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 originally installed.
- The pump could be running at a faster speed than the original design.
- 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 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.
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.
- 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 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:
The next two charts show where the dimensions came from: