#### 6.3.4 Water header wall thickness

For the purpose of finding the wall thickness of a hemispherical head heater, the water head shape is assumed to approximate to half a sphere and the formula from BS5500: 'Minimum thickness for pressure loading only for spherical shells', Clause 3.5.1.2 [11] is used.

Using the same notation as on Fig 3.49, the wall thickness (e) is given by pD_{1}/(f - 1.2p) where D_{1} = internal diameter.

In practice, the calculated wall thickness is increased by the manufacturer as appropriate to the method of construction, i.e., to allow for tolerances on forgings or the geometry needed to produce a sound casting.