How to Include Suction Tank in Hydraulic Calc?

7/1/2022

We are bidding a job that may possibly need a 1,250 gpm rated fire pump due to not enough flow from the city. Our test was 43 psi static, 35 psi residual at 872 gpm. We will need to include a ground suction tank.

How do I correctly include or simulate a ground suction tank in my hydraulic calculations?

We use the Sigma Hydraulic Calculation program.

Thanks in advance.

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5 Comments

Alex

7/1/2022 07:31:48 am

Hi,

Fire pumps are installed when you dont have enough pressure. Tanks/wells/or the use of other water sources are required when you are short on flow.

I've never used Sigma Hydraulic so I cant help with the specifics in terms of running the calc. Is your water source above or below the pump inlet? If below, are you using a vertical turbine pump?

Thanks,
Alex

Jesse

7/1/2022 08:11:55 am

The fire pump will be you driver for calculation. You can put your water supply node where the tank will be, and set the pump node to the pump ratings: i.e. churn is static, rated capacity is residual / flow and follow the pump curve out with the 150% rating. If your tank has a higher elevation than the pump you can add a couple psi for the water column.

I've never used Sigma, but the hydraulic software suites I have used before were all relatively similar in this regard.

Steven

7/1/2022 08:33:35 am

Jesse has it right. You won't need to include your hydrant flow test results in the calculation whatsoever. I also haven't used Sigma before, but other calculation packages will let you include a pump curve as the only water supply source. I recommend doing that if the software will let you. If the software does force you to include something for the City supply, then do just what Jesse said, and include the pump flow/pressure curve as your Static / Residual / Flow values.

Although the weight of the water in the tank does provide *some* additional pressure into the suction of the pump, I would just ignore it for simplicity and to give a more conservative calculation result (unless there is a significant elevation difference between the tank discharge and the pump suction).

Franck

7/1/2022 09:41:23 am

You can take the theoretical curve (commercial curve) provided by the supplier as a first approach.
But best way is to have at least the test results from the actual fire pump, bench test ot commisioning test.
Of course, the values from the commissioning test are the more accurate to reflect what you really have on site.

Normally, the hydraulic calculation is for your sprinkler system, so independent from the water supply. It is only at the end that you compare the hydraulic demand to the available water supply.
Whatever your water supply, you will have the same hydraulic demand.
Then, depending on the water supply, you will have some margin (or not).

Jody Aycock link

7/2/2022 09:12:37 am

I question whether a tank is needed. What size is the
city's water storage tank? Does it feed the under ground water mains you are connecting to? Typically a city water storage tanks capacity is a million gallons or more so if this is the case, then there should be ample water for the duration times required by nfpa 13 and all you would need to do is add the appropriate fire pump to the system. A call to the local water dept should clear any questions up about the water supply and just run a hyd calc to find out your gpm, then you multiply by however long nfpa 13 requires the water duration to be and you have the total amount of gallons of water you need for the system to operate properly. As long as you have enough water to cover the duration requirement with the water available in the city system you are good. If in the end, if you do need a water storage tank, then if you make it your single supply, then the connection point to the tank would be your source and a hose allowance wouldn't be required. And when you calc with the tank and your hyd calc program won't queue up, then try adding 1 psi as your residual pressure and re-run.

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How to Include Suction Tank in Hydraulic Calc?

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