Page 233 - Valve Selection Handbook
P. 233
220 Valve Selection Handbook
The graphite commonly used for rupture discs is made from low ash
petroleum cokes, calcined at high temperatures. It is then mixed with
pitch, formed into blocks, and then heat treated. The result is porous,
brittle material that requires sealing for use in rupture discs. This is com-
monly done by impregnating the graphite under vacuum with either phe-
nolic or furane resins.
Less frequently used is pure graphite. This is exfoliated graphite, origi-
nally in powder form. When suitably compressed, the graphite forms into
an impervious flexible sheet. Restricted to some instances, however, pure
graphite will absorb some liquid. This problem can be overcome by
applying a suitable coating to the process side only. The maximum per-
missible operating temperature of the disc is limited in this case by the
temperature resistance of the coating.
Temperature and Burst Pressure Relationships
Temperature influences the strength of the disc materials so that there
is a relationship between temperature and burst pressure. The relation-
ship varies between rupture discs of identical material but different con-
struction.
The temperature and burst pressure relationships shown in Figure 6-1
apply to solid-metal forward-domed rupture discs as made by one manu-
Figure 6-1. Temperature/Burst Pressure Relationship of Solid-Metal Forward-Domed
Rupture Discs Made of a Variety of Materials. (Courtesy of Continental Disc
Corporation.)