Except for check and pressure-relief valves, industrial valves need a stem to operate. An inadequately sealed stem is a source of fugitive gas emissions. Valves with emissions greater than 500 ppmv are considered leakers. The 500-ppmv-limit threshold can place excessive demands on many low-end valves. Figure 5.24.1 shows the primary maintenance points for a packed, stemmed valve. The cost of keeping these valves in compliance can be expensive over time compared to another valve of greater initial cost.
The following special valves are designed to control fugitive emissions:
Bellows-type stems for both rising stem and quarter-turn valves show almost zero leakage and require no maintenance during their service life (Gumstrup 1992). However, because bellows seals are more costly than packed seals, they are typically used in lethal and hazardous service. Figure 5.24.2 shows a typical design of a bellows-sealed valve. Diaphragm valves and magnetically actuated, hermetically sealed control valves are two other valves least prone
to leaking. Figure 5.24.3 shows a typical diaphragm valve.
For services not requiring zero leakage on rising and rotary valve stem seals, a new group of improved packed seals is available. Most new seals claim to reduce fugitive emissions to a maximum leakage of less than 100 ppmv (Ritz 1993). Gardner (1991) discusses other types of valves for emission control.
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