VORTEXING LIQUID V026
Vortexing of the fluid in a suction sump or pit sounds a lot like cavitation problems and will cause excessive shaft deflection that is harmful to:
- Mechanical seals
- The pump intake structure and piping.
One way to tell if you have 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 or sump design can eliminate this vortexing problem, but what do you do if the installation is not new and the problem exists? There could be several things that could have caused the 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 originally installed.
- The pump could be running at a faster speed than original design.
- 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.
- The return line is giving a water fall affect.
- A clogged trash rack or screen can restrict some of the incoming liquid.
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 available.
- 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 possible.
- 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.
The hydraulic Institute ANSI recommendation: ANSI/HI 9.8-1998 Section 9.8.7 recommends the following formula to determine the minimum submergence of a submersible pump::
- S = Minimum submergence to prevent vortexing, in inches
- D = Pump suction inlet diameter, in inches
- Q = Pump design flow rate, in USGPM
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 vortices.
- Float large spheres on the surface to break up vortices.
- Move the pump away from the wall.
- Reduce the inlet velocity by spreading the flow over a larger area, or change the direction and velocity of the flow by the use of baffles.
- Eliminate the separating wall between pumps.
- Keep the inlet flow to the pit below 2 feet/second (0.7 meters/sec)
- Keep the flow in the pit below 1 foot/sec (0.3 meters/sec)
- Any type of a logical flow straightener will help reduce velocity.
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:
- 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 “S”. 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.