Subject: Pumps installed in pits. How to correct some existing
suction problems. 14-12
Vortexing at the pump suction looks like a whirlpool of liquid in
a sump or pit and sounds a lot like cavitation. Vortexing will cause
a loss of the pump's capacity and excessive pump shaft deflection
that is harmful to:
- Mechanical seals
- Pump bearings.
- The pump intake structure and piping.
- The coupling.
- The pump driver.
One way to tell the difference between a cavitation
or vortexing problem is to remember that vortexing problems are
intermittent as the vortices form. Cavitation once started tends to
stay with you. Proper pit design can eliminate this vortexing
problem, but what do you do if the installation is not new and the
problem already exists? There could be several things that could have
caused the recent vortexing problem:
- The pump capacity has increased
- If the discharge head of a centrifugal pump is reduced the
capacity will increase.
- Maybe a larger pump has replaced a smaller pump that was
- The pump could be running at a faster speed than the original
- The system has been altered to accommodate a change in the
operation, the pump is producing more capacity.
- Additional pumps have been installed in the pit.
- The flow or volume to the pump inlet has changed.
- The fluids/ solids mixer has changed.
- The pit inlet has been reduced. The line is restricted with
solids of some type
- You have more air in the liquid. Maybe the return line is
giving a water fall affect in the pit or sump.
- A clogged trash rack or screen can restrict some of the
Maybe the original design was bad and that is causing the problem.
Although this is a very large subject there are a few guide lines you
might check out:
- To prevent vortexing, the minimum submergence for a continuous
running pump is 1.75 times the diameter of the bell (not the pump)
inlet . This can vary with pump manufacturers because there is
also the possibility of cavitating if you do not have enough NPSH
- The pump suction bell should be a minimum of 0.5 diameters off
the sump or pit floor.
- The pit inlet should be as far away from the pump suction as
- The usable pit volume should equal or exceed the maximum
capacity to be pumped in two minutes. If the pumps are on a float
switch they should be sized to allow no more than four starts per
hour per pump.
Now we will take a look at what you can do with an existing
installation. Remember that a low velocity and straight line flow to
all pumps is always desired. If you are getting vortexing problems
you might be able to:
- Place a cone under the bell.
- Use diffuser screens.
- Use floating rafts around the pump column to break up the
- Float large spheres on the surface to break up vortices.
- Move the pump away from the wall.
- Increase the size of the inlet piping.
- Reduce the inlet velocity by spreading the flow over a larger
area, or change the direction and velocity of the flow by the use
- Eliminate the separating wall between pumps.
- Keep the inlet flow to the pit below 2 feet/second (0.7
- Keep the flow in the pit below 1 foot/sec (0.3
- Any type of a logical flow straightener will help reduce
In the next few illustrations I will show you the recommended sump
dimensions to prevent vortexing and eddy flows. The first chart shows
the recommended dimensions:
The next two charts show where the dimensions came from:
- Dimensions "Y and A" are recommended minimum values.
They can be as large as desired but should be limited to
the restrictions shown on the chart.
- If the design does not include a screen, or if the
channel has a sloping approach, dimension "A" should be
up to two times as long.
- If the channel approach has a down slope the angle
should not be more than 15 degrees
- About the screens:
- The screen or gate width should not be less than
- Heights should not be less than "H".
- Use dimension "S" for the width of an individual pump
cell, or the center to center distance of two pumps if no
division walls are present.
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