By
Nazmul B. Zaman, Senior Product Specialist, ITT Goulds Pumps, Seneca Falls, NY It is a best practice to consider the total cost of ownership or the life cycle cost of a pump. While there are many major components included in the cost of ownership, one important segment is the life cycle cost associated with the cost of sealing the pump. Click here to Read Part I.
A sealing method is required for most pumps except for magnetically driven pumps and canned rotor pumps. A sealing method must be incorporated to seal between the rotating shaft and the static stuffing box cover in order to prevent pressurized liquid from escaping.
No-flush mechanical seal vane particle ejector ring: In the early 1990s, ITT Goulds Pumps developed a TaperBoreTM PLUS seal chamber with Vane Particle Ejector (VPETM ring). The VPE ring is a set of static vanes against the rotating shaft. The static vanes of the VPE ring induce the fluid from the seal chamber and eliminate solid particles, vapor and air bubbles from it. The process also refurbishes the seal chamber with clean liquid.
With the success of the VPE ring in chemical and general industry applications, in 1999 ITT Goulds Pumps initiated tests of the VPE ring in a paper stock application. A 500-hour test was performed on an end-suction pump in Goulds Pumps' R&D paper stock test loop.
The pump was fitted with a slurry seal with tungsten carbide faces and a VPE ring. The test included simulation of a mill outage by shutting down the pump during the weekend and turning it on Monday morning. There was no visual leakage at any point of the test. The seal was disassembled and under a thorough analysis the seal revealed no signs of abnormal wear.
Packing 3-inch single cartridge seal with silicon carbide faces, carbon restricting bushing and Plan 32 3-inch split seal with 316SS special bushing, internal groove and Plan 32 Dynamic seal TaperBore™ PLUS seal chamber with VPE™ ring and John Crane 5870 cartridge seal—no flush Initial cost Packing—$80 $1,675 Single cartridge with split mechanical seal and silicon carbide faces—$2,848 Dynamic seal—$5,611 Slurry seal—$3,162 Replacement interval Packing—6 months 2 years 4 years 5 years 6 years Replacement labor—hours Packing—1 Sleeve and lantern ring—4 4 4 None (Replaced during major overhauls) 4 Sealing water—gallons per minute* .5 2 .25 None None Seal drag** None .38 HP at .38 HP at 1.4 HP at .38 HP at *U.S. Environmental Protection Agency
Eight years ago, one pump in the field pumping 5% paper stock was retrofitted with a slurry seal (John Crane 5870) and the VPE ring. A few years later, several pumps were converted with the same sealing arrangement. The original pump is still running after eight years. The pumps that were retrofitted later have now been running for six years. (The six-year average is not due to the mean time between failures (MTBF); most of the conversions are not older than six years.)
To compare the economics of each sealing method a thorough lifecycle cost of all sealing methods was calculated and graphed to present a visual comparison.
Life cycle cost calculation methods
Sealing costs are calculated for an end-suction pump with single seal and 3-in. sleeve OD. Pump operates 24 hr/day, 7 days per week. Life of the pump is 20 years. Cumulative cost or life cycle is defined as the total expenses related to sealing the pump for the 20-year estimated life. This includes the initial cost of the sealing device, and its operational costs including power consumption and the cost of flush water. It also includes the replacement cost of the sealing device, any periodic maintenance costs, and the cost of any spares directly related to the sealing method. A spreadsheet was created to calculate the costs. A simple series was assumed without calculating the Net Present Value of the costs. 70% efficiency was assumed for the plant water (seal water) pump to calculate power consumption for producing 80 PSI flush water. The chart was created using Years of Operation as the horizontal axis and Cumulative Cost as the vertical axis. It is important to be open minded about these cost assumptions. Costs can vary according to pump size, manufacturer and customer discount levels. In addition, the cost of water and utilities may vary from country to country, state to state and plant to plant. However, the relative trends revealed by this analysis provide an accurate guideline.
Conclusions
Over a 20-year period of operation, running a single seal with throat bushing and Plan 32 is the most expensive sealing method.
Next in cumulative cost is for single split with SpiralTracTM and Plan 32. Although packing shows low initial cost and lower operating cost, packing requires constant attention by mill personnel. It leaks continually and causes wet flooring around the pump. Initial cost of dynamic seal is higher. Dynamic seal also consumes higher power. But the 20-year life cycle cost is actually lower than single seal and packing. However, there are limitations on suction pressure with dynamic seal, especially when the pump is driven by VFD or at lower speed. TaperBoreTM PLUS Seal chamber with VPE ring and John Crane 5870 Cartridge Seal requires moderate initial investment, but the 20-year life cycle cost is lower than the other sealing methods TaperBoreTM PLUS Seal chamber with VPE ring and John Crane 5870 cartridge seal advantages:
20-year life cycle cost is lowest among the other sealing methods Lower initial investment than dynamic seal High P-V (Pressure-Velocity) Limit. Can handle much higher suction pressure No flush water requirement No instrument air or nitrogen bottle requirement Pump can be remotely located without worry No freezing or clogging issues related to flush water Much lower power consumption than dynamic seal, resulting in higher overall pump efficiency No secondary sealing. No lip seal or elastomeric diaphragm seal Regardless of the variation in initial cost for each sealing method, power and water costs, and use of any flush, will increase the cost of sealing in the long run. That is the ultimate finding of this exercise.
1. SpiralTracTM TM TM
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