6.3.1 To find tube thickness
All modern CEGB HP feedheaters use 19.05 mm (0.75 in) outside diameter low carbon manganese steel tubes. Tube thickness is found from application of Clauses 220.127.116.11 and 18.104.22.168 of BS1113 .
From Clause 22.214.171.124 but using the same notation as BS5500 , Clause 126.96.36.199:
e = minimum thickness of straight tube, mm
p = calculation pressure, N/mm2
f = allowable design stress, N/mm2
D = outside diameter of tube, mm
For a typical 660 MW unit
p = 217.2 bar or 21.72 N/mm2
D = 19.05 mm outside diameter
The heater tubes are manufactured to conform to ESI Standard 23-4 82  (which supersedes GDCD Standard 56-80 ).
The tube material conforms to the Type 400' which appears in the allowable design stress Table 2.3 (k) in BS5500 , giving 96 N/mm2 at 300°C and 88 N/mm2 at 350°C for this material.
If the temperatures used to determine the allowable stress in the tubes are taken as that experienced by the ultimate heater, then the worst case is considered.
Steam temperature (assumed) - 370°C
Outlet feed temperature - 253°C
Average temperature - 312°C
By interpolation f at 312°C - 97.8 N/mm2
Then tube wall thickness:
t = (21.72 x 19.05)/[(2 x 97.8) + 21.72] = 1.90 mm
From BS1113 Clause 3.7.3, the thickness (eb) allowing for bends is given by eb = 1.125 e, so eb = 1.90 x 1.125 = 2.14 mm.
Using the nearest standard tube thickness of 203 mm, tube bore = 19.05 - 4.06 = 14.99 mm, say 15 mm. Having determined the tube thickness and hence the bore, the area for flow per tube is known and so the number of tubes required for a specified HP feed flow can now be calculated.