John Dee Warwick is a beef processor (Abattoir) located in south-east Queensland.
They wanted to upgrade their waste water pumping system to enable them to handle the day-to-day waste water 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. The class of the piping was unknown, so you will see we have used 3 different system head curves to ensure we had all possibilities covered.
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 following options were considered for the application. These were:
- A smaller pump to handle "day-to-day" flows, with the second pump being a larger one to handle higher "storm condition" flows.
- Two "medium sized" pumps to operate alternately during normal conditions, but both pumps running in parallel during higher flow demands.
- Two "medium sized" pumps to operate alternately during normal conditions, but both pumps operating in series during the higher flow demands.
One small and one large 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 Trash pump. It could operate at 1100 rpm with a 5.5kW electric motor to deliver 25 l/s. See the performance curve of this pump (fig.1)
John Dee needed the second 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 pump to be controlled with a Variable Frequency Drive (VFD).
This duty suited the Gorman-Rupp V6A60-B, which can run at 1550 rpm with a 37kW electric motor to deliver 61 l/s in the system. See the performance curve on the 2nd following page (fig.2).
As can be seen above, the T4A3S-B will deliver 25 l/s if piping is thin walled, and approx 22.5 l/s if piping has higher pressure rating with thicker walls.
The above shows that the V6A60-B will deliver 61 l/s if piping has a lower pressure rating, but will still deliver approx 53 l/s if piping has a higher pressure rating.
Pros and cons
The main advantage of this system is its flexibility with regard to flow rate.
With the VFD, operators can ramp the pump speed up during higher flows and back down during lower inflow periods. If John Dee wanted to, they could opt for a larger motor (75kW maximum), which would then give them the capacity to ramp flow up to 70 l/s even if piping was the higher pressure rating.
The disadvantages of the system are as follows:
- The capital costs are much higher because of the cost of the higher performance pump, but mainly due to the higher cost of the VFD controls. If John Dee went for maximum flexibility, this system would be the most expensive by up to $30,000 (or approximately 50 per cent more) not including what might be the added cost for running power to a 75kW motor.
- There is no spare parts compatibility between the pumps, so any emergency parts stock the plant might want to carry, would need to be for two different pump models.
- Pumps would not wear evenly, and the plant might find that their larger pump is in "near new" condition in 5 years but the smaller pump needs an upgrade because it’s been doing all the work.
Two "medium sized" pumps in parallel
To try to get maximum flow from this option, we need to start with a higher "normal" flow rate, because bringing the 2nd 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 pumps. A single pump will produce 44 l/s at a calculated head of 17.5m.
This is slightly more than desirable, but because the pumps can operate alternatively, shorter pump cycles will pose no problems for the motors in the "starts per hour" area. See the performance curve for this pump (fig.3).
Our problem here is that 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. See the performance (fig.4)
As can be seen from the above, the Gorman-Rupp T6A3S-B does a very good job at "normal flow" conditions.
As can be seen from this curve, there is very little extra flow (44 l/s up to 48 l/s) when the second pump is brought on to operate in parallel. Both pumps are now delivering 24 l/s and operating at 20.5m of TDH. If we look back at Fig.3, we can see that this operating point is at the extreme left hand side of the operating range, so is inefficient and will probably be noisy.
Pros and cons
The advantages of this system are as follows:
- The cost is substantially cheaper than Option A because:
- Motors are much smaller (2 x 18.5kW vs a 5.5kW and 75kW of Option A)
- There is no VFD
- Pumps can be set up to alternate so that starts per hour is not an issue, and pumps wear evenly.
- The big disadvantage of this system is that it will not deliver the required higher flows for peak flow conditions, and when it does try to do this with two pumps running, they are inefficient and "not happy" at their operating duty point.
Two "medium sized pumps, with parallel/series piping
This option is exactly the same as Option B (in that the pump selection and motor selection is exactly the same) except for the piping arrangement.
Instead of both pumps only being able to pump into the common discharge line, interconnecting piping is added along with an additional non-return valve and an additional isolation valve. Gorman-Rupp call this "Parallel/Series Piping".
This additional piping now allows the pumps to operate as series connected pumps when the second pump is called upon to operate.
When a single pump operates it will perform exactly the same as Option B (see performance in Fig. 3). But look what happens when the second pump is brought on in this system (see Fig. 5)
As can be seen, if we look at the lower pressure piping, flow for the single pump operation is approximately 44 l/s, but when the second pump comes on, the flow is increased to approx. 61 l/s (27 per cent more flow than standard parallel connected pumps).
Pros and cons
The only disadvantage with this system is that it is not as flexible as Option A in that the flow cannot be ramped up and down to match in-flows. Although this can be added with the addition of a VFD to the "discharge pump" (in this system, one of the pumps always acts as the "suction pump" or "first stage" and the other acts as the second stage during dual pump operation). This will also mean that the VFD is much smaller than with Option A (18.5kW vs the 75kW of Option A).
The advantages of this system are as follows:
- The cost is substantially cheaper than Option A
- There is spare parts compatibility between the two pumps.
- Pumps can alternate after every pumping cycle.
- One pump running can handle the regular flow rate.
- When higher inflows demand a higher output, both pumps operating in series can deliver 61 l/s (or 38.6 per cent more than when a single pump is operating).
John Hart of John Dee Warwick elected to go with option 3 as the best engineered solution.