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.emperature. The temperature of the liquid being pumped is important because it affects the seal-face material selection as well as face wear life. This is primarily a result of changes in lubricity of the fluid with changes in temperature.
.ommon seal designs may handle fluid temperatures in the 0 oFto +200 oF(-17 o.-93 o.) range. When temperatures are above the +200 oF (93 o.)range， special metal bellows seals may be used up to the +650 oF (343 o.) range. Low temperature (-100 oFto0 oF) (-73 o.to -17 o.) alsorequires special arrangements， since most hydrocarbons have little lubricity in this range.
The most important consideration concerning temperature is to avoidoperating close to a temperature， which will allow flashing of the liquid. Mechanical seals work well on many liquids; they work poorly on most gases.
.ubricity. In any mechanical seal design there is rubbing motion between the dynamic seal faces. This rubbing motion is most often lubri-cated by the fluid being pumped. Therefore， the lubricity of the pumped liquid at the given operating temperature must be considered to determine if the chosen seal design and face combination will perform satisfactorily.
Most seal manufacturers limit the speed of their seals to 90 fps (27.4 mps) with good lubrication of the faces. This is primarily due to the centrifugal forces acting on the seal which tend to restrict its axial flexibility.
Abrasion. When evaluating the possibility of installing a seal in a liquid that hasentrained solids， several factors must be considered. Is the seal constructed in such a way that the dynamic motion of the seal will be restricted by fouling of the seal parts. The seal arrangement that is usually preferred when abrasives are present is a flushed single inside typewith a face combination of very hard material. However， factors such as toxicity or corrosiveness of the material may dictate that other arrangements be used.
Corrosion. When considering the corrosiveness of the material beingpumped， one must determine what metals will be acceptable for the sealbody， what spring material may beused， what face material will be compa-tible with the liquid being pumped (thatis， whether the binder or the carbonor tungsten carbide will be attacked， or whether the base metal of the platedseal-face will be attacked)， and what type of elastomer or gasket material can be used. The corrosion rate will affect the decision of whether to use a single or multiple-spring design because the spring can tolerate a greater amount of corrosion without weakening it appreciably.
.oxicity. This factor is becoming an increasingly important considera-tion in the design of mechanical seals. Since the rubbing seal faces requireliquid penetration to cool and lubricatethem， 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.
1. Is the product thermosensitive. The heat generated by the seal faces may cause polymerization.
Additional Product Considerations
2. Is
the product shear sensitive，i.e.， will it harden due to turbulence.
3. If the product ishighly flammable， be aware of possible ignition sources.
4. In-hazardous services plan for personnel protection in the event of seal leakage.
5.
Products with dissolved gas must be properly vented. In mostcases， vent the stuffing box back to pump action.
6. Seals in cold services are extremely sensitive to moisture. There must be a way to ""dry out the system'' after repair.
7. .onsideration must be given to the pressure and temperature that theseal will see during normal operation，startup， shutdown， and upset conditions.
8. .apor pressure of the product must be known to prevent vaporization in the stuffing box.
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