The document summarizes the basics of pressure relief devices, including why they are required, common components, classification and types. It provides examples of relief scenarios and causes of overpressure. The key steps in relief device sizing calculations are outlined. An example calculation is shown for checking the adequacy of installed relief devices for a reactor system during an emergency relief scenario involving an external fire.

1. M+W Group-India
Pressure Relief Devices-Basics &
Sample Calculation
14th Jan2016

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Relief Device requirement & components.
Classification & Types of relief devices.
Rupture Disc & types of RD’s
Relief Scenarios & cause of over pressure.
Relief Device Calculation Steps.
Relief Device Selection
Agenda

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Safety Moment

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Pressure Relief Devices
Why Relief Devices are Required?
Relief Devices are required for following reasons:
To protect personnel from the dangers of over pressurizing equipment
To minimize chemical losses during pressure upsets
To prevent damage to equipment
To prevent damage to adjoining property
To reduce insurance premiums, and
To comply with governmental regulations
Components of Relief System
Relief Device, and
Associated lines and process equipment to safely handle the material ejected.

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Knock out Drum Cyclone Separator Condenser
Scrubber Flare Incinerator
Relief Discharges- Atmosphere &
Effluent System

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Relief Valve
Opens normally in proportion to the pressure increase
Used primarily with incompressible fluids.
Safety Valve
Characterized by rapid opening or pop action
Normally used with compressible fluids
Safety Relief Valve
Spring loaded pressure relief valve that may be used either as safety or relief depending on
the type of application
Set Pressure
Inlet gauge pressure at which the device is set to open
Overpressure
Pressure increase over the set pressure of the device to achieve rated flow
Overpressure = Accumulation, when the Set pressure = MAWP
Terminologies

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Classification of Relief Devices
Pressure
Relief Device
Non-
Reclosing
type
Rupture Disk
Pin Actuated
Type
Re-closing
type
Relief
Valve
Conventional Balanced
Bellows Piston
Pilot
Operated
Pop Action Modulating Diaphragm
Safety
Valve
Safety Relief
Valve
Combination

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Reclosing Type Relief Device
Losing entire contents is unacceptable
Toxic and Hazardous Service
Return to normal operation quickly
Non-Reclosing Type Relief Device
Capital and maintenance saving
Losing the contents is not an issue
Benign service (non-toxic, non-hazardous)
Need for fast acting device
Potential for valve plugging
Combination Type Relief Device
Need a positive seal
Protect safety valve from corrosion
System contains solids
Choice of Relief Device

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Conventional Safety Valve

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Balanced Bellow Spring Loaded SRV

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Pilot Operated Safety Relief Valve

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Pilot Operated Safety Relief Valve
POP ACTION & NON - FLOWING TYPE POP ACTION & FLOWING TYPE

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Pilot Operated Safety Relief Valve
MODULATING & NON-FLOWING TYPE MODULATING & FLOWING TYPE

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Terminologies
Operating pressure
MAWP
Design pressure
Set pressure
Accumulation
Overpressure
Blowdown

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A rupture disc is a thin diaphragm (generally a solid metal disc) designed to rupture
(or burst) at a designated pressure. It is used as a weak element to protect vessels
and piping against excessive pressure (positive or negative).
Reduced fugitive emissions - no simmering or leakage prior to bursting.
Protect against rapid pressure rise.
Less expensive to provide corrosion resistance.
Less tendency to foul or plug.
Types of Rupture Disc
Conventional Tension-Loaded Rupture Disc
Pre-Scored Tension-Loaded Rupture Disc
Rupture Disc

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Types of Rupture Disc
Conventional Tension-Loaded Rupture Disc Pre-Scored Tension-Loaded Rupture Disc

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Relief Scenarios/ Causes of Overpressure
Relief Load Calculations
Relief Valve Sizing
Inlet/ Outlet Line Sizing
Relief Valve Sizing Calculations - Steps

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All the Relief scenarios/ causes are a specific example of
the following variation or multiple variations:
An increase in heat input to a system
A decrease in heat removal from a system
An increase in mass input to a system
A decrease in mass removal from a system
Blocked Outlets
Control Valve Malfunction
Check valve leakage or failure
Utility failure
Electrical/ Power Failure
Cooling Water Failure
Instrument Air Failure
Steam Failure
Inert Gas Failure
Highlighted Scenarios are encountered normally.
Relief Scenarios/ Causes of Overpressure
Loss of Heat
Loss of Instrument air or Electric Instrument
power
Reflux Failure
Abnormal Heat Input
Heat Exchanger Tube Failure
External Fire
Hydraulic Expansion
Process changes/ chemical reactions

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Blocked Outlets
Outlet valve closed
All other valves that are normally open and not affected by primary cause of failure
are open
Consider only the inlet streams having sufficient pressure to open the pressure
relief valve
Capacity to be determined at relieving condition
Control Valve Malfunction
One inlet valve fully open irrespective of its fail safe position
All other valves that are normally open and not affected by primary cause of failure
are open
Capacity to be determined at relieving condition
Vapour blow-by scenario to be checked for liquid level control valves
Scenarios details

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Check valve leakage or failure
All check valves to be considered to fail full open
In case of multiple check valves, one to be considered to fail full open, and the
other(s) shall be considered to leak
Check for available information from vendor OR
Assume 1 CFM/Inch of line ID/ 100 PSI pressure differential
Loss of Instrument air or Electric Instrument power
Valves to attend “Failure” position
For “Fail Last Position” valves, the valves should be assumed to go to a position
which will maximize the relief load
Refer “Blocked Outlet” or “Control Valve Malfunction” scenarios
Scenarios details

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Steam Failure
Steam failure to Turbine drives
Steam failure to Exchangers/ Reboilers
Steam failure to Ejectors
Inert Gas Failure
To Compressor seals
To Catalytic reactors
To Instrument/ equipment purging
Reflux failure
Due to failure of Reflux pump or Closure of valve on reflux line or Loss of duty of
Partial/ Total condenser
Overpressure in Column due to loss of coolant
Calculation of column without reflux is required
Scenarios details

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Abnormal Heat input
Failure of Heat input control device – leading to higher than normal heat input
Clean heat transfer coefficient
Maximum normal temperature of heating medium
Maximum rate of Heater design heat input or burner overdesign
Heat Exchanger Tube failure
The design pressure, of the low pressure side, is less than maximum operating
pressure, of the high pressure side
High pressure fluid is either a vapour or a liquid that will flash on the low pressure
side at relieving conditions
Review chemical reaction, if any
The sudden sharp break of one heat exchanger tube
Flow of high pressure fluid through an opening equal to twice the inside cross
sectional area of a tube
Scenarios details

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External Fire: Effect of Fire on Wetted surface of a vessel
Basis/ assumptions
Flow to/from equipment is stopped
The vessel absorbs heat only through the wetted area walls
All absorbed heat goes into vaporising the contents
No credit is taken for heat removal by condensers or coolers
Equipment wetted surface upto and less than 7.6 m (25 ft) above the source of
flame (exception: spheres)
Fire zone: 2500 to 5000 ft2 ≈ 230 to 460 m2 ≈ 17.2 m to 24.3 m dia circle
Scenarios details

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Set Pressure & Accumulation Limits for PRV’s
Contingency Single Device Installation Multiple Device Installation
Max. Set Pr. % Max. Acc. Pr. % Max. Set Pr. % Max. Acc. Pr. %
Non-fire case
First Relief
device
100 110 100 116
Additional Relief
device(s)
- - 105 116
Fire case
First Relief
device
100 121 100 121
Additional Relief
device(s)
- - 105 121
Supplemental
device
- - 110 121

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Design Procedure for Fire Case
Q = 21,000 x F x A 0.82
Where adequate drainage or firefighting measures do not exist, then the following API
521 equation should be used for calculating Q:
Q = 34,500 x F x A 0.82.
Q = total heat absorption to the wetted surface in BTU/hr (imperial units)
F = environmental factor
A = total wetted surface area in ft2 (imperial units)
F = an environment factor (= 1.0 for bare vessel)
Relief Load Calculations

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Typical Example
Scope:- To Check the Adequacy of the Installed Relief
Device during Emergency Relief with THF fill up & identify
all the events that lead to overpressure for the Reactor
system.
Basis and Assumptions:-
Calculation for reactor will be based on THF.
The Reactor filling is considered upto 80%
For conservative results Design pressure of weakest item in
reactor system is considered as maximum allowable
pressure in system.
Adequate drainage or firefighting measures are exists at
site.
Fire insulation is not considered for reactor.
Safety factor of 20% is considered for calculation.

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External Fire Sizing Basis
Vessel is fill upto 80% fill level, this volume corresponds to a level of 1932mm from
bottom dish fire can impinge on the vessel up to this point.
Calculation

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Head Volume or Volume of
the frustum of a right cone
pi * h * (D^2 + D*d + d^2)/12 (Perry chap-3, p 3-11)
Where h = height, D = large diameter, d = small
diameter
Cone Angle ATAN ( h / (D/2 - d/2)) (Form. Trigonometry)
Surface Area for vessel
(m2) corresponds to 80%
fill level is calculated by
If (Overall Height is <= base depth, then
Vol =
(pi*x*(d^2+d*(d+2*x/TANalpha)+(d+2*x/TANalpha)^2))/
13
Multiplied to Vol =
(pi*x*(3*d^2+6*d*x/TANalpha+4*x^2/TANalpha^2))/12.
So vessel surface area comes out to be 4.5 m2 @ 80% Fill Level.
Calculation

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Heat input due to the external fire is calculated from Q = 21000 F A^0.82.
Area = 4.5 * 10763 = 47.06 ft2.
Q = 494072 BTU/hr or 144 KW.
Control Valve Failure Case:-
The maximum flow of nitrogen through the pressure regulating valve is given by:
Vo =P1 x Cg x 1.018
Assuming critical flow, 1.018 factor applied to convert from air to nitrogen
Vo = Volumetric flow rate of nitrogen (SCFH),P1 = Upstream pressure, Cg = Wide
open gas sizing coefficient. (Refer CRR 136/1998, Workbook for Chemical Reactor
Relief Sizing, HSE.)
Vo = The Relief flow rate for wide open PCV is 73.3 kg/hr . This is quite less than
calculated for fire case and hence relief load calculated based on 'external fire case
supersedes the above case.
Calculation

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DIERS Calculation Methodology for Two Phase flow onset and Disengagement (for
non -foamy Churn Turbulent Fluid/Bubbly flow and Vertical vessels) is used below in
the calculations.
Relieving Pressure:- (2*1.21+1.01325) = 3.433
Liquid/vapour Properties of THF at Relieving Pressure:-
Heat Input Due to fire is Q= 494072 BUT/hr.
Crosssectional Area of vessel A – in ft2
Constant – K ,If the Stability Parameters Kf >0.3 the 1.53 or else 1.18.
Correlating Parameter C0 If the Stability Parameters Kf >0.3 the 1.0 or else 1.01.
Vessel Average Void Fraction :- α (Volume upto tan level-volume at 80% fill
level)/(Volume upto tan level)
Enterainment Check

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Boil off Rate Fr – Q/λ, Heat input/Latent Heat
Superficial Vapor Velocity Jgx
Bubble rise Velocity (ft/sec)
Calculate Dimensionless Superficial vapor
velocity due to flow.
Calculate Dimensionless Superficial vapor
velocity at which two phase vapor-liquid flow
commences.
Design Criteria
ψf >= ψ, Two-phase venting is predicted.
ψf < ψ, All vapor venting is predicted.
ψf > ψ, Two-phase flow is in progress,
complete disengagement is predicted.
DIERS Calculation Methodology for two phase flow
onset & Disengagement
Where, Jgx Superficial vapou velocity in ft/sec.
F is Vapor Flow rate lb/hr
A Vessel Cross Sectional Area ft2
Ρ Vapor Density lb/ft3
Where, Ux Bubble Rise Velocity ft/sec.
S is surface tension dynes/cm
ρg Vapor Density lb/ft3
ρv Liquid Density lb/ft3
Where, α Vessel Average void fraction
VT, Total Vessel Volume
VL Vessel Filled Volume
Co, Coorelating Parameter
REF 09 (I-B7, APPENDIX I-B ,
DIERS MANUAL). For Vertical
Vessels.

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From The DIERS Methodology & Entrainment Check – Single Phase is observed.
Relief flow rate or Boil off Rate = 3034* 0.453 is 1374 Kg/hr
RV set pressure = 2.0 Barg, Max. relief pressure = 3.433 bara
Check for critical / Sub-critical flow through RV using following (Ref 1 - Section 3.6.1.4
Eq 3.1) : .
Pcf/P1 = (2/K+1)^(K/K-1)
Minimum Value of P1 allowing for accumulation is 3.433
Then Pcf 3.433 * 0.57 =1.98 bara This is above atmos. Pressure, so flow regime
through the valve is critical.
Use equation 3.2 from (Ref. API RP 520, Seventh Edition, January 2000) for critical
flow sizing. Area in m2
RV Sizing
Where, K ratio of specific heats .
Pcf is minimum downstream pressure (bara) giving
rise to critical flow
P1 is upstream pressure (bara).
M
TZ
KKPKC
W
cbd 1
13160

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Where C is the flow coefficient, Fig 32 API RP
520, Seventh Edition, January 2000
Required Relief Rate W = 1374 kg/hr
Coefficient of discharge Kd = 0.62 constant
Backpressure correction Kb = 1.0
Combination correction factor Kc = 1.0 for BD
& 0.9 for Relief Valve
Pressure upstream of BD (P1) = 343 Kpa abs
Compressibility factor (Z) = 1.0
Temperature of inlet gas (T) = 382.3
Molecular Weight of Vapour (M) = 72.1
RV Sizing
( ) ( )1/1
1
2
520
−+






+
kk
k
k
Required Area = 593 mm2
Safety Factor 20%
Installed Size = 100 mm

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Relief Valve Selection
Orifice
designation
Orifice area Standard
PSV size
Alternate
PSV sizein² mm²
1 0.062 40.00 3/4 x 1 1 x 1
D 0.110 70.97 1 x 2 1.5 x 2
E 0.196 126.45 1 x 2 1.5 x 2
F 0.307 198.06 1.5 x 2 1.5 x 2.5
G 0.503 324.52 1.5 x 2.5 2 x 3
H 0.785 506.45 1.5 x 3 2 x 3
J 1.287 830.32 2 x 3 3 x 4
K 1.838 1185.80 3 x 4 3 x 6
L 2.853 1840.64 3 x 4 4 x 6
M 3.60 2322.58 4 x 6 -
N 4.34 2799.99 4 x 6 -
P 6.38 4116.12 4 x 6 -
Q 11.05 7129.02 6 x 8 -
R 16.0 10322.6 6 x 8 6 x 10
T 26.0 16774.2 8 x 10 -

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API RP 520, 'Sizing, Selection, and Installation of Pressure-Relieving Devices in
Refineries, Part 1 - Sizing and Selection', Seventh Edition, January 2000.
API RP 521, 'Guide for Pressure-Relieving and Depressuring Systems' Fourth Edition,
March 1997.
PID & GA Drawings
Aspen for Physical Properties
CRR 136/1998, Workbook for Chemical Reactor Relief Sizing, HSE.
DIERs Manual " A perspective on Emergency relief system" by DIER Techincal
Committee.
Guide to Pressure Relief (PSG 8), Part C:Section 5, 1999.
Chemical Engineer's Handbook - Perry, Seventh Edition.
References

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Thank You