pressure relief devices_presenation
TRANSCRIPT
M+W Group-India
Pressure Relief Devices-Basics & Sample Calculation14th 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 DiskPin 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, thenVol = (pi*x*(d^2+d*(d+2*x/TANalpha)+(d+2*x/TANalpha)^2))/13Multiplied 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
2520
−+
+
kk
kk
� 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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