouble, as this was identified as the obvious reason for the incorrect working length. Our artisans were able to identify this concern during the assembling of the mechanical seal and while installing it into the pump. (Convert contact component seals, info cartridge seals, will be published in April)
To our entire problem solving surprise, the mechanical seal started to leak after the same run time and with the same failure mode. We now had a complete cartridge seal that was installed which we could bring back to the refurbishment centre to pressure test. The mechanical seal failed the pressure test, which had passed the test a week before.
The seal was taken apart and the secondary seals were changed and subsequently followed by a pressure test, of which the seal held pressure. We had a look at the selected o-rings as secondary seals, and all looked to be in perfect condition. However, as the only parts of the mechanical seal that causes the pressure test failure, we started to zoom into the sizes of the o-rings.
Firstly, we identified that the non-static O-ring, on the face retainer, was one size smaller, as shown above.
Secondly the sectional size was also smaller, as seen above.
To eliminate all the possibilities of a non-conformance when assembling and/or selection of the secondary seals for the mechanical seal, we performed a test. We took two different materials of o-rings, which were according to the selection chart, compatible with the product and put it in the product for a week. In the EPDM bin we found two o-rings from the correct material but the one was not marked with a blue dot, as it should be for EPDM.
Test Conclusion:
The non-marked EPDM O-ring decreased more in size than the one with the blue dot. The reduction in size was both too small for the sealing arrangement and failed the pressure test when installed in the mechanical seal. The ISO-LAST (Perfluoro-elastomer) was still the same size as selected from the store.
RCA (Root Cause Analysis) Conclusion
The reason for the failure of this sealing arrangement was that the EPDM O-ring on the face retainer was not compatible with the product, even if the O-ring compatibility handbook confirms that it should have been compatible. These failure modes were overlooked several times, due to the fact that the visual condition of the O-ring appeared to be in good condition.
Recommendation and seal material upgrade
The face retainer non-static O-ring was changed to ISO-LAST (Perfluoro-elastomer). The pump ran for 3 months with no evidence of any seal leakage, and is still running successfully.
Summary
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As the smallest and cheapest part of the mechanical seal, the secondary seals / o-rings, are often times over looked as the biggest contributor to premature seal failures.
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Always go for the best O-ring compatibility compound option,
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Please try to send your material selection for mechanical seals, either to Johannesburg, Spartan or the UK.
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When analysing secondary seals with any seal failure analysis, always measure and compare to the sizes to what it should be.
This case history / information bulletin was compiled by Pieta Engels; please feel free to contact me.