Items covered in this Annexure are :
1 Relief devices on process side 2 Relief devices on limpet/jacket 3 Relief devices on hold tanks 4 Types of R D & sizing 5 Types of SRV & sizing
A) Relief devices on process side of reactors:
As per factories act under rule 65 relief devices are to be provided on all equipments Which are operating under pressure above atmospheric pressure .GCL has applied stricter criterion by which all vessels having a valve on vent line even if vessel is not opreating under pressure are considred as pressure vessels & are to be provided with relief devices . Type and number of relief devices depend on operating conditions . 1. Reactions under section 73 H + Pressure + Runaway possible :
Section 73 H of factories act refers to special safety precautions for certain highly hazardous chemical process as listed under :.
a. Nitro or Amino processes b. Halogenation reactions
c. Aromatization & Isomerisation process .
For the processes covered under section 73 H factory act specifies that R.D is to be provided in addition to usual spring loaded SRV.
Runaway reaction : In a runaway reaction heat generated due to exothermicity is not removed at the rate of generation resulting in increase in temp. & further acceleration of reaction.The exotherm may be in main intended reaction or side reaction .Whether a reaction is runaway or not is determined by lab. either based on expt. /reference to literature or by generating data from outside lab. such as CISRA.
It is proposed to install on separate nozzles i) R.D ( R D 1 )& SRV in series
ii) Two R.D in series ( R D 2 & R D 3)
Pressure rating of R D 1 = Max. operating pressure + 10 % of Max. operating pressure + burst tolerance of disk (which is equal to 5% of burst pressure ).
Pressure rating of R D 2 & R D 3 : MWAP of vessel - burst tolerance of disk
Pressure rating of SRV = Max. operating pressure + 10 % of Max. operating pressure ).
Note : 1) RD below SRV should of non fragmenting type.
2) Reactions under 73 H + Pressure + no Runaway : R.D & SRV in series is to be installed .
Pressure rating of R D & SRV : Max. operating pressure + 10 % of Max. operating pressure + burst tolerance of disk (which is equal to 5% of burst pressure ).
3) Reactions under 73 H at atm . pressure + Runaway present or absent: It is proposed to install one safety device provided
a) that vessel is connected to scrubber which is provided with R D or SRV . The scrubber’s SRV or R D is effectively vessel’s relief device during operation.
e.g acid chloride prep.
b) with open vent e.g Cumene Nitration
Note :Rule 65 which refers safety measures for vessels operated under pressure above atmospheric specifies that “ suitable safety valve or other effective device (OED ) “ is to be provided on pressure plant or vessel .We can provide R.D in place of safety valve since R. D qualifies as OED.
In all above cases vapour load is calculated based on combined heat generated due to
i) Exothermicity : Maximum heat liberated is calculated either from lab. data taking into account possible deviations in operating condiions or from studies at CISRA .In absence of data maximum heat liberated is taken = 10 times the average rate .
ii) Heat input to limpet coil .This is calculated based on steam pr., & pressure drop in line.
iii) Heat absorbed by reaction mass due to external fire.
Note :
2. Other reactions :
For reactors carrying out reactions other than those specified under section 73 H and also for reactors used for distillation ,mixing , crystallisation e.t.c it is proposed to install only R.D .
-Burst pressure of disk = MAWP of reactor – burst tolerance – mfg. Tolerance (whereever applicable ) Vapour load is calculated on the basis of combined heat input to reactor due to
i) Heat of reaction :
ii) Heat input to limpet coil .This is calculated based on steam pr., & pressure drop in line.
iii) Heat absorbed by reaction mass due to external fire.
Relief devices on limpet/jacket of reactors Reactors other than GLV:
In case valves on CW lines are closed ,any raise in temp. of process medium will result in increase in CW temp. & hence increase in CW pressure . SRV/TRV is to be installed on CW return line. Since only pressure relief (no specific capacity ) is required ¾ * 1 “ SRV is adequate.
SRV set pressure = MAWP of limpet of reactor .
For GLV in case stem pr. < MAWP of jacket , valve is required to relieve hydraulic pressure only & ¾*1 SRV is adequate .
Pressure rating = MAWP of jacket
In case steam pressure exceeds MAWP of jacket
,SRV is to be designed for flow rate of steam calculated as per line size .SRV is to be provided with lifting lever in case of IBR approved lines .
Relief devices for hold tanks : A. Tanks containing pressurized liquid : Safety devices to be installed are:
1. R D 2. SRV
3. R D @ the outlet of SRV
R D is installed before & after SRV to protect SRV from corrosion , solid particles e.t.c The pressure rating of R D = MAWP of tank less burst pressure margin .
The pressure rating of SRV = MAWP of tank R D is to be of non fragmenting type .
R D & SRV are to be sized based on external fire conditions.
Heat absorbed due to external fire
Q = 21000*( Aw) ^0.82* F Ref: Ludwig Vol. 1 page 260 Where Q is heat absorbed in Btu/hr
Aw is total wetted area in sq. Feet F is the environmental factor .
F= 1 for uninsulated tank
F = 0.3 for tank with 1 “ thk. insulation F = 0.15 for tank with 2 “ thk. insulation F = 0.075 for tank with 4 “ thk. insulation
Venting rate V = Q /Heat of vapourisation
Tank is not be insulated unless required for purpose other than relief device sizing.
B. Tanks operating at atmospheric pressure/under vacuum
R. D to be sized for external fire conditions . Tank is not insulated unless required for purpose other than relief device sizing. R D to be suitable for vacuuum conditions.
C. Relief devices on limpet/jacket :
For limpet/jacket provided with CW/CHW (no steam) 1/2*3/4 “ S RV is to be installed .For limpet/jacket provided with no steam .relief valve is not required if max. Pressure of steam is less than MAWP of limpet/jacket .
If steam pr. is > limpet/jacket ,SRV is to be sized for maximum flow rate possible which is calculated based on line size ,length & number of fittings.
Types of R D :
1. Conventional Prebulged tension loaded solid metal disks .
a. Max. operating of vessel /tank = 70 % of rated pressure of disk for non pulsating service. For pulsating service max.. operating of vessel /tank = 50 % of rated pressure of disk .Disk is installed with dome facing away from process side .
b. Disk can be used for liquid service also.
c. Least expensive of all disks.
d. Drawbacks : i)Disk fragments on bursting & hence cannot be installed below SRV
ii) Vacuum support is required if vacuum exceeds 600 mm Hg. Because of vacuum support if disk is installed upside down it will burst at pressure higher than rated pressure.
2. Prescored tension loaded disks :
a. his disk is a solid metal disk prescored in a specific pattern to weaken the disk.
b. Max.. operating of vessel /tank = 80 to 85 % of rated pressure of disk
c. They avoid disadvantage of conventional tension loaded disk viz.fragmentation & need for vacuuum support. Since there is no vacuum support ,disk with PTFE on process side is available. However they are costlier than conventional solid metal disks.
d. Disk installed incorrectly will burst at rated pressure or lower.
3 . Composite disk :
a. This is prebulged disk & has a slotted top metal section wherein burst pressure is controlled by size &
location of slots & perforations.The disk is isolated from process by PTFE membrane . Since burst pressure is not controlled by thickness , disk is available at lower burst pressure than conventional disks .
b. Max.. operating of vessel /tank = 80 % of rated pressure of disk.
c. Drawbacks : Similar to conventional metal disks viz.need for vacuum support ; fragmentation & bursts at higher pressure if installed upside down.
4. Reverse buckling disks
Installed with dome facing towards process side & hence disk is in compression. The bursting action of disks is due to either knife blades on downstream or prescoring on disks.
Advantage over tension loaded disks :
b. Disk does not fragment & hence can be installed below SRV .
a. Vacuum support is not required .Disk is available with PTFE seal facing process & hence can be used for corrosive applications.
b. Disk can be used up to 90 % of operating pressure.
c. Manufacturing range is zero.
d. Disadvantage :Cost is higher than conventional tension loaded disks.Except RLS other types are not suitable for liquid service .
5. Non metallic disks :
Rupture disks in graphite are available which are useful in highly corrosive services .The drawbacks of graphite disks are :
a. Cannot be installed below SRV because of fragmentation ; b. lower shelf life due to resin bonding .
Effect of temperature :
The burst pressure of disk is function of temperature .The temp. :pressure relation for RD is different than that for parent metal .In order to minimise effect of temp. it is recommended to install R D at least one metre away from vessel nozzle so that disk faces much lower temp. than the vessel. In this case operating temp. of disk can be specified as 50 ° C.
Sizing of R D :
For vapour or gas service area of R D is calculated from formula : W = C*K*A*P*(M/T)^0.5
Ref: Loss prevention in process industries by F Lees Page 12/67 Where W is venting rate in Kg/hr
A is area of disk in sq. mm P is absolute pressure in bars
M is molecular weight of gas or vapour T is temperature in ° K
K is Coeff. of discharge = 0.6 C is constant = 2.7
From A , diameter of disk is calculated .