Need help reading Pump Curve for Sewage

[JimFromAustin]

I'm not quite sure how to read this graph? What does TDH (Total Dynamic Head) mean? This is probably my main confusion point, not sure what TDH means? Does this chart tell me the Green Pump is the most powerful, and the orange pump the least powerful?

Here are their instructions: Multiple pumps may be used in your application. The dark blue curve on the graph below shows the Total Dynamic Head (TDH) for the application you specified. The point where the TDH curve and a pump curve intersect show the estimated gallons per minute that will be pumped by that series.

 

[Fitter30]

There some good videos on tdh on you tube and how to pick the right sewage pump.

 

[Sponsor]

 

[Breplum]

Always choose duplex pumping system, as pumps always fail and the two pump systems give you best lifetime results of a difficult situation.

 

[wwhitney]

I'm not quite sure how to read this graph? What does TDH (Total Dynamic Head) mean?

Head is another word for pressure. The units are convenient: if you are pumping water uphill 20 ft, then the static pressure you have to pump against is 20 ft of head. 1 ft of head = 0.433 psi.

The static part of that statement is for the case water isn't flowing or is flowing so slowly you can ignore any dynamic effect. In practice when pumping significant gpm through a fairly small pipe, the water flow sees drag from the pipe and that creates extra pressure you have to pump against. So dynamic head, for a given flow rate through a given piping configuration, would be how much extra pressure that drag creates, which your pump has to overcome. Dynamic head strictly increases with increasing flow rate.

Total dynamic head is the sum of dynamic head and static head. If the graph you posted is from some online app that required you to put in some info about the static head and about the piping configuration, then the blue line on that graph shows what the total dynamic head is for a given flow rate through that system (if your input are accurate). It is an increasing function, as again dynamic head increases with flow rate.

Now a pump is going to be able to push water/sewage with a certain amount of oomph. Meaning it can push a lot of water against a small head, or less water against a bigger head. So if for the pump you graph "pressure the pump can push against" vs "the flow rate the pump can move at that pressure," you get a graph where head decreases with flow rate. That's the pump curve for the pump.

If you graph the pump curve and the system curve on the same graph, the point where they cross tells you where they will jointly operate if you connect that pump to that system. It's the point of equilibrium, where the pump can handle creating that much pressure at that flow rate, while the system itself requires that much pressure to allow that much flow to move through it.

Your graph has multiple pump curves, and the intersection between any one of them and the system curve (the one marked TDH) tells you what that pump will do. So for example the orange line and the blue line cross at about 15 ft of head and about 55 gpm (estimating from the graph). If you're happy with a pump that can move 55 gpm, you can go for that orange pump. At the other end of the options shown, the green line and the blue line cross at about 52 ft of head and 115 gpm. So if you want 115 gpm of flow, you'd use the green pump. And then you have various choices in between.

Note that with the larger pump and the higher flow rate comes a higher head of 52 ft. That means if you choose that, the pump will pressurize the pipe connected to it at 52 ft of head. So that might require a sturdier pipe able to resist 52 ft of head than if you chose the smallest pump that only would be making 15 ft of head connected to your system.

Cheers, Wayne

 

[JimFromAustin]

Head is another word for pressure. The units are convenient: if you are pumping water uphill 20 ft, then the static pressure you have to pump against is 20 ft of head. 1 ft of head = 0.433 psi.

The static part of that statement is for the case water isn't flowing or is flowing so slowly you can ignore any dynamic effect. In practice when pumping significant gpm through a fairly small pipe, the water flow sees drag from the pipe and that creates extra pressure you have to pump against. So dynamic head, for a given flow rate through a given piping configuration, would be how much extra pressure that drag creates, which your pump has to overcome. Dynamic head strictly increases with increasing flow rate.

Total dynamic head is the sum of dynamic head and static head. If the graph you posted is from some online app that required you to put in some info about the static head and about the piping configuration, then the blue line on that graph shows what the total dynamic head is for a given flow rate through that system (if your input are accurate). It is an increasing function, as again dynamic head increases with flow rate.

Now a pump is going to be able to push water/sewage with a certain amount of oomph. Meaning it can push a lot of water against a small head, or less water against a bigger head. So if for the pump you graph "pressure the pump can push against" vs "the flow rate the pump can move at that pressure," you get a graph where head decreases with flow rate. That's the pump curve for the pump.

If you graph the pump curve and the system curve on the same graph, the point where they cross tells you where they will jointly operate if you connect that pump to that system. It's the point of equilibrium, where the pump can handle creating that much pressure at that flow rate, while the system itself requires that much pressure to allow that much flow to move through it.

Your graph has multiple pump curves, and the intersection between any one of them and the system curve (the one marked TDH) tells you what that pump will do. So for example the orange line and the blue line cross at about 15 ft of head and about 55 gpm (estimating from the graph). If you're happy with a pump that can move 55 gpm, you can go for that orange pump. At the other end of the options shown, the green line and the blue line cross at about 52 ft of head and 115 gpm. So if you want 115 gpm of flow, you'd use the green pump. And then you have various choice in between.

Note that with the larger pump and the higher flow rate comes a higher head of 52 ft. That means if you choose that, the pump will pressurize the pipe connected to it at 52 ft of head. So that might require a sturdier pipe able to resist 52 ft of head than if you chose the smallest pump that only would be making 15 ft of head connected to your system.

Cheers, Wayne

Excellent Explaination Wayne! Is Dynamic Head = distance + pipe size + flow rate? Or only a function of flow rate regardless of distance or pipe size?

In my situation, I have a guest house in the back yard. The pump is buried just below the guest house sewer line. The sewer line exits the guest house and drops into the pump basin. After that, it has to pump 1 or 2 feet up out of the basin where a 2" (more or less level) drain line comes out of the basin and connects with the main house sewer system. Once the 2" line connects with main house up front then gravity should start taking over.

Do I have 2 ft of static head?

 

[wwhitney]

Excellent Explaination Wayne! Is Dynamic Head = distance + pipe size + flow rate?

If you replace "=" with "is a function of", then yes. I think there may also be a subtle terminological difference, in that distance, pipe size, and flow rate determine the pressure loss due to flow through that pipe. The pump needs to develop sufficient dynamic head to satisfy that pressure demand. So the term "dynamic head" may only be used in the context of the pump side of things, not sure.

Do I have 2 ft of static head?

If the high elevation point of the pumped flow is 2 ft above the liquid level in the basin when the pump turns on, then that's 2 ft of static head. Or maybe instead of the high point of the pumped path, we should use the transition point from pumped flow to gravity flow; not sure about that.

BTW, I don't have any experience with pumped sewage, my background is more about fully pressurized systems like domestic water or natural gas. So for example I have no idea what criteria you'd use to choose between a pump that would do 55 gpm and a pump that would do 115 gpm.

Cheers, Wayne