Hydraulic Calculations with Two Water Supplies?

9/25/2020

How do I properly perform hydraulic calculations for a sprinkler system with two water sources?

The system we have is supplied by two independent street mains. Each service has a slightly different static and residual pressure. The calculations need to show both supplies. Thanks in advance.

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11 Comments

Colin Lusher

9/25/2020 10:13:05 am

Dual water supplies are normally provided for redundancy/reliability. If that is the case, then you should pick only the weakest water supply, and perform a calculation with only the one water supply, ignoring the redundant supply.

Or does your system require BOTH supplies to meet your demand?

Ivonn

9/25/2020 10:52:02 am

Agree with the comments and question of Colin Lusher

The connection of each supply are in different position? Consider this in the selection of the remote area and most demand criteria.

When you choose with what supply consider flow and pressure maybe one have more pressure but the flow is just enough or the other have a bigger flow but the pressure are too low may be you can do the calc with booth (separately).

Justin Milne

9/25/2020 10:23:22 am

I have this same question!

Hector Ramirez link

9/25/2020 10:50:17 am

For the simplest results, calculate to the weaker of the 2 water supplies. However to properly calculate 2 water supplies you will be required to balance the flow from both until the pressure loss from both supplies to your demand system are the same or close enough (within ~0.5 psi of each other). You do this by running 2 calculations, one for each of the supply, making an assumption about the flow entering from the other supply. Compare the pressure losses and make adjustments on your initial assumptions and repeat the calculations until both calculations' pressure losses to the system are close.

Paul G

9/25/2020 11:24:10 am

This^ Better than I was trying to explain it. You can also complete your sprinkler calculations to a common point where the supplies connect to get your estimate demand for balancing.

Are you looking to do this via hand calculations or are you using a specific program? Some programs only allow one source, others permit multiple.

Additionally, if using two street mains, are they truly independent? Or do they connect somewhere before they reach their source?

If so, I think you will need to analyze the water supply data, as the flow test for each were most likely taken at separate times and may not be truly accurate.

As others have said, depending on the requirements, like for high-rises in my area, you need two sources, however you only calculate to the lesser of two sources. The second is redundant in case you lose one.

Brian Gerdwagen FPE

9/25/2020 12:31:22 pm

I use the weaker water supply, but at both locations as if it was supplying the underground from two locations.

UGT to CONN
UGT to UGT2
UGT2 to CONN

This way the supply is not duplicated and it balances itself.

Franck

9/25/2020 01:03:51 pm

As indicated above, it is wise to compare to the weakest and use the second one as a back up (to avoid a water supply impairment in case of works on one water supply).

If you want to compare anyway to both curves at the same time, you can do it in several steps by using hazen williams (for friction losses) ans Q1.85 graph for the flow curves.
It is pretty easy, but difficult to explain without examples and curves to show you how it works.

1st step : you calculate your friction losses (hazen williams) and elevation difference between the test point and the common connection to your system.
You have then :
For first supply : Static pressure 1 - flow 1 and residual pressure 1 at connection point.
For second supply : Static pressure 2 - flow 2 and residual pressure 2 at connection point.
With these points, you draw your flow curves for each supply (straight line)
To have the combine curve, you add the flows at the same pressures.
It is in fact like having 2 fire pumps in parralel at your connection point.
As long as P2 > P1 static ( or vice versa) you have the flow curve from water supply 2 only (backflow / check valves prevent supply 1 to be operational) and when P2 = P1 Static and below, you add both curves.

As a good guess, and considering than you have straight lines with Q1.85 graph, you can make your curve with 3 points :
Again if you consider P 2 static > P1 static
P stat = P2 Stat
From P2 stat to P1 stat = supply 2 curve. You will end at (flow 2 bis) corresponding at that pressure.
This is your first part of the curve

The second part will start at P1 stat and (flow 2 bis) and finish at a pressure based on the higher residual flow measured
Again, if P residual 2 > P residual 1, the pressure for the third point would be P residual 2 and the flow obtained by adding the flows at P residual 2 for both water supplies

Sorry for this long explanation, it would be pretty easy with a simple drawing.

Mark H

9/25/2020 04:56:32 pm

All great comments. Only thing I have to add is if the two connections supply common site private fire hydrants you could do on site flow test and use that for your supply but I would agree with all the comments about using weakest of the two supplies and consider the second tap redundancy. Would also confirm elevation between the two tests is not a factor.

9/25/2020 10:53:48 pm

In practical, required water can not be drawn from both sources, Hydraulically it'll draw water from Stronger supply if both are connected, adequate and fulfill the required demand. So if you want validate viability of your system, design/check with weaker one, and make other alternative supply, the purpose would be solve.

Also, never forget to design check for maximum possible static pressure which will still be from stronger one and system need to accommodate it.

Franck

9/27/2020 08:22:10 am

Although it is a good practice to consider one water supply and the other as a back up, it is also true that if you have 2 water supplies, water will be delivered by both (adding the flow at the same pressure).
It is that principle that enables having warm water by mixing hot water and cold water from 2 separate supplies.
The same way, on a loop system, water will come from both legs at the point of connection.
The short leg is the strong supply and the long leg is the weak supply, but water will come from both (with more water coming from the strong supply).
Considering only one supply is a conservative approach
It is a good practice, but if one supply is not enough to meet the system demand, it could be interesting to check with both supplies

Peter Glodic link

9/29/2020 06:22:12 pm

The method I would use to hydraulically calculate the system is to first use HYENA+ (hydraulic analysis computer software - disclosure: I own the company) to calculate the required Input Point Flow/Pressure (with one input point).

If the applicable code requires the building to have a dual water supply, I would use HYENA+ to calculate each of the water supplies independently to prove that each supply is capable of supplying the system without the other operating (this is what we are typically required to do here in Australia).

If the designer wanted to know what the net effect was of the system with both supplies running, one could simply include both water supplies as separate inputs and running the HYENA+ calculation to work out the ‘Discharges for given input’.

HYENA+ will then balance out the actual flow being drawn from each supply relevant to the duty and associate pressure losses in the pipework. Depending on the actual pressure & flow being discharged from each supply, one supply may discharge the majority of the required system demand.

This concept is also reflected above far more eloquently by Franck

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