Dynamic Seals Lower Life Costs of Wastewater Pumps
Traditional packing and mechanical seals both require clean water for the lubrication of the packing or the seal faces. Finding clean water in a sewage plant can be difficult.
By Nazmul Zaman
Traditional packing and mechanical seals both require clean water for the lubrication of the packing or the seal faces. Finding clean water in a sewage plant can be difficult. Even if you can find clean water in a sewage plant, it may be unadvisable to hook up a potable or non potable water line to a sewage pump. This is due to the fact that the sewage water may flow backward in case of a power failure, malfunctions, or interruption of the clean water pump.
When a plant water supply is used for sealing purposes, an air gap between the incoming water supply and seal water system is required to prevent contamination. Hence, sewage plants often use an independent seal water system. This seal water system normally consists of a tank with overflow connections and a float to maintain the air gap, two small pumps with duplex control panel, Y-strainer, relief valves and isolation valves. The cost of a seal water system with installation and piping cost can be from $10,000 to $20,000.
In addition to the cost of a separate sealing system, the sewage plant also has to consider the cost of power to run the seal water pump and the cost of clean water. A 4X4 sewage pump with 2" sleeve OD can consume 0.5 gpm to 2.5 gpm depending on packing type and lantern ring position. On the other hand, a single mechanical seal can also consume 0.5 gpm to 1.5 gpm of flush water. This means if the pump runs for 24 hours a day, the total consumption of sealing water for just one 4X4 pump can be as high as 1000 to 2500 tons of water per year. Since this sealing water goes back to the process, these 1000 to 2500 tons of extra water need to be treated again before it goes back to environment.
To help the water and wastewater community, ITT A-C Pump introduced a patented dynamic seal for sewage pumps. The dynamic seal is not a stranger to the pump industry. It has been successfully used in the Pulp and Paper Industry for years. Functionally, it is just an auxiliary expeller in the stuffing box mounted on the same shaft.
As the pumped liquid enters the dynamic seal chamber, it is opposed by the centrifugal force of the expeller, thus preventing the leakage through around the shaft. The expeller creates a ring of liquid and air phase and maintains equilibrium between the two phases, hence the name Equiseal®.
Two lip seals prevent any leaking during the idle condition of the pump. A bypass line connects the dynamic seal chamber to the suction of the pump, which assures the seal chamber pressure remains equal to the pump's suction pressure.
Proper operation and sealing capability of a dynamic seal depends on the speed of the pump and the suction pressure. Since a wastewater or a non-clog pump can run very slow or be driven by a variable speed drive, often a single expeller may not be enough to insure sealing. To solve this issue, ITT A-C Pump offers double or two stage Equiseal® options.
Since the dynamic seal works like another small impeller, it consumes power. In equilibrium stage the expeller actually pumps very little water and hence the power consumption is very low. Many operators may be concern about the cost of power draw by a dynamic seal. Although they appear to have higher initial cost and seem to have higher power consumption, the systems can be less expensive to operate if total cumulative cost is considered over the life of the pump.
Cost of a dynamic seal for a 4X4 pump is $2500. Cost of a mechanical seal is $1200. Since most of pump manufacturers offer the packing as standard, the price is already built in the pump price. Hence the initial cost of packing is zero. However the cost of sealing water, replacement of packing twice a year, and changing the sleeve and lantern ring every year can be significant. Following is a comparison between the operating costs of conventional packing, mechanical seal and dynamic seal.
These simple assumptions were used to create the Life Cycle Cost chart.
1. Sleeve OD is 2''.
2. Pump runs 24 hrs year around.
3. Life of the Pump is 20 years.
4. Packing needs replacement twice a year and the sleeve and lantern ring need to be replaced once a year. Packing costs $100, sleeve costs $250, lantern ring $100 and labor rate $25/Hr.
5. Replacing just the packing takes 1 hr and replacing the sleeve and lantern ring takes 4 hr.
6. 0.5 gpm of sealing water require for lubricating the packing.
7. Cost of clean water and treating it back is $2.50 per 1000 gallon.
8. Packing draws 0 HP powers.
9. Initial cost of a single cartridge mechanical seal is $1200.
10. Seal needs to be replaced every two years.
11. It takes 4 hr to replace the seal.
12. Seal requires 0.5 gpm for flush water.
13. Seal drag is 0.24 hp for a 2" seal at 1750 rpm.
14. Initial cost of a dynamic seal is $2500.
15. Dynamic seal needs to be replaced every 5 years.
16. Cost of expeller, sleeve and lip seals are $1500.
17. Dynamic seal is replaced during major overhaul of the pump. Hence there is no labor cost added for replacement of the seal.
18. Dynamic seal consumes 1.3 hp power at 1750 rpm.
19. No flush water require for dynamic seal.
From the chart it is obvious that the total cumulative operating cost of the dynamic seal overcomes the cumulative operating cost of mechanical seal after the second year and the cumulative operating costs of packing after the fifth year. After the fifth year a dynamic seal saves money over both mechanical seal and packing.
About the Author:
Nazmul Zaman has been a Senior Application Engineer with A-C Pump and Goulds Pumps Water / Wastewater Marketing Group in Seneca Falls, New York for the past six years. He is a graduate Mechanical Engineer with 13 years of previous experience as a Maintenance Engineer in the Paper industry and as an Application Engineer with a major east coast pump distributorship.