BS 750:2006 pdf download – Specification for underground fire hydrants and surface box frames and covers
5 Screw-down (type 2)
5.1 Classification Screw-down hydrants of type 2 shall conform to one of the following patterns.
a) Type 2a: The bonnets and/or outlets of this pattern shall be secured in such a manner that they cannot be removed when installed in a pipeline. With the exception of stem sealings, it shall be made clear in the manufacturer’s literature that the hydrant cannot be maintained whilst in situ.
b) Type 2b: Designed to facilitate maintenance of obturator, stem, outlet and their seals, in situ, whilst mounted in a chamber having a surface box frame and cover conforming to Section 3.
They shall have:
i) Y and Z flanges DN 50, PN 16 or PN 25, compatible with the mating dimensions given in BS EN 1092-2 and BS EN 1092-3 as appropriate, and using bolts, studs or set screws specified. They need not be circular and are not required to have raised faces. NOTE Y or Z flanges may have a groove in the face to accommodate an “O” ring. The bolt holes may be slotted and the slots may extend to the periphery of the flange.
ii) A bolted connection at the body/bonnet joint with a minimum of three bolts.
c) Type 2c: Designed to facilitate maintenance as type 2b. They shall have bolted connections between both the outlet and body and the bonnet and body. These connections shall each have a minimum of three bolts. d) Type 2d: Designed to facilitate maintenance as type 2b, but having means of connection of the outlet and/or bonnet to the hydrant body other than type 2c (see 6.3, 6.4, and 5.5). Type 2b, type 2c and type 2d hydrants shall be capable of being dismantled in situ using standard, readily available tools.
5.2.1 Hydrants of the screw-down type shall conform to the dimensions shown in Figure 2. 5.2.2 The clearance between the back of flange X (where used) and the body shall be not less than 17 mm at the position of any bolt hole.
5.2.3 The mating dimensions of flange X (where used) shown in Figure 2 shall be in accordance with BS EN 1092-2, DN 80 and PN 16 or PN 25. NOTE It need not be circular or full form and may be provided with additional holes, which may be slotted, to suit other flange designs.
5.3.1 All copper alloy materials used in the construction of hydrants shall conform to BS EN 1982, BS EN 12163, BS EN 12164, BS EN 12165, BS EN 12167 or BS EN 12168.
5.3.2 The threaded part of the valve which engages with the stem shall be a copper alloy.
5.3.3 Stems of screw-down type hydrants shall be manufactured from stainless steel conforming to BS EN 10088-1 with a minimum chromium content of 13% or copper alloy (see 5.3.1).
5.3.4 Stainless steel outlets shall be manufactured from grades conforming to BS EN 10088-1, with a minimum chromium content of 13%.”
5.4.1 Stem sealings shall be of one of the following two types:
a) toroidal sealing ring (“O” ring) type;
b) other pressure actuated types of seal.
5.4.2 Stem seals of screw-down hydrants shall be capable of being replaced with the valve closed and under operating pressure.
5.4.3 Where stem sealing is of the toroidal sealing ring (“O” ring) type, then two such seals shall be used.
5.4.4 A wiper ring shall be positioned above the seals to prevent the ingress of foreign matter.
5.5.1 Bonnets connected by means other than that described for type 2b and type 2c shall be secured by two independent and functionally different means. These different means shall each be capable of preventing the bonnet becoming unintentionally detached in the absence of the other. Components involved in the attachment/retention of hydrant bonnets shall not be made from plastics.
5.5.2 Bonnets connected by means other than that described for type 2b and type 2c shall conform to the type requirements in Clause 11 with:
a) both security systems in place;
b) only one of the security systems in place; c) only the other security system in place.