Page 231 - Valve Selection Handbook

218 Valve Selection Handbook

4. Rupture disc downstream of the pressure relief valve:
These discs are used to prevent corrosive fluids in the vent sys-
tem entering and corroding the valve.
5. Rupture discs upstream and downstream of the pressure relief
valve:
These discs are used to combine the advantages of upstream and
downstream installation of rupture discs.
Limitations of Rupture Discs in Liquid Systems

When ductile rupture discs burst in gas service, the expanding gas
forces the disc open in milliseconds.
When used in liquid service, ductile rupture discs will burst in this
manner only if there is a large enough gas pocket between the liquid and
the rupture disc. If rapid full opening in liquid service is required, the
minimum gas volume to be maintained upstream of the rupture disc is
commonly recommended to be equivalent to at least 10 diameters of pipe
to which the rupture disc is connected.
If the system is totally full of liquid and excess pressure is due to ther-
mal expansion, the pressure will initially only deform the rupture disc.
The resultant volume increase of the pressure system may be sufficient to
initially prevent any further pressure rise. If the system pressure contin-
ues to rise, forward-acting rupture discs become finally so highly
stressed that they fail at their rated burst pressure.
In the case of reverse-buckling discs, however, only a limited number
of types are capable of bursting in liquid-full systems at the rated pres-
sure. When planning to employ reverse buckling discs in liquid full sys-
tems, the manufacturer should be consulted on the selection.
Graphite rupture discs, being brittle, give instantaneously full opening
upon bursting, irrespective of the type of service.
Independent of the type of rupture disc, the maintenance of a gas
pocket in liquid service is advantageous for other reasons. The gas pock-
et minimizes the pressure rise due to volume change of the liquid and
dampens peak impulse loads in pulsating service, resulting in a reduction
in the frequency of disc failure.
Construction Materials of Rupture Discs

Table 6-1, Ductile Construction Materials, shows a range of ductile
construction materials used for rupture discs.