Nazmul B. Zaman, Senior Product Specialist, ITT Goulds Pumps, Seneca Falls, NY
Feb. 19, 2013
It is a best practice to consider the total cost of ownership or the life cycle cost of a pump. The major components included in the cost of ownership are the initial cost, installation cost, operating cost and maintenance cost. One sub-segment of the life cycle cost is associated with the cost of sealing the pump. This white paper will identify the costs associated with sealing a pump, different sealing methods, and will compare their respective costs during the pump's life cycle.
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.
Option 1: Packing: Packing is the original method of sealing a pump. The compressible fiber is formed to stuff the annular space between the stuffing box cover and the shaft. Packing requires lubrication and cooling to prevent excess heat generation by friction. It is normally recommended by the pump manufacturer to flush the packing with water and let it leak at a rate of 50 to 60 drops per minute. Packing requires a constant supply of clean water and periodic attention of an operator.
Options 2 and 3: Single and double mechanical seals: Mechanical seals are another method of sealing a pump; special attention is needed to seal a paper stock pump. A regular single seal will most likely fail in a paper stock environment without a clean water flush, which provides a clean environment and lubrication between the seal faces. A carbon or Teflon throat bushing is used at the bottom of the stuffing box cover in order to prevent stock fiber from entering the seal chamber. A throat bushing alone will not prevent the fiber from reentering the seal chamber. A good rule of thumb is to maintain a flush water flow rate that provides a 15-ft/sec velocity across the throat bushing in order to keep stock fiber or other solids from migrating into the seal chamber. Theoretically, a 3-in. seal requires about 2.45 gal/min of water to maintain 15 ft/sec velocity through the bushing clearance. Some seal manufacturers recommend using 1 gal/min per inch of seal size as a rule of thumb. Other seal manufacturers have invented special throat bushings with spiral internal grooves to remove solids from the seal chamber. They also claim that the flush water requirement can be drastically reduced by using such bushings. One example is the SpiralTrac TM by EnviroSeal.
The external clean liquid flush to a single seal is often referred as the CPI Plan 7332 or simply the Flush Plan 32. A simple Plan 32 consists of a flow-regulating valve, Y-strainer and piping. It may also include optional pressure and temperature gauges, a flow indicator, a shut off valve, and a check valve.
Double mechanical seals are also successfully used in sealing paper stock pumps. A double mechanical seal consists of two single seals mounted back to back. Pressurized barrier fluid is required for the double seal arrangement, therefore Flush Plan 54 or Plan 53 are used in conjunction with a double seal.
Plan 32 Plan 53—Pressurized air Plan 54 Pump is remotely located Pump is remotely located Pump is remotely located Freezing problems Limitation of types of barrier fluid (only water can be used) Freezing problems Clogging problems Clogging problems Compressed air often contains moisture which can freeze and block air supply Fluctuation of air pressure If the seal chamber pressure is higher than supply air pressure, a pressurized nitrogen bottle will be required. The barrier water outlet from double seal normally goes to the drain. Any water that goes in the drain also has to be treated before discharging to nature (river or lake).
Flush Plan 54 includes piping and fittings to provide barrier fluid to the double mechanical seal. It may also include the combination of a flow meter, flow or pressure control valves, and a flow indicator. Flush Plan 54 is the responsibility of the customer.
Plan 53, normally the responsibility of the pump manufacturer, consists of a pressurized reservoir that usually contains a fluid such as Glycol, heat transfer fluid or vegetable oil. However, none of these fluids are allowed in paper stock applications as they can contaminate an entire batch of pulp if the inner seal fails. Using plain water as a barrier fluid might also be prohibited in geographical locations where the ambient temperature can reach below freezing.
Option 4: Dynamic seals: Dynamic seals are another means of sealing without flush water. Hydrodynamic sealing, or dynamic seals, were developed in the 1940s by the leading pump manufacturers of the day. It is a zero flush sealing method for paper stock pumps that is now preferred by many customers.
In a hydrodynamic sealing method, an expeller or repeller is installed behind the impeller inside the cavity of a modified stuffing box cover, and normally separated by a backplate. During the pump operation the pumping action of the expeller reduces the pressure at the stuffing box cover. Depending on the suction pressure, the centrifugal action of the expeller can create a slight vacuum and draw a small amount of air into the expeller cavity. As the air fills the cavity, the pumping action is reduced and equilibrium between the liquid and air phase is established.
Hydrodynamic sealing is only possible during the operation of the pump. During the idle condition, a secondary sealing method must be incorporated to prevent leakage from the static pressure of the suction head. Typical secondary sealing may include diaphragm seal, check-seal, lip-seal, or three rings of self-lubricating packing.
A dynamic seal eliminates the need for external flush water, but it is very limited in sealing higher suction pressure. It becomes impractical when the pump speed is too low or driven by a variable frequency drive (VFD). A standby or idle pump with a dynamic seal may dewater and dry between the small clearances. A water flush may be required during a restart of the pump. Hence a pump with a dynamic seal may not be considered completely flushless. Issues with flush water line clogging and freezing remain a concern. In addition, since the expeller is like a small impeller, it absorbs power and therefore increases operating costs.
To be continued ... Click here to read Part II.
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