Case Gasket Leakage
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?By E. M. Araza
Q – We have an axially-split, 5-stage, between-bearings pump (BB3). During a scheduled maintenance, we replaced the 1st stage impeller that had cavitation damage. When the pump was put back in service it leaked at the gasket area in the midpart of the casing. The operating conditions have not changed – the case maximum working pressure is at 2150 PSIG. Other than changing the impeller and case gasket nothing else was done on the pump. What could be the reason/s for the leakage at the split line?
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A – Not all flat gasket sheets are created equal although they may look alike. Check that the replacement gasket used has the correct material and pressure-temperature (PxT) rating especially because the pump working pressure is very high. The PxT limit of the gasket should not be exceeded. The gasket must also be of the correct thickness. (Note that the use of gaskets with asbestos filler is strictly not allowed, and even illegal to use in many jurisdictions.) If in doubt, consult your gasket supplier for proper advice.
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Check that your inhouse procedure for replacing the gasket was strictly followed. Careless cutting of gasket around areas of the volute throats and stud holes can damage the gasket and create leak path. Improper replacement of the gasket can result in unexpected leakage.
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Verify that proper bolt tightening sequence and torquing were completely followed. Bolt tightening sequence is not done consecutively to adjacent bolt locations, nor haphazardly; rather it should be done crisscrossing the bolts properly, as recommended by the pump manufacturer. Ensure that proper torque values are used depending on the bolt size, bolt material, and whether the bolts are lubricated or dry. Full torque values are not to be applied on one pass, rather the bolts should just be pre-stress on the first pass. A minimum of two passes, ideally three passes, should be made before reaching the full torque values.
领英推荐
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There are instances where a casing is applied with adhesive at the split line to prevent the gasket from moving during pump assembly. It is possible that the gasketed surface may not have been cleaned thoroughly with solvent where microscopic size adhesive particles are retained creating a leakage path. Make sure that the gasketed surface is thoroughly cleaned.
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See related articles by the author:
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Recommended procedure for replacing case gasket
Recommended bolting tightening sequence
Recommended bolt torque values
Knighten Machine Director of Machining
8 个月Very Well Stated
Prime Equipment Consultant
8 个月I agree with Morg. There are multiple verifications that should be completed before just replacing a gasket. Not to mention the thickness of the gasket. I would also be concerned about the balance of the rotating assembly. Cutting a gasket requires specific training , it’s not an easy task.
Retired Pump Hydraulic Specialist at Flowserve Corp.
9 个月Thank you all for your comments and likes. Please keep them coming. This is exactly what I hope to achieve when I created this Centrifugal Pump group and newsletter here in LinkedIn - to provide a forum for the exchange of ideas coming from a diverse group of pump people so that we may learn something new and useful from each other. Please join or follow this group, and encourage others to do the same. While we're on the topic of case gasket, I received a couple of follow-up questions: 1. Would you recommend to apply adhesive to the pump parting flange to hold the gasket in place, and why? 2. If you would recommend to do it, should the adhesive be applied on the lower half casing, or on the upper half? What do you think?
Principal at Hydraulic, Measurement, and Inspection Consulting, LLC
9 个月API RP 697 infers a surface finish of 63 to 125 Ra for the horizontal split line. So, I agree with Steve and Dave. The words "except for changing impellers nothing else was done" worry me because this infers that no one checked the parting flanges for flatness. This is an area of concern.
Sales Manager - Specialty Products - Hydraulic Decoking Systems, Specialty Mechanical Seals at Flowserve Corporation
9 个月Surface finish at split line is also critical to sealing. Surface finishes of above 63 uin are often used depending on manufacturer preference. The rougher finish allows case to grip gasket better. Care in cutting the gasket adjacent to stationary element components is critical to avoid premature wash out. In addition to proper torque verifying gaskets compression can be achieved by cutting out windows in the gasket near outside of case split. This allows use of feeler gauge during and/or at end of torquing process.