Page 139 - Valve Selection Handbook

126 Valve Selection Handbook

open, thereby permitting the seal element to flex radially. The rigidity in
the closed position is required to achieve the desired seat interference for
a tight shut-off against high and low pressures, including vacuum.

Butterfly Valves for Fire Exposure

Butterfly valves, which may be exposed to plant fire when handling
flammable fluids, must remain essentially fluid-tight internally and exter-
nally and be operable during and after a fire. These conditions may be
met by fire-tested versions of high-performance butterfly valves.
The requirements for testing and evaluating the performance of butter-
fly valves when exposed to fire are similar to those for ball valves,
described on page 108.

Body Configurations

The preferred body configuration for butterfly valves is the wafer,
which is clamped between two pipeline flanges. An important advantage
of this construction is that the bolts pulling the mating flanges together
carry all the tensile stresses induced by the line strains and put the wafer
in compression. This compressive stress is eased by the tensile stresses
imposed by the internal fluid pressure. Flanged bodies, on the other hand,
have to carry all the tensile stresses imposed by the line strains, and the
tensile stresses from the line pressure are cumulative. This fact, together
with the ability of most metals to handle compressive loads of up to
twice their limit for tensile loads, strongly recommends the use of the
wafer body.
However, if the downstream side of the butterfly valve serves also as a
point of disconnection while the upstream side is still under pressure, the
cross-bolted wafer body is unsuitable unless provided with a false flange.
A flanged body or a lugged wafer body in which the lugs are threaded at
each end to receive screws from the adjacent flanges is commonly used.

Torque Characteristic of Butterfly Valves

The torque required to operate butterfly valves consists of three main
components:

T h = hydrodynamic torque that is created by the flowing fluid acting
on the disc