Calculate the location for a break tank

[Spainers87]

Hi All,

I have what should be a simple problem but I can't get my head around it.

I have a high level tank at elevation approx 1600m. A 350mm pipe runs down the valley to a low point of approx 600m and up to another tank on the other side at approx 800m.

I would like to calculate the hydraulic grade line that tracks the pipe so that I can determine where a break tank may be installed to keep the pressure in the pipe acceptable (e.g. below 16bar). A pumping station will be installed in the valley to get the water back up the the last tank.

What information do I need and how do I go about doing this. I get stuck at step 1, calculating loss due to entry into the pipe from the first tank (0.5 v^2/g) as I do not know the velocity.

Please help, its been a long time since I did any hydraulics but its in my head somewhere.

Spainers

 

[bimr]

The break tank location will be determined by the static (elevation) head, not the friction head. After you determine the elevation of the break tank, you can then determine the piping size that will give you the required capacity.

Here is a publication that has an example similar to your problem. See page 2.

http://www.nesc.wvu.edu/pdf/dw/publications/ontap/2010_tb/hydraulics_pressure_DWFSOM147.pdf

 

[Spainers87]

Thank you for the reply and the link.

I would like to include friction losses at pipe entry and in the pipe itself. Any help for expanding on the information given in your solution?

Thanks.

 

[cvg]

you will need a pump at each break tank also
estimate your pumping rate
estimate your pipe size and length
use any one of several equations to calculate the pipe friction loss (such as hazen-williams)
add that to the static to get the required pump head at the pump station
note that your hgl lines are drawing incorrectly

 

[Spainers87]

Thanks for the reply. The flow is downhill for the most part (from left to right) the break tanks are to ensure that the pressure doesn't get too high (pn16 flanges). Flow from each break tank to the next is downhill via gravity. To use haen-williams, do I not require the flow rate?

 

[cvg]

A pumping station will be installed in the valley to get the water back up...

the high pressure is when you pump back up, not when you drain down. gravity flow down hill does not create the highest pressures. and yes, step 2 - estimate pumping flow rate or gravity flow rate before calculating the head loss.

 

[Spainers87]

As it currently stands, the downhill pipe is fitted with PRVs at various points to keep the pressure down. (a 1000m head of water must have a significant pressure head no?) But the PRVs are cheap and of dubious source. If 1 fails then it could spell disaster.

I'm in asset management and we are proposing the installation of several break tanks to keep the pressure low (inlet to the break tank is via a valve and hydro turbine to power the pumping station(s) at the bottom.

Surely the large static head results in large forces in the pipe when the inlet to the break tank is closed?

I am quite comfortable with doing the two reservoir problems where the flow rate in a pipe between two reservoirs is calculated. Could I do the same for this problem but set the height of the second reservoir as H + Hf?

Thanks for your help

 

[bimr]

Transmission pipes are usually sized with low velocities to minimize pumping costs. In your case, you would want the transmission pipes sized to minimize friction so that you can recover the energy with the proposed hydro turbine. At this point, your project seems to be a planning exercise, so it is probably best to ignore the friction losses at this point. The pipe entry losses are not typically significant when the velocities are low.

The original consultant probably sized the 350mm pipe for the maximum fire flow capacity so the pipe velocity should be on the low side (with minimal friction). To determine the velocity, you need to know the maximum system capacity.

PRV's are also used to create pressure zones. If you have any users that are located on the hillside, you have a choice between using a PRV and installing a pump on the break tank. If you just use the hydro turbine, the discharge pressure would be too low for hillside distribution and you would forced to repump.

The choice between using a break tank and PRV depends on many factors, not just cost. You have considerations for fire flow, fire water storage, flow, size of system, etc. This is not a simple evaluation and you have not provided enough information to understand the application.

PRV's are generally reliable and you do not have to worry about them failing. You will have to do a Hazop analysis to determine the requirements for pressure relieving devices on the various tanks and pipe segments.

You will need to install a break tank at each elevation drop of 165 meters according to your proposed design concept. 165 meters of elevations equates to approximately 16 bar of pressure. You will probably understand why the original consultant used PRV's. It will be costly to construct so many break tanks, especially on a hillside.

 

[Spainers87]

Bimr, that was a great answer. To clarify,I amin asset management so pipe condition and lifetime assessment is my area. The original consultant designed along with your thinking. Unfortunately the contracted installed extremely sub standard assets. E.g consultant designed for 25 bar connections, contactor installed 16.

We are trying to add value to the project in addition to delivering original requirements e.g CSR, sustainability etc....

Your are right and we are in initial project readability stage but I am anticipating client questions.

I think your answer is perfect for now so I need to work from here. I will post back with clients comments for posterity purposes

Thanks all for your input