Flowserve Vogt Forged Steel Gate - Manual (Page 13)

Flow Control
Vogt Valves
FCD VVENIM2000-02 Forged Steel Gate, Globe and Check Valves 13
2.7 Fire Safety
Flowserve Vogt Valves all metal-seated gate, globe and check valves
come standard with flexible graphite packing and gaskets and are of
a fire-safe design. If these valves are supplied with low melting point
packing, or gaskets, such as fluoropolymer or elastomeric seals
for sealing, the fire-safe design and full shell pressure/temperature
ratings are sacrificed.
The purchase order should indicate when the customer requires
fire-safe designs.
2.8 Pressure Surge
Closure of a valve in a flowing fluid line causes the velocity of the
fluid to be reduced to zero. If the fluid is a relatively incompressible
liquid, the inertia of an upstream column produces a pressure surge
at the valve whose magnitude is inversely proportional to the time
required for closure. The surge pressure is also proportional to the
length of the upstream fluid column and the fluid velocity prior to
closure initiation. If the application involves a long upstream line,
a long downstream line, high velocity, or rapid closure, singly or in
any combination, the possibility of an unacceptable pressure surge
should be investigated.
(1)
Also to be considered are condensation-induced pressure surges
which occur when a fluid velocity change is caused by rapid conden-
sation or when a slug of water is accelerated by contact with steam.
An example would be when condensate collects on one side of a
closed valve that has steam on the other side, then opening the valve
will cause collapsing steam voids, sharp pressure surges and accel-
eration of condensate slugs. Condensation-induced pressure waves
can result in pressure pulses that are significantly higher than those
produced by a sudden valve closure. In such events, non-shock
rated gray iron valves installed in steel piping systems are particu-
larly vulnerable to catastrophic failure. Traps are required to prevent
condensate accumulation and “blow-off” valves located at the low
point in the system are needed to ensure condensate drainage.
Operation and maintenance personnel must be aware of the function
of these devices in relationship to the “shut-off” valve operation and
the necessity for their being in proper working order.
(1)
(1) Paragraph extracted from MSS SP 92.
2.9 Throttling Service
Flow control (throttling) valves may be subject to extreme fluid
turbulence that may create a high-energy conversion within the valve
and associated piping system. High noise levels usually indicate this
energy conversion, either by cavitation of liquids or by shock waves
from gases.
Therefore, it is possible to damage the valve and piping system when
the throttling of liquid flow results in severe and continuous cavita-
tion conditions. Likewise, with gas flow under severe throttling con-
ditions, shock waves can possibly result in damage to the system.
Vogt “flow control valves” should be used when the user requires
true flow regulation. Flowserve Vogt Valves should be consulted on
proper valve selection for throttling and/or flow control applications.
2.10 Temperature Changes
Forged steel expands and contracts with rising and falling tem-
peratures, and in general, an increase in temperature results in a
decrease of mechanical strength that is regained on return to a lower
temperature. A temperature gradient in a steel part may impose
significant thermal stresses or deformity with a possible negative
effect on valve performance.
Frequent temperature cycling may result in development of thermal
stress fatigue. Any one or a combination of the following increases
this possibility:
An increase in temperature range, level, rate of change, an
increase in the metal section thickness, or number of cycles.
Thermal cycling has been determined to increase the tendency
for stem seal leakage.