(FREQUENTLY MADE MISTAKES)
In my 34 years of quoting engineered wastewater pumping systems, I found that some mistakes were often repeated. This is understandable, since wastewater systems only appear on a limited number of projects and the specifying engineers may not be familiar with the components of a system and how these components interact. This section will address mistakes involving pumps and controls
PUMPS
The following are the most frequent but not the only mistakes we see. We try to help designers, suppliers and installers avoid these mistakes.
1. PUMPS DO NOT MATCH THE SUPPLY VOLTAGE: Most submersible wastewater pumps are single voltage and cannot be altered in the field. The designer should check the available voltage at the construction site before listing voltage and phase in the specification. If pumps with the wrong voltage or phase arrive at the job site, there are only two possible fixes: re-order the pumps with the correct voltage or a buck-boost transformer (if the voltage supply is lower than the pump voltage)
Both options are expensive solutions.
The pump manufacturer’s specifications should also be checked, as some larger pumps are not available in all voltages) (most frequently in 208/1/60).
2. PUMPS SELECTED BASED ON INCOMPLETE OR INCORRECT INFORMATION: We see these mistakes more often in after market applications rather than written specifications. There are several areas where this can cause problems:
a) Important aspects of the nature of the wastewater are omitted. It is important to clarify the source and content of the wastewater being pumped. (I once had a premature failure of a high head pump, and on a visit to the job site, I saw the liquid was bright red. When I asked what was being pumped, I was told “just water-soluble paint”. The contractor was un-aware that water-soluble paint is extremely abrasive and the carbon-ceramic seals in the pump did not last two weeks.)
b) The wrong type of pump is selected.
· An effluent pump that passed a maximum of ¾” solids should not be used in a sewage application.
· A grinder pump should not be used in any application other than sewage applications (they will not “grind up” sticks, leaves etc.)
· It is not recommended that a grinder pump is installed in front of a septic tank. (it can impede the process in the tank by raising the T.S.S)
c) The system design is different than the conditions on site. We see this problem when there are changes in the design from the initial specifications. We also see this in after market applications when we are told the head is “around “20 ft. (or some other vague dimension.)
3. CONFUSION IN THE SPECIFICATIONS: We have seen different pump model numbers in the written specifications, the pump schedule and on the print. Often the three different models vary so much that it is difficult to determine which is the proper pump with the required performance. There would be far less confusion if the specification lists the flow, head, electrical and other requirements rather than spell out specific model numbers.
4. PUMPS ARE OVER OR UNDER SIZED: We do see pumps that will not meet the head or flow rate required. People in the industry often oversize the pump to make sure they will have enough flow or head to meet the requirements. This approach can lead to serious problems.
We had a project at a university that had pumps that were too large for the application, but we were unable to persuade the engineers to downsize the pumps. At the start-up for the pumps, the was an extremely violent noise and shaking when the pumps were turned off. The large flow rate and the piping configuration combined to cause extreme water hammer. We made note on the start-up forms, and two weeks later one of the 4” cast iron check valves broke from the water hammer and both check valves had to be replaced with a “surge buster” type check. This was a costly fix that could have been avoided.
These are the situations we most frequently see. If you have any questions or cases you have seen, let me know at gwilliams@geofwilliams.com or at williamsgeoffrey06@gmail.com
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