After last issue, I got many suggestions, two of which weren’t obscene or impossible or both (I don’t think it is physically possible to do that to myself, Tom). But being the megalomaniacal dictator wannabe that I am, I shall ignore them and do what I want (like most politicians). So here goes.
How many times have you asked yourself “I wonder how thick the footing needs to be for this pump”? Chances are you have never asked yourself that question, but I won’t let that deter me from pontificating on the subject.
Small pumps up to a few kilowatts don’t need anything special – the one I have at home on my rainwater tank just sits happily on the slab without any bolts holding it down. However, big pumps certainly do need consideration. When you have big motors in the hundreds-of-kilowatts range, there is a lot of energy and even small out-of-balance forces can become big vibration problems with a consequent dramatic reduction in pump life between refits. There are a few basic rules of thumb to observe. These are:
- The mass of the pump concrete foundation should be five times the mass of the pump, motor and base plate being supported (to provide mass damping)
- The foundation should be 75 mm wider than the base plate, all around, up to 375 KW and 150 mm above 375 KW
- Imaginary lines, extended downward 30 degrees to either side of a vertical through the pump shaft, should pass through the bottom of the foundation and not the sides
- Ensure there is a good quality flowable grout between the pump frame and the plinth. Epoxy grout is recommended for pumps above 150 kW
- Pump-motor frames must be levelled using wedges and shims, not levelling nuts. Once the grout is set, the hold-down bolts then need to be tensioned to the foundation (which is not possible if a levelling nut is
installed – it just tensions the bolt between the two nuts)
Pump slab stiffness – As a bare minimum, the slab should be stiffer and stronger than the pump frame. If it is on a reactive site, as another minimum it should be as stiff as a masonry structure slab under AS2870.
If the pump is up on a structure, then using mass damping is not really possible. In this case, make the support structure very stiff with a natural frequency at least 1.75 times the primary excitation frequency of the pump.