Concrete underground tank - safe pressure at inlet? 1

[TomaszKruk]

Hello,

I'm at an conceptual stage of a long water line problem and hope the forums can help me. I don't have to design anything specific just yet. I'm hoping someone could give me some general guidelines.

I am to design a water pipeline and assumed high pressure losses due to contaminants settling on pipe's walls. I've done the darcy-weisbach calculation and decided the Customer would need to build intermediate pumping stations. After some work I've selected what I think are the best locations given circumsatnces (away from houses, near roads, etc), but it turns out I might have a problem with the first location - it's in a depression in terrain, which means overpressure might go near 0 just before it, and with some bad luck (with decreased flow or during transients) the pressure might go into the negative.

Air realease - vacuum valves will be installed on the line, but I've been thinking if I shouldn't rise the pressure at the "0" pumping station anyway. The deal is - what is the maximum safe pressure I can assume at pumping station "1" (the one near 30 000 m mark)? I don't want to assume anything unreal or non-standard. The values given on the graph already includes 5 m of overpressure at the pumping station "1" tank. I would say this scenario would be solved if I assume pressure head of 20 m at tank's inlet. I am, however, worried about the mass of water eroding the tank (it's around 1500 m3/h).

Just to complicate things - if I assume lower flow, and operation with intermediate pumping stations not bypassed, the local pressure minimums become even worse, and I'd need to increase pressure at the starting pumping station even further. What maximum could you say is still considered standard?

Having done some research previously I would prefer to avoid any slack line. I'm also afraid that throttling the flow at the tanks inlet might cause problems with transients and the design of control equipment.

My company will not design pumping station "0" (the one at 0 meters). We might, hopefuly, be the ones to design the intermediate pumping stations together with the tanks themselves (underground concrete slabs).

I would also be really grateful for anyone pointing me to books, documents, or manufacturers that might provide guidelines to design either means of reducing pressure at the inlet or diffusing it.

Regards,
Thomas

 

[LittleInch]

Are you assuming you need underground tanks and then pumps??

Why not just connect the pump suction direct to the pipe?

Then you just control either pump speed or discharge pressure / flow to maintain a set inlet pressure or outlet pressure.

I don't know what water pipelines work to but I like at least 3 bar above atmospheric pressure at any point. 1 bar should be minimum.

Over that length you won't have issues with transients and maintaining a back pressure into your tank, if indeed you want them is no problem.

If you have a tank then just install a control valve. Very simple and straightforward.

You might have got more interest in the pipelines forum, but this will get some responses.

i didn't understand your issue about lower flow.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[TomaszKruk]

Thank you for your answer. I will contact the pump manufacturer and ask if he can help with setting a system that works with pumps connected in series (it takes time sadly). I was aware that buidling a tank would be a waste of pressure and money, but thought it would give us better control of the system, and increase the system's reliability. Idea of making the pumps work in series is definately worth exploring as the "0" pumping station would be the first constructed and would do all the work, while the line is still new.

About lower flow - what I meant is that lower friction losses would make me choose between going into pressure way below 0 or overpressure at the tank around 5 bar.

What makes me afraid of going into higher pressure on the line is transients happening with that much of the water pumped under such a pressure. The graph above doesn't show that, but we are to consider working pressure just under 60 bar if we only find the necessary equipment.

When it comes to pressure at tank I was similarly afraid of assuming higher pressure, as the mass of water hitting concrete / steel elements under such pressure would be destructive.

About control valves - I did ask a manufacturer for an offer but didn't get one yet. It would be the easiest option. I have some experience with malfunctioning control equipment and I can see some problems arising when it comes to our fluid's purity. I'd much rather double check here on forums, that assume I can use a control valve that stops working properly at first sight of suspended solids.

 

[MartinLe]

What is the highest and lowest flow you need to design the system for?
The pipes will always be completele filled pressure pipes?

 

[TomaszKruk]

Yeah - always completely filled pressure pipes. Max flow is as stated on the graph - 1550 m3/h. Minimum flow is tricky. Minimum demand at receiver is way too low to meet minimum velocity we assumed. We plan to negotiate for 600 m3/h.

 

[LittleInch]

I don't understand why you're asking a pump manufacturer - you are the designer, not the pump guy.

You cannot over pressure an atmospheric tank.

I don't know why you're so worried about the flow rate - sure it's a reasonable number but just make the pipes big enough and you won't have a problem.

Is this "dirty" or clean water here?

with a break tank you need all the pumps to be working all the time to flow at any rate, not just the maximum rate.

You need to describe a bit more your role in this as it doesn't appear you have much experience and real need someone local to explain things. There really isn't anything here which would worry me - you just need to design it properly.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[TomaszKruk]

Thanks for the answer. And yeah - I'm inexperienced so it might be the reason why I'm worried about stuff I should not be.

1) When it comes to manufacturers - the practice here is to make the manufacturers select the pumps. The designer gives them working points, and they themselves figure out what they would like to offer. I want their cooperation when it comes to pump selection, because they are my best bet on getting some more info.

2) I understand - I need to, however, as far as I understand, assume some overpressure at the end of the line. Even if it is just a safety precaution (a little bit extra to acommodate unexpected losses and so on).

3) I have some experience in standard water distribution systems, where pressures are way lower. The PN of 40 and 63 worry me, because I assume higher pressure can make transients that much more severe. So far we're on planning stage or conceptual stage (depends how you call it). I will have some time to get more information and consult on this topic. I'm still worried about making false assumptions that will cause me problems later.

4) Water with suspended solids, that did not sediment readily.

5) What I meant when saying that pumps at point "0" will need to do all the work at the beginning I meant that we plan the line with long life cycle in mind. At the beginning there will be no need for extra pumping station as the colebrook-white rougness will be sufficiently low to allow for the pumps to pump the water all the way. Second pumping station will be installed later. The customer wants the line and firt pumping station built ASAP, and then to start design process for the other one.

6) Aye - we have some consulting engineer I already wrote an email today. Thought I'd manage with some help online, but yeah - I'd need to substitue experience with research and online help, and it would both take too long and in the end it could turn out I was too inexperienced to even try to prepare for everything.


Cheers,

 

[LittleInch]

Ok,

1) Yes vendors select the pumps based on a data sheet that YOU fill in. That's not the same as asking them how their pumps can be incorporated into a system - That's down to you as the designer.

2)you normally allow a little bit of losses in a system but to create a larger back pressure to keep the line "tight" then you normally use control valves

3) Pressure isn't the issue - it's velocity. higher than 2.5 to 3 m/sec and it can get interesting, 5+ and it gets very interesting.

5) I don't get this bit. Are you suggesting adding more pump stations just because the pipeline has silted up a bit?? I think some pipe cleaning or internal coating would cost much less than an extra pump station never mind the increased OPEX costs

6) you never stop learning and nowadays that comes in many forms. feel free to look here, but try and figure out a way first then ask if its a good idea or if you're stuck.

A pump station every 20km tells me your pipe is too small...

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[TomaszKruk]

1) That's what I meant

2) thanks - that's what I wanted to know.

3) Thanks! Those are low in my pipe. Can you reccomend what to read to get more into this?

4) N/A

5) That's what I meant. I will forward your info about coating. We did consider it but someone decided against it. I will try to revisit.

6) I cannot tell for sure if I've done enough or if I'm wasting somebody's time. I did think I hit a wall, and decided it's worth it to at least ask. I like to chat about technical stuff, as I find it both fun and instructive, and I hope it's the same for people reading it.

About the distance between pumping stations - very much worth thinking about. I was afraid of any high pressure at long water line. From what you're saying it should not be avoided but handled properly. There's still time to grow into the project - hope it works out

Again - thanks for your time. I appreciate the input and understand your comment about inexperience.