Duty/Standby & Duty/Assist: What’s the difference?

We are surprised we don’t get asked this question more often as it is an essential part of a pump station design. The difference between Duty/Assist and Duty/Standby is subtle, but it is important to the correct operation of all pump stations, be it a wastewater pump station, surface water pump station, or even a booster pump station.
To understand the difference between Duty/Standby and Duty/Assist, it would be beneficial to explain the basic operation of both a foul wastewater pump station and a surface water system.
All twin pump stations, also known as dual pump stations, require an electrical control panel to manage the operation of the pump cycles. This control panel will enable either one of the two pumps to perform its duties when the water level within the pump chamber reaches a predetermined height. A set of suspended electrical floats, or probes, inside the chamber, will signal to the control panel that the level of wastewater, or surface water, is high enough that pumping is required. Once the water level has been lowered, another float will signal to the control panel that the pump can be turned off. The control panel determines which of the two pumps should be operating at any given time.
At this stage it doesn’t matter if the pump station is Duty/Assist or Duty/Standby, as the control panel will ensure that a different pump is used on each pumping cycle in both scenarios. The alternating of the two pumps ensures an even amount of wear and tear. The “Duty” pump refers to whichever of the two pumps is being used. The other pump is then referred to as either the “Standby” or “Assist” pump.
Duty Standby Pump Stations
With a Duty/Standby set-up, it is only possible for one of the two pumps to be triggered at any one moment. If, for whatever reason, the Duty pump fails, the Standby pump will then be instructed by the control panel to take over. The pump station will then work as a single pump system until the damaged/blocked pump is fixed.
Duty Assist Pump Stations
With a Duty/Assist system, both pumps can operate at the same time. The day-to-day operation of the system would be the same as a Duty/Standby, where one pump acts as the duty pump, whilst the other is on standby. If the pump fails for any reason, the 2nd pump is instructed to take over. However, with a Duty/Assist set-up, if the duty pump simply cannot keep up with the flow rate discharging into the chamber, the standby pump would be told to “assist” the duty pump. Both pumps would then produce a flow rate which is almost double that of the initial rate. In practice, because of frictional losses, the combined flow rate isn’t quite double, but it is not far off.
Applications of Duty Assist and Duty Standby Pump Stations
There are not any rules as to when you should or should not select one over the other, however, here at Solid Links, we would suggest Duty/Standby for foul wastewater, whilst we prefer to specify Duty/Assist for surface water applications.
For foul wastewater applications, a pump is required which is capable of achieving the necessary duties, and, pump with a flow rate sufficient enough to meet “self-cleansing velocity”. For example, a flow rate of at least 3.8 litres per second is required to reach the self-cleansing velocity, when using a 3” discharge pipe. With Duty/Standby, there will be two identical pumps, each with the ability to pump above the required minimum flow rate. If they were both to operate at the same time, like Duty/Assist pumps, the combined flow rate would probably be too high for the receiving drain to accept. So the design must allow for each pump to be capable of performing the required duties on its own.
For surface water applications, we would propose a pump station which will have two identical pumps so, when they are both running at the same time, the requisite flow rate is met. When there are not any solids being pumped, self-cleansing velocity isn’t necessary. As a result, the Duty pump is normally only required to discharge at a slower rate. It is only when the rainfall increases that there is a necessity for faster pumps, so the 2nd pump is used to assist. Duty/Assist tends to be a more cost-effective means of dealing with surface water as each of the two pumps are smaller.
However, do not think that the only options are Duty/Standby or Duty/Assist. Here at Solid Links, we have designed and supplied pump stations with more than 2 pumps. These can be Duty/Assist/Assist (three pumps) or even Duty/Assist/Assist/Assist (four pumps), or any other combinations. These are normally used for large-scale surface water projects where the system needs to be designed for very heavy rainfall spread over a large surface area, but when the discharge rate also needs to allow for only light rainfall situations.
Bespoke Pump Station Designs
Whether you’re a business owner, or homeowner, and would like to discuss how a bespoke pump station design can benefit you, please reach out to any of the Solid Links team:
T: 01227 200801

Our Pump Stations
At Solid Links, our team provide individually designed pump stations specific to your project and its needs, saving you time and money on excavation expenses.
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Benefits of a “bespoke” pump station design over “off-the-shelf” systems
Not many applications involving a pump station are the same. Everything from the chamber size to the pumping distances/lifts will differ from site to site, even if it is only a slight change.
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