Centrifugal pump mechanical seals

There have been tremendous advances in the design of centrifugal pump mechanical seals, yet many new pumps employ ancient rope-type packing to prevent product leakage around the shaft. 

Well, something has changed. I ve changed. Since Liz and I authored the third edition, we have both become smarter. Smarter in the sense that we have learned a lot about our misconceptions about how process equipment works. Much of our imderstanding about vacuum ejectors, centrifugal compressors, pump mechanical seals, and distillation tray hydraulics were not correct. We re both learning all the time. So we must be getting smarter. 

NonmetalHc centrifugal pumps utilize polymers for all components coming into contact with the pumping media. Even the steel (qv) shaft is sleeved with plastic to isolate the shaft from the fluid. Mechanical seals are reversed so that the metal face does not come into contact with the corrosive... 

Lubricity. In any mechanical seal design, there is rubbing motion between the dynamic seal faces. This rubbing motion is often lubricated by the fluid being pumped. Most seal mauufac turers hmit the speed of their seals to 90 ft/sec (30 m/sec). This is primarily due to centrifugal forces acting on the seal, which tends to restrict the seal s axial flexibihty. 

The most common maintenance problem with centrifugal pumps is with the seals. Mechanical seal problems account for most of the pump repairs in a chemical plant, with bearing failures a distant second. The absence of an external motor (on canned pumps) and a seal is appeahng to those experienced with mechanical seal pumps. 

You can t begin to resolve problems in your centrifugal pumps, bearings and mechanical seals, until you learn the numbers 60°, and 240°, with respect to your pump cutwater. 

Actually, everything we said about bearings, mechanical seals, piping, TDH, system curves and mating the pump curve to the system curve, the affinity laws, cavitation, horsepower and efficiency arc as applicable to PD pumps as centrifugal pumps. 

There are trade-offs for magnetic drive and canned pumps versus centrifugal pumps with double-mechanical seals. The former have no seals to leak, but need active interlocks to prevent high temperature for temperature sensitive materials. Similarly, diaphragm pumps, that have... 

Static seals are standard provision from many suppliers therefore seals for the moving elements in pumps will be discussed. For centrifugal pumps, both stuffing box and mechanical seals will be considered and the discussion will be extended to reciprocating and rotary machines. 

Shaft seal requirements and two common types of seals, packed stuffing boxes and simple mechanical seals, are described and discussed in this section. A packed box typically is used on slow- to moderate-speed machinery where a slight amount of leakage is permissible. A mechanical seal is used on centrifugal pumps or other type of fluid handling equipment where shaft sealing is critical. 

The shaft-seal development for high-pressure centrifugal pumps is quite similar to that explained for compressors. The traditional mechanical seals lubricated by liquids are now being partly replaced, for obvious reasons, by gas-lubricated ones. 

Answer—there is an error in the design of this pump. The error is that this is a self-flushed pump. The mechanical seals on centrifugal pumps require a lubricant or seal flush material to keep the seal faces from touching and rubbing. In most pumps, this seal flush fluid comes from the discharge of the pump itself. Such pumps are called self-flushed pumps. 

Fig. 11 Differential pressure relief of mechanical seal on a semi-hermetic high-pressure canned-motor centrifugal pump...

Mechanical seals have fully established themselves for all types of centrifugal pumps in a variety of services. Though more costly than packing, the mechanical seal reduces pump maintenance costs. 

This process differs from others in several engineering details. The severe duty placed on the reboiler of the aluminum chloride recovery tower due to fouling was a problem which was solved by a vertical thermosyphon reboiler operating ivith sufficient head to provide a high rate of liquid circulation. Another problem was suitable pumps to recycle aluminum chloride slurry to the top of the reactor. Centrifugal pumps with mechanical seals using butane flush were most satisfactory. 

WATER LUBRICANT - Water used as a lubricant for example, in a mechanical seal on a centrifugal water pump. 

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