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Residential Fire Sprinkler Water Supply Options

Residential Fire Sprinkler Water Supply Options

I want to run a sprinkler system direct from the mains supply.

What options do I have?

The following information is drawn from our experience of residential fire sprinkler water supply options gained from operating for a number of years in the South-East . It applies mainly to a consumer with an existing water supply who is considering the possibilities of installing a fire sprinkler system into a refurbished property. It can also apply to a small infill development of one or two properties. It does not necessarily apply to large scale developments for which development departments of the Water companies are involved. Most problems occur when the fire supply is supplied at a local level. It must be appreciated that there is no common strategy shared by individual water companies so this information should be considered as very general and the customer should make their own investigations. We will endeavour to provide as much information as possible draw from years of experience.

 

  • There is a new 25mm MDPE mains feed entering the property. Can this be used for a sprinkler system?

Extremely unlikely: A standard domestic water supply is around 30 litres per minute delivered through a 25mm connection. At a flow rate of 30 litres per minute, the hydraulic losses are minimal assuming that the main water supply is situated relatively close to the property boundary and the length of feed within the boundary are not excessive. A fire sprinkler system however, is designed to deliver more than 3 times this quantity of water.  At these flow rates, hydraulic losses increase exponentially rendering a 25mm feed impractical.

  • There is a new 32mm MDPE mains feed entering the property. Can this be used for a sprinkler system?

If the supply is to be used for a combined domestic and fire sprinkler system, theoretically the answer is yes. However, this configuration does pose a problem because water companies are unlikely to provide an unmetered domestic supply. A standard domestic rated water meter will not pass the quantity of water needed for a domestic sprinkler system without introducing excessive hydraulic losses. A connection could be taken from behind the meter but fewer water companies will allow this nowadays. However if, for some reason, there is no meter on the supply it may be possible to combine the fire supply with the domestic water supply as long as a priority demand valve is connected to the domestic supply in order to divert all the available water to the sprinkler system during an activation. Thames Water will now only provide a combined 50mm supply with high-flow meter which may be expensive, it does reduce the risk of an unsuitable dynamic flow reading

  • If our water company insists on metering our new 32mm supply and we want a combined domestic/sprinkler feed, could it be a meter that will allow high flow rates to pass through it enabling the same supply to be shared between the domestic and sprinkler system?

Water companies are reluctant to allow this for two reasons. Firstly, meters which register high flow rates are expensive and secondly and more importantly, they do not register low flow rates related to domestic use so the water companies loose revenue. There are non-invasive meters (magnetic resonance) available but at the time of writing, they have not been passed by Weight & Measures for the purpose of metering for charging and are also expensive.

  • The Standard for Domestic Fire Sprinklers BS9251 states that the supply should be un-metered. Is this the case?

Although all water companies once agreed in principle to a protocol agreement formulated by Water UK some years ago agreeing to provide un-metered fire sprinkler supplies, most Water companies insist on metering fire supplies to guard against fraudulent use. They will not charge for the water however.

  • There is a dedicated 32mm sprinkler supply with 3.5 bar pressure in the main and the fire sprinkler system requirement is only 2 bar, could the sprinkler run direct from the main supply?

All Water Companies will undertake to provide a 32mm fire supply to a property (except Thames Water) when required. There is a problem however:

A  Water company may provide a 32 mm ferrule but then they may only drill a reduced tapping into the main supply This will not affect a normal domestic supply (30 litres per minute) but this will introduce a large dynamic pressure loss when delivering a large quantity of water to a fire sprinkler system. The result is that a great deal of the static pressure in the main supply is wasted by trying to force the water through this restriction. This will render the fire supply inadequate unless there is an extremely high pressure in the mains supply.

It should also be pointed out that even if there is a 32mm connection with a full tapping, then large hydraulic losses will be introduced by inappropriate valves (28mm brass stop cocks for example)/too may elbows etc which may have been used by the water company before the supply has reached the property boundary itself. This will also affect the dynamic pressure available in the system. Generally, ground work contractors working on behalf of any Water utilities company in the South-East will be unaware of the special requirements of fire supplies and will normally use the wrong type.

 

  • We have a new 32mm supply. There is 2.2 bar pressure in the mains but the fire sprinkler pressure requirement is 2.4 bar. Can we use a small pump on the main to increase the pressure?

Pumps can be utilised directly on a new mains water supply subject to Water Board consent however, the pump will only increase the pressure. It will not increase the flow rate.

 

Sometimes we recommend the use of a storage based system but if it is not possible to accommodate this type of system and a direct connection is required, you should:

  • Notify the water authority that an unmetered 50mm (minimum in most cases) supply is required for a fire sprinkler system with a ‘full tapping’ into the main supply and keep a copy of the application.
  • If there is a long distance from the boundary to the property provide a 50mm feed and associated fittings will reduce friction losses.
  • Inform them of the approximate pressure and flow requirements of the system making sure you stress that these are dynamic not static requirements.
  • Ensure that the Water company does NOT fit a standard 32mm brass stop tap.
  • Ensure that the Water company does NOT install an ‘Adplas’ box and unscrew the associated meter!
  • Be on site when they make the connection – make sure they have provided what you have specified before they back fill.
  • Find out if the Water Company will be installing a check valve adjacent to the stop-cock. If so, we recommend both fittings should be 50mm.

 

These notes are intended purely as a guide only for Residential Fire Sprinkler Supply Options.

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