Mechanical seals Lubrication

Tandem mechanical seals, lubricated and blocked by a seal gas with slightly higher pressure than the gas discharged by the compressor offer major advantages. There is no contami... 

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. 

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. 

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 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... 

The seal must also resist the vibrations from the explosions of internal combustion in the engine, chassis and wheel vibrations, and even potholes in the road. This seal must resist strong chemicals (anti freeze, anti-rust agents, radiator stop-leak and sealant chemicals, gasoline and lubricant residuals), and also solid particles (rust, iron slag, minerals, asbestos fibers, and silica from the engine casting mold). In spite of all this, the mechanical seal on the water pump of your car can run 7, 10, even 15 years without problems. 

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. 

Figure 6-46A. Exploded view of rotary lobe gas pump with mechanical seals and pressure lubrication for bearings. By permission, Roots Division Dresser Industries, Inc.

As in the compressed packing gland, lubrication also must be provided in mechanical seals. The sealing area surfaces should be lubricated and cooled with pumped fluid (if it is clean enough) or an outside source of clean fluid. However, much less lubrication is required with this type of seal because the frictional surface area is smaller... 

Maximum Allowable Working Pressure, code, 399, 405, 406, 408 Mechanical seals, 171-177 Fundamentals, 172 Inside, 172 Installations, 173 Lubrication, 174 Outside, 173 Seal flush system, 176 Tandem, 177... 

The model has been applied successfully to predicting the performances of bearings, gears, seals, and engines [10-12]. A fundamental limitation of the statistic models is their inability to provide detailed information about local pressure distribution, film thickness fluctuation, and asperity deformation, which are crucial for understanding the mechanisms of lubrication, friction, and surface failure. As an alternative, researchers paid a great interest to the deterministic ML model. 

Mechanical seals are preferable to packing boxes since packing material is usually dilficult to sterilize and often requires a lubricant which m gain access to the product. The product must also be protected ftom lubricant used on other moving... 

Mechanical seals are classified as inside or outside, depending on whether, the primary (rotating ring) is located inside the housing running in the fluid, or, outside. Outside seals are easier to maintain, but inside seals are more commonly used in the process industries, as it is easier to lubricate and flush this type. 

BEARINGS AND LUBRICATION (cont) 0 OIL HEATER REQ D O ELECTRIC OsTEAM (2.9.2.9/5.2.6.3) 0 OIL PRESS TO BE GREATER THAN COOLANT PRESS (5.2.6.2.b) REMARKS MECHANICAL SEAL OR PACKING (CONT) 0 VAPOR PRESSURE 1PSIA) B) <° Pi 0 HAZARDOUS O FLAMMABLE O OTHER... 

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. 

Figure 7.13. Types of seals for pump shafts, (a) Packed stuffing box the sealing liquid may be from the pump discharge or from an independent source, (b) Water cooled stuffing box. (c) Internal assembly mechanical seal the rotating and fixed surfaces are held together by the pressure of the pump liquid which also serves as lubricant a slight leakage occurs, (d) Double mechanical seal with independent sealing liquid for handling toxic or inflammable liquids.

Mechanical seals prevent leakage at the rotating shaft by sliding metal on metal lubricated by a slight flow of pump liquid or an independent liquid [Figs. 7.4(c) and (d)]. 

Table V gives a typical values for estimation purposes of blending horsepower required for various low-and medium viscosity situations. Mixers may be either top entering or side entering. Again, a side-entering mixer requires more power and less capital dollars, and this must be evaluated in looking at practical equipment. Side-entering mixers have a stuffing box or mechanical seal and are limited for use on materials that are naturally lubricating, noncorrosive, or nonabrasive.

Fig. 10. Arrangements of mechanical seals. A, Single seal. It is the most common and handles most applications. B, Inside mounted seal. It operates better because it has positive lubrication the entire seal is surrounded by fluid. C, Outside mounted seal. It is more easily accessible for maintenance and less of the seal is exposed to corrosive fluid.

Because of its elastic recovery properties, PTFE, a self-lubricating compound, is used in sealing and flexing applications such as sanitary seals, valve diaphragms, o-rings in mechanical seals, and seals and seats of stem and rotary valves such as ball valves. It is also commonly used as a liner or coating material for valves and pumps for the pharmaceutical industry. 

The system operating pressure is contained by a double-row cooled and lubricated carbon mechanical seal between the casing and rotating shaft in the vapor space at the upper end of the centrifuge assembly. 

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