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Sealed Boxes

Mechanical Seals Introduction

Mechanical seal is being used increasingly on fluid pumps to replace packed glands and lip seals.Pumps with mechanical seals perform more efficiently and generally perform more reliably for extended periods of time.

Mechanical seals are provided to prevent pumped fluids from leaking out along the drive shafts.The controlled leakage path is between two flat surfaces associated with the rotating shaft and the housing respectively.The leakage path gap varies as the faces are subject to varying external loads which tend the move the faces relative to each other.

The mechanical seal requires a different shaft housing design arrangement compared to that for the other type of seals because the seal is a more complicated arrangement and the mechanical seal does not provide any support to the shaft.

In order for the mechanical seal to perform over an extended time period with low frictional the faces are generally hydrodynamically lubricated.The fluid film will need to carry substantial load.If the load becomes to high for the film surface contact will take place with consequent bearing failure. This lubricating film is generally of the order of 3 micrometres thick , or less. This thickness is critical to the required sealing function.Mechanical seals often have one face of a suitable solid lubricant such that the seal can still operate for a period without the fluid film.

Pressure Balance Mechanical Seals
It is possible to reduce the seal contact pressure by using a pressure balanced seal design of off-set a proportion of the force generated by the pumped fluid pressure.This principle is illustrated in the sketch below.

Design Features
The mechanical seal generally includes a three static seals.

The sleeve seal - this is usually an O-Ring
The seal between the moving seal member and the shaft or sleeve.- This is often an o-ring but can be a wedge or vee seal. This seal may not be used for bellows type mechanical seals
The housing seal is generally an o-ring of a gasket.
All of these seal must be compatible with the fluid being contained and the associated environment.These seals may limit the design for high temperature applications. In this case the bellows type alternative may be the best option.

The sealing faces are generally pressed together using some form of spring loading. Several different spring loading systems are available.

Single spring
Multiple springs distributed around seal body
Disc Springs
Disc Springs
Bellows
Magnetic
For conventional mechanical seals the single spring arrangements is used.The other spring arrangements are used in the space is restricted.

It is vitally important that the sealing surfaces perfectly flat and are parallel.

The seal faces are usually dissimilar materials with the softer face being the narrower surface. For abrasive applications similar hard materials are used e.g tungsten carbide. The seal surfaces must have sufficient strength to withstand the hydrostatic fluid forces and must be able to remove the heat generated by sliding action.Carbon is often used against bronze, cast iron, stainless steel etc.

The seal surface must be flat, smooth and square to the shaft.  Both surfaces a normally lapped to a high quality finish. The harder surface is most important because the softer surface is designed to run-in over the initial operating period.

The shaft design is critical. It must be rigid enough to support the seal in the correct position and the shaft surface finish must be suitable to ensure good sealing on the static seals (0.4 micrometers CLA or better). The shaft Total Indicated Runout (TIR) should not exceed 0.125mm.There should be minimum shaft vibration.The shaft may be subject to fretting corrosion as a result of micro-movements of the seal and is is often desireable to have locally hardened surfaces or to use sleeves.

Assembly Options
There are a number of mechanical seal options

External Seal.. This design is installed on the outside of the stuffing box with the sealed pressure inside. This provides good access allowing the seal components to be be cleaned.
Internal Seal.. Generally mechanical seals are mounted inside the stuffing box with the sealed pressure outside the seal.
Double Seals.. Mechanical seals mounted in pairs are used for sealing hazardous, toxic or abrasiv fluids and are often provided with clean flushing fluid between the seals.Double seals also provide an additional degree of safety were the pressure differentials are likely to reverse and/or there is a high risk of the sealing failing.  There are a number of double seal assembly options as listed below
In Series - Used primarily to overcome the risk of failure of a single seal.

Face to Face - Used when a cooling fluid interface is required . One seal is used for the process fluid the other seal is used for the coolant.

Back to Back - Used when an abrasive fluid is being contained and both seals are flushed with a clean buffer fluid.The flushing fluid is introduced at a higher pressure the process fluid

The are a large number of variant mechanical seals e.g split seals.Improved systems are constantly being introduced onto the market

Additional Equipment
The use of mechanical seal generally involve the use of additional equipment primarily for the flushing /coolant systems.This includes pumps, coolers, strainers, filters etc.

About the Author

Ningbo Yinzhou Guowei, mechanical seal Component Factory is a professional engaged in the design, development, manufacture, sale of various cartridge seal and flygt seals business. Production and testing are well-equipped with a high-quality staff.

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Jabsco Sealed Macerator SelfPriming Pump $203.98 Sealed MaceratorSelfPriming Macerator Pump with RunDry ProtectionWaste EvacuationThe Jabsco 18590 series DC macerator pump unit is the ideal solution for emptying marine holding tanks when not in a discharge restricted area. Self priming to 5ft this pump will empty a typical 30 gallon (115 Liter) holding tank in less than 3 minutes. Duty cycle of the pump is 15 minutes continuous which means a 180 gallon (680 liter) can be emptied in a single operationIt is recommended that holding tanks be flushed with several gallons of water after each pump out. This will reduce the possibility of a continued build up of sediment in the tank which could cause a potential blockage.Fish Box and Livewell Tank EvacuationThe Jabsco Macerator Pump is the ideal pump for the emptying of Fish box and livewell receptacles. Very often the livewell tank will be above the waterline of the boat when this is the case gravity can be used to empty the livewell tank. If not installed in this format then it is beneficial to use the Jabsco macerator pump for evacuation due to its selfpriming capabilities and also it s grinding properties.Fish boxes are typically mounted into the floor of the vessel and are very often below the waterline or only partially above the waterline. Again the Jabsco Macerator pump has the characteristics that lend themselves to this application. The bait residue particles of ice fish scales and anything else that could potentially be found in the FishBox will be easily passed buy the Jabsco Macerator Pump and the self priming capabilities are perfect for fish box installations common to many of today s sports fishing boats.Features:Impeller: Jabsco Nitrile compound Macerator: Stainless Steel Cutter reduces particle size to 1/8 maximum. New 4 blade designSeal: Lip TypeProduct : JABSCO 12V SEALED MACERATOR PUMP 11/2 IN 1 OUT Manufacturer : JABSCO Manufacturer Part No : 185902092 UPC : 671880564373

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Jabsco Self Priming Sealed Macerator Pump $200.17 Run-Dry Macerator Waste Pump WASTE EVACUATION The Jabsco 18590 series DC macerator pump unit is the ideal solution for emptying marine holding tanks when not in a discharge restricted area. Self priming to 5ft this pump will empty a typical 30 gallon - 115 Liter - holding tank in less than 3 minutes. Duty cycle of the pump is 15 minutes continuous which means a 180 gallon - 680 liter - can be emptied in a single operation It is recommended that holding tanks be flushed with several gallons of water after each pump out. This will reduce the possibility of a continued build up of sediment in the tank which could cause a potential blockage. FISH BOX AND LIVEWELL TANK EVACUATION The Jabsco Macerator Pump is the ideal pump for the emptying of Fish box and livewell receptacles. Very often the livewell tank will be above the waterline of the boat, when this is the case gravity can be used to empty the livewell tank. If not installed in this format then it is beneficial to use the Jabsco macerator pump for evacuation due to its self-priming capabilities and also its grinding properties. Fish boxes are typically mounted into the floor of the vessel and are very often below the waterline or only partially above the waterline. Again the Jabsco Macerator pump has the characteristics that lend themselves to this application. The bait residue particles of ice, fish scales and anything else that could potentially be found in the Fish-Box will be easily passed buy the Jabsco Macerator Pump, and the self priming capabilities are perfect for fish box installations common to many of todays sports fishing boats. Triple sealed motor to prevent both internal and external motor corrosion New Four-bladed chopper reduces clogging Detects the absence of fluid then shuts-off pump before impeller is damaged Sensor provides adequate run-dry period to allow initial priming of the pump Interrupting power supply to the macerator resets the Run-Dry Device Flow rate to 12 gpm Self-priming to 4ft. Fitting: Inlet Port: 1-1/2 Hose Barb plus 1-1/2 NPT Discharge Port: 1 Hose Barb

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