Solving Pump Selection Dilemma for Reticulation System

[James3849]

Hi all,

I'm stuggling with a design problem to do with pump selection for a reticulation system at a mine site.

Essentially I have to reticulate water from a storage dam to the following areas with the following minimum discharge pressures.

- Conveyor washboxes and spraybars ( constant 8-11 L/s @ 600kPa depending on no. of operating conveyors)
- Stockpile dust suppression sprinklers (intermittent 15L/s @ 700kPa)
- A vehicle washdown facility (twice a day filling a elevated storage tank at around 1.5L/s @ atm)

Note that there is intensly varying demand throughout the day, especially the stockpile dust suppression sprinklers which only operate for a few minutes each throughout the day.

Can anyone help suggest an efficient way to hydraulically size the system and select ideal pumps considering that there will be different system curves due to varying flow demand scenarios?

 

[Danger]

Not that I can help you at all, but I do have a question for clarification purposes. Are you thinking of an individual pump (or pumps) for each system, or distributing the outflow from one or more huge pumps that run the whole thing?

 

[Q_Goest]

Fluid flow through pipes, fittings and valves generally is done using the method outlined in the Crane paper 410. You can purchase a copy online, or even pick up a free paper (Pipe-Flow Professional) that is very similar and has most of the same information here:
http://www.eng-software.com/products/methodology/pipe_flo.pdf

I'd suggest setting up a spreadsheet or similar program to do these calculations and provide for documentation that would be maintained in your company's project files.

You've given flow rate and pressure at a point (ie: spraybars, sprinklers, etc...) in your OP. Given the geometry of the pipe from your pump to the point of known pressure and flow rate, you can then determine the pressure drop through the pipe which gives you the pressure at the pump. Do this for each leg individually. Where the flow for various legs goes through a common pipe, you have to make some decisions as to how the flow is controlled.

To start, try sketching out a flow diagram showing pipe lengths, diameters and valves for the various flow cases. If you need more help, post your flow sketch.

 

[James3849]

Danger: There will be one pump station to serivce all of the demands I mentioned in my first post. I am unsure of the budget alotted to this area, but we should be aiming to minimize the number of pumps necessary.

Q Goest: I have worked on other reticulation networks under fixed speed pumps and also gravity fed and I also have the technical paper you mentioned. Calculating head loss through pipe systems is not my weakness, it is mainly pump selection and how to tackle modelling this pipe network under varying demand.

As I am not experienced with sizing VSD pumps or VSD pump systems (which is most likely what will be used), are there any online resources (or otherwise) that outline how best to size VSD pumps, especially for applications similar to this?

 

[Q_Goest]

As I am not experienced with sizing VSD pumps or VSD pump systems (which is most likely what will be used), are there any online resources (or otherwise) that outline how best to size VSD pumps, especially for applications similar to this?

You can download a free copy of a pump book here:
http://www.lightmypump.com/pump_book.htm

What you're looking for is called an "operating point". See section 4.5.

In short, you will have various system configurations. Each configuration will provide the given flow as required to a portion of your piping network. For example, you may have one system configuration that provides flow to your spraybars with all other flow shut off. A second configuration will have flow to your sprinklers and shutdown facility. Etc...

Each configuration will have some required pressure at the pump outlet. The required pressure will give you the correct flow rate for each demand. The pump outlet pressure determined by the pressure drop/flow rate through the system, has to match the pump flow capacity at this outlet pressure. System flow will only match the pump flow at the operating point.

That may sound difficult, but it's not that bad. All you need to do is have valves (glove valves, preferably with linear or equal percentage plugs) designed into your system to produce just the right pressure drop so that the operating point is obtained. In the manual, they point out how to adjust a valve to do just this. You can also have controls set up to measure flow and adjust the flow accordingly.

Doing this iteratively by hand is painful to say the least. As mentioned previously, you need to either set up your equations using a computer program or get yourself some software if you don't know how to do it yourself. I believe Crane even sells software for this purpose that would meet your needs, but I'd suggest you slog through it and make your own as it will help you understand the fundamentals if you can write your own program.