John Dee Warwick is a beef processor (Abattoir) located in South East Queensland. They wanted to upgrade their wastewater pump system to enable them to handle the day-to-day wastewater flows from their plant (estimated to be 25 to 35 litres per second (l/s), but then to be able to ramp up to meet additional flow demands during storm conditions (estimated to be at least 60 l/s).
The static head in the system is only 3 metres, with discharged effluent running through 450m of 150mm PVC piping.
John Dee wanted to use self-priming pumps to minimise their occupational health and safety issues and to minimise service costs over the life of the installation.
The Options
The following options were considered for the application. These were:
a. A smaller wastewater pump to handle “day-to-day” flows, with the second pump being a larger one to handle higher “storm condition” flows.
b. Two “medium-sized” wastewater pumps to operate alternately during normal conditions, but both pumps running in parallel during higher flow demands.
c. Two “medium sized” wastewater pumps to operate alternately during normal conditions, but both pumps operating in series during the higher flow demands.
a. One Small and One Large Wastewater Pump; It was ideal for John Dee to operate at 25 to 35 litres per second (l/s) during normal operation. This perfectly suited the Gorman-Rupp T4A3S-B Super T Series Sewage pump. It could operate at 1100 rpm with a 5.5kW electric motor to deliver 25 l/s.
John Dee needed the second sewage pump to deliver at least 60 l/s, but wanted to be able to pump at less than this flow occasionally to cover some “peak” normal conditions that the smaller pump was not coping with.
It was therefore ideal for this sewage pump to be controlled with a Variable Frequency Drive (VFD). This duty suited the Gorman-Rupp V6A60-B self-priming pump, which can run at 1550 rpm with a 37kW electric motor to deliver 61 l/s in the system.
The advantage of this system is its capacity to handle the wide flow range, but the disadvantages are cost [because of the large pump and need for VFD] and parts incompatibility between the two different size pumps required.
b. Two “Medium Sized” Wastewater Pumps in Parallel;
To try to get maximum flow from this option [without going to a VFD], we need to start with a higher “normal” flow rate, because bringing the 2nd wastewater pump in when there is a long discharge main does not often result in a great deal of increased flow. For this option, we have selected a pair of Gorman-Rupp T6A3S-B Super T Series wastewater pumps. A single pump will produce 44 l/s at a calculated head of 17.5m. This is slightly more than desirable, but because these self-priming pumps can operate alternatively, shorter pump cycles will pose no problems for the motors in the “starts per hour” area.
Our problem here is when we bring the 2nd pump on and operate the pair in parallel, the flow rate only increases to 48 l/s because of the steepness of the system head curve. The big disadvantage of this system is that it will not deliver the required higher flows for peak flow conditions, and when operating at maximum flow, the individual pumps operate at inefficient points on their respective curves.
c.Two “Medium Sized Wastewater Pumps, with Parallel/Series Piping; This option is exactly the same as Option B (Gorman-Rupp T6A60S-B pumps) except for the piping arrangement. Instead of both wastewater pumps discharging into the common discharge line, interconnecting piping is added along with an additional non-return valve and an additional isolation valve. Gorman-Rupp calls this Parallel/Series Piping”.
This innovative piping system allows a single pump to deliver the required minimum flow, but when higher flows are required and the 2nd pump is energised, the pumps operate as series-connected pumps, enabling them to double their pressure and overcome the friction losses in the discharge line and deliver the required maximum flow rate.
The Solution
Mr John Hart of John Dee Warwick elected to go with option 3 as the best-engineered solution.
The advantages of this system are as follows:
- The cost is substantially cheaper than Option A.
- There is spare parts compatibility between the two sewage pumps.
- Pumps can alternate after every pumping cycle.
- One wastewater pump running can handle the regular flow rate.
- When higher inflows demand a higher output, both wastewater pumps operating in series can deliver 1 l/s (or 38.6% more than when a single pump is operating).
The Pumps
The Gorman-Rupp Super T Series self priming centrifugal wastewater/sewage pump has been the world benchmark for this technology since 1963 (over 50 years!). They have been designed for economical, trouble-free operation, with superior solids handling capability and a large inspection cover to access pump internals for service or blockage removal.