Does Deluge System Use Supply or Demand Calc?

2/11/2021

This question pertains to hydraulic calculations at the supply and sprinkler sides of a deluge system:

Our example using rough numbers; the deluge system requires 4,000 gpm based on density requirements through the building. This is the system demand.

Now suppose you have a strong water supply. The water supply actually provides 8,000 gpm based on main capacity testing. This is the system supply.

Now my question is, the pipe is wet up to the deluge valve. There is about 50-feet between the deluge valve and the hazard. It is understood that the system demand is 4,000 gpm, but within the pipe network, is it necessary to size the pipe for a deluge system based on 8,000 gpm on the supply side?

Until the water begins discharging through the deluge nozzles, the 8,000 gpm will be surging through the pipe network to reach the nozzles, correct?

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7 Comments

Glenn Berger

2/11/2021 09:19:30 am

Pipe sizes are based upon the demand requirements. Some may alter the pipe sizes to allow for greater pressure usage, if available.

Dan Wilder

2/11/2021 09:23:47 am

Supply capacity of the system is not a consideration (for this specific question, if you are working towards a containment, foam usage result, tank/break tank sizing, or a balancing requirement then a supply calc is needed). A demand calculation provides the minimum required pipe/valve sizes to make the system work. The fact the supply can provide 8000GPM (surging water has to have an outlet) and still keep the adequate pressure is just an additional safety factor to the system.

Think of it this way, assuming a light hazard calc requires 5.6K-7PSI-14.8GPM at the "hydraulically most demanding sprinkler". From that sprinkler using known calculation methods (Hazen-Williams), the riser GPM/PSI can be determined via pipe sizing and lengths based on however many sprinklers are within the calculated area - Lets call it a system demand of 247GPM @ 32PSI. Now, if that one sprinkler every does get activated, it will not see just that 7PSI, it's going to see (for example) 80PSI (50 GPM - thats 1/5 of the system demand already). As additional sprinklers activate, the GPM/PSI equalizes and it is fairly easy to have the demand calc requirements be exceeded by only a few activated sprinklers. If the entire calculated area sees the available water supply, it may actually discharge 431GPM @ 80 PSI (weirdly accurate since I ran a quick test calc..). The demand calc also takes into account a safety margin (PSI or % of PSI depending on AHJ) which the supply calc does not account for.

I'm not sizing valves or worried about velocities (if that is still a thing were you are) based on the 431 GPM running through the system, only the 247 GPM just as I am not sizing the system for the actual supply.

I hope that made sense....?

Justin Milne

2/11/2021 11:48:36 am

It makes sense, yes, I just know that there Unified Facilities Criteria codes required 10 ft/s for underground lines if you want to use ductile iron pipe. Otherwise you'd have to use PVC for the underground. Therefore, the velocity in the pipe might dictate thrust block requirements for underground depending on velocity and GPM, right?

CJ Bonczyk

2/11/2021 09:49:22 am

If its open nozzle/sprinkler style your deluge valve and piping network only needs to be sized based on the simultaneous flow of all devices downstream of the valve which as you state is the 4,000 gpm demand/the entire area protected by the deluge system. Hydraulic calculations and sizing of the pipe for system demand will show how much water is being used based off of system demand and what is still available via the supply. You don't need to account for the additional leftover available water unless its a water storage tank design with refill rates or hangar supplying a foam systems.

If you have high pressures in the water supply and are worried about the pressure surges in the deluge system piping then you have a different issue. High pressure surges causing heavy water hammer in deluge systems can be corrected with the use of items such as surge tanks (Amtrol or Young Engineering).

2/11/2021 11:52:18 am

At what pressure are is water hammer going to start to be an issue? Only have 70 psi currently and normally don't have issues with water hammer. Pump startup would jump that 70 psi to 184 psi.

Franck

2/12/2021 02:48:55 am

Just one comment on waterhammer.

Waterhammer is an effect of the pressure rise (surge) that accompanies a sudden change in the velocity of water flowing in a pipe.

Pressure surges may be initiated by:
• The sudden closure of a valve
• The quick increase of pressure inside a given system
• The sudden stopping of a pump
• The sudden development of abnormal water demand when a water main breaks

It happens, for example, upon a pump start (sudden pressure variation in the system) when you have a closed volume that can not really compress to compensate.
So a pipe full of water will have water hammer if you have a sudden high pressure.

Discharge lines from pumps are subjected to waterhammer caused by water column separation. This may occur when a pump suddenly stops (due to power failure, manual shutdown…) or if the discharge valve is closed too quickly with the pump operating. This is one of the main reason for not having ¼ turn valves on fire mains.

The principal factors contributing to water column separation are:
• The rate of flow stoppage, either by rapid closing of a valve or the fast deceleration of a pump
• The length of pipe system (this factor determine the time that pressure continues to fall before positive-pressure waves returning from the far end of the line counteract the initial pressure drop)
• The normal operating pressure at critical points, such as the crest of hills
• The velocity of the water just before pump stoppage or valve closure occurs: the greater the velocity, the larger the size of the void, reverse flow velocity, and the final pressure rise.

Normally, as deluge systems have many open nozzles downstream the deluge valve and the fire pump starts only once the valve has activated, there is limited water hammer effect has the water compression is less a problem (until all pipes are full of water with nozzles discharging, but in that case, because of friction losses to bring the water, the residual pressure in the system is lower than the one out of the fire pump at no flow.

Note that to avoid having water hammer in sprinkler systems, NFPA indicates the following for the start/stop setting of fire pumps (in the annex section of NFPA 20) :
a) Cut-out Jockey Pump : Pressure of the fire pump at churn (= pressure when the pump is running and there is no flow delivered)
b) Cut-in Jockey Pump : Cut-out Pressure of Jockey pump – 10 psi
c) Cut-in Pump #1 : Cut-in Jockey Pump – 10 psi
d) Cut-in Pump #2 : Cut-in Pump #1 – 10 psi
e) Etc.

+ it is also sometimes advisable, if water requirements call for more than one pumping unit to operate, that the units should start at intervals of 5 to 10 seconds.

This sequential starting is absolutely necessary, otherwise in case of a sudden sharp drop in pressure, all the switches would trip at the same time.
Where this rule is not adhered to, pump houses have been destroyed when the surge of tons of water breaks the headers or elbows in the underground piping.

2/12/2021 02:58:04 am

Another additional comment to emphasize the excellent explanations by Dan.
The flow requirement from the system and the available flow from the water supply are not the only parameters.
The required pressure and available pressure are also key factors, whether it is a deluge system of a sprinkler system.

If your demand is 4000 gpm (need to be hydraulically calculated to add additional flow from friction losses from one nozzle to another) at 65 psi, and your 8000 gpm water supply can only deliver 4000 gpm @ 45 psi (you can have this value on the flow curve from your water supply - Good guess with Static pressure, residual pressure @ flow and use of a Q1.85 graph for city water supplies)), then you have a problem.
Your 8000 gpm water supply will not be able to deliver 4000 gpm inside your system because of the too low pressure available.
In other words, the friction losses in your system will prevent 4000 gpm to run into your pipes...

And the demand point of a system is not obtained by multiplying the density by the area of coverage (this will give the theoretical minimum flow, not a demand for the system), but by calculating the demand flow AND pressure from all the sprinkler/nozzles operating at the same time. And if you need 7 psi for the most remote nozzle/sprinkler, you may need 8 psi fore the next one... and so on, because of friction losses to bring water from one nozzle to the next one through the piping.

Does Deluge System Use Supply or Demand Calc?

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