Water tanks for sprinkler systems
Automatic fire sprinkler, wet riser and mist system tanks are installed for two main reasons; property protection or life safety.
In both cases their efficiency in controlling and extinguishing fires has for over one hundred and twenty years protected lives and the environment and it should be accepted that all sprinkler systems will protect lives by restricting fire spread and protection escape routes.
Property protection systems are often installed at the request of the building occupier’s insurer to protect the business by ensuring the building and contents are protected against fire.
Some systems are installed to comply with building or fire regulations primarily to protect employees, the public and fire fighters from the risks of fire. This type of system should be more accurately referred to as ‘enhanced availability’ sprinkler systems but are more commonly called ‘life safety’ sprinkler systems and include a range of added features which reduce the possibility that the system will ever be non-operational due to scheduled service and maintenance being carried out.
In both of these cases an essential part of the system is the water supply. This can take the form of a direct supply from the local water service main with or without a booster pump or, more reliably, having water stored in a tank with a pump or pumps to deliver the water to the sprinkler system.
The most common arrangement is a single water storage tank with two fire pumps, each capable of meeting the needs of the sprinkler system.
For enhanced reliability, the water storage tank can be split into two half-capacity tanks. This ensures that there is always a water supply available to the sprinkler system, even when one tank (or any of its equipment) is being serviced or maintained.
Standard of construction for fire fighting tanks
It is essential that the water storage tank is of robust construction and is designed and constructed to need little maintenance or servicing. Balmoral fire fighting water storage tanks are designed in accordance with best practice and technical information relating to corrosion protection and design considerations. They are installed such that the need for emptying for maintenance is reduced to a period of not less than 10 years.
Where sprinkler systems are designed to the LPC Rules for Automatic Sprinkler Installations incorporating BS EN 12845:2009 the most commonly used water storage tanks have LPCB approval. LPCB listed tanks must comply with the rigorous manufacturing and test standard LPS 1276.
Type ‘A’ tanks with a 15-year maintenance free period can still be provided for contracts designed to the now obsolete BS 5306 Part 2:1990.
A list of LPCB-approved tanks can be found in the LPCB ‘Red Book’ and on their website. These tanks come in sizes from 5m3-1300m3 depending on the hazard category of the sprinkler system. If a larger capacity is required a combination of approved tanks can be used.
The relevant sections of the LPC Rules for Automatic Sprinkler Installations incorporating BS EN 12845:2009 that apply to water storage tanks for sprinklers are:
- TB 203 - Care and maintenance of automatic sprinkler systems
- TB 204 - Sprinkler system grading
- TB 209 - ESFR sprinkler protection
- TB 218 - Water supply diagrams
- TB 221 - Sprinkler in schools
- TB 224 - Water storage tanks (cisterns)
- TB 229 - Variations to BS EN 12845 rules
- TB 233 - Water supplies for life safety systems
Sprinkler tanks are also listed by the US insurers’ certification body UL for use on specified sites. For contracts designed to NFPA 13 (US National Fire Protection Association Codes) either an FM Approvals or LPCB listed tank can be used subject to insurers’ approval.
Balmoral fire fighting water storage tanks
This is the most common type of tank used for sprinkler systems. Construction is by galvanised steel or aluminium sheets that are bolted together to form a cylinder. A mastic seal is applied between each metal sheet or a butyl rubber or EDPM liner is fitted. These tanks are usually installed close to the protected premises on a prepared concrete base or occasionally inside the protected premises.
These tanks are constructed using square steel panels that are bolted together to form a cube shape to suit the space available. They can be installed inside or outside the premises.
These tanks are formed by bolting together square GRP panels to form a cube to suit the space available. They can be installed inside or outside the premises.
Can be any of the above options installed at elevated heights in a tall building, on a hill or on a separate high level platform. Depending on the tank’s elevation, the ‘static’ pressure or ‘head’ can be sufficient to supply the needs of the sprinkler system.
Positive head or suction lift
Section 10.6 of BS EN 12845:2009 requires the following (see also LPC Rules TB 210:2008). Wherever possible, horizontal centrifugal pumps shall be used, installed with a positive suction head in accordance with the following:
- At least two thirds of the effective capacity of the suction tank shall be above the level of the pump centre line
- The pump centre line shall be no more than 2m above the low water level of the suction tank (level X in 9.3.5)
If this is not feasible, the pump may be installed under suction lift conditions or vertical turbine pumps may be used.
Note: Suction lift and submersible pump arrangements should be avoided and only used when it is not practicable to arrange positive suction head.
Corrosion protection of steel tanks
The ‘weight’ of galvanised coating on steel panels and sheets used in the construction of superior quality sprinkler tanks can be up to 610g/m2 on both sides of tank wall.
An alternative corrosion protection system which has been specifically approved and listed by the LPCB can be used. In the LPCB listings of approved equipment, these ranks will have either a ‘g’ or ‘e’ respectively in their approval number, ie, 123c/06g or 123c/02e.
Balmoral Tanks provides LPCB-approved fire fighting water tanks up to 1300m3 in capacity. Greater capacity tanks are available.
All LPCB listed tanks must bear a plate stating:
- The name and address of the manufacturer
- The date of installation
- The LPCB Reference Number including the type suffix (‘Superior’ or ‘Single’) which has been specified by the purchaser and agreed by the manufacturer
- The maximum capacity in cubic metres
- Multi-supply tanks must indicate the capacity for the automatic sprinkler system separately
The preferred source of water to supply the sprinkler tank is from the service mains as it is clean and free from extraneous matter that could cause clogging of the sprinkler pipes.
How much water is needed?
This depends on the hazard classification of the sprinkler system and the sprinkler rules used. Usually the greater the hazard the larger the volume of water required to control or extinguish the fire. The duration of water application also increases with the hazard.
What affects the amount of water required?
- The sprinkler design standard being used
- Hazard classification and design density
- Type of system, wet or dry pipe
- Height of the highest sprinkler above the lowest sprinkler
- Duration of water supply
- Whether the tank is a Full holding or Reduced capacity
- The capability of mains supply to fill the tank in an acceptable time (36 hours for BS EN 12845)
Full holding or reduced capacity
A tank is classified as ‘full holding capacity’ when all water necessary for the sprinkler system is stored on site. For example an Ordinary Hazard 15m (OH3 15m) wet-type sprinkler system would have a 135m3 tank.
A ‘reduced capacity’ tank for the same hazard could be as small as 30m3 provided the inflow from the service mains could make up the shortfall of 105m3 in 60 minutes.
What duration of water should be provided?
- BS EN 12845 Light Hazard - 30 minutes
- BS EN 12845 Ordinary Hazard - 60 minutes
- BS EN 12845 High Hazard - 90 minutes
- BS 9251 Residential - 30 minutes
- BS 9251 Domestic - 10 minutes
- Schools (TB221) OH1 - 30 minutes
- Schools (TB221) OH2+3 - 60 minutes
- NFPA - Varies; 30-240 minutes
Tank support base
The support base for the sprinkler tank will vary depending on the type of tank installed. The most common types are circular or rectangular concrete bases, low level concrete walls or steel beams. When the tank is ordered, the tank manufacturer will issue detailed drawings of the type of base required for their particular tank. These plans must be strictly adhered to and should not be modified without written permission from the manufacturer.
Generally speaking, cylindrical tanks should be installed on a new concrete base. Where existing slabs are used, they must be free from any contaminants that may affect the liner.
The external surface of the tank can be painted to choice by the client but the tank manufacturer should be consulted to ensure that any coating used does not damage the tank wall and does not invalidate the LPCB listing and any manufacturers’ warranty.
Whenever possible the sprinkler tank should be located within 5m of the sprinkler pump house.
Careful consideration must be given to the location of the sprinkler tank. Areas to be avoided are:
- Near trees where roots could damage the tank base
- In areas where water can accumulate near the tank base and accelerate corrosion or in time undermine the tank base
- Near pipes or underground services where the weight of the tank may damage them. For every metre of tank height the weight of water on the ground will be one tonne per square metre
- Near hot or corrosive exhaust extracts/pipes where the tank’s protective coating could be damaged
During the planning stage drawings and details of the location of external sprinkler tanks should be submitted to the insurer for their approval and comment before installation take place.
FM Approvals is an international leader in third-party testing and certification services. The organisation tests property loss prevention products and services - for use in commercial and industrial facilities - to verify they meet rigorous loss prevention standards of quality, technical integrity and performance.
- LPCB Red Book: List of Approved Fire and Security Products and Services
- LPS1276 Issue 1.1: Requirements for the LPCB Certification and listing of above ground suction tanks for sprinkler system
- WIS4-25-01: Water Industry Specification for the use of steel tanks in the water industry
- BS4211: Ladders for Permanent Access
- BS5493: Protective coating of iron and steel structure against corrosion
- BS8007: Code of practice for the design of concrete structures for retaining aqueous liquids
- BS EN ISO 12944: Paints and varnishes – Corrosion protection of steel structures by protective paint systems
- LPC Rules for Automatic Sprinkler Installations: Incorporating BS EN 12845 fixed fire fighting system – Automatic Sprinkler systems – Design, installation and maintenance
- BS9251: Sprinkler system for residential and domestic occupancies: Code of practice
- LPCB Scheme Document SD037: Suction tanks for fire pumps for automatic sprinkler pumps
- FM Approvals Data sheets
- National Fire Protection Association Standards
Tanks sheets must have a minimum thickness of 2.5mm with bolted joints designed in accordance with AWWA-D103 formulae. Tanks shall be of bolted construction to withstand liquid load of 1000kg/m³ and a minimum wind load of 862 N/m² (45m/s) which may increase if local conditions require it.
Seismic loading will be determined on a site by site basis and call on one of the five zones: 50-year, 100-year, 250-year, 500-year and >500-year.
The roof must be able to withstand a live load of 0.75kN/m² and a wind speed of 45m/s. Local conditions may dictate higher loads. No part of the roof structure can be below the water level.
Shell manway to be a minimum Ø600mm and placed in the first full ring from ground level.
Screened roof vent shall have at least 1.5 times the cross-sectional area of the suction or infill line, whichever is larger. The screen shall have 10mm openings and be manufactured in non-corrosive metal.
Infill line capable of filling tank in less than eight hours. Suction line to penetrate tank shell and terminate with downward pointing 90° elbow and 6mm anti-vortex plate at least twice the diameter of the suction line. The anti-vortex plate must be situated 150mm, or half the suction line, above the foundation.
Test return line to discharge in different quadrant to the suction line.
Exterior ladder to begin 2.4m above the foundation, with a gated section to enable access.
Shell plates must have a:
- Galvanised coating with a minimum thickness of 305g/m² per exposed surface and an internal Butyl or EPDM rubber liner, or
- Galvanised coating with a minimum thickness of 610g/m² per exposed surface, or
- Galvanised coating with a minimum thickness of 305g/m² per exposed surface, with the internal surface coated with two layers of bituminous coating having a total thickness of between 0.2mm and 0.5mm.
Tank heating and insulation
Should it be required, heating must be of such capacity so that the temperature of the coldest water in the tank shall be maintained above 5.6°C during the coldest weather.
External insulation can be provided by a metal jacket to protect it from the weather.
Internal insulation is permitted provided it is fitted in accordance with FM Approvals guidelines and conforms to the following characteristics:
- Have minimum density of 30kg/m³
- Be flame retardant with a minimum Euroclass E fire classification
- Have minimum compressive strength of 1% nominal compression at 100kPa
Balmoral Tanks gratefully acknowledges BAFSA for its kind assistance with the text used on this page