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PRESSURE SAFETY VALVES &
EARLY SIZING
AMOL DUDHATE
INSTRUMENTATION & CONTROLS
15 July 2016

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SAFETY

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AGENDA IMPORTANT TERMINOLOGIES
TYPES & PRINCIPLE OF PSV
EARLY SIZING FOR PSV
PSV DOCUMENTATION
INTER-DISCIPLINE INTERFACE
ALERTS & LESSONS LEARNED

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NEED OF SAFETY VALVES

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SIZING BASIS
• External fire
• Thermal expansion
• Blocked outlet
• CV failure
• Abnormal process conditions
• Utility Failure

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IMPORTANT TERMINOLOGIES
• Safety Valve : It is device actuated by the static pressure upstream of the
valve and characterized by rapid opening or pop action. (normally used with
compressible fluids)
• Relief Valve : A pressure relief device designed to open and relieve excess
pressure and to reclose and prevent the further flow of fluid after normal
conditions have been restored. (normally used with non-compressible fluids)
• Backpressure : The pressure that exists at the outlet of a pressure relief
device as a result of the pressure in the discharge system.
Total Backpressure = Superimposed + Built-up

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IMPORTANT TERMINOLOGIES
• Superimposed Back Pressure : The
static pressure that exists at the outlet
of a pressure relief device at the time
the device is required to operate.
• Superimposed backpressure is the
result of pressure in the discharge
system coming from other sources.
(constant or variable)
• Built Up Back Pressure : The
increase in pressure at the outlet of a
pressure relief device that develops as
a result of flow after the pressure relief
device opens.

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IMPORTANT TERMINOLOGIES
• Set Pressure : The inlet gauge
pressure at which the pressure
relief device is set to open under
service conditions.
• Closing Pressure : The value of
decreasing inlet static pressure at
which the valve disc reestablishes
contact with the seat or at which lift
becomes zero as determined by
seeing, feeling or hearing.
• Blowdown : The difference
between the set pressure and the
closing pressure of a pressure
relief valve, expressed as a
percentage of the set pressure or
in pressure units.

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IMPORTANT TERMINOLOGIES
• Accumulation : The pressure increase over the maximum allowable
working pressure of the vessel, expressed in pressure units or as a
percentage of maximum allowable working pressure (MAWP) or design
pressure.
• Overpressure : The pressure increase over the set pressure of the
relieving device. Overpressure is expressed in pressure units or as a
percentage of set pressure.
• Overpressure is the same as Accumulation only when the relieving device
is set to open at the maximum allowable working pressure of the vessel.
• Coefficient of Discharge : The coefficient of discharge is used for
calculating flow through a pressure relief device.
• C.D.T.P. : The pressure at which a pressure safety valve is adjusted to
open on the test stand. The cold differential test pressure includes
corrections for the service conditions of backpressure or temperature or
both.

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APPLICABLE STANDARDS
• API STD 520, Sizing, Selection, and Installation of Pressure-
relieving Devices in Refineries, Part II—Installation
• API STD 521 / ISO 23251, Guide for Pressure-relieving and
Depressuring Systems
• API STD 526, Flanged Steel Pressure Relief Valves
• API STD 527, Seat Tightness of Pressure Relief Valves
• ASME Boiler and Pressure Vessel Code, Section VIII—Pressure
Vessels, Division 1

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Types of
Safety
Valves
Conventional
Valves
Balanced
Bellow
Valves
Pilot
operated
Valves

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• Self-actuated spring-loaded PSV
• Basic elements include :
 Inlet Nozzle
o Full Nozzle
o Semi Nozzle
 Bonnet
o Open
o Closed
 Spring
 Adjusting Screw
 Adjusting Ring
 Disc
CONVENTIONAL PSV

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CONVENTIONAL PSV

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CONVENTIONAL PSV
• Advantages
 Most reliable type if properly sized Simple Design
• Disadvantages
 Relieving pressure affected by back pressure
• Selection Criteria
 The superimposed backpressure is not variable (otherwise the pressure at which the
valve will open will vary)
 Built-up backpressure should not exceed 10 % of the set pressure at 10 % allowable
overpressure.

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BALANCED BELLOW PSV
• A balanced PSV is a spring-loaded PSV
which incorporates a bellows of balancing
the valve disc to minimize the effects of
backpressure on the performance
characteristics of the valve.
• For conventional safety valve, total
backpressure should not exceed 10% of the
set pressure at 10% allowable pressure.
However, it is possible to get valve with
balanced bellows if total back pressure
(superimposed + built-up) is up till 50% of
the set pressure.

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BALANCED BELLOW PSV
• Advantages
 Relieving pressure not affected by back pressure
 Can handle higher built-up back pressure
 Protects spring and guiding surface from corrosion
• Disadvantages
 Bellows susceptible to fatigue/rupture
 Will release flammables/toxics to atmosphere in case of
bellows rupture
 Requires extended venting system for Bonnet vent to
safe location
• Selection Criteria
 Where the total backpressure (superimposed plus built-
up) does not exceed approximately 50 % of the set
pressure

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PILOT OPERATED PSV

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PILOT OPERATED PSV
• Advantages
 Relieving pressure not affected by backpressure
 Can operate at up to 98% of set pressure
 Smaller, lighter valves at higher pressure and/or with larger orifice size
• Disadvantages
 Pilot is susceptible to plugging by fouling fluids, hydrate formation etc.
 Vapor condensation and liquid accumulation above the piston may cause problems
• Selection Criteria
 When back pressure can not be met by Bellows type
 Very low margin between Max operating pressure and Set pressure

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PSV OPERATION - VAPOR / GAS SERVICE

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PSV OPERATION – LIQUID SERVICE

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PRESSURE LEVEL RELATIONSHIPS

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EARLY SIZING FOR PSV
• Early Sizing Involves :
 Selection of possible orifice area to meet required capacity.
 Maximum capacity with selected orifice area.
• Objective of Early Sizing :
 Removal of holds in the P&ID.
 Early release of ‘hold’ on piping lines and piping expanders and reducers.

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PROCEDURE FOR EARLY SIZING
• Early sizing can be done immediately after the release of process data.
• Vendors to be identified for early sizing.
• Identification of requirement of valves :
 With rupture disc.
 Without rupture disc.
 Acceptable Sizes.
• Overpressure considerations :
 10% for vessels equipped with a single pressure relief device
 16% for vessels equipped with multiple pressure relief devices
 21% for fire contingency

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PROCEDURE FOR EARLY SIZING
InstruCalc
Kd(Liq.)=0.62
Kd(Gas)=0.975
Dresser
Kd(Liq.)=0.6696
Kd(Gas)=0.8775
Farris
Kd(Liq.)=0.652
Kd(Gas)=0.858
Tai-
Milano
Obtain From
Graph.
Obtain From
Graph.

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PROCEDURE FOR EARLY SIZING

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PROCESS
DATA AS
PER IDIT
DATASHEET
FOR
INQUIRY
VENDOR
OFFERS
•EARLY SIZING
•VENDOR
SELECTION
FOR SIZING
•VENDOR
SIZIES /
RATING
/CAPACITY
TQ STAGE
TBE
PR
VDR
•GA
•CALCULATION
FROM
VENDOR
•NOISE VALUES
TO HSE
•REACTION
FORCES &
CAPACITY TO
PIPING &
PROCESS
PSV DOCUMENTATION
• PSV DATASHEET
• CALCULATIONS
• PSV COMPARISION SHEET

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P&ID REPRESENTATION & RELATED NOTES
• Isolation valve requirement
– Valve shall be full bore.
– Valve shall have capability of being locked or car-sealed open.
• Venting to safe location
– Tubing MTO for Balanced bellow PSV

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INTER-DISCIPLINE INTERFACE
• PROCESS
 Calculations
 Rated relieving capacity to Process for final relief system design (inlet/outlet pressure
drop, line sizing, flare header sizing)
 Size & rating change mark-ups
• PIPING
 Reaction force value for piping considerations in Reaction Forces/ Stress Analysis/ Pipe
support design
 GA drawings: Dimensional details and weight
 Acoustic insulation requirement for Noise limitation
• HSE
 Noise Calculations

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ALERTS & LESSONS LEARNED
• Selection of MOC :
 Usually a conventional pressure relief valve is specified as ‘full nozzle’. For a full nozzle relief
valve, the inlet fluid is in contact with the nozzle and the nozzle internals (disc, disc holder
etc.) and these are always specified in line with the inlet piping material specification / vessel
trim. This makes the inlet section (i.e. nozzle, disc, disc holder etc.) compatible with the
process fluid.
 It may be noted that sometimes the downstream piping may be of CRA or may have
corrosion allowance. The selection of relief valve body and bonnet material in line with
downstream piping material will automatically satisfy the corrosion criteria.
 In case the FEED / P&ID specification break calls for full nozzle relief valves with the body
material as per the inlet piping spec. or vessel trim, the same may be reviewed with Process /
Client to establish why it cannot be as per the downstream piping specification.

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ALERTS & LESSONS LEARNED
• Safety Valve Bellow Failure :
 FREQUENT CHATTERING
 EXCESSIVE BACKPRESSURE
 LACK OF MAINTENANCE
• If Bellow type PSV is installed in
hazardous area, then the vent should be
routed to a safe location by tubing or
piping.

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API AREA VS ASME AREA (VENDOR SPECIFIC)
ORIFICE
DESIGNATION
AREA as per
API (sq.in)
AREA as per FARRIS
(2600) (sq.in)
AREA as per DRESSER
(1900) (sq.in)
D 0.110 0.150 0.1279
E 0.196 0.225 0.2279
F 0.307 0.371 0.3568
G 0.503 0.559 0.5849
H 0.785 0.873 0.9127
J 1.287 1.430 1.4960
K 1.838 2.042 2.1380
L 2.853 3.170 3.3170
M 3.60 4.000 4.1860
N 4.34 4.822 5.0470
P 6.38 7.087 7.4170
Q 11.05 12.27 12.8500
R 16.0 17.78 18.6000
T 26.0 28.94 30.2100

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FLANGE RATINGS AS PER API 526 VS ASME B 16.34

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MODEL SNAPS

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STRESS DUE TO PSV REACTION FORCES
PEC-EN-VSS-L-10746 : PSV Reaction Force Calculation

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QMS DOCUMENTS
• PEC-EN-PRO-J-14000 : Early sizing of Pressure Relief Valves by
InstruCalc
• PEC-EN-GDE-J-09021 : Relieving Devices Sizing & Selection

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THANK YOU