Centrifugal Pumps Design and Application 2nd ed - Val S. Lobanoff, Robert R. Ross (Butterworth-Heinemann, 1992)
Chemical Pumps Metallic and Nonmetallic 297 and the casing cover relative to the axial O-ring. Because the axial O-ring has a greater sealing diameter, it requires additional bolting. Corner O- rings take more tolerancing but are a good compromise between radial and axial machining. The O-rings are made out of EP (ethylene propylene) for hot water, buna for hydrocarbons, viton for general chemicals, and Kalrez for highly corrosive chemicals. They are usually color coded to designate materials. Stuffing Box Casing Covers The casing cover, sometimes called a stuffing box extension, encloses the back end of the casing. The casing cover also includes the stuffing box or seal chamber. Originally, the ANSI standards required that the minimum stuffing box packing size be 5 /ie in., 3 /s in., or 7 /i6 in. depending on pump size. The stuffing box was designed to handle both mechanical seals and packing; however, through years of experience, it was found that the mechanical seal's outside diameter had too small clearance between it and the bore of the box. This limited the amount of cooling that a seal could obtain, especially in double seals. So even though there was cooling injection into the gland and out of the box for double mechanical seals, there were frequent failures at the outboard seal. As a result, the specification includes optional large bores that only accommodate mechanical seals. This should give adequate cooling of the box to increase the life of the seal. If a customer requires a box for both seals and packing, he will require a box to the original specification. The taps into the stuffing box may be l k in. minimum, but 3 /8 in. is the preferred NPT size, Depending on fluid temperature, the option of a cooled or heated stuffing box is usually offered. The type of mechanical seals offered on these pumps are single seals, double seals, and tandem seals. Mechanical seals used in ANSI pump applications are discussed in detail in Chapter 17. Frame The frame for the horizontal pump is composed of the support head and bearing housing. Depending on pump size, this can be one integral component or two separate pieces. Some manufacturers refer to the bearing frame housing as the bearing housing. The bearing frame or housing consists of the housing, the shaft, bearings, bearing end cover, flinger, and feet. On pump sizes AA and AB, the feet are usually cast integral with the bearing housing support head combination.
298 Centrifugal Pumps: Design and Application Support Head The support head is the member that aligns and fixes the casing to the bearing housing. ANSI requires the support head to be made of ductile iron or carbon steel. This requirement stems from concern that a system upset could subject the pump to excessive pressure and result in catastrophic failure of the cast iron support head. Support heads are also offered in stainless steel as an optional feature. This is done for three reasons: • To reduce corrosion due to leakage from packing or mechanical seals. « To reduce thermal conductivity in very high or very low temperature applications. • To have a material that has high impact properties for temperatures below 40°F. On vertical in-line pumps, the support heads are larger than horizontal pumps because they must allow the rotor to be passed out of the support head during disassembly and also must adequately support the weight of the vertical motor. When a bearing housing vertical in-line is employed, the support head is at least 50% higher in height than with the rigid coupling design. Depending on size and motor horsepower, the support heads are made out of cast iron, ductile iron, or fabricated steel. When the vertical support heads are made out of ductile iron or carbon steel, extra care has to be taken in machining the toleranced dimensions because of the release of residual stresses in these materials. Bearing Housing This is usually made out of cast iron. After machining, it is protected internally with a rust preventative such as a paint or clear material to prevent rust particles from forming internally in the housing during storage or shut down. The housing is designed to hold a reservoir of oil that is approximately a half pint on the small pumps, and 3 to 4V2 pints on the large pumps. The housings have vents, drains, and a tap for an oiler. The vent and drain should be a minimum of Va in. so the oil can readily flow during filling or draining. The vent should be designed in a way that water cannot enter into the housing. Sometimes the vent is made up of a pipe that comes up through the bottom of the housing; other times it is composed of a nipple and cap with a sixteen-hole or commercial vent. The oiler is located so mat movement of the oil from the rotation of the shaft does not prevent the oil from entering into the housing. Oilers are sup-
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298 <strong>Centrifugal</strong> <strong>Pumps</strong>: <strong>Design</strong> <strong>and</strong> <strong>Application</strong><br />
Support Head<br />
The support head is the member that aligns <strong>and</strong> fixes the casing to the<br />
bearing housing. ANSI requires the support head to be made of ductile<br />
iron or carbon steel. This requirement stems from concern that a system<br />
upset could subject the pump to excessive pressure <strong>and</strong> result in catastrophic<br />
failure of the cast iron support head. Support heads are also offer<strong>ed</strong><br />
in stainless steel as an optional feature. This is done for three reasons:<br />
• To r<strong>ed</strong>uce corrosion due to leakage from packing or mechanical seals.<br />
« To r<strong>ed</strong>uce thermal conductivity in very high or very low temperature<br />
applications.<br />
• To have a material that has high impact properties for temperatures below<br />
40°F.<br />
On vertical in-line pumps, the support heads are larger than horizontal<br />
pumps because they must allow the rotor to be pass<strong>ed</strong> out of the support<br />
head during disassembly <strong>and</strong> also must adequately support the weight of<br />
the vertical motor. When a bearing housing vertical in-line is employ<strong>ed</strong>,<br />
the support head is at least 50% higher in height than with the rigid coupling<br />
design. Depending on size <strong>and</strong> motor horsepower, the support<br />
heads are made out of cast iron, ductile iron, or fabricat<strong>ed</strong> steel. When<br />
the vertical support heads are made out of ductile iron or carbon steel,<br />
extra care has to be taken in machining the toleranc<strong>ed</strong> dimensions because<br />
of the release of residual stresses in these materials.<br />
Bearing Housing<br />
This is usually made out of cast iron. After machining, it is protect<strong>ed</strong><br />
internally with a rust preventative such as a paint or clear material to prevent<br />
rust particles from forming internally in the housing during storage<br />
or shut down. The housing is design<strong>ed</strong> to hold a reservoir of oil that is<br />
approximately a half pint on the small pumps, <strong>and</strong> 3 to 4V2 pints on the<br />
large pumps. The housings have vents, drains, <strong>and</strong> a tap for an oiler. The<br />
vent <strong>and</strong> drain should be a minimum of Va in. so the oil can readily flow<br />
during filling or draining. The vent should be design<strong>ed</strong> in a way that water<br />
cannot enter into the housing. Sometimes the vent is made up of a pipe<br />
that comes up through the bottom of the housing; other times it is compos<strong>ed</strong><br />
of a nipple <strong>and</strong> cap with a sixteen-hole or commercial vent. The<br />
oiler is locat<strong>ed</strong> so mat movement of the oil from the rotation of the shaft<br />
does not prevent the oil from entering into the housing. Oilers are sup-