Flow rate confusion on fire hydrant landing valves

[Asisraja D]

Hi all
I have a doubt on flow rates for fire hydrant landing valves. we have selected 100 mm X 80 mm Oblique type single head landing valves for our plant and we get 63mm orifice diameter but as per vendor specification it shows testing flow rate around 900 liters per minute at 7 bar pressure but for my understanding how can we get that much flow rate at 63mm nozzle and it is possible only if we consider 4.8 m/s velocity for the hydrant flow but i am bit confusing here at what will be the use of pressure here at 7 bar when we achieve this much flow rate ? or this pressure may be conserved at the discharge side in terms of 900 lpm ? can anyone please explain me in this.

 

[katmar]

The pressure at the hydrant (sometimes called residual pressure) is required to generate sufficient velocity in the monitor or nozzle that will be used to fight the fire. The velocity from a fire fighting nozzle will typically be 25 to 30 m/s to allow the sprayed water to reach a reasonable distance. The energy to accelerate the water to these velocities comes from the residual pressure.

 

[LittleInch]

The flow rate and velocity is high, but not unreasonable for a hydrant. The 14 bar / 7 bar thing might be 14 bar working pressure at the hydrant and 7 bar at the end of the hose. (though without knowning how long the hose is that's not easy to state.

 

[Asisraja D]

katmar sir​

if so can i consider this flow rate at discharge side for designing my fire hydrant system ? for each nozzles of 63mm is 900 LPM at 7 bar.

 

[Asisraja D]

LittleInch sir​

How long it could be is 15 meters so my point is at the discharge nozzle flow rate. The outlet of the single headed landing valve not the hose nozzle. so can i consider flow rate of 900 lpm for 63mm nozzle ?

 

[katmar]

Without a sketch of what is confusing you, I am also a bit confused. There are two separate aspects to this calculation. The first is the system consisting of the fire water pump, the piping to the landing valve and the landing valve itself. It might well be that the piping is a ring main or a network and there are several valves that could be opened simultaneously, but to keep it simple and to illustrate the principles I will assume that you have a single pump feeding a single valve through a direct pipe.

You need to size the pump and piping so that at the desired flowrate of 900 l/min the pressure available at the outlet of the valve would be 7 kgf/cm2. If the line from the pump to the valve is 50 m of 4" Sch 40 pipe, the valve has a Cv of 150 and is mounted 5 m above the pump then the overall pressure drop would be just less than 1 kgf/cm2. If you provide a pump with a capacity of 900 l/min at 8 kgf/cm2 you will have 7 kgf/cm2 available for whatever is downstream of the landing valve. Remember that if you have more than 1 landing valve that could be open at the same time the total flowrate will be higher, but each valve will need 7 kgf/cm2 available at its outlet. It will be more complicated but the principle is the same.

The second aspect of the calculation is the equipment connected downstream of the landing valve. It is standard practice in the fire industry to specify the 7 kgf/cm2 that you have reported for the pressure available for this equipment, but in general this is a bit conservative. You don't want to be short of pressure when fighting a fire.

Typical downsteam equipment might consist of a 20 m long fire hose of 65 mm ID with a 25 mm nozzle. The pressure drop across this with a flowrate of 900 l/min would be about 6 kgf/m2, so the provision of 7 kgf/cm2 by the landing valve supplier is reasonable.

If these calculations do not eliminate your confusion you will need to provide us with a sketch and clear details of what is confusing you.

 

[Asisraja D]

katmar sir​

You need to size the pump and piping so that at the desired flowrate of 900 l/min the pressure available at the outlet of the valve would be 7 kgf/cm2.

yes. i will design my pump for the flow rate.

If you provide a pump with a capacity of 900 l/min at 8 kgf/cm2 you will have 7 kgf/cm2 available for whatever is downstream of the landing valve.

I understand this would work if i select the desired pump to get the 900 lpm flow rate at 7 kg/cm2.

i can understand that i will consider for my design for piping to this flow rate because the valve has capacity of giving 900 lpm at 7 kg/cm2 at downstream landing valve outlet.

Typical downsteam equipment might consist of a 20 m long fire hose of 65 mm ID with a 25 mm nozzle. The pressure drop across this with a flowrate of 900 l/min would be about 6 kgf/m2, so the provision of 7 kgf/cm2 by the landing valve supplier is reasonable.

so the final flow rate i will achieve at the end of 20 meters hose reel from the 25 mm dia nozzle is 900lpm.

i will send you the conceptual drawing. it will be easy understanding.

 

[Asisraja D]

@katmar sir​

pls just have a look at this dwg.

 

[katmar]

Your sketch is exactly what I had in mind, so we are talking of the same problem now.

In line with the principle of conservation of mass, if the pump is delivering 900 LPM and there is only 1 spray nozzle open then there must be 900 LPM flowing out of that valve. The problem of getting balanced flows to each valve when you have multiple valves open is a different problem, but that was not what you were asking (I don't think).

 

[Asisraja D]

katmar sir​

but that was not what you were asking (I don't think).

yes. but anyhow we could not run the plant with single landing valve so if i have to design it for 20 numbers for the same type of nozzles.
(20 Numbers of single headed landing valve for same flow rate of 900 lpm.

then how much capacity of the pump should i need to select ?

 

[katmar]

That depends on how many of the 20 you want to be able to use at the same time. That is why these valves are rated for 14 bar. If you have fewer valves open the pressure will increase (assuming you are using a centrifugal pump). If you decide that the most valves you want open at the same time is 5, then the pump must be rated to give 5 x 900 = 4500 LPM. The pressure specification for the pump will come from an analysis of your piping pressure losses at 4500 LPM. When the flow is less than this then the pressure will be higher than at 4500 LPM and each valve will deliver a bit more then 900 LPM.

 

[Asisraja D]

katmar sir ​

Thanks for your comments and it is really helpful for me. i will come there with new thread after i design our conceptual fire hydrant system.