Improved Mean Time Between Planned Maintenance (MTBPM) for Mechanical Seals in Process Pumps

Transcription

1 Flow Solutions Division Improved Mean Time Between Planned Maintenance (MTBPM) for Mechanical Seals in Process Pumps Seal Chamber Design Introduction Attention is being focused on the need to increase the mean time between planned maintenance of pumps used in the chemical process and refinery industries. Of the many factors affecting the MTBPM of pumps, performance of end faced mechanical seals is a major contributor. Only a small percentage of seals achieve their design life. One of the critical factors affecting seal life is the temperature at the seal faces. It is more important than the seal chamber temperature itself. In fact, seal chamber temperature readings can often be misleading when used in seal life studies. Advanced Tapered Seal Chamber Figure 1 Minimum 4 tapered chamber bore assures proper gas venting and allows proper seal heat removal without flush Enlarged radial space, 0.88 to 1.00 in. (22.3 to 25.4 mm) typical, for improved heat removal Research conducted at Flowserve shows that the design of the seal chamber, the axial location of the seal faces within the chamber, the throat design, and the venting of the chamber all are important design factors relating to seal face temperature. Also, testing verifies the importance of seal chamber design in handling abrasive slurries. Through the efforts of ANSI, API, users, OEM pump companies, and seal manufacturers, significant design improvements in seal chambers and sealing systems are now available. See Figure 1. This paper reviews the subject of seal chamber design and the benefits that can be gained in MTBPM when improved chamber designs are combined with correct material selection, environmental controls, and seal design. Improvements in MTBPM can be achieved now by implementing new seal chambers and improved seal designs in existing pumps. Copyright 1 of 8 Confined O-ring gasket for positive sealing and proper alignment of seal (

2 Conventional Stuffing Box Limitations Interchangeability of seals and packing in the same stuffing box forces compromises in the seal design. The typical seal O.D. to stuffing box bore radial clearances of 0.06 inch (1.5 mm) to less than 0.03 inch (0.8 mm) results in severe seal performance penalties such as: 1. Inadequate removal of seal-generated heat. 2. Vulnerability of seals to damage from abrasives. Large Cylindrical and Tapered Chambers A seal chamber, such as that shown in Figure 3, offers one solution to the shortcomings of the conventional stuffing box. It eliminates the wasted space in the box, provides more clearance over the seal O.D., and can provide more room outside of the seal chamber for double and tandem seal arrangements. Optional Enlarged Cylindrical Seal Chamber Figure 3 3. Installation difficulties with double and tandem seal arrangements that are necessary for safety and environmental reasons. 2.00" (50.8 mm) 1.12" 1.06" (28.4 mm) (26.9 mm) 3.25" (82.6 mm) Two Inch (50.8 mm) Single Seal in a Conventional Process Pump Stuffing Box Figure 2 Confined Gasket 2.75" (69.8 mm) 1.12" 1.06" (28.4 mm) (26.9 mm) 2.50" (63.5 mm).12" -.25" ( mm) Clearance Seal O.D. to Box Bore.88" " ( mm) Typical Radial Space Wasted Space The enlarged tapered option, shown in Figure 4, has the added advantages of being statically self venting..31" -.38" ( mm) Typical Radial Space.03" -.06" (0.03" mm) Clearance Seal O.D. to Box Bore These "seal only" chambers are not interchangeable with packing. However, it is estimated that 85-90% of the ANSI/API process pumps use some type of seal. Optional Enlarged Tapered Seal Chamber Figure 4 Minimum Figure 2 illustrates a typical single seal installed in a conventional process pump stuffing box that was designed to be used for packing. There is wasted space in the area between the seal and the bottom of the stuffing box because the box was designed for five to seven rings of packing. This design also limits the distance outside of the box to the first obstruction, distance needed for double or tandem seal designs required for safety or protection of the environment. 4 Taper 2.00" (50.8 mm) 1.12" (28.4 mm) 1.06" (26.9 mm) 3.25" (82.6 mm) Confined Gasket.12" -.25" ( mm) Clearance Seal O.D. to Box Bore.88" " ( mm) Typical Radial Space 2 of 8

3 Industry Standards - ASME (ANSI) B73 and API 6 Over the past few years, committees have reviewed the ASME B73 and API 6 pump standards. The revisions to these standards provide for optional cylindrical and tapered seal chambers with increased radial clearances. Tables 1 and 2 summarize the previous and existing ASME (ANSI) B73 and API 6 standards for radial clearances as shown in Figure 5. Radial Clearance per ASME B73.1 and API Standard 6 Figure 5 Seal Chamber Radial Clearance Shaft or Sleeve O.D. Table 1 ASME B73.1M Seal Chamber Dimensions* Specifications for Horizontal End Suction Centrifugal Pumps for Chemical Process Revised Minimum Bearing Frame Previous Radial Radial Clearance Size Clearances (Shaft or Sleeve O.D. to Bore) AA - AB 5/16 in.(7.94 mm) 3/4 in.(19.09 mm) A05 - A80 3/8 in.(9.52 mm) 7/8 in.(22.22 mm) A90 - A1 7/16 in.(11.11 mm) 1.0 in.(25.40 mm) * ASME B73 shows tapered and cylindrical bore seal chambers as optional. Table 2 API Standard 6 Centrifugal Pumps for General Refinery Service Minimum Dimensions for Seal Chambers Overhung Pumps Furnished with Shaft Sleeves 6th Edition 7th Edition Minimum Packing Space Minimum Radial Clearance Shaft Size Radial Clearance (Shaft O.D. to Bore) in.( mm) 3/8 in.(9.52 mm) in.(25.40 mm) in.( mm) 3/8 in.(9.52 mm) in.(28.58 mm) in.( mm) 3/8 in.(9.52 mm) in.(31.75 mm) Copyright 1998 Flowserve Corporation 3 of 8

4 Flowserve Seal Chamber Performance Studies Studies were completed in Flowserve laboratories using standard size industrial pumps equipped with: 1. Standard Stuffing Boxes, Single Seal in Conventional Stuffing Box Figure 6 Chamber Seal Face 2. Enlarged Cylindrical Seal Chambers, and 3. Enlarged Tapered Seal Chambers. Figure 6 shows a standard single seal in a conventional stuffing box. probes were installed in the stuffing box and 0.06 inch (1.5 mm) back from the face of the stationary insert. Figure 8 depicts an enlarged cylindrical seal chamber. Note that: Single Seal in Enlarged Cylindrical Seal Chamber Figure 8 Chamber Seal Face 1. There is increased radial clearance over the seal, however, 2. The seal chamber is not self-venting. Figure depicts a commercially available enlarged cylindrical seal chamber available as a pump component or as an add-on feature. Note that the: Single Seal in "C" Style Enlarged Cylindrical Seal Chamber Figure Chamber Seal Face 1. Seal chamber is not self-venting, and the 2. Seal faces are located in a recessed bore in the gland. Figure 12 shows an enlarged tapered self-venting seal chamber with an open throat per new ASME B73 Seal Chamber Specification. The chamber is: Single Seal in Enlarged Tapered Seal Chamber Figure 12 Chamber Seal Face Self venting, Has a minimum 4 taper for better heat dissipation, and Has increased radial clearances over the seal. 4 of 8

5 Plot of Figure 6 Test Figure 7 F F 40 Seal Face T Seal Face T Chamber T Time, hours Chamber T Time, hours C Plot of Figure 8 Test Figure 9 C The tests performed by Flowserve are summarized below. In all cases, the upper curve, labeled "seal face T", shows the differential temperature of the seal faces over the pump suction temperature. The lower curve, labeled "chamber T", shows the temperature increase in the chamber over the pump suction temperature. Figure 7 summarizes the temperature rises experienced with a standard single seal installed in a conventional stuffing box. Note the temperature build-up in the stuffing box that contributes to high sealing fluid temperature with resulting low viscosity, poor lubricating properties, and high seal face wear; all resulting in reduced seal life. In many cases, seal life is further reduced by the design limitations imposed by the narrow 0.03 to 0.06 in. ( mm) clearances typically found in the standard stuffing box. In abrasive service, these tight clearances can result in damage to seal hardware. Enlarging the seal chamber bore reduces the seal face and chamber temperatures as shown in Figure 9. However, this configuration is not self venting and gases can be trapped in the seal chamber. Plot of Figure Test Figure F Seal Face T Chamber T Time, hours C Opening up the seal chamber as done in the commercially available "C" chamber design shown in Figure also lowers the temperature in the seal chamber. However, note the high temperatures at the seal faces and the erratic chamber temperature surges shown in Figure 11. These result from the poor design of the chamber. The seal chamber becomes a natural trap for gases and entrained air in liquids. The seal chamber will not fail, but the seal will. Plot of Figure 12 Test Figure 13 The low seal face and chamber temperatures shown in Figure 13 are consistent with extended seal life F 40 Seal Face T Chamber T Time, hours C Tapered seal chambers such as shown in Figure 12 are also self venting on shutdown. Flowserve expects this seal chamber configuration, combined with the latest seal technology contained in Flowserve design, to be a significant contributor to improving the MTBPM of rotating equipment in the chemical process, refinery, pulp and paper, and other industries. 5 of 8

6 Improved MTBPM Available Now Field evaluations of mechanical seals in the new seal chambers demonstrate a marked increase in seal life. The average seal life doubles when operated in the enlarged seal chambers with no other changes. Several pump OEMs offer improved seal chamber designs that allow retrofitting of existing pumps. These improved seal chambers used with currently available state-of-the-art Flowserve seal designs can improve MTBPM now. Enlarged Tapered Bore Seal Chamber with X-1 Seal Design Figure 14 X-Series Seal Design Features 1. Available as single, double, or tandem seals with common parts to reduce inventories. 2. The cartridge design eliminates installation errors. 3. No dynamic O-rings, no fretting, no hang-ups. 4. Hard-vs-carbon or hard-vs-hard faces available for extended seal life. 5. Product is at O.D. where centrifugal forces keep debris away from seal faces for improved life. 6. Optimum seal face design to reduce heatloads and extend seal life. 7. High seal stability factor. Faces remain closed during upset conditions. Withstands pressure reversals. 8. High seal strength factor. Seal face deflection due to pressure, temperature, and heat flux are minimized. 9. Vibration dampener eliminates hang-up.. Bi-directional flow inducer standard. Provides built-in circulation for cooling of double and tandem seals. 11. Buffer fluid is on the seal face I.D. for double seals providing positive seal face lubrication. Figure 14 illustrates a seal chamber arrangement that is typical of improvements that can be implemented immediately. The enlarged tapered seal chamber provides the greater radial space mentioned earlier in this paper. Increased clearance between the seal O.D. and seal chamber bore allows the seal to run cooler. The tapered bore and the open throat vent the chamber prior to start-up of the equipment. X-Series, SL-Series, and other Flowserve seal designs specifically built for these new state-of-theart large bore seal chambers are available now from Flowserve Corporation. 12. X-1/1 design eliminates multiple sleeve-onsleeve fits with less wobble and wear of seal hardware. A Flowserve X-0/1 seal design utilizing an improved seal chamber and supply tank can provide a cost effective double or tandem sealing system for handling hazardous, toxic, or abrasive applications. See Figure 15. Ask for Flowserve Form 539. Flowserve Supply Tank Figure 15 The design characteristics of Flowserve seals such as the X-Series cartridge metal bellows and SL- Series cartridge designs include many of the requisites of the extended MTBPM mechanical seal. For more information on the Flowserve X-Series seal designs ask for Flowserve Form 566. For information on the Flowserve SLM-Series seal designs, ask for Flowserve Form of 8

7 Enlarged Seal Chamber Case Histories Large East Coast Chemical Plant Equipment: Seal: Duriron Mark II Pumps CBR Seal Design Reference Drawing: 2F CBR Seal Design in a Duriron Mark II Pump, Enlarged Cylindrical Seal Chamber Figure 16 Several pumps installed in an incinerator waste service were operating single CBR seal design in conventional stuffing boxes achieving approximately six weeks seal life. The six weeks were an improvement over other seal designs and materials tried previously. New seal cover plates were supplied to the plant with opened up radial clearances and the same CBR seal designs were used. Currently, these CBR seal designs are now providing over one year average life. See Figure 16. Central States Power Plant Equipment: Seal: A. S. H., DG-9-5 Slurry Pump SL-5000 Seal Design Reference Drawing: 2D These pumps circulate % lime slurry through the absorber spray system in a flue gas desulfurization system. SL-5000 Seal Design in an A. S. H. Frame DG-9-5 Slurry Pump Enlarged Tapered Seal Chamber Figure 17 Flush The rugged SL-5000 seal is designed to fit enlarged seal chambers. Large cross-section components provide long, trouble-free, seal life. There are 16 of these 6.250" SL-5000 designs in service at this plant plus two 4.000" SL-5000 seals in Warman lime slurry pumps. One of the seals has been running successfully for over two years. As the lime tends to build-up on the chamber surfaces, the seals are flushed with a minimum of water, API Flush Plan 32. Further Studies This paper deals with actions that can be taken today to improve MTBPM through seal chamber design. Flowserve's R&D efforts are continuing to investigate improved seal and chamber designs that will advance MTBPM even further in the years to come. There will be an evolution of advanced sealing systems as the new ASME B73 and API 6l0 standards become integrated in new equipment purchases. Advanced sealing systems allow retrofitting of existing pumps to provide the same benefits. Flowserve can provide assistance through plant surveys and an analysis of MTBPM history to help identify equipment that offers immediate potential for improvement and recommend appropriate action. Contact your local Flowserve representative. References 1. ANSI B73, American National Standards Institute, American Society of Mechanical Engineers, New York, NY. 2. API 6 7th Edition, American Petroleum Institute, Washington, DC. 3. The Effects of Seal Chamber Design on Seal Performance, M. Davison, May 1989 Pump Symposium, Houston, TX. 4. Life Extension Possibilities for ANSI and API Pumps, R. Gabriel, Oct 1988, Pacific Energy Association. 7 of 8

8 The information and specifications presented in this product brochure are believed to be accurate, but are supplied for information purposes only and should not be considered certified or as a guarantee of satisfactory results by reliance thereon. Nothing contained herein is to be construed as a warranty or guarantee, express or implied, with respect to the product. Although Flowserve Corporation can provide general application guidelines, it cannot provide specific information for all possible applications. The purchaser/user must therefore assume the ultimate responsibility for the proper selection, installation, operation and maintenance of Flowserve products. Because Flowserve Corporation is continually improving and upgrading its product design, the specifications, dimensions and information contained herein are subject to change without notice. Primary Worldwide Flow Solutions Division Locations Licensees, authorized agents, and affiliated companies located worldwide. United States Kalamazoo, MI Temecula, CA Phone Phone Fax Fax Edmonton, Alberta Phone Fax Canada St. Thomas, Ontario Phone Fax Netherlands Roosendaal Phone Fax Argentina Mendoza Phone Fax Singapore Phone Fax ORIG 01/99 USA Mexico Tlaxcala Phone Fax Brazil Sao Paulo Phone Fax Japan Osaka Phone Fax ISO 9000 Certified Germany Dortmund Phone Fax Australia Marayong NSW Phone Fax FTA4 Printed In USA 8 of 8 Copyright 1999 Flowserve Corporation