Startup/Shutdown
Valve
When fluid in motion is abruptly stopped, a Hydraulic Surge is created
in the system. Hydraulic Surge is often referred to as "Water Hammer".
The kinetic energy, released as pressure, can spike up to six times
the system's operating pressure--destroying system instrumentation,
pumps, pipes, fittings, and valves. Without a suppression device,
the shock wave travels the length of the pipe back to the pump,
then reverses again, oscillating back and forth until friction dissipates
the pressure spike or a system component fails.
There are several major culprits that produce this "Water Hammer"
effect--quick closing valves, back surge, pump start up and pump
shut down. Quick closing valves can be defined as valves that close
within one and one-half seconds. Quick closing valves have the potential
of stopping large volumes of energized fluid, producing violent
water hammer. Pump start-up also stops fluid in motion. During pump
start-up, fluid in a pipe is static and must be accelerated; the
pumped fluid is abruptly stopped when it contacts the static fluid
in the pipe, again creating a shock wave. SENTRY Surge Suppressors
will absorb the resistance to acceleration and/or the Water Hammer
surge created in each situation. As the surge enters the Suppressor,
the gas inside is compressed, the fluid is accumulated and the shock
wave is absorbed. When steady system flow rate is achieved, pressure
and fluid are slowly released back into the system by the suppressor.
During pump shut-down and in back surge situations fluid is reversed.
When a pump is shut off, fluid will reverse direction due to the
differential pressure created by the momentum of the fluid in motion.
In fact, if the pressure differential is below the vapor pressure
of the fluid, a vapor pocket will form creating even higher transient
pressures. The reversed flow creates "Water Hammer" as it slams
into the pump's check valve. This effect is compounded in a back
flow situation, where fluid is pumped over an elevation or vertically,
creating an increase in power due to the fluid's accelerated velocity.
