TDH - Complex Rising Main 2

[SMIAH]

Profile attached.

I want to check the pump total head.

Should I average the
minimum head = 143.4-140.5 + total pipe flow friction losses
and
maximum head = ((154.4-144.4)+(149.0-147.0)+(149.0-148.5) + total pipe flow friction losses)?

Or simply TDH = 154.4 - 143.4 + pipe flow friction losses from this line.

Thanks!

 

[SMIAH]

(I feel like taking the maximum head btw).

 

[LittleInch]

The thing that will make the difference is what is on the end of the line and what your total pipe friction loss is. Given that this is a pumped system abut 1 km long by the look of it, my guess is that your friction losses are a lot more than 10m. Your exit point is lower than your inlet (I asusme you're flowing left to right?), therefore your first equation is the one to use. Providing that your pump develops more than 10m you will make it over the first hill. Once you have more than 10m , forget about the highest point in respect of pump head and just look at the end point elevation.

The pipe friction loss on the other hand should be such that it allows the pressure in the pipe to be above atmospheric pressure at all times. Otherwise the fluid will pull a vacuum on the outlet leg. Also if you have no valve on the outlet and stop pumping, the pipe will self drain to a certian extent.

Draw your results on a graph ( see attached) and if you have or need a back pressure on the end point include that in your calcualtion and just add it to the line friction to obtain your required pump head above the start point.

Remember pump head as shown on apump curve / data sheet is actually differential head so if you have positive pressure or head on the suction of the pump this needs to be subtracted from the required pump discharge head to get the "pump differential head". Sorry if this is obvious.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[SMIAH]

Flowing left to right (and wastewater) - it will end in a manhole with a discharge 2 m below the end point (140.5).
With an air valve on the top of the hill (154.4).
Static head calculated with the stop water level.
Adding losses from the aspiration line to the discharge line.

I'm not sure I get things straight.
Geometric Head = 11 meters (154.4-143.4)?
Friction losses = Total length of the pipe

 

[LittleInch]

I probably confused things, but geometric head is now 143.4 - 140.5 - 2. Friction loss is friction loss. Looks like you have atmosperic pressure at the end point?

Providing your friction losses are more than about 12m for the entire line, then forget about the first high point.

However if all you are doing is pumping the water up to the top of the first hill (i.e total friction losses less than 10m), the all your pump needs to do is 10m plus friction losses in the first 193m.

I ask again - what are your calculated friction losses (m/100m) or total and then I can draw the head loss onto your profile and tell you a bit more.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[SMIAH]

Geometric Head is 143.4 - 140.5, no? (as 140.5 is the pipe outlet then it falls straight (2 m) in a manhole with a discharge at 138.5).

For a peak flow of 35 L/s, I get 18.8 meters of losses (150 mm CHW = 150 length = 883 m + EK = 3.0 for bends and valve).
Losses = 1.8 m in the aspiration line.

I was thinking that for 35 L/s, the TDH would be 34.4 m (1.8 + 18.8 + 13.9)
You seem to tell me that Geometric head is not 11 meter (water level to the hill) but rather... 3.1 m.

 

[SMIAH]

So is it ?
13.9 m (sum of all the hills)
11 m
3.1 m
or even 1.1 m

 

[SMIAH]

(3.1 but 2.9)
(1.1 but 0.9)

 

[SMIAH]

Pump adding 23.5 m with 34 L/s.

Pressure at each points.

 

[LittleInch]

geometric head is 143.5 minus 140.5 if the pipe ends in an open manhole. What happens after that is irrelevant to the pump.

See attached graph.

Your problem is that because you have an open end, there isn't sufficient back pressure in your pipe to keep the pipe full of liquid past the 149m high point at kp 723m. This point now effectively become the end of your pipe for hydraulic purposes as the flow will be gravity from that point onwards and try anf low faster than the incoming flow (called slack flow). Thus moving back from that high point with your calcualted losses, your pump discharge head to do your flow is 168m-144.4m = 24m assuming that your pump is at the 144.4 m level - the yellow line.

Forget all about the other head losses and concentrate on friction only, plus the high point at the end of the pipeline which actually defines your hydraulic end point, hence static head is actually 149 - 144.4 plus friction of say 18 allowing for the reduced length = 23m - essentially the same as the graphical solution ( I added 1m for graphical purposes).

What you seem to neglect is that for a pipe which is filled with liquid and kept above 0 barg at all times, whilst there is effort to move the liquid up the hill, this is balanced by gravity helping it down the hill on the far side. Think of this as continuous train of trucks for the entire length all joined together. Once you've got trucks from one end to the other then the ones going down the hill will help to pull those going up the hill. The problem you have is that at the 149m mark, the trucks become detached and run down the hill under their own volition, faster than the trucks being pushed along. Thus gaps start to appear between the trucks.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[SMIAH]

So TDH 24 m. I'll get pressure below 15 PSI (atmospheric) in the system
It'll be able to get over the hillpoint (154.4 m).
(?)

 

[LittleInch]

Yes (24m).

You will get pressure below 15 psia in the section beyond the last high point, so it might not be smooth flow going into the manhole at the end point.

You will be able to get over all the hills.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[SMIAH]

You know what? this is an actual pumping system (real).
It has a 34 L/s pump with a TDH of .. 17 m.

I was tempted to put a pump with a TDH of 31.4 m to avoid getting pressure below atmospheric in the system.
Could be more $.

Thanks for your time.

 

[SMIAH]

I'll check operation time and real flow measurement.

My conclusion is that it doens't pass 34 L/s now with a 17 meter pump.

 

[LittleInch]

The only way as i see it to avoid sub atmospheric pressure is to instal a back pressure control valve on your exit point. Is it worth it? Up to you. Most pipeline engineers would say yes, but in reality this is a low pressure water system and often works at this sort of pressure.

There are fine margins so one or two metres either way will make a big difference, e.g. is the high point measurement the pipe or ground level?. The head at the high point could easily be-2 to -3 m and hence drag down the requited head at the inlet. Friction losses are notoriously conservative so you could easily get close to your flow with a 17m pump.

It's just not the best way to do it.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[SMIAH]

I Think that the 17 m pump can't pump 34 L/s that's all.

 

[LittleInch]

Quite possibly not, but I doubt it's that far away, prob somewhere between 25 to 30 l/sec. Will be interesting to see.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[SMIAH]

I'm gonna do a drop test.
No flow recorded for that small pump station.

It's probably not dramatic though as you say (pumping more often).

 

[SMIAH]

LittleInch,

Is it important to maintain pressure above 0 PSI in the system? I wouldn't take the risk to rely on the siphon effect...

I'm thinking about maintaining it above 14.7 PSI.

 

[LittleInch]

Yes I think it is important to avoid creating vapour / vacuum which collapses later on and leads to unstable flow. Above 14.7 psig is fine. The pipeline won't stop working if you go below this, but it is generally recognised as good practise to operate at all times > 0 psig

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way