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Centrifugal Pump Components,Wearing Rings,Stuffing Box,Lantern Ring,Mechanical Seals

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Centrifugal Pump Components
Centrifugal pumps vary in design and construction from simple pumps with relatively few parts
to extremely complicated pumps with hundreds of individual parts. Some of the most common
components found in centrifugal pumps are wearing rings, stuffing boxes, packing, and lantern
rings. These components are shown in Figure 10 and described on the following pages.

Wearing Rings
Centrifugal pumps contain rotating impellers within stationary pump casings. To allow
the impeller to rotate freely within the pump casing, a small clearance is designed to be
maintained between the impeller and the pump casing. To maximize the efficiency of a
centrifugal pump, it is necessary to minimize the amount of liquid leaking through this
clearance from the high pressure or discharge side of the pump back to the low pressure
or suction side.
Some wear or erosion will occur at the point where the impeller and the pump casing
nearly come into contact. This wear is due to the erosion caused by liquid leaking
through this tight clearance and other causes. As wear occurs, the clearances become
larger and the rate of leakage increases. Eventually, the leakage could become
unacceptably large and maintenance would be required on the pump.
To minimize the cost of pump maintenance, many centrifugal pumps are designed with
wearing rings. Wearing rings are replaceable rings that are attached to the impeller and/or
the pump casing to allow a small running clearance between the impeller and the pump
casing without causing wear of the actual impeller or pump casing material. These
wearing rings are designed to be replaced periodically during the life of a pump and
prevent the more costly replacement of the impeller or the casing. 

Stuffing Box
In almost all centrifugal pumps, the rotating shaft that drives the impeller penetrates the
pressure boundary of the pump casing. It is important that the pump is designed properly
to control the amount of liquid that leaks along the shaft at the point that the shaft
penetrates the pump casing. There are many different methods of sealing the shaft
penetration of the pump casing. Factors considered when choosing a method include the
pressure and temperature of the fluid being pumped, the size of the pump, and the
chemical and physical characteristics of the fluid being pumped.
One of the simplest types of shaft seal is the stuffing box. The stuffing box is a
cylindrical space in the pump casing surrounding the shaft. Rings of packing material
are placed in this space. Packing is material in the form of rings or strands that is placed
in the stuffing box to form a seal to control the rate of leakage along the shaft. The
packing rings are held in place by a gland. The gland is, in turn, held in place by studs
with adjusting nuts. As the adjusting nuts are tightened, they move the gland in and
compress the packing. This axial compression causes the packing to expand radially,
forming a tight seal between the rotating shaft and the inside wall of the stuffing box.
The high speed rotation of the shaft generates a significant amount of heat as it rubs
against the packing rings. If no lubrication and cooling are provided to the packing, the
temperature of the packing increases to the point where damage occurs to the packing,
the pump shaft, and possibly nearby pump bearings. Stuffing boxes are normally
designed to allow a small amount of controlled leakage along the shaft to provide
lubrication and cooling to the packing. The leakage rate can be adjusted by tightening
and loosening the packing gland.

Lantern Ring
It is not always possible to use a standard stuffing box to seal the shaft of a centrifugal
pump. The pump suction may be under a vacuum so that outward leakage is impossible
or the fluid may be too hot to provide adequate cooling of the packing. These conditions
require a modification to the standard stuffing box.
One method of adequately cooling the packing under these conditions is to include a
lantern ring. A lantern ring is a perforated hollow ring located near the center of the
packing box that receives relatively cool, clean liquid from either the discharge of the
pump or from an external source and distributes the liquid uniformly around the shaft to
provide lubrication and cooling. The fluid entering the lantern ring can cool the shaft and
packing, lubricate the packing, or seal the joint between the shaft and packing against
leakage of air into the pump in the event the pump suction pressure is less than that of
the atmosphere. 

 Mechanical Seals
In some situations, packing material is not adequate for sealing the shaft. One common
alternative method for sealing the shaft is with mechanical seals. Mechanical seals
consist of two basic parts, a rotating element attached to the pump shaft and a stationary
element attached to the pump casing. Each of these elements has a highly polished
sealing surface. The polished faces of the rotating and stationary elements come into
contact with each other to form a seal that prevents leakage along the shaft.


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