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INDUSTRIAL DIGEST
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INDUSTRIAL DIGEST
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Article #64: Enhanced Methods of Pump Sealing:
by Pumping Machinery, LLC
Real world pump installations are not always pretty. Two main pump components are known to cause maintenance headaches – seals and bearings. Seal failure can shut down a critical piece of equipment and bring the plant operation to a halt. The long-lasting challenge has been to reliably isolate the liquid end of the pump from the environment. In addition, reliability is sought to be accomplished at a reasonable cost.
Main elements of a pump Failed seals or packings are a mess
A pump wet end must be sealed off the environment: either by packing or by mechanical seal
1. Benefits and Drawbacks of Packings: * low cost / simple and quick installation / failure is usually gradual allowing time to react – but… * wear by friction with the sleeve or shaft / leakage is required to lubricate packings: messy floor or wasted injection water
To enhance packing performance, several optional auxiliaries can be used: * lantern ring / injection of clean fluid / flush flow monitoring, as shown below:
2. Benefits and Drawbacks of Mechanical Seals: * essentially no leakage when initially installed - but… * expensive / failure is catastrophic / reliability is compromised over time
Seal performance can be improved by proper flushing arrangement, as shown on the illustration above – throttling bushing is installed at the bottom of the sealing chamber bore, and seal glad is additionally equipped with the throat bushing. Still, a failure of mechanical seal is typically catastrophic and quick, requiring immediate action, and expensive repair and downtime. Double mechanical seals help somewhat in this aspect, but at a significant cost, and reliability issues, while improved, still remain significant problem.
3. Boiler Feed and High Temperature pumps experience In power generation industry, throttling bushings have been used for many years. For hot water boiler feed pumps, neither packings nor mechanical seals offer a desirable degree of reliability. While advances in packings and seals technology have been made over years, labyrinth throttle bushings still provide a more reliable, simple and economical way to keep the pumpage from leaking to the environment. The length of the bushing, number of serrations, and clearances depend on pump operating pressure and other operating considerations. Typically, a cold condensate is injected in the middle of the serrated bushing, thus eliminating hot water/steam leakage from the bushing end:
4. API applications Refineries have very critical demands on pump designs. While in power generation, as critical as it is, labyrinth bushings solve most of the sealing problems, they do not solve them entirely, as some of the leakage still remains. In case of refineries, no leakage is allowed. API-610 8th Edition, a governing pump specification for the refineries, states that: (from spec) 2.7.3.18 Throat bushings shall be provided: a. To function as a replaceable wearing part b. To establish differential hardness between rotating and stationary parts, and 2.7.3.20 …floating bushing shall be installed in the gland and positively retained against pressure blowout to minimize leakage if the seal fails
A close diametric clearance is required by API for such applications, typically not to accede 0.007” for shafts to 2” diameter. Throat bushing in such arrangements is typically referred to as a disaster bushing.
5. “SLICKER”TM Pump Sealing Design
This design combines simplicity and reliability of the packings with tight sealing ability of the mechanical seal, but without worries of catastrophic failures of mechanical seals.
A tight clearance of throttling bushing and throat bushing ensure low leakage rate, and reliable operation not only at failure, but eliminating such failure as a root cause. A diagram and principle of its operation is shown on a sketch below:
Clean water is injected via one of the two connections, provide via a panel that is provide with the design. During normal operation, a “fine flow” (typically a fraction of a gallon per minute) is supplied to the tight clearance (2), with flow measured by flow meter FMF with adjustment available via valve VF. System pressure is controlled by the panel valve VISO and injection pressure is measured by the gage Pmain. A very small amount of water lubricates the packing, and is additionally throttled by series of tight and open clearances 3-8, similar to methods used in boiler feed pump applications, as described earlier. To ensure that even this minute (but clean!) leakage does not flow along the shaft sleeve to the atmosphere, the gland is provided with leak-out port, at the bottom, to allow controlled direction of a trickle of clean water. The gland is piloted against the seal box, and can be either of solid construction, or split construction, for installation where axial distance to the obstruction is tight.
For dirty applications, such as wastewater, for example, a “rough” flush can be periodically engaged (provided at the panel as standard), to achieve higher flush rate, for a brief period of time. Most of the rough flush water will flow into the pump, and some under the packing, cleaning up accumulated debris and particulates. Still, no water, nor waste, will be coming out of the gland, as is the case for conventionally packed pump stuffing boxes.
Packing rings thus remain well lubricated and operate with low friction and long time, with no catastrophic failures. Eventual replacement of packing is simple, and is no different as compared to replacement of conventional packing rings.
Summary SLICKERTM pump sealing arrangement combines simplicity and reliability of packing with seal-off capabilities of mechanical seals. The design is simple, inexpensive and reliable. It makes life of maintained personnel easier with reduced emergencies and significantly improved reliability. From the management standpoint, SLICKERTM saves money, enhances plan uptime, and conserves water.
References
e. Zeidan F., at al, “Experience in the use of flexure pivot tilt pad bearings in boiler feed pumps”, Proceeding of International Pump Users Symposium, Houston, TX, 1999
For more information, call Pumping Machinery, LLC at: 770-310-0866 (section www.pumpingmachinery.com/pump_magazine/maintenance/maintenance.htm)
Pumping Machinery, LLC – saving plants energy, improving reliability, reducing pollution and helping environment
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