Seal En.ironment
Once an adequate definition of the product is made， the design of the seal environment can be selected. There are four general parameters that an environmental system may regulate or change:
1. Pressure control
2. Temperature control
3. Fluid replacement
4. Atmospheric air elimination
The most common environmental control systems includeflushing，barrier fluids， quenching， and heating/cooling systems. Each has its use in regulating the parameters mentioned previously.
Seal Arrangement Considerations
There are four considerations:
1. .ouble seals have been the standard with toxic and lethal products， but maintenance problems and the seal design contribute to poor reliability. The double face-to-face seal should be looked at more closely.
2. .o not use a double seal in dirty service-the inside seal will hang up.
3. The API standard is a good guide to the use of balanced and unba-lanced seals. Application of a balanced seal at too low a pressure may encourage face lift-off.
4. The number of arrangements and auxiliary features are more than
100. Regardless of the seal vendor， the arrangement will generally determine success.
Equipment
Too few people consider the equipment with the seal selection. In mostcases， poor equipment will give poor seal performance， regardless of the sealor arrangement chosen.Also， beware that different pumps with the same shaft diameter and T.H may present different sealing problems. (Note: These same considerations may be used for troubleshooting.)
Secondary Packing
More emphasis should be placed on secondary packing than itreceives， especially if these members involve Teflon. Most seal designs using an O-ring for shaft packing give similar performance. A wide variation in performance is seen between various seal vendor designs when Teflon shaft packing is used. .epending onthe seal arrangement， there can be a difference in mating-ring (stationary) packing performance when Teflon is used.
Seal-Face Combinations
.hoices of seal-face combinations have come a long way in the last 8.10 years. Stellite is being phased out in petroleum and petrochemical seal applications. Better grades of ceramic are being offered as the standard material. The cost of tungsten carbide has decreased considerably. Relap-ping services for tungsten are available near most industrial areas. Siliconcarbide is gaining a hold on the market， especially in abrasive service. The technology of manufacturing tungsten carbide in a composite or overlay arrangement is offered by all of the major seal manufacturers. The dynamics of seal faces are better understood today.
Seal Gland Plate
The seal gland plate is an item that is caught in-between the pump vendorand the seal vendor. The pump vendors can furnish good， reasonably priced alloyglands， but they are also limited because the gland is cast and must fit several seal designs. There are also some glands furnished by pump vendors that can beeasily distorted by bolting. Special glands requiring heating，quench， and drain with a floating-throat bushing on ANSI pumps should be furnished by the seal vendor. Gland designs on several ANSI pumps are not that impressive.
.ain Seal Body
.esigns differ considerably from one manufacturer to another. The term""seal body'' makes reference to all rotating parts on a pusherseal， excluding shaft packing and the seal ring. The configuration or options offered on the seal body may be the chief reason to avoid the design for that particular service.
Seal Systems
In recent years， these systems have become more sophisticated to meet modern chemical process requirements and government restrictions. A simple seal system is the buffered and educted restrictive-ring seal system.This type of system， as shown in Figure13-31， must operate with buffering pressure greater than the process and eductor pressure. The eductor pressure must in turn be below the atmospheric pressure. Problems with these systems are common because the eductor system does not have a large enoughcapacity， the buffered gas pressure is not higher than the process pressure， and in many cases the rings are installed backward.
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