Seals, mechanical

A new countercurrent continuous centrifugal extractor developed in the former USSR (214) has the feature that mechanical seals are replaced by Hquid seals with the result that operation and maintenance are simplified the mechanical seals are an operating weak point in most centrifugal extractors. The operating units range between 400 and 1200 mm in diameter, and a capacity of 70 m /h has been reported in service. The extractors have been appHed in coke-oven refining (see Coal conversion processes), erythromycin production, lube oil refining, etc. 

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... 

Continuous-Flow Stirred-Tank Reactor. In a continuous-flow stirred-tank reactor (CSTR), reactants and products are continuously added and withdrawn. In practice, mechanical or hydrauHc agitation is required to achieve uniform composition and temperature, a choice strongly influenced by process considerations, ie, multiple specialty product requirements and mechanical seal pressure limitations. The CSTR is the idealized opposite of the weU-stirred batch and tubular plug-flow reactors. Analysis of selected combinations of these reactor types can be useful in quantitatively evaluating more complex gas-, Hquid-, and soHd-flow behaviors. 

Crane Packing Co., Packing and Mechanical Seals, 2nd ed., Morton Grove, HI., 1966. 

Mechanical seal in suction system under vacuum is leaking air into system, causing pump curve to drop. 

Stuffing Box. Stuffing box shall he suitable for packing, single (inside or outside) or double-inside mechanical seal without modifications. Stuffing box shall he accurately centered by machined rabbit fits on case and frame adapter. 

Stuffing Box with Mechanical Seal. Mechanical seal shall he of the (single inside) (single outside) (double inside) (cartridge) type and (balanced) (unbalanced). 

Luhtication—Stuffing Box with Mechanical Seals. Suitable tapped connections shall he provided to effectively lubricate, cool, flush, quench, etc., as required by the application or recommendations of the mechanical seal manufacturer. 

Mechanical Seal Selection There are many factors that govern the selection of seals. These factors apply to any type of seal ... 

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. 

Basically, most mechanical seals have the following components as seen in Fig. 10-111. 

Interna] and External Seals Mechanical seals are classified broadly as internal or external. Internal seals (Fig. 10-112) are installed with all seal components exposed to the fluid sealed. The advantages of this arrangement are (1) the ability to seal against high pressure, since the hydrostatic force is normally in the same direction as the spring force (2) protection of seal parts from external mechanical damage and (3) reduction in the shaft length required. 

Materials Springs and other metalhc components are available in a wide variety of alloys and are usually selected on the basis of temperature and corrosion conditions. The use of a particular mechanical seal is frequently restricted by the temperature limitations of the organic materials used in the static seals. Most elastomers are hmited to about 121°C (250°F). Teflon will withstand temperatures of 260°C (500°F) but softens appreciably above 204°C (400°F). Glass-filled Teflon is dimensionally stable up to 232 to 260°C (450 to 500°F). 

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. 

Eliminate water as a mechanical seal barrier fluid... 

Not in-kind Ensure that the replacement satisfies the require-replacements (e.g., ments of duty gaskets, rupture, Implement management of change review disks, packing, mechanical seals) resulting in failure. Possibility of hazardous release. CCPS G-22 CCPS G-27 CCPS Y-28 OSHA 1910.119... 

Develop alternative agitation methods to eliminate shaft seal as a potential hot spot Train operators to visually check mechanical seal fluid on regular basis Inert vapor space... 

Install mechanical seal fluid reservoir low level sensor with alarm... 

Use double or tandem mechanical seal with inert seal fluid... 

The bearings are W ater lubricated. The typical materials of construction are carbon, copper alloys, bakelite and ceramics. The mechanical seals, like a double oil seal protected w ith a cap called a Sand Guard, are robust and perfect in sealing the inotor to prevent the entry of pumped... 

Oil-filled motors Instead of water oil is now used with oil and mechanical seals at the bearings to prevent leakage of oil through them. 

This is the pressure measured in the stuffing box or seal chamber. This is the pressure to be sealed by the mechanical seal or packing. The seal chamber pressure must be within the limits of the mechanical seal. This... 

O-rings and other secondary seals on single mechanical seals. 

Some solutions may not be practical, or economical, or timely and consistent with production. You could be forced to live with cavitation until the next plant shutdown to make the neces.sary corrections. In the meantime, the cavitation shock waves and vibrations will travel through the impeller, down the shaft to the mechanical seal faces, and onto the shaft bearings. We offer some. specific recommendations for surviving cavitation shock waves and vibrations in Chapters 13 and 14 on Mechanical Seals. 

Mechanical losses - Bearings, lip seals, mechanical seals, packings, etc., all consume energy and reduce the pump s efficiency. Small pumps (less than 15 HP) are particularly susceptible. 

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