Fire Hydrants

[rod stanyon]

What is the minimum water flowrate for a fire hydrant

[John Webb]

The last edition of BS750 I saw some years ago required hydrants to be capable of 2000 litres/minute at 1.7bar, but I think this was a test of the hydrant's performance rather than the minimum expected when installed. A hydrant on a large main in a town may be able to give much more; one in a rural area at the end of a 100mm main may not be able to supply more than a few hundred litres/min at best. That is why FRS often check hydrants for both working properly and the amount of water they can deliver.

On industrial sites insurers may require that certain flowrates are achieved depending on the use of the site. For high-risk sites the checks may need to be made with several hydrants in use at once.
Hope this is of help.

[fireftrm]

Joh is quite correct, there is no laid down minimum flow rate for a hydrant. The hydrant needs to be capable of 2000lpm, but the main does not.

[Mike Buckley]

Another point to be borne in mind is that the Water Authroities are now deliberately reducing the pressure in their mains in an attempt to lower the amount of water they are losing through leaks. However the Companies are obliged to increase the pressure if requested by the FRS in the case of a fire.

The other problem is the provision of water for fire fighting is the responsibility of the FRA liasing with the Water Authorities. So if the FRA is happy with the flow rate from the public main I dont see what the insurance companies can do.

[Chris Houston]

The other problem is the provision of water for fire fighting is the responsibility of the FRA liasing with the Water Authorities. So if the FRA is happy with the flow rate from the public main I dont see what the insurance companies can do.

There is not much an insurance company can do about this other than to assume that there will be low pressure at night.  This one of the reasons why they often need sprinkler systems to have tanks and need non combustible buildings with good compartmentation and good risk management.

[kurnal]

I am sure you are right fireftrm but what therefore is the point of BS750 referring to a running pressure of 1.7 Bar?
The only relevance I can see is a very obscure relationship to the minimum size of orifice thrugh which the water must flow through the relationship with the velocity of the water.

[John Webb]

I think the BS750 figure is to ensure the hydrant's resistance to water flow is not excessive. At the Fire Research Station we had a mixture of sluice and screw-down hydrants around the site - we could always get a bit more out of the sluice hydrants than the screw-down ones although both were coming off the same mains system. I don't have any figures for the flowrates we did measure. Fortunately we never had anything which rated 2000 lpm to put it out!