June 2005
vol.2_iss 6
All major manufacturers of industrial pumps and mechanical seals produce quality products that are designed to work together for trouble-free operation. Yet mechanical seal failure is a common cause of premature pump failures and subsequent repairs. This issue of Run Times provides a brief description of common failure contributors, as well as some insight regarding practices to avoid these problems.
Dale B. Andrews – Editor
Misalignment Pump bearing housings, shafts, and seal chambers are manufactured with close tolerances. As a pump ages, these tolerances open as a result of wear, corrosion, erosion and, in some cases, improper maintenance. Misalignment resulting from excessive clearance in each of these areas, compounds to create an ever increasing misalignment at the seal faces, accelerating seal wear, resulting in premature seal-pump failure. Seal misalignment also originates from sources other than component wear. Misalignment may also be the result of high nozzle loads. If the suction or discharge piping needs to be mechanically leveraged into position in order to be bolted to the pump, there is a chance the resultant forces and moments acting on the pump will be sufficient to move the seal chamber out of alignment with the pump shaft. Excessive nozzle loads may also result from thermal growth of the suction and discharge piping piping during operation. If excessive nozzle load is a suspected problem, mount a dial indicator to the shaft and check for seal chamber misalignment prior to start-up. Table 1 lists allowable misalignments misalignments as set forth in APIAPI1 682, Shaft Sealing Systems for Centrifugal and Rotary Pumps . 1
Shaft TIR – Measured at the sleeve
< 0.002” 0,05mm
Seal chamber centering fit
<0.005” 0,127mm .0005 in/in 0,5µm/mm of seal chamber bore diameter
Seal Chamber face angularity
Table 1 1
API standards can be obtained at http://api-ep.api.org/publications/ ©2005 Lawrence Pumps Inc. 371
Market St., Lawrence. MA. 01843. Tel : (978) 682-5248 Fax:(978) 975-4291 Web http://www.lawrencepumps.com http://www.lawrencepumps.com Contact: Contact:
[email protected]
Page 1 of 3
Cavitation or Gas Entrainment Seals perform best when operating in a stable environment. Either gas entrainment or cavitation may result in tremendous side loads on a pump. A pump often begins to cavitate when the process system demands increased flow. Our October 2004 issue of Run Times 2 provides information about cavitation and its causes. As we pointed out in last month’s 3 issue , modifying a pump to bring its operating point within a stable operating range is often less expensive than dealing with the repair and downtime costs associated with a pump that is operating too far off design. Temperature The heat load on the seal is a combination of heat soak from the process and heat generated by friction of the fluid between the seal faces. Seal chamber temperature, as a rule of o o thumb, should be at least 50 F (28 C) below the temperature limit of any seal material and o o at least 50 F (28 C) below the boiling or flash point temperature of the buffer fluid. Temperature rise of the buffer fluid across the seal should be limited to less than 40 oF (22oC). Release of entrained gasses or boiling of the buffer fluid between the seal faces can spall material from the seal faces as well as disrupt the fluid film separating the faces. The mechanical seal buffer fluid delivery system design must ensure the buffer fluid is delivered at a temperature and flow rate to manage the heat load. The American Petroleum Institute provides guidelines for seal support systems in both the API-682 and 4 the API 610 standards. Flowserve Inc. publishes their “Piping Plan Pocket Pal ”, a very easy to use reference for the selection and application of seal piping plans.
2
http://www.lawrencepumps.com/newsletter/news_v04_i5_oct.html
3
http://www.lawrencepumps.com/newsletter/news_v02_i5_may05.html
4
http://www.flowserve.com/Seals/literature/Piping_Plan_Pocket_Pal.pdf . This may also be requested in hardcopy form at http://www.flowserve.com/seals/flipbook/
©2005 Lawrence Pumps Inc. 371
Market St., Lawrence. MA. 01843. Tel : (978) 682-5248 Fax:(978) 975-4291 Web http://www.lawrencepumps.com Contact:
[email protected]
Page 2 of 3
Buffer Fluid Compatibility For the seal support system to be effective, the buffer fluid must be compatible with the process; compatible with the seal materials, including the elastomers; environmentally safe; stable under the operating conditions for long periods; and, have viscosity and heat transfer characteristics that allow for good lubrication of the seal faces. John Crane Inc. publishes a white paper titled “Buffer and Barrier Fluids5” that provides an excellent overview of proper buffer fluid selection, as well as analysis of the relative performance of some more common fluids. Adequate Throat Bushing Flush Rates Some pumps rely on a flushed close tolerance throat bushing on the process side of the seal to keep solids away from the seal faces. The velocity through the throat bushing must be adequate to keep process solids from migrating into the seal chamber and building up around the seal. A good rule of thumb is to maintain a flush flow rate that provides a 15ft/sec (4,6M/sec) velocity across the throat bushing. The throat bushing should be inspected as part of each maintenance period and replaced if it shows wear. Finally, whenever a seal experiences premature failure, data describing the actual operating conditions, and any the worn parts, should be retained. Both the pump and the seal manufacturer should be involved in a failure analysis. Most failed seals provide many clues about their failure mode. A rigorous failure analysis may lead to the identification of the root cause, process or system improvements, and extended pump operating life.
5
http://www.lawrencepumps.com/documents/news_05_06_buffer and barrier fluids.pdf
©2005 Lawrence Pumps Inc. 371
Market St., Lawrence. MA. 01843. Tel : (978) 682-5248 Fax:(978) 975-4291 Web http://www.lawrencepumps.com Contact:
[email protected]
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