Page 245 - Valve Selection Handbook

232 Valve Selection Handbook

easier to produce to close tolerances over a wide temperature range than
rupture discs that fail in tension.
The buckling pressure is determined not only by the properties of the
material but also by the shape of the dome. When the disc is exposed to
rising temperature, the strength of the material falls while the dome
expands and gains strength. This gain in strength partially compensates
for loss in material strength due to rising metal temperature. Buckling
discs are, therefore, less sensitive to temperature changes than forward
domed rupture discs.
Because reverse-buckling discs function at low stress levels, there is
no permanent deformation until the disc starts to buckle. This buckling
process proceeds exceedingly fast. By itself, the disc does not burst open
on reversal. This is achieved either by a cutting device against which the
disc must be slammed, or by scoring the disc, or by expelling the disc
from the holder.
Outstanding advantages of reverse-buckling discs are low burst pres-
sure capabilities; operating ratio up to 90% and higher; on request, zero
manufacturing range and reduced burst tolerance, excellent for cyclic and
pulsating pressures; extended service life due to being less affected by
fatigue than forward-domed rupture discs.
The following shows a cross section of numerous variants of reverse-
buckling discs that have been developed.

Reverse buckling disc with knife blades. Figure 6-14 shows a reverse
buckling disc in combination with knife blades that are designed to cut
the disc open upon reversal. For this to happen, the disc must strike the
knife blades with high energy. The disc may therefore be used in gas ser-
vice only and in liquid service if there exists a substantial gas volume
between the liquid and the disc. In totally full-liquid systems, reversal
speed will be too slow to cut the disc. In this case, the disc comes initial-
ly to rest on the knife blades before being cut open after a substantial
pressure rise. In the past, this situation has led to a number of recorded
pressure vessel ruptures.
It is essential that the edges of the knife blades are kept sharp. The cut-
ting edges must therefore be checked on a regular basis and must be
resharpened if necessary. Care must be taken not to change the blade
location or configuration. In most cases, the manufacturer should per-
form repair or replacement.
The advantages of this type of disc are that it can be designed for low
burst pressures, it does not require vacuum support, it is excellent for