Beef processor selects wastewater pump solution for dual purpose

John Dee Warwick is a beef processor (abattoir) located in South East Queensland. The processor wanted to upgrade its wastewater pump system to enable it to handle the day-to-day wastewater flows from its 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 m, with discharged effluent running through 450 m of 150 mm PVC piping.

John Dee wanted to use self-priming pumps to minimise its 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:

• Option 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.
• Option B: Two “medium-sized” wastewater pumps to operate alternately during normal conditions, but both pumps running in parallel during higher flow demands.
• Option C: Two “medium-sized” wastewater pumps to operate alternately during normal conditions, but both pumps operating in series during the higher flow demands.

Option A: One small and one large wastewater pump

It was ideal for John Dee to operate at 25–35 L/s during normal operation. This suited the Gorman-Rupp T4A3S-B Super T Series Sewage pump. It could operate at 1100 rpm with a 5.5 kW 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 37 kW 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 sized pumps required.

Option B: Two “medium-sized” wastewater pumps in parallel

To try to get maximum flow from this option (without going to a VFD), a higher ‘normal’ flow rate would be needed to start with, because bringing the second wastewater pump in when there is a long discharge main does not often result in a great deal of increased flow. For this option, a pair of Gorman-Rupp T6A3S-B Super T Series wastewater pumps was selected. A single pump will produce 44 L/s at a calculated head of 17.5 m. 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.

The problem here is when they bring the second 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 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.

Option 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 second 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

John Hart of John Dee Warwick elected to go with option C as the best-engineered solution.

The advantages of this system are as follows:

• The cost is less 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).