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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