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Why
check valves slam and what to do about it
Author: Sara Peters
Check
valves are simple valves.
They
use forward flow to open the valve, then automatically close when flow stops,
preventing reverse flow from making its way back through the valve.
They
don’t require power to function, and they don’t require a whole lot of
attention or maintenance.
They
do get a lot of attention, however, when they slam.
What
causes the slam?
A
check valve relies on gravity and/or reverse flow to close completely.
Opportunity
for check valves to slam frequently occurs at pump shutdown.
When
a pump stops, gravity and reverse flow slam the check valve shut.
Because
fluid is non-compressible, it creates a pressure or shock wave (water hammer).
The
fluid continues to flow back and forth until friction losses cause the wave to
settle.
Slamming
check valves cause serious pressure surges in a system that rattle pipes and
damage equipment.
Some
people assume that’s just what check valves do. "No matter what, they’re
going to slam."
But
that’s just not the case. Usually, the real cause of these problems, stems from
poor sizing and selection, not the type of valve itself.
Minimizing
Slam Through Proper Sizing
Oftentimes
check valves are selected based on the pipe size and the desire for the largest
Cv possible.
(CV
stands for “coefficient of flow” for valves and demonstrates a valve’s capacity
for liquid to flow through it.)
Of
course, you want the ability to put as much fluid through the valve as
possible, right?
Makes
sense, except this completely disregards the fact that flow conditions
determine the internal performance of a check valve because the disc is
directly in the flow path.
If
there is not enough flow to keep the valve fully open with the disc reaching
the ceiling of the check valve, the disc will move up and down, resulting in
pre-mature wear, potential for failure, and a higher pressure drop.
For proper check valve selection, consider the following:
· Line sizing
· Application data
(fluid characteristics, including temperature and pressure)
· Seat type
· Installation
(horizontal or vertical)
· End connection
· Valve rating
· Material
compatibility with the medium
· Envelope dimensions
· Leakage
requirements
Selecting
a valve of the proper size will have an impact on the amount of water hammer
felt by the piping system.
Selecting
the right type of check valve
Another
solution to slamming check valves is to look at the type of check valve used in
the application.
Silent,
ball, and resilient hinge are alternatives to the traditional swing check
valve.
They
each carry different characteristics with varying degrees of pressure drop and
closure speed.
Silent
Check Valves
Silent
or spring–assisted check valves do not rely on gravity or reverse flow to
close.
Instead,
the spring closes in about one-tenth of a second, the valve before reverse flow
has an opportunity to slam the valve shut.
It
is able to close quickly because it has a shorter distance to travel than the
flap on a traditional check valve.
This
video from DFT illustrates how pressure spikes in the system are greatly
reduced with a silent check valve.
Ball
Check Valves
Ball
check valves are economical and very simple valves.
They
sometimes utilize a rubber coated ball that floats up and out of the way while
flow passes through the valve, and seats when flow stops.
This
particular valve closes slowly, so the potential for slamming is high.
There
are other styles of ball check valves that work similarly to silent or in-line
check valves. These are called cone check valves.
Resilient
Hinge Check Valves
Resilient
hinge check valves are very similar to the traditional swing check valve.
The
major difference is that a rubber molded disc flexes up and out of the way when
fluid flows through the valve. The flap does not rotate around a hinge pin.
It
has a shorter stroke than a traditional swing check valve, reducing the closing
time of the valve and minimizing the potential for water hammer to occur.
Conclusion
Each
of these valves have different characteristics, features and benefits that
should be carefully selected for just the right fit into a system.
If
you’re experiencing issues with a check valve or need to select a new one for
your process, talk to an engineer experienced in selection/sizing of these
valves.
Doing
so will help your system perform at its highest efficiency while reducing slam
and requiring less maintenance.
Sara Peters leads Crane Engineering's blogging team, coming up
with fresh stories and insights for our readers to apply to their every day
work.
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