undertan1995
Civil/Environmental
- Jul 23, 2020
- 4
I'm seeking some advice for something that should be pretty simple. I have a tendency to over-complicate, but here goes:
Background info:
I've got a project in which a water treatment plant receives water at an average rate of 1 MGD, with a max rate of 3 MGD. After standard chlorination math I've found that the chlorination system will need to be able to feed a maximum of 125 PPD. However, the plant currently has a chlorinator rated for 250 PPD. For conservative reasons I've decided to ensure that the chlorine ejector has enough supply pressure and flow to output 250 PPD, even though it really only seems like 125 PPD is sufficient.
The system has a booster pump which withdraws a water supply from the incoming water main (conveying 1-3 MGD), boosts across the ejector, and injects in the water supply main at a point downstream from the withdrawal point. I have determined that the pressure at the injection point is approximately 46 ft (20 psi). The 1" solution line from the ejector assembly to the injection point is approximately 85' with several bends and fittings.
Problem:
It's my understanding that the backpressure seen on the ejector depends on the system flow (injection point pressure + headloss from injection point to ejector). Obviously you can't calculate the headloss without knowing the system flow. So that leads me to selecting a pump. The chlorinator companies don't really dabble in booster pumps. They just say you need a particular flow and head, and I have yet to find an example online where this is gone over in detail.
I plotted the system head curve looking from the pump discharge to the injection point. I assumed that the static head condition over the pump is essentially zero (technically slightly negative), since it's taking water from a pipe under pressure and discharging it right back into that pipe. So the system head is literally controlled by friction losses. So I started plugging in flows and heads along my developed system curve to see which would be me the proper combination of ejector backpressure and supply pressure/flow. Since the system head curve will only show me the head instantaneously at the booster pump, I had to factor in flow and headlosses to the ejector inlet.
My question is, am I overcomplicating this? Or does this seem like a reasonable way to design?
Background info:
I've got a project in which a water treatment plant receives water at an average rate of 1 MGD, with a max rate of 3 MGD. After standard chlorination math I've found that the chlorination system will need to be able to feed a maximum of 125 PPD. However, the plant currently has a chlorinator rated for 250 PPD. For conservative reasons I've decided to ensure that the chlorine ejector has enough supply pressure and flow to output 250 PPD, even though it really only seems like 125 PPD is sufficient.
The system has a booster pump which withdraws a water supply from the incoming water main (conveying 1-3 MGD), boosts across the ejector, and injects in the water supply main at a point downstream from the withdrawal point. I have determined that the pressure at the injection point is approximately 46 ft (20 psi). The 1" solution line from the ejector assembly to the injection point is approximately 85' with several bends and fittings.
Problem:
It's my understanding that the backpressure seen on the ejector depends on the system flow (injection point pressure + headloss from injection point to ejector). Obviously you can't calculate the headloss without knowing the system flow. So that leads me to selecting a pump. The chlorinator companies don't really dabble in booster pumps. They just say you need a particular flow and head, and I have yet to find an example online where this is gone over in detail.
I plotted the system head curve looking from the pump discharge to the injection point. I assumed that the static head condition over the pump is essentially zero (technically slightly negative), since it's taking water from a pipe under pressure and discharging it right back into that pipe. So the system head is literally controlled by friction losses. So I started plugging in flows and heads along my developed system curve to see which would be me the proper combination of ejector backpressure and supply pressure/flow. Since the system head curve will only show me the head instantaneously at the booster pump, I had to factor in flow and headlosses to the ejector inlet.
My question is, am I overcomplicating this? Or does this seem like a reasonable way to design?