BS 746:2014 Specification for gas meter unions and adaptors

BS 746:2014 pdf free.Specification for gas meter unions and adaptors.
4 Materials
4.1 Metal fittings
Metal fittings shall be made from one ot the tollowing materials:
a) brass for hot pressing, grade CW6O2N or CW617N to BS EN 12165 or BS EN 12420;
b) brass for casting, grade CC49OK, CC491K, CC750S, CC7545 or CC7605 to BS EN 1982;
c) brass rod, grade CW6O2N, CW614N or CW617N to BS EN 12164, BS EN 12167 or BS EN 12168;
d) brass sheet and strip, grade CW5O8L to BS EN 1652;
e) copper, grade CROO4A to 85 EN 1976;
f) malleable cast iron to BS EN 1562;
g) carbon steel of any grade, with a content not exceeding 0.01% for lead, 0.06% for sulfur and 0.06% for phosphorous and which is suitable for welding, and forming or machining, as appropriate, to PD 970; or
h) austenitic stainless steel to BS EN 10088-3:2005, grades 1.4404 and 1.4541.
4.2 Elastomeric washers
4.2.1 General
Elastomeric washers shall be tested in accordance with BS EN 549. When tested in accordance with BS EN 549, the material from which elastomeric washers are made shall:
a) be of a hardness 91±3 IRHD 2);
b) otherwise conform to BS EN 549, grade H3. A2.
4.2.2 Leak rate at low pressure
4.2.2.1 Leak rate
Each size of washer (see Table 8 and Table 9) shall be tested separately in accordance with 4.2.2.2. When tested in accordance with 4.2.2.2. the total leak rate of the assembly shall not exceed 0.2 1/h and the release torque of any union nut shall be not less than given in Table 1.
4.2.2.2 Test method
4.2.2.2.1 Select 60 washers and divide these into four groups of 15. Erect four separate assemblies, each:
a) comprising 15 washers, nuts, liners and bosses, together with suitable
apparatus that includes means of allowing the internal pressure of each assembly to be changed and measured; and
b) having an internal volume not exceeding 0.035 m3. Calculate the internal volume of each assembly.
4.2.2.2.2 Allow the apparatus, washers, nuts, liners and bosses to stabilize at
(20 ±3) °C for a period of 1 h. Assemble the washers, nuts, liners and bosses and tighten each nut to the applicable torque given in Table 1.

4.2.2.2.3 Subject each assembly to the following environmental conditions, as applicable.
Assembly 1
Leave at room temperature of (20 ±3) °C for a period of not less than 4 h.
Assembly 2
Leave at room temperature of (20 ±3) °C for a period of 1 h, then place the assembly in an environmental chamber at a temperature of (—20 ±3) °C and leave for a period of not less than 4 h. Remove the assembly from the environmental chamber and leave the assembly at room temperature for a period of 2 h.
Assembly 3
Leave at room temperature of (20 ±3) °C for a period of 1 h, then place the assembly in an environmental chamber at a temperature of (60 ±3) °C and leave for a period of not less than 4 h. Remove the assembly from the environmental chamber and leave the assembly at room temperature of (20 ±3) °C for a period of 2 h.
Assembly 4
Leave at room temperature of (20 ±3) °C for a period of 1 h then place the assembly in an environmental chamber at a temperature of (—20 ±3) °C and leave for a period of not less than 4 h. Increase the temperature within the chamber to (60 ±3) °C, and once a temperature of (60 ±3) 0C is reached maintain the temperature for a period of not less than 4 h. Remove the assembly from the environmental chamber; leave the assembly at room temperature of (20 ±3) °C for a period of 2 h.
NOTE If any of the groups are split into subgroups, due for example to the environmental chamber being unable to accommodate 15 samples, it is necessary to be able to show that the test conditions were the same for each subgroup.
4.2.2.2.4 After each assembly has undergone the environmental conditioning specified in 4.2.2.2.3, set the pressure in the assembly to 75 mbar. Isolate the pressure supply, then observe a 1 mm temperature stabilization. Take the pressure (P1) within the assembly and observe a test period of 2 mm. Following this, take a second pressure reading (P2). Calculate the rate of leakage during the test period using the total volume of the assembly under test and the two pressures P1 and P2. The gauge used shall be a digital pressure gauge with a resolution of 2 dcp.
NOTE 1 When undertaking the gas tightness test care has to be taken not to affect the gas tightness of the assembly, e.g. avoid tightening against it or otherwise clamping the joint.
4.2.3 One-off test for complete assembly with new washers
4.2.3.1 Each size of washer (see Table 8 and Table 9) shall be subjected separately, as part of a complete assembly, to the gas tightness test in 4.2.3.2. When tested in accordance with 4.2.3.2 • no air bubbles shall be released from the assembly.
4.2.3.2 Select five new washers of each size and assemble the washers, nuts, liners and bosses. Tighten each nut to the applicable torque given in Table 1. Pressurize the assembly to 350 mbar and seal off the ends. Immerse the assembly in a water bath which is at ambient temperature and with a depth not exceeding 300 mm. Observe for a period of 30 s after any external trapped air has been dispersed.
5 Dimensions
5.1 General
Fittings shall conform to the appropriate dimensions specified in Table 3 to Table 10, as applicable.
NOTE Reference numbers have been added to Table 4 and Table 6 to Table 10 to assist the ordering and purchase of fittings.
5.2 Undercuts
The thread undercuts shall be Grade 1, Form A, Width 2p in accordance with BS 1936-1, except that undercut radius shall be <0.5 mm.
6 Machined surfaces
The machined surface of a fitting shall have:
a) a concentricity tolerance on all threads and diameters ot U 0.4 mm; and b) a surface roughness of not more than 1.6 pm on the sealing faces.
7 Screw threads
Screw threads shall conform to the dimensions given in Table 11 and Table 12, and shall be of basic Whitworth form as illustrated in Figure 1.
8 Strength of nuts and female adaptor ends
8.1 Nuts and adaptors shall be tested in accordance with 8.2. When tested in accordance with 8.2, nuts and the female ends of adaptors shall not fracture.
8.2 Screw a nut fitted with a liner or adaptor, but without the washer, onto a suitable boss component and tighten to the appropriate torque given in Table 2.
9 Finish
Metal fittings shall be clean and free from scale.
Metal components, excluding meter bosses, conforming to 4.10 and 4.lg) shall be protected from corrosion in accordance with a) and b). as applicable. The protective finish shall be such as to not affect the dimensional conformity of threads in Table 11 and Table 12 or the efficacy of sealing surfaces (as specified in Clause 10).
a) Where a hot dipped zinc corrosion protection coating is applied to malleable cast iron conforming to 4.11), this shall conform to BS EN 10242 or BS 7371-6.
b) Where an electroplated zinc and chromate passivated corrosion protection coating is applied to carbon steel conforming to 4.lg), the coating designation shall conform to BS EN 12329 and, as a minimum, be suitable for service condition 3, as defined in BS EN ISO 27830, or as agreed with the purchaser. The chromate conversion coating shall be trivalent, rather than hexavalent, and conform to the corrosion resistance test requirements for the corresponding chromate conversion code in BS EN 12329:2000, Table 2.
NOTE Requirements for the corrosion protection of meter bosses are given in
gas meter standards, e.g. 85 EN 1359 and 85 EN 14236.BS 746 pdf download.BS 746:2014 Specification for gas meter unions and adaptors