Steels_ Metallurgy and Applications, Third Edition

Stainless steels 353

Pressure vessel codes

In an earlier chapter, an introduction was given to the British Standard

Specification relating to the construction of bridges (BS 5400) and steelwork

in buildings (BS 5950). In a similar vein, specifications have also been prepared

for the construction of boilers and pressure vessels, e.g.:

1. BS 1113:1992 Design and manufacture of water tube steam generation plant.


2. BS 5500:1997 Unfired fusion welded pressure vessels.

Both specifications have a materials section which defines the steels that can be

used and the associated design stress values. An example is shown in Table 4.13

which has been reproduced from BS 1113. The steels identified in this table are

from BS 3059 (Steel boiler and superheat tubes) and cover a range of compo-

sitions from 0.3% Mo up to 12% Cr-Mo-V. Against each steel grade, a series

of design stress values (f) is provided for various operating temperatures up to

a maximum of 620"C in this particular table. As illustrated shortly, these design

stresses are calculated on the basis of short-term tensile properties at low to

moderate temperatures, but at higher operating temperatures design is based on

creep rupture strength. Where the latter is the case, the values in Table 4.13 are

shown in italics.

Thus specifications such as BS 1113 and BS 5500 provide design engineers

with clearly defined data on the allowable design stress values at different oper-

ating temperatures for the various steel specifications and product forms identified

in the previous section.

Steels for elevated-temperature applications

Prior to dealing with the derivation of design stress values, the following termi-

nology must be defined:

9 Rm min. TS at room temperature

9 Re(T) rain. 0.2% PS (1.0% PS for austenitic steels) at temperature T

9 Sat mean value of stress to cause rupture in time t at temperature T

* fE design strength based on short-term tensile strength properties

9 f F design strength based on creep rupture characteristics

In BS 1113 and BS 5500, the allowable design stresses at elevated temperatures

are calculated in the following manner:

1. C, C-Mn and low-alloy steels
   BS 1113 BS 5500

   
   

   2500C fE = Re(T) or Rm _> 150 ~ fE = Re(T) or Rm
   1.5 2.7 1.5 2.35
   whichever gives the lower value.