Mechanical seal factors

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

Toxicity. This factor is becoming an increasingly important consideration in the design of mechanical seals. Since the rubbing seal faces require liquid penetration to cool and lubricate them, it is reasonable to expect that there will be some vapor passing across the faces. This is in fact the case. A normal seal can be expected to leak from a few ppm to 10 cc/min. It is also generally accepted that the seal leakage rate will increase with speed. 

The sealing of process gas along the rotating shaft is a delicate and important prohlem. Many factors enter into the selection of the type of mechanical seal hest for the service. ... 

I. Welded joints would be used on ammonia service and mechanical seals on pumps. Use minimum factor as full equipment details are not known at the flow-sheet stage, factor = 0.1. 

This pump uses a closed impeller, mechanical seals, and is constructed of stainless steel 304L. The small power load of approximately 3.5 kW is supplied by an electric motor drive employing a horizontal shaft. The design casing pressure is sufficient to give a 30% safety factor above the total head required. Mechanical seals are preferred to a packed box arrangement due to their superior ability to contain the corrosive and toxic acid liquor. 

Figure 8.8 costs include the pumping unit, coupling and gear, mechanical seal, and the base plate. All are FOB fabrication shop costs for 2001. The basis of material is cast iron. Use Table 8.24 for Fm. See Table 8.25 for associated cost factors. 

The additional cost of the material for a larger-diameter shaft is rarely a major factor for carbon or stainless steel, but may be sizeable for special alloys. The additional cost of a larger mechanical seal, for pressurized applications, may add considerable cost regardless of the shaft material. 

Both the CNIS/Gc ratio (Fig. 6.14a) and the seale factor (Fig. 6.14b) decrease substantially with the MAH concentration for the composites with complete separation. This behavior suggests that single values of CNIS are of little value for a comparison of relative toughness of materials of different eomposition or as a parameter for material selection and product design. In these eases, a fracture mechanics approach should be carried out to make comparisons of materials with different degree of adhesion relative to an equivalent state of stress. 

Chemical attack (i.e., corrosion or chemical reaction with the liquid being sealed) is another primary source of mechanical seal problems. Generally, two primary factors cause chemical attack misapplication or improper flushing of the seal. 

The factors that cause mechanical seals to leak are numerous ... 

The mechanical design of the idler roUs is a function of the particular service under which the conveyor operates. Minimum industrial standards for roU dimensions, bearings, and appHcation criteria for different service conditions have been estabHshed (14). Idler life is deterrnined by a combination of factors such as bearings, seals, shell thickness, load density, and the operating environment. 

The API mechanical standards for the rotary and centrifugal compressor have a test specified for proof of rotor insensitivity. This would normally be the test invoked at this point to ultimately prove the rotor. The fifth edition of API 617 expanded on this test and changed the acceptance criteria from those based on amplification factor to an acceptance level based on internal seal clearances. 

Rotational speed of shaft and peripheral speeds of seal. Mechanical limitations—dimensions of space required versus space available, shaft deflection and whip, shaft end play, shaft diameter, and maintenance. Miscellaneous factors—cost, allowable by-pass or out-leakage, allowable contamination of gas with air, inert gas, oil, and other fluid. 

Compression set and durometer hardness are also important mechanical properties. Compression set is defined as the amount by which an elastomer fails to return to its original thickness after being subjected to a standard compressive load or deflection for a specified time at a specified temperature. A low percent compression set typifies a more compression resistant elastomeric formulation. Compression set of a closure on a sealed vial is a factor in maintaining the sterility and potency of the drug itself. 

More complex environmental rigs can be illustrated by an arrangement where pipe line components, seals, valves, etc., are mounted as in service and pressurised cyclically with the service fluid. The rig is contained in an enclosure so that elevated or subnormal temperatures can be applied. The degree of acceleration can be varied for both mechanical and environmental factors. 

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