CMS Doc ID
SOP-1061
SOP Title
Standard Operating Procedure for RV sizing until
vendor data sheet approval
Date Issued
25/07/2022
Directorate
UED
Sub Team
UEFP
Security
Restricted
Page No
9
Version
02
Document Authorisation
Document Owner (CFDH) / Dept.
Head
Document Custodian
Document Author
Niyaz, Salman UEF
Hadhrami, Siham UEFP
Busaidi, Mustafa UEFP25
Date : 23-06-2021 7:23 AM
Date : 22-06-2021 4:34 PM
Date : 21-06-2021 12:27 PM
Printed copies are UNCONTROLLED
Number of People involved___5____
 Target Cycle Time_9 weeks_
Special
Requirements
Key Point Images
Procedure Overview Instructions and Explanations
Understanding the
1. Identify all credible 1.1 Identify credible reliving
Project scope from
scenarios and arrive
scenarios based on system
Process Design Basis at governing
boundary/process units, see
(PDB).
scenario.
the attached guide which
shows credible scenario
Understand Process By: Process
identification (attachmentSystem /boundary
1).
Engineer, TA-2
limits using Process Process Engineer
1.2 Provide required Design
Flow Scheme (PFS) /
data. Refer to Checklist for
Process Engineering Via: Standard
Process Calculation – PSV
Flow Scheme
sizing (attachment-2).
Calculation Sheet For
(PEFS).
1.3 Enter the required input for
Relief Valve Sizing
all RV sizing scenarios.
Use UniSim
Inputs cells are highlighted
Time: 2-3 days
Simulation model to
in yellow.
take the required fluid
1.4 Evaluate relieving load
properties.
resulted from each scenario
reflected in the summary
Obtain inlet/outlet
sheet
piping lengths
1.5 Identify required relief area
estimate from piping
for credible scenarios.
engineer.
1.6 Select the scenario which
gives highest orifice area as
governing scenario for
PEFS and PSFS.
1.7 Perform isenthalpic (JT)
flash from credible high inlet
pressure and lowest inlet
temperature to flare / vent
system superimposed back
pressure, which is normally
atmospheric pressure. Note,
the inlet pressure and
temperature condition do
not need to be a relief
condition, instead it may be
low ambient temperature
Input to RV data
sheet template
(Provided by
Mechanical Piping
Engineer).
2. Process inputs to
Mechanical RV data
sheet.
By: Process
Engineer, TA-2
Process Engineer
Via: Mechanical RV
data sheet
Time: 1-2 days
and trip pressure in the RV
leakage scenario. Value
from this step is used as
minimum
outlet
temperature.
1.8 Perform isentropic flash
from credible high inlet
pressure and lowest inlet
temperature combination to
the choked pressure at the
RV nozzle. Relief valve
body design temperature is
lowest of temperature
calculated from 1.7 and 1.8
1.9 RV inlet line sizing to be
performed along with RV
sizing. The pressure loss
between equipment with an
RV and the inlet of an RV
during a relief case must be
less than 3% of the RV set
pressure (in gauge units)
value based on the rated
flow to prevent “chattering”.
Refer to SOP-1062
1.10 Calculation to be checked
and approved by competent
persons as per checking and
approval schedules PR1768.
1.11 After checker and approver
comments are incorporated,
Calculation sheet to be
signed off.
2.1 Obtain latest template for
RV
datasheet
from
Mechanical.
2.2 Process Engineer to fill all
highlighted cells in yellow
(Process inputs) based on
All cases (including fire
case) from the calculation
sheet.
2.3 For gas / liquid and three
phase relief scenarios,
provide the liquid viscosity
and mass flux, but no other
properties are required.
2.4 The
template
from
Mechanical Piping may have
a tab for composition data. It
is not required to fill this tab
for any scenarios.
2.5 The superimposed backpressure to be determined
for the different relief valve
sizing scenarios and the
different facility relieving
scenarios. Refer to Annex
for examples of scenarios
when superimposed backpressure may be >0.
2.6 Enter the incremental backpressure caused by flow
from this relief valve in the
built-up back-pressure box.
2.7 Total back pressure is a
calculated
value
in
datasheet. Total back
pressure is calculated by
adding
built-up
and
superimposed
back
pressure.
Piping Class
3. Completed inputs 3.1
Selection report to
to Mechanical Data /
reflected selected
Preparation of the
material based on
MR report /
3.2
the Material Selection Procurement Process
Report
/ Review vendor
offers / documents.
By:
Mechanical piping
Engineer, TA-2
Mechanical piping
Engineer,
Procurement
Engineer
3.3
Via: Mechanical RV
data sheet
3.4
3.5
Time: 6-8 weeks
3.6
3.7
3.8
Inputs to be provided on the
RV data sheet by
Mechanical Engineer.
Mechanical
Engineer
prepares
Material
Requisition for the RV, RV
data sheet is included as
attachment.
MR to be sent for IDC / IFR
Process to confirm sizing
scenarios and provide
mark-up of datasheet
containing new scenarios if
required by HAZOP, IPF or
process
design
development.
Procurement team to share
AFT MR to approved
vendors
Vendor offers will be
received.
Disciplines to review
submitted offer by vendors
and start TQ cycle.
Process Engineer to
ensure that all the process
inputs given in data sheets
are correctly selected by
the vendor and also check
the calculation done by
vendor is acceptable and
agrees with company
calculation.
Differences
need to be raised in TQ.
Checklist to be developed
in future for guidance.
CDTP is provided by
vendor in their datasheet,
Process Engineer is to
ensure that CTDP = set
pressure - superimposed
back pressure (Refer
Annexure
for
CDTP
description).
3.9 Obtain the inlet / outlet
piping equivalent lengths
from piping isometrics and
repeat inlet/outlet pressure
drop calculations (See
Section 1.9). Inlet piping
sizing to be performed
based on vendor rated
flow.
3.10 Vendor to list all deviations
from
datasheet
and
technical standards in the
deviation list and the list to
be reviewed by disciplines.
3.11 Once TQ stage completed,
TBE will be prepared by
mechanical discipline and
issued.
3.12 Confirm all vendor data that
may affect relief valve
sizing; control valve sizes,
pump and compressor
curves, vessel sizes,
Class-1 check valve sizes.
Revise RV calculations.
3.13 For relief valves where
Class-1 check valves limit
the relieving, flow confirm
check valve size from
piping.
3.14 Approved TBE to be signed
by
discipline
Lead
Engineers.
3.15 Online bidding to be
performed
for
the
acceptable vendors.
3.16 PO will be placed for the
selected vendor.
3.17 Piping will prepare AFP MR
and send to selected
vendor.
3.18 Vendor to provide GA,
datasheet and sizing
calculation.
3.19 GA / datasheet will be
reviewed by Piping and
sizing
calculation
/
datasheet will be reviewed
by Process.
3.20 Process engineer to review
vendor data sheet / GA
drawings against the
vendor sizing calculations
Approved Vendor
documents - Code A
and make sure all
information is included and
is consistent.
3.21 Comments to be provided
in SPO and shared to
vendor
until
vendor
documents have no further
comments and declared
code A.
4. Update Process / 4.1 Piping engineer to provide
piping documents
mark-up of the PEFS
based on the agreed
showing the final RV size /
vendor data.
flange rating / insulating
gaskets and material.
By: Process
4.2 Final RV size to be updated in
Engineer, TA-2
the PEFS, SM and Flare and
Process Engineer
Vent Report. HOLDs on
these documents to be
Piping engineer, TA-2
removed.
On certain
Piping Engineer
projects PSFS may also
show RV sizes.
4.3 RV inlet line size to be reTime: 3-5 days
checked based final vendor
rated flowrate in case any
change in the process data
between TQ stage (AFT MR
for RV) and approved vendor
documents after PO issue).
4.4 Piping to reflect vendor
details in the 3D model
 Pre-Checks
Key Point Images
Things to Do Before Starting the Process
☐ Approved Process Design Basis
☐ Approved final Simulation model
☐ Process Flow Scheme (PFS)
☐ Process Engineering Flow Scheme (PEFS)
☐ Inlet piping length from piping engineer
 Tools and Materials
Step
☐ Validated approved RV calculation sheet
☐ UNISIM Software
1.1-1.6
1.3
Things You Need Before Starting the Process
Key Point Images
People
Who Is Required to Be Notified
☐ Process Engineer
☐ Process Engineer (TA-2)
☐ Piping Engineer
☐ Piping Engineer (TA-2)
☐ Procurement Engineer
 Key Learnings; Performance Points 
Key Point Images
What Have We Learned That We Should Share
Abbreviation:
AFP
Approved for Purchase
AFT
Approve for Tendering
IDC
Inter-Discipline Checking
IFR
Issued for Review
MR
Material Requisition
MW
Molecular Weight
PSV
Pressure Safety Valve
Key Point Images
RV
SM
SOP
TA
TBE
TQ
Relief Valve
Safeguarding Memorandum
Standard Operating Procedure
Technical Authority
Technical Bid Evaluation
Technical Queries
References and links:




Relief Valves – Selection, Sizing and Specification DEP 80.36.00.30-Gen
Sizing, selection and installation of pressure-relieving devices in refineries Part I – Sizing and Selection API
Standard 520
API Standard 521 Pressure-relieving and Depressuring Systems.
API Standard 526 Flanged Steel Pressure – Relief Valves



DEP 80.45.10.10 Design of Pressure Relief , Flare and Vent Systems
DEP 80.45.10.11 Overpressure and Underpressure – Prevention and Protection
FEED-QM-FOM-144, Rev.01 Checklist for Process Calculation – RV sizing.
WHAT IS CDTP?
Relief valves are tested every 2 years to ensure that they open at the correct pressure. This test is performed in the
workshop with no back-pressure on the outlet side of the relief valve and at ambient temperature.
The test conditions differ from the installed conditions of the relief valve due to different back-pressure imposed by
the flare / vent and different temperature of the process fluid.
Cold Differential Test Pressure (CDTP) is the differential pressure that a relief valve must be shown to open at under
test conditions to ensure that it opens correctly at relieving conditions when installed. CDTP value may be different
to set-pressure. There are two reasons why CDTP is different to set-pressure, these are pressure correction and
temperature correction.
Pressure Correction
The pressure correction is normally straightforward. The back-pressure in the disposal system caused by other
events when the relief valve is about to open is determined. This back-pressure is caused by other relief valves,
EDP or PCV to flare that may be flowing at the same time or simply due to the purge flowrate. The CDTP is lowered
to account for this. Pressure correction only applies to conventional spring relief valves.
Temperature Correction
This is less straightforward and can only be determined by the vendor. Temperature correction may apply to all
types of relief valve. Note, temperature correction is not required for fire case relief valves.
CDTP = (Set-pressure - superimposed back-pressure) * temp correction
CDTP = Set-pressure x temperature correction
Conventional Spring Types
Pilot and Balanced Bellows
Examples
1. Steam relief valve with fluid temperature at the relief valve inlet of 300 deg C relieving to atmosphere through
short tailpipe. In this case, there is no superimposed back-pressure (0 kPag). Therefore, back-pressure at
opening condition and test conditions are the same. However, the relieving temperature is much higher than
ambient temperature. This reduces the strength of the spring and therefore, the temperature correction requires
the CDTP to be higher than set-pressure. Vendor will determine the CDTP, CDTP cannot be checked.
2. High pressure vessel operating at 70 deg C relieving to low pressure vessel operating at 200 kPag. The
superimposed back-pressure on the installed relief valve is higher than atmospheric pressure during the test.
Therefore, the CDTP must be lower than set-pressure to ensure that the relief valve opens at the correct
pressure when the process pressure is superimposed on the relief valve as back-pressure. However, the
relieving temperature is close to ambient temperature, so no temperature correction is required. Vendor will
determine the CDTP, but it can be checked by FO Process Engineers.
Considerations
If CDTP is lower than set-pressure, it may be necessary to lower pressure trip settings to ensure that the trip always
happens at a lower pressure than the relief valve opens, or increase the design pressure / set-pressure to ensure
sufficient margin above trip pressure. As this may affect project schedule, increase pressure rating of the piping and
increase project cost, it may be simpler and more cost effective to specify a balanced bellows relief valve.
WHAT IS SUPERIMPOSED BACK-PRESSURE?
Superimposed back-pressure is the back-pressure imposed on a relief valve by other events in the relief valve
discharge system. Examples:



Simultaneous relief and depressurization. An example could be partial relief from a station inlet occurring at
the same time plant depressurization.
Flare gas recovery where the flare system is kept at slightly elevated pressure by a control valve to provide
driving force to route flare gas to a compressor.
Where the relief valve discharge is routed back to a process system operating at lower pressure. For
example, routing a pump discharge relief valve back to the suction vessel. The suction vessel operating
pressure acts as superimposed back-pressure on the relief valve.
Why is Superimposed Back-Pressure Important?
Any back-pressure on a relief valve will reduce the relief valve capacity and needs to be accounted for when sizing
the relief valve.
For conventional spring type relief valve, superimposed back-pressure acts on the reverse side of the disc. This
superimposed back-pressure tries to prevent the relief valve opening. This is shown in Figure 8 from API-520 Part 1
(2020). Bursting discs are affected in the same way.
Balanced bellows relief valves are not affected by superimposed back-pressure. pilot operated relief valves may be
affected, it depends on the exact design of the pilot.
Considerations
Superimposed back-pressure values will be used by the supplier to calculate the CDTP for the relief valve. If the
superimposed back-pressure is not present in operational setting, the relief valve will open early because the CDTP
is less than the set-pressure. Therefore if the superimposed back-pressure is variable, the potential for opening early
when the superimposed back-pressure is less than maximum must be evaluated.
WHAT IS BUILT-UP BACK-PRESSURE?
Built-up back-pressure is the increase in back-pressure caused by flow from the specific relief valve being
considered. Built-up back-pressure is only present when the relief valve is open.
Why is Built-up Back-Pressure Important?
Built-up back-pressure on a relief valve will reduce the relief valve capacity and needs to