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# Fire flow analysis

Dear All,

After  a long gap,  i am coming again to the forum. Now, my issue is with fire flow analysis.

The fire flow guidelines given by NFPA  for my project (residential area) criteria is 1000 GPM flow for minimum of 2 hours. and it is to be operated with  two fire hydrants. ie to hydrants to be run in parallell with 32 GPM each. Now, how to simulate this scenario in the watergems model fire flow analysis.

kindly help me out. Since, now i am feeding the values in  each node and checking for fire flow.

Further, i am using watergems V8i version.

With Regards,

Sreepathy

• There are really two steps to determining the available fire flow. First, you determine how much water you can deliver to the area and then you check if there are sufficient hydrants to get the water out of the pipes.

What WaterGEMS/CAD calculates is the amount of water that can be delivered to the area. Then you need to check whether there are enough hydrants. For example, you may be able to bring 50,000 gpm to the intersection of two 48 in. pipes, but if there are no hydrants there, the available fire flow many be zero.

For the type of hydrants we have in the US, according to the Insurance Service Office Fire Suppression Rating Schedule, you can credit 1000 gpm for every hydrant within 300 ft of the location, 670 gpm for hydrants 201 to 600 ft and 250 gpm for every hydrant between 601 and 1000 ft.

I’m not sure about what kind of hydrants you have in the area where you work.

The flow you mention of 32 gpm seems very low. Do you mean 32 L/s (about 500 gpm)?

• I guess RSP means how to run the model with two hydrant at the same time, i.e. the automated fire flow to check two hydrant opened at the same time for two hours.

Unfortunately the automated fire scenario is design to check only one hydrant, and I guess you should do it manually at one hydrant and automated for the second one; unless somebody else has other way.

I wish if WaterGEMS/WaterCAD has capability to run automated fire scenario at two locations.

• As I explained, the fire flow analysis does not tell you how much water comes out of one hydrant or two hydrants or ten hydrants. It tells you how much water can be delivered to that node at the required residual.

The actual flow that you get out may be limited by the number of hydrants that you have. If you look at the kinds of equations that the fire flow analysis uses, there is no input for number of hydrants. It just determines how much water is available. That number may be reduced if there are not enough hydrants.

That's all you can really do because the real fire flow will be determined by the laying length of hose, how many hoses and hose streams,the size of the hoses, what size pump is on the fire truck, what nozzles the fire fighters will be using, etc. and the engineer has no knowledge of that at the time he is doing design.

• Yes, as mentioned by Mohammed, i need to run a fire flow analysis for total 64 lps but with 2 fire hydrants running in parallel with 32 lps. this is as per the NFPA -14 guidelines.

Is there any option in Water gems to perform this option.

• You folks aren't listening. The WaterGEMS fire flow analysis is a two hydrants analysis and it is a one hydrant analysis and it is a three hydrant analysis... What WaterGEMS does is calculate the amount of water that can be delivered to a node in the system at the specified residual pressure (usually 20 psi in the US). After that, it is up to you to check that you have enough hydrants to get the water out of the node.

In your case, it sounds as if they only allow you to take credit for 500 gpm for each hydrant so that you need to have two hydrants within a reasonable distance of the node.

This is a somewhat conservative number but it is pretty accurate.

Can you send me the exact wording from NFPA-14 that you are referring to? Send it to tom.walski@bentley.com.