Reference Manual

1−10
Figure 1-16. Extension Bonnet
W0667-2
Figure 1-17. Valve Body with
Fabricated Extension Bonnet
W1416
carbon steel because of its lower coefficient of
thermal conductivity. On cold service applications,
insulation can be added around the extension to
protect further against heat influx.
Figure 1-18. ENVIRO-SEALt Bellows
Seal Bonnet
W6434
Bellows Seal Bonnets
Bellows seal bonnets (figure 1-18) are used when
no leakage (less than 1x10−6 cc/sec of helium)
along the stem can be tolerated. They are often
used when the process fluid is toxic, volatile,
radioactive, or highly expensive. This special
bonnet construction protects both the stem and the
valve packing from contact with the process fluid.
Standard or environmental packing box
constructions above the bellows seal unit will
prevent catastrophic failure in case of rupture or
failure of the bellows.
As with other control valve pressure/ temperature
limitations, these pressure ratings decrease with
increasing temperature. Selection of a bellows
seal design should be carefully considered, and
particular attention should be paid to proper
inspection and maintenance after installation. The
bellows material should be carefully considered to
ensure the maximum cycle life.
Two types of bellows seal designs are used for
control valves:
D Mechanically formed as shown in figure 1-19
D Welded leaf bellows as shown in figure 1-20
The welded-leaf design offers a shorter total
package height. Due to its method of manufacture
and inherent design, service life may be limited.