Page 126 - Valve Selection Handbook

Manual Valves 113

As more chemical-resistant elastomers became available, the use of
butterfly valves spread to wide areas of the process industries. The elas-
tomers used for these purposes must not only be corrosion-resistant but
also abrasion-resistant, stable in size, and resiliency-retentive, that is,
they must not harden. If one of these properties is missing, the elastomer
may be unsuitable. Valve manufacturers can advise on the selection and
limitations of elastomers for a given application.
Efforts to overcome some of the limitations of elastomers led to the
development of butterfly valves with PTFE seats. Other efforts led to the
development of tight shut-off butterfly valves with metal seatings. By
these developments, butterfly valves became available for a wide range
of pressures and temperatures, based on a variety of sealing principles.
Butterfly valves give little resistance to flow when fully open and sen-
sitive flow control when open between about 15° and 70°. Severe throt-
tling of liquids may, of course, lead to cavitation, depending on the vapor
pressure of the liquid and the downstream pressure. Any tendency of the
liquid to cavitate as a result of throttling may be combated partly by siz-
ing the butterfly valve smaller than the pipeline so that throttling occurs
in the near half-open position, and/or by letting the pressure drop occur
in steps, using a number of valves, as discussed in Chapter 2. Also, if the
butterfly valve is closed too fast in liquid service, waterhammer may
become excessive. By closing the butterfly valve slowly, excessive
waterhammer can be avoided, as discussed in Chapter 2.
Because the disc of butterfly valves moves into the seat with a wiping
motion, most butterfly valves are capable of handling fluids with solids
in suspension and, depending on the robustness of the seatings, also pow-
ders and granules. In horizontal pipelines, butterfly valves should be
mounted with the stem in the horizontal. Furthermore, when opening the
valve, the bottom of the disc should lift away from solids that may have
accumulated on the upstream side of the disc.

Seating Designs

From the point of seat tightness, butterfly valves may be divided into
nominal-leakage valves, low-leakage valves, and tight shut-off valves.
The nominal- and low-leakage valves are used mainly for throttling or
flow control duty, while tight shut-off butterfly valves may be used for
tight shut-off, throttling, or flow control duty.
The butterfly valves shown in Figure 3-69 are examples of nominal-
and low-leakage butterfly valves in which both the seat and disc are