Sizing a waste water rising main

[alfaromeo99]

I have a rising main of DN300. It is presently passing forward 100L/s. i want to pass more forward by installing new pumps. How is this calculated so i can still be within design requirements and not go above design limits for this particular pipe. I need to pass as much forward as possible in a bid not to have to upsize the pipework.
Cheers
A

 

[gerhardl]

At 100l/s you will already have a velocity of about 1,42m/s, which is fairly high. Others will have more practical insight her than me, but you have to be careful.

What is your situation?

Domestic, industrial or other? Pressure, length and type of outlet and branches. Limitations here, especially on pressure? Increase for normal use, or only firewater?

What are your pressure, and how much do you have to increase the pressure?

Some points:

By increasing the velocity, you also increases the possibillities of higher pressure peaks and water-hammer in the complete system.

There will be limits for valves and piping, highly depending on pressure class, type and not at least quality and actual present condition.

In my experience 4m/s would be a maximum for firewater conditions, 2 m/s for normal distributing mains, and in between you will find the normal flow speed for flushing of water main pipes!

I would not have increased the capacity at all, if there was an including clause that everything attached could withstand the increase.

If this is not the case I would put the sensible limit at 2m/s - giving a 40% increase to 140l/s.

Although other water systems uses much higher velocities, the great question here is the state of the existing pipeline and attached equipment, and the limits it actually was dimensioned for, and if on this base an increase is sensible at all.

I would like comments from others on this case!

 

[gerhardl]

Sorry! Missed the heading WASTE WATER rising main, which of course modifies my previous answer. All about pressure and conditions stand, a bit more unsure about velocity limitations.

Recomment: Still seems to be high as is!

Sorry about the mistake!

 

[Artisi]

First step is to calculate the head loss thru the pipe line for a range of flows rates round the anticipated new flow rate - this will give you an indication if head loss / velocity it is within the "normal" range for this pipe diameter. Once a flow rate is decided on you can then go about selecting the appropriate pump.

Bear in mind that the head loss thru the pipe line will increase at the square of the flow rate - increasing the flow to 150l/s will increase the head loss by a factor of 2.25

Supplying more info would give people a much better chance to give you further advice.

 

[rconner]

Since it appears you're thinking about a significant expense anyway in installing "new pumps" etc., you might want to look at also (with engineering economic analyses and as best you can estimate electricity rate for pumping costs etc.) the present value of long-term energy costs of going with quite high flow velocity. There is a quite helpful (and free) tool for looking at that sort of thing included in the "Computer Program" download package at

http://www.dipra.org/.

"With energy costs rising at an even greater rate than construction costs, it may be cost effective over the life of the transmission line to select a larger pipe size than would traditionally be used."
Mr. Tom Walski, Journal AWWA June 1980

 

[rconner]

[p.s. in 1980 I think electricity was at a staggering rate of about a U.S. nickel per kWh]

 

[alfaromeo99]

Ok, let me feed in some more detail.
I am looking at a waste water rising main. The existing flow is 100L/s, i want to pass more forward by installing a third pump that will bring the pass forward flow to a max of 155L/s. The static head is 15m and the total dynamic head is 18m.
The existing rising main is DN300 CICL. Can anyone show me the calculations to work out if i can pass more flow through the existing pipework. I haven't done this for years and need a refresher.
If more info is needed, just ask.
Thanks,
A

 

[Artisi]

The total head for 155l/s becomes
(18-15)^2 + 15 = 24m

The next problem you have is the existing pumps, are they capable of pumping the required flow rate at 24m head

 

[Artisi]

Ignore my earlier post- it's 0630 and the brain is still sleeping.

For the new flow of 155l/s the total head becomes

(155/100)^2 x 3 + 15 = 22.2m

Can the pumps achieve this head at the new flow rates.

 

[alfaromeo99]

I am waiting for more detail on the flygt pumps. How do i work out the velocity for the existing 100L/sec and the 155L/sec. I could really do with some worked examples i suppose.
Thanks for your help Artisi!

 

[Artisi]

V = 21.22 x l/m / d^2 (pipeID mm)