POWER January 2011 - 19

rapidly closing are the most common
causes. Fluids at liquid state are largely
noncompressible. This is what allows
you to apply pressure at one end of a
pipeline and attain pressure throughout
the system.
When an outlet valve suddenly closes,
the energy contained in the water
flow compresses the water nearest the
valve. Like a spring, this energy then
reverses flow, sending a shockwave at
a sudden drop in pressure, the gas expands
to force the fluid back out of the
surge suppressor, so there is no danger
of causing a column separation.
Other than size, one key difference
between a pulsation dampener and a
surge suppressor is where they are installed.
The pulsation dampener should
be placed as close to the pump outlet
as possible. Surge suppressors, on the
other hand, will be needed at various
When an outlet valve suddently closes,
the energy contained in the water flow
compresses the water nearest the valve.
Like a spring, this energy then reverses
flow, sending a shockwave at the speed of
sound back upstream.
the speed of sound back upstream until
it hits an obstruction, such as a joint,
another closed valve, or the impeller
in the pump. Most of the energy from
that shockwave then bounces off that
obstruction and returns to hammer the
valve again. The wave travels back and
forth between the obstruction and the
valve until friction finally dissipates
the energy.
Another problem can occur when a
pump suddenly shuts down, perhaps due
to a power outage. When this happens,
the fluid column may experience a sudden
drop in pressure, causing a separation
in the fluid column, with some
of the fluid becoming vapor. When the
pressure increases once again above
the vapor pressure point, the collapse
of the vapor pocket sends a shockwave
through the system.
Surge suppressors are similar in design
to pulsation dampeners, but they
are built to address much larger pressure
and volume fluctuations. The bladder
is precharged to a level lower than
the minimum allowable system pressure
so that there will always be some liquid
within the surge suppressor. When there
is a pressure surge, most of the fluid
under higher-than-average pressure in
the system flows into the surge suppressor
and, consequently, dissipates
the compression wave. When there is
18
points throughout the system. They can
be installed at the pump outlet to prevent
damage if the pump loses power.
Others can be installed at critical points
in the piping network where pressure
surges may happen, such as upstream
from a quickly closing valve.
Bladder or Diaphragm
Accumulators
Though surge suppressors and pulsation
dampeners are designed to minimize
the damage caused by an increase
in pressure, accumulators are designed
to prevent a pressure drop. A common
application is called a LOSA (lube oil
system accumulator), which provides a
temporary source of oil in the event of a
flow disruption. Hydraulic accumulators
are energy storage devices that smooth
the pulsation of oil pumps and provide
short-term oil pressure when there is
a power outage or during switchover
between oil pumps. Accumulators also
help maintain a constant oil pressure
during temporary changes of demand
(Figure 6).
An accumulator is essentially a pressure
vessel that stores oil and contains
a mechanical means of maintaining
pressure when the pump shuts down,
thus cushioning fluctuations in oil
pressure.
Accumulators differ in the type of
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mechanical means used, such as spring,
gravity, and gas load. Gas-loaded accumulators
use compressed gas to provide
pressure and are one of two types: separator
and nonseparator accumulators.
Nonseparator accumulators do not have
any barrier between the gas and the
liquid. This is the simplest design and
can store the greatest amount of oil.
However, because there is no barrier
separating the gas from the oil, the gas
may become absorbed by the fluid, particularly
at high pressures. Then, as the
pressure drops, the absorbed gas forms
bubbles in the oil, causing sponginess
in the system that may damage the
pump through cavitation.
Bladder-type accumulators consist
of a metal cylinder containing a pressurized
bladder. They are designed in
accordance with American Petroleum
Institute Standard 614/ISO 10438,
which covers lubrication systems, and
ASME Pressure Vessel and Boiler Code,
Section VIII, Division 1. In accordance
with the standards, these accumulator
vessels are made of 300 series stainless
steel and can withstand maximum pressures
of about 1,500 psi.
Because of its high flexibility and
low weight, the bladder has a rapid
response time, allowing the accumulator
to quickly compensate for pressure
drops in the system and prevent damage
to bearings and other components.
Diaphragm accumulators serve a similar
function, and like the diaphragm
pulsation dampeners, the accumulator
vessel is also divided into two halves
by a diaphragm. The basic construction
of a diaphragm accumulator is similar
to the diaphragm pulsation dampener.
Suction Stabilizers
Suction stabilizers perform a similar
function to that of surge suppressors
and pulsation dampeners, but they
protect against a pressure drop on the
inlet of pumps. Rather than absorbing
excess liquid, they supply it when
there is a pressure drop, such as during
pump start-up. This lowers the risk
of cavitation, head loss, and pulsation
that can otherwise occur on the inlet of
the pump and may damage the pump.
It also prevents the low-pressure transients
and spillback that injects frothing
into the suction pipe, destabilizing
the flow. ■
-Contributed by Joe Cheema, senior
project engineer for accumulator manufacturer
Fluid Energy Controls Inc. in Los
Angeles, California.
POWER | January 2011
http://www.powermag.com
POWER January 2011

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