Page 459 - APPLIED PROCESS DESIGN FOR CHEMICAL AND PETROCHEMICAL PLANTS, Volume 1, 3rd Edition

Process Safety and Pressure-Relieving Devices 425

disk and valve have been established by test in Footnotes to ASME Code
accordance with UG-132.
Note that in lieu of testing, Par (b) 2 and (b) 47. The minimum net flow area is the calculated net
3 above allows the use of a capacity factor of 0.80 area after a complete burst of the disk with appro-
as a multiplier on the stamped capacity of the priate allowance for any structural members which
spring loaded safety relief valve (nozzle type). may reduce the net flow through the rupture disk
Some manufacturers test specific valve/rupture device. The net flow area for sizing purposes shall
disk combinations and determine the actual not exceed the nominal pipe size area of the rup-
capacity factor for the combination, and then ture disk device.
use this for the net capacity' determination. See 48. Use of a rupture disk device in combination with a
Figures 7-10, 7-11, 7-12, 7-13A and 7-13B. safety or safety relief valve shall be carefully evalu-
c. A rupture disk device may be installed on the out- ated to ensure that the media being handled and
let side 50 of a spring loaded safety relief valve the valve operational characteristics will result in
which is opened by direct action of the pressure in pop action of the valve coincident with the burst-
ing of the rupture disk.
the vessel provided. (Figure 7-12)
49. Users are warned that a rupture disk will not burst
at its design pressure if back pressure builds up in
l. The valve is so designed that it will not fail to the space between the disk and the safety or safety
open at its proper pressure setting regardless of relief valve which will occur should leakage develop
any back pressure that can accumulate between in the rupture disk due to corrosion or other cause.
the valve disk and the rupture disk. The space 50. This use of a rupture disk device in series with the
between the valve disk and rupture disk shall be safety or safety relief valve is permitted to minimize
vented or drained to prevent accumulation of the loss by leakage through the valve of valuable or
pressure due to a small amount of leakage from of noxious or otherwise hazardous materials and
the valve. 51
where a rupture disk alone or disk located on the
2. The valve is ample in capacity to meet the inlet side of the valve is impracticable, or to prevent
requirements of UG-133 (a) and (b). corrosive gases from a common discharge line
3. The stamped bursting pressure of the rupture from reaching the valve internals.
disk at the coincident disk temperature plus any 51. Users are warned that an ordinary spring loaded
pressure in the outlet piping shall not exceed safety relief valve will not open at its set pressure if
the design pressure of the outlet portion of the back pressure builds up in the space between the
safety or safety relief valve and any pipe or fit- valve and rupture disk. A specially designed valve is
ting between the valve and the rupture disk required, such as a diaphragm valve or a valve
device. However, in no case shall the stamped equipped with a bellows above the disk.
bursting pressure of the rupture disk al the
coincident operating temperature plus any *Reprinted with ASME permission. ASME Pressure Vessel Code, Section
pressure in the outlet piping exceed the maxi- Vlll, Division I, UG-127, 1989 Edition, pp. 8�8.
mum allowable working pressure of the safety or
safety relief valve. Establishing Relieving or Set Pressures
4. The opening provided through the rupture The pressure at which the valve is expected to open
disk device after breakage is sufficient lo permit (set pressure) is usually selected as high as possible con-
a flow equal to the rated capacity of the sistent with the effect of possible high pressure on tl1e
attached safety or safety relief valve without
exceeding the allowable overpressure. process as well as the containing vessel. Some reactions
have a rapid increase in temperature when pressure
5. Any piping beyond the rupture disk cannot be increases, and this may fix the maximum allowable
obstructed by the rupture disk or fragment. process pressure. In other situations the pressure rise
6. The contents of the vessel are clean fluids, free above operating must be kept to some differential, and
from gumming or clogging matter, so that accu- the safety valve must relieve at the peak value. A set pres-
mulation in the space between the valve inlet sure at the maximum value (whether maximum allowable
and the rupture disk ( or in any other outlet working pressure of vessel, or other, but insuring protec-
that may be provided) will not clog the outlet. tion to the weakest part of the system) requires the small-
7. The bonnet of the safety relief valve shall be est valve. Consult manufacturers for set pressure compen-
vented to prevent accumulation of pressure. sation (valve related) for temperatures >200°F.