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STHE Overpressure Protection
Colin Deddis, Senior Process Engineer, EPT
22 March 2010
STHE Overpressure Protection
STHE Overpressure Protection
• Changes in guidance & practice since 2000
• Response times of relief devices
• Dynamic analysis of STHE overpressure and relief
• Defining the problem with implementation
• Incident examples
• Design & operational issues
• JIP Proposal
EPT
Changes in Guidance – API521/BS EN ISO 23251
STHE Overpressure Protection
• Two-thirds rule replaced with:
− “Loss of containment of the low-pressure side to atmosphere is unlikely to
result from a tube rupture where the pressure in the low-pressure side
(including upstream and downstream systems) during the tube rupture
does not exceed the corrected hydrotest pressure”
− “Pressure relief for tube rupture is not required where the low-pressure
exchanger side (including upstream and downstream systems) does not
exceed the criteria noted above.”
• Dynamic analysis:
EPT
− “This type of analysis is recommended, in addition to the steady-state
approach, where there is a wide difference in design pressure between
the two exchanger sides [e.g. 7 000 kPa (approx. 1 000 psi) or more],
especially where the low-pressure side is liquid-full and the high-pressure
side contains a gas or a fluid that flashes across the rupture. Modelling
has shown that, under these circumstances, transient conditions can
produce overpressure above the test pressure, even when protected by a
pressure-relief device [64], [65], [66]. In these cases, additional protection
measures should be considered.”
Changes in Guidance – API521/BS EN ISO 23251
STHE Overpressure Protection
• Tube rupture design basis:
− “The user may perform a detailed analysis and/or appropriately design
the heat exchanger to determine the design basis other than a full-bore
tube rupture. However, each exchanger type should be evaluated for a
small tube leak.
The detailed analysis should consider
a) tube vibration,
b) tube material,
c) tube wall thickness,
d) tube erosion,
e) brittle fracture potential,
f) fatigue or creep,
g) corrosion or degradation of tubes and tubesheets,
h) tube inspection programme,
i) tube to baffle chafing.”
EPT
Current Practice
STHE Overpressure Protection
• API521/BS EN ISO 23251 allows use of relief valves or bursting disks but
states:
− “The opening time for the device used…..should also be compatible with
the requirements of the system.”
• Opening times of relief valves considered to be too slow, hence bursting
disks commonly used.
• Advances in heat exchanger design practice e.g. vibration analysis,
materials etc. have decreased likelihood of tube rupture
EPT
Response Times of Relief Devices
STHE Overpressure Protection
• Bruce Ewan, University of Sheffield
EPT
Summary of test conditions and test numbers – phase 1
Relief device

Relief
diameter
(in)
Open tube
4mm orifice 8mm orifice 15mm orifice
Relief pressure
(bar)
39
38
37
0
4
51
50
49
10
6
55
54
53
10
Stainless steel disc
4
41
42
40
15
(reversed dome)
8
48
47
46
15
2” Spring loaded pop
action RV
-
59
58
57
15
2” Bellows RV
-
62
61
60
15
2” Pilot operated RV
-
66
65
64
15
Graphite disc
High pressure test
4” graphite disc. Rupture time = 1.9 ms
1. 10
Test 49p 1.9ms Rupture
Pressure (bar)
45
40
35
30
25
20
15
10
5
0
0
0.002
0.004
0.006
0.008
K4 Pressure
0.01
0.012
Time (seconds)
Max = 40.87
Sheffield Model - Re-Calibrated after T est 39p
Min = 0
Low pressure test
2” spring loaded RV. 110% open capacity in 6 ms
6. 8
Test 59p 10% Capacity in 6ms
Pressure (bar)
50
40
30
20
10
0
0
0.002
0.004
0.006
0.008
0.01
0.012
-10
K4 Pressure
Time (seconds)
Max = 27.38
Sheffield Model - T aken from T est 51p
Min = 0
High pressure test
2” spring loaded RV. 110% open capacity in 4 ms
5. 8
Test 57p 10% Capacity in 4ms
Pressure (bar)
120
100
80
60
40
20
0
-20
0
0.002
0.004
0.006
K4 Pressure
0.008
0.01
0.012
Time (seconds)
Max = 100.75
Sheffield Model - T aken from T est 51p
Min = 0
Low pressure test
2” pilot operated RV. 110% open capacity in 4 ms
8. 8
Test 66p 10% Capacity in 4ms
Pressure (bar)
30
25
20
15
10
5
0
-5
0
0.002
0.004
0.006
K4 Pressure
0.008
0.01
0.012
Time (seconds)
Max = 22.11
Sheffield Model - T aken from T est 51p
Min = 0
High pressure test
2” pilot operated RV. 110% open capacity in 2.5 ms
7. 8
Test 64p 10% Capacity in 2.5ms
Pressure (bar)
100
80
60
40
20
0
0
0.002
0.004
0.006
0.008
0.01
0.012
-20
K4 Pressure
Time (seconds)
Standard
Max = 84.07
Sheffield Model - T aken from T est 51p
Min = 0
Summary of test conditions – phase 2
Driver
Relief Device
Size
Pressure
(barg)
4mm
8mm
15mm
Orifice
Orifice
Orifice
Safety Valve
(SRV)
2H3
100
Test no. 2
Test no. 1
Test no. 3
4L6
100
Test no. 23
Test no. 24
Test no. 25
Relief Valve
4 in
100
Test no. 21
Test no. 20
Test no. 19
Stainless Steel
Disc
3 in
100
Test no. 6
Test no. 5
Test no. 4
4 in
100
Test no. 7
Test no. 8
Test no. 9
3 in
20
Test no. 18
Test no. 16
Test no. 15
4 in
20
Test no. 13
Test no. 12
Test no. 11
Graphite Disc
4L6 safety relief valve
4” relief valve
Low pressure test
4L6 safety. 110% open capacity in 10 ms
3. 15
Test Vent 23
Pressure (barg)
35
30
25
20
15
10
5
0
0
0.002
0.004
K4 to Device Pressure
0.006
0.008
0.01
0.012
Time (seconds)
Standard T est
Max = 27.34
Sheffield Shock T ube
Min = .45
High pressure test
4L6 safety. 110% open capacity in 4 ms
1. 15
Test Vent 25
Pressure (barg)
100
80
60
40
20
0
0
0.002
0.004
K4 to Device Pressure
0.006
0.008
0.01
0.012
Time (seconds)
Standard T est
Max = 91.39
Sheffield Shock T ube
Min = .18
SRV, RV and Graphite Disc at High Pressure
Dynamic Analysis of Tube Rupture
STHE Overpressure Protection
• Ian Wyatt, Atkins
EPT
Dynamic Modelling of Tube
Rupture
Ian Wyatt - Atkins
JIP on Bursting Disks for Shell & Tube Exchangers – 1st Stakeholders Meeting
API-521/BS EN ISO 23251 – 5.19
API-521.BS EN ISO 23251 does not dictate what has to be done:
• If a steady-state method is used, the relief-device size should be
based on the gas and/or liquid flow passing through the rupture.
• A one-dimensional dynamic model can be used …
• This type of analysis is recommended, in addition to the steady-state
approach,
• where there is a wide difference in design pressure [e.g. 7 000 kPa …
There is a warning at the bottom:
• Modelling has shown that, under these circumstances, transient
conditions can produce overpressure above the test pressure,
even when protected by a pressure-relief device ...
Different Exchanger Configurations
Gas
Similar Tube Rupture consequences
apply to all of these configurations:
• Single pass gas, single pass liquid
• Multiple pass gas and/or multiple pass
liquid
• HP Gas on tube side or shell side
• Cooling Duty or Heating Duty
• Horizontal or Vertical or Angled
Gas
Liquid
Liquid
Gas
Liquid
Gas
Liquid
Liquid
Gas
Liquid
Gas
Stages to Tube Rupture
For all configurations there are four phases to the consequences of a
Tube Rupture – identified in the tube rupture tests performed as part
of the previous JIP:
Phase I – Percussive Shock
Phase II – Fast Transient
Phase III – Liquid Discharge
Phase IV – Gas Discharge
Phase I – Percussive Shock
•
•
•
•
Rapid rupture creates percussive shock wave
Extremely short lived <0.1ms
Shell does not ‘feel’ the pressure spikes
Not Model
Gas
Liquid
Flare Header
Gas
Liquid
Phase II – Fast Transient
•
•
•
•
•
•
Gas entering shell is faster than time to overcome liquid momentum
Fast transient pressure wave results travelling at sonic velocity
Pressure wave usually breaks bursting disc
Shell and pipework overpressures possible
Simulated using software with necessary fast transient capability
Shell baffle path ‘straightened’ – 1D Model
Gas
Liquid
Flare Header
Gas
Liquid
Phase II – Fast Transient
•
•
•
•
•
•
Gas entering shell is faster than time to overcome liquid momentum
Fast transient pressure wave results travelling at sonic velocity
Pressure wave usually breaks bursting disc
Shell and pipework overpressures possible
Simulated using software with necessary fast transient capability
Shell baffle path ‘straightened’ – 1D Model
Flare Header
Liquid
Pressure
Liquid
Phase III – Liquid Discharge
Gas
Liquid
Flare Header
•
•
•
•
•
•
Gas
Gas bubble grows towards exits
Liquid displaced through available exits
Volume flow balance between bubble and
displaced liquid
Possible to over pressurise Shell and
connected pipework
Gas-Liquid interfaces affect pipe supports
Shell baffle path ‘straightened’ – 1D Model
Gas
Gas
Liquid
Liquid
Flare Header
Gas
Gas
Liquid
Liquid
Flare Header
Liquid
Flare Header
Gas
Gas
Liquid
Liquid
Flare Header
Gas
Gas
Liquid
Liquid
Phase IV – Gas Discharge
•
•
•
Gas from rupture passes out of system
Pseudo steady state depending on gas supply
Usually not modelled
Gas
Liquid
Flare Header
Gas
Liquid
Results
Relief device does not always protect against over pressure
Even some below 2/3rds rule exceed limits – two of them lower
pipework design pressures
800%
700%
600%
%Peak/LP Design Pressure
•
•
500%
400%
300%
200%
100%
0%
0%
100%
200%
300%
400%
500%
600%
700%
800%
%HP/LP Design Pressure
Results
LP Short Term Design Limit
"2/3rds Rule"
Max HP
900%
1000%
STHE Overpressure Protection – the “problem”
STHE Overpressure Protection
• Increased use of bursting disks to protect STHEs over past 10 to 15
years
• Estimated frequency of guillotine tube rupture
− 0.0009 per unit per year (~1 per 1,100 years)[1]
• Frequency of bursting disk failures protecting STHEs
− 7 incidents in 13 years (~50 exchangers)
− 0.011 per unit per year (~1 per 90 years)[2]
• Future growth in numbers of high pressure STHEs requiring
overpressure protection
• Has the balance of risk shifted?
EPT
1.
2.
IP Guidelines for the Design and Sae Operation of Shell & Tube Heat Exchangers to Withstand the Impact of Tube Failure, Aug 2000
Estimate based on incidents known to BP
STHE Overpressure Protection – the “problem”
STHE Overpressure Protection
Two major hazards associated with bursting disk failures:
• Impairment of relief system – liquid inflow & overfill
• Incident escalation - reverse rupture leads to uncontrolled
hydrocarbon release from relief system
EPT
Incident #1 – liquid overfill
Flare
STHE Overpressure Protection
Relief Header
PSHH
Flare Knockout Drum
• Bursting disk rupture in forward direction
• PSHH in void space of bursting disk assembly fails to isolate exchanger
• Sustained cooling medium flow into relief system
• Liquid overfill & potential overpressure of knockout drum
EPT
EPT
80 psig Burst
STHE Overpressure Protection
Incident #2 – excessive backpressure
80 psig Burst
50 psig
100 psig
225 psig
225 psig
Note: The top disc impacted bottom disc causing it to also rupture
STHE Overpressure Protection
Incident #2 ctd.
EPT
STHE Overpressure Protection
Any other incidents……?
EPT
???
Design & Operational Issues
STHE Overpressure Protection
• HSE Safety alert 01/2008 Steve Murray, HSE
EPT
Health and Safety
Executive
Bursting disc failure: flare
system impairment
Stephen Murray
HSE Inspector, Offshore Division
HSE Safety Alert 01/2008
http://www.hse.gov.uk/offshore/alerts/sa_01_08.htm
Alerts:
•
•
•
to advise industry of incidents
enable lessons to be learned
industry takes appropriate action to avoid
similar incidents
HSE Safety Alert 01/2008
SWR
Heat Exch.
gas
SWS
HP Flare
Drum
HSE Safety Alert 01/2008
SWR
PAH
HP Flare
Drum
Heat Exch.
LAH
LP flare
drum
ESDV
gas
SWS
ESD
Closed
drain
ESDV
Overboard
HSE Safety Alert 01/2008
press =
4 barg
(no alarm)
What happened?
disc failure
water
enters
drum
tell-tail blocked?
SWR
PAH no alarm
overfills
HP Flare
Drum
Heat Exch.
liquid @+40m
does not
trip
no level seawater
>LAH
pumps
LAH
LP flare
drum
ESDV
gas
SWS
not tight
shut-off
ESD
fills
Closed
drain
fills
ESDV
Overboard
closed
HSE Safety Alert 01/2008
Summary
•
•
•
uncontrolled flow of seawater into flare system
several hours to identify source
flaring event may have lead to serious gas
release
HSE Safety Alert 01/2008
Lessons
•
Be aware of potential for impairment of
flare/relief system from uncontrolled cooling
medium flow from ruptured bursting disc
•
Ensure disc rupture will initiate measures to
ensure isolation of cooling medium so that
flare/relief system is not compromised
HSE Safety Alert 01/2008
Legal requirements
•
Provision and use of Work Equipment Regs
1998
•
Management of Health & Safety at Work Regs
1999
•
Offshore Installations (Prevention of Fire &
Explosion and ER) Regs 1995
Health and Safety
Executive
Bursting disc failure: flare
system impairment
Stephen Murray
HSE Inspector, OSD
Design & Operational Issues
• Bursting disks utilised for overpressure protection of STHEs
STHE Overpressure Protection
− Once opened, they maintain an open flow path from the
process/utility system to the relief system.
EPT
− A sufficient margin (~30%) must be maintained between
operating and set pressure to avoid rupture. In STHE
applications, they are often located on cooling medium systems
which can be susceptible to pressure surges.
− Failure in the reverse direction due to superimposed
backpressures from the relief system.
Design & Operational Issues
• Bursting disks utilised for overpressure protection of STHEs
STHE Overpressure Protection
− Once opened, they maintain an open flow path from the
process/utility system to the relief system.
EPT
− A sufficient margin (~30%) must be maintained between
operating and set pressure to avoid rupture. In STHE
applications, they are often located on cooling medium systems
which can be susceptible to pressure surges.
− Failure in the reverse direction due to superimposed
backpressures from the relief system.
Design & Operational Issues
• Selection of relief route
STHE Overpressure Protection
− Multiphase – high velocity liquid slugs
− HP or LP flare system (high pressure gas under relief
conditions but large liquid volumes under a failure case)
− Should relief from STHEs be segregated from other relief
routes?
• Is HAZOP effective at identifying potential failure modes and
consequences?
• Additional protective measures required for failure cases.
EPT
Gaps in current guidance
STHE Overpressure Protection
• Broader design requirements associated with bursting disks and interface
with relief systems not addressed
• At what pressure ratio are relief valves acceptable?
− Large differential pressure may actually favour relief valve – extent of
overpressure may yield sufficiently rapid response
− Lower differential pressures – shell & nozzles may survive overpressure.
• What extent and duration of overpressure is acceptable?
EPT
Aims of JIP
STHE Overpressure Protection
• Eliminate or mitigate hazards associated with overpressure
protection of STHEs
• Develop revised set of design guidelines for overpressure protection
of STHEs principally to address:
EPT
− Heat exchanger design.
− Relief device selection.
Heat Exchanger Design (1)
STHE Overpressure Protection
• Determine criteria to assess if guillotine fracture is possible based
on the mechanical properties of the materials of construction used in
heat exchanger tubes.
• Determine any minimum tube thickness specification required to
prevent guillotine fracture.
• Define the vibration analysis requirements that need to be applied to
ensure that the likelihood of guillotine fracture is minimised.
• Define any sensitivity analysis of process variations which should be
carried out to ensure that the design is robust.
EPT
Heat Exchanger Design (2)
STHE Overpressure Protection
• Determine if differential pressure limits can be established below
which transient effects can be ignored.
• Determine the maximum allowable transient overpressures in the
shell under tube rupture conditions to cater for peak pressures. This
will require experimental and analytical work.
• Determine the impact of transient loads on the piping systems if
bursting disks are not applied for overpressure and develop
appropriate design guidelines to ensure that the piping design is
robust but not overly conservative.
EPT
Relief Device Selection
STHE Overpressure Protection
• Develop a rule-set for relief device selection to accommodate the
tube rupture case
EPT
− Scale-up to typical relief device sizes encountered in real
applications.
− Testing of response times of a variety of relief valves to a range
of overpressures .
− Establish mechanical integrity criteria for relief valves for use in
tube rupture service.
− Establish the range of process conditions for which conventional
relief valves could be utilised to protect against tube rupture and
those for which bursting disks are required. This needs to
consider aspects such as differential design pressure between
low and high pressure side of exchanger etc.
Shopping list – issues captured in Stakeholders’
meeting
• Deliverable – software
STHE Overpressure Protection
• Criteria for selecting RDs
• Set points – selection criteria
• Overpressure/overpressure times
• Testing and inspection
• Type/size/etc of RD and response capability, affecting selection
• Design issues including
− Instrumentation
− Seawater system
• Learning from experience – what went wrong: capture findings/lessons
learned
• Construction/operation/maintenance etc of whole system
• HAZOP – pertinent guide words (like RABS guidelines)
• HE stress distribution – revisit/extend Sheff Uni work
EPT