'To determine the right pump in a sewer, you need an expert'

Know what questions you should ask when making your pump choice

"Specification texts that prescribe certain pumps, wells and valves, can you blindly trust them as a contractor? If only it were such a party..." Speaking is Richard Janssen, director and owner of Nering Bögel of Weert. "Specifications are often unclear. It then says, for example, 'Capacity to be determined by the contractor.' Do you recognize this? How do you determine the required capacity? How do you determine what head is needed? You will also have to determine the type of pump/type of impeller and the dimensions of the well itself. The check valve is a separate story, which one should you choose and why? How do you prevent air entry into the pump? How important is the construction of the manhole and what role does the control box play?"

So how do you determine capacity?

"There are basic guidelines for this, as described for example in the Leidraad Riolering or the KNMI's rain duration lines. In practice, however, there are often different views on this, resulting in some parties calculating with the peak load of the downpour and others with the average load during the entire duration of the downpour," Janssen explains. Because of this substantial difference, different systems are offered in practice, with different capacities and sizes.

What can be said about the head required?

The pump has to carry the water somewhere. So how far from its well does the pump have to transport the water through a pipe, and what kind of pipe do you need to do that? Janssen: "Often you can't get to your drainage point in a straight line and the necessary bends have to be made in the pipe. This creates additional resistance. The height difference is also very decisive, so the net vertical height between pump flange and end point, basically the outflow moment.

The resistance of all the piping plus the static head difference determine the pump pressure (head) that the pump should be able to produce at the required capacity. This is then calculated in the pump curve by the supplier and plotted (working point) in the pump characteristic curve." 

So what is the right type of pump (impeller)?

"Now it gets a little more exciting," Janssen said. "The rule of thumb is: the coarser the solids in the water, the wider the passage the impeller must have to prevent clogging. The common cry of 'cutting impeller' is the wrong choice in most requested cases. For pumping practically clean HWA rainwater we do not need to use a cutting impeller. For pumping DWA sewage water through a long, thin pipe (pressure sewage through which toilet paper, bandages, cleaning cloths et cetera must pass), a cutting impeller is required. Cutting impeller pumps use a lot of power relative to other type of pump(s)."

What do the dimensions of the well have to do with it?

If you place a pump in a well that is too narrow or small, the pump will start at "every gallon of water" and stop immediately, we know. "That's right," Janssen says. "So the pump is commuting far too often, at the expense of the motor and control box. A properly sized sump has enough water that goes with the proper starting frequency of the pump(s). There are rules for this. In addition, the pump will still have to remain under water with its suction side sufficiently deep when shut off to prevent air entrapment."

Which check valve do you apply and why is it needed?

"You should always install a check valve or balancing valve, otherwise the water from your pressure line will flow back into the well, which will then partially fill up again. Or worse, siphoning occurs with the sewer system. The maxim here is: what the pump has pumped should stay there. But the choice of a check valve/ball check valve is important. Of course, the free flow of this valve must always be greater than the free passage in the pump impeller and equal to or greater than the internal diameter of the discharge line. Avoid additional resistance and clogging caused by a check valve/ball check valve that is too tight.

Preventing air ingress into the pump

If the pump with its cochlea housing is not submerged far enough, then when the pump is started, it will be able to trap air. If this air entrapment is not taken out, the pump cannot pump liquid (the air continues to circulate in the cochlea). "So the pump runs and only little or no water comes out. Solution: slowly pull the submerged pump up (chain) until the air bubble can escape from the cochlea housing. By installing the float/measuring probe at proper heights, you prevent air entry."

How important is proper manhole construction?

Janssen: "Then the pump is neatly built into a large pit equipped with guide rods and chains, but then they brick the shaft at height equipped with a shaft neck and pit cover that is too small. This has consequences: You then have difficulty getting the pump out, because the modified manhole does not have guide rods. Moreover, this is against the design guidelines from NEN-EN 476 (= general requirements for sewer components) and makes it difficult to carry out any maintenance work in the sump. So for pump wells, do not brick up the shaft to adjust to height, but choose a proper well size with a spacious removable lid."

How important is a switch box?

"This is your control center, you can make it as comprehensive as possible, but purely functional is a quiet switching time and the use of high-quality components, which are still available later for any repairs. You can also include any potential-free contacts for external control and monitoring. One step further, frequency control, especially for heavy pumps, would be a welcome option for preserving your (expensive) pumps."

In short: the subject matter is too complex to leave to the good intentions of a contractor, who has only a limited frame of reference regarding everything involved in wells, valves and pumps. That is also the reason why at Nering Bögel in Weert the door is always open to anyone who needs sound advice. "We have that knowledge in-house, just use our expertise. We assemble the pump pit including all components ourselves in our 'pump hall'. We source our pumps from HOMA Pumps BV for its proven qualities. Of course, we test and check everything before the pump installation goes onto the truck," Janssen says in conclusion.