Centrifugal Pumps Design and Application 2nd ed - Val S. Lobanoff, Robert R. Ross (Butterworth-Heinemann, 1992)
Mechanical Seats 38? rather than a few gpm with a conventional flush. Figure 17-24 illustrates a seal with the upstream pumping concept at the inboard seal. Early experiments demonstrated that this 1.750-inch diameter seal operating in water at 1800 rpm can move 0.6 cc/min to a stuffing box pressure of 800 psig. This concept when applied to multiple seals would allow a plant operator to bring a supply of water or a neutral liquid directly to the space between the seals. The low pressure liquid can be made to flow in small quantities to higher pressures through the pumping action of the seal faces. This improvement in seal design has resulted in a new way to flush a seal, which can be applied to abrasive slurry service, hot water, hazardous liquids, heat transfer, and liquids with poor lubricating properties. This is a significant development resulting in a non-contacting, non-leaking seal for liquid service. The upstream pumping concept is a patented development by John Crane Inc. Mechanical Seals for Chemical Service Traditionally, mechanical seals for this type of service are fitted to pumps that meet American National Standards Institute (ANSI) Specification B73.1. This specification defined the stuffing box cross section by the size of the packing that could be used on a given application. Standard sizes of cross section were 5 /i6, 3 /s, and 7 /i6 inches. The idea behind the specification was that if seals were used and had to be replaced they could be replaced by packing. However, in practice when a seal is worn out or Figure 17-24. Package symmetrical seal with upstream pumping feature (courtesy of John Crane).
388 Centrifugal Pumps: Design and Application failed, it was replaced by another seal. Fitting a seal to packing cross sections was sometimes difficult, particularly when multiple seal ar rangements were required. In addition, the smaller the space around the seal, the warmer it would run. Figure 17-25 shows the original stuffing box and the new revised box that allows more than two times the clearance over the pump shaft. The new box dimensions allow a current ANSI seal to run 25 °F cooler than the original design. By opening up the back of the stuffing box and providing a slight taper to the bore, the stuffing box will run approximately 40°F cooler than the original design. This data was developed testing conventional seals that are considered standard in the original stuffing box. These include elastomeric bellows, wedge, and O-ring seals. Stuffing box design plays an important part in providing a good environment for the mechanical seal. Figure 17-25. Basic stuffing box designs.
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388 <strong>Centrifugal</strong> <strong>Pumps</strong>: <strong>Design</strong> <strong>and</strong> <strong>Application</strong><br />
fail<strong>ed</strong>, it was replac<strong>ed</strong> by another seal. Fitting a seal to packing cross<br />
sections was sometimes difficult, particularly when multiple seal ar<br />
rangements were requir<strong>ed</strong>. In addition, the smaller the space around the<br />
seal, the warmer it would run.<br />
Figure 17-25 shows the original stuffing box <strong>and</strong> the new revis<strong>ed</strong> box<br />
that allows more than two times the clearance over the pump shaft. The<br />
new box dimensions allow a current ANSI seal to run 25 °F cooler than<br />
the original design. By opening up the back of the stuffing box <strong>and</strong> providing<br />
a slight taper to the bore, the stuffing box will run approximately<br />
40°F cooler than the original design. This data was develop<strong>ed</strong> testing<br />
conventional seals that are consider<strong>ed</strong> st<strong>and</strong>ard in the original stuffing<br />
box. These include elastomeric bellows, w<strong>ed</strong>ge, <strong>and</strong> O-ring seals. Stuffing<br />
box design plays an important part in providing a good environment<br />
for the mechanical seal.<br />
Figure 17-25. Basic stuffing box designs.
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