Flowserve Nordstrom Dynamic Balance Plug Valve and Double DB Plug Valve - Manual (Page 5)

Flow Control
Nordstrom Valves
FCD NVENIM2005-01 Dynamic Balance ® Plug Valve and Double DB ® Double Isolation Plug Valve 5
such standards as API 607, “Fire Test for Soft-Seated Quarter-Turn
Valves,” and API 6FA “Specification for Fire Test of Valves.” Consult
a Flowserve customer representative for information regarding
qualification of valves to fire test standards.
1.5 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, the magnitude of which 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, and/or rapid closure, singly
or in any combination, the possibility of an unacceptable pressure
surge should be investigated.
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.
For example, when condensate collects on one side of a closed
valve that has steam on the other side, opening the valve will cause
collapsing steam voids, sharp pressure surges, and acceleration 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 particularly 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 familiar with both the function of
these devices in relationship to the shutoff valve operation and how
to keep these valves in proper working order.
1.6 Throttling Service
Valves used to control the rate of fluid flow may be subject to severe
fluid turbulence that can have the effect of creating a high-energy
conversion within the valve and piping system. This energy conver-
sion is usually indicated by high noise levels, either by cavitation of
liquids or by shock waves from gases. (The noise in a water faucet is
an example of a low-level cavitation noise.)
The possibility exists for mechanical damage to the valve and piping
system when throttling of liquid flow results in severe and continu-
ous cavitation conditions. Likewise, with gas flow under severe
throttling conditions, shock waves can result in damage to the
system.
Flowserve personnel should be consulted on proper valve selection
for throttling applications.
1.7 Temperature Changes
Valve structural materials expand with rising temperatures and
contract with falling temperatures. Generally, increasing temperature
causes a decrease of mechanical strength that is regained on return
to a lower temperature. A condition of nonuniform temperature in a
structure may impose significant thermal stresses or distortion with
possible adverse effect on valve performance.
The possibility of thermal stress fatigue should be considered in
applications involving frequent temperature cycling. This possibility
is increased by any one or a combination of the following:
• increasing temperature range
• increasing temperature level
• increasing rate of temperature change
• increasing thermal conductivity of the fluid
• increasing thickness of metal sections
• increasing the number of cycles
In some cases, thermal cycling may also increase the tendency for
stem seal leakage.
1.8 Other Pressurization
Dynamic Balance Plug Valves are not provided with a pressure-relief
device to protect them from overpressurization from line pressure.
It is the user’s responsibility to provide a relief device as part of the
system in which the valve will be used.
1.9 Trapped Pressure
When a closed valve containing liquid is heated (e.g., from pro-
cess condition, radiation, or solar heating) the cavity pressure will
increase due to volumetric expansion or vaporization of the liquid.
Conversely, cooling an undrained cavity below the freezing point may
also result in volumetric expansion of the media. These expansions
can result in extremely high pressures in the valve.
The purchaser should consider providing positive means for preven-
tion of such overpressurization when these conditions are antici-
pated.
1.10 Material Compatibility
It is important that valve structural materials and lubricants be chem-
ically compatible with the other piping system components, line
fluids, and the environment. When compatibility is a concern, guid-
ance should be obtained from informed sources such as Flowserve
or the system’s engineers.