Shell & Tube Heat Exchanger (STHE)
Overpressure Protection
from Tube Rupture
Colin Deddis, BP Exploration Operating Co Ltd
19 July 2011
The views expressed in this paper are those of the individual authors / presenters and are not intended to represent the view s or position of BP on the matters
discussed
STHE Overpressure Protection
Acknowledgements
• Ian Wyatt, Atkins Ltd
• Stephen Murray, Health and Safety Executive
• Bruce Ewan, University of Sheffield
• Colin Weil, Consultant
• Mark Scanlon, Energy Institute
The views expressed in this paper are those of the individual authors / presenters and are not intended to represent the view s or position of BP on the matters
discussed
STHE Overpressure Protection
• Tube rupture scenario
• Guidance & practice – changes since 2000
• Dynamic analysis of STHE overpressure and relief
• Current industry design practices
• Design & operational issues with bursting disks in
this service
• Energy Institute JIP Proposal
The views expressed in this paper are those of the individual authors / presenters and are not intended to represent the view s or position of BP on the matters
discussed
Different Exchanger Configurations
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
Gas
Liquid
Gas
Liquid
Liquid
Gas
Liquid
Gas
Courtesy of Ian Wyatt - Atkins
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 by the Institute of Petroleum:
Phase I – Percussive Shock
Phase II – Fast Transient
Phase III – Liquid Discharge
Phase IV – Gas Discharge
Courtesy of Ian Wyatt - Atkins
Phase I – Percussive Shock
•
•
•
•
Rapid rupture creates percussive shock wave
Extremely short lived <0.1ms
Shell does not ‘feel’ the pressure spikes
Not Modelled
Gas
Liquid
Flare Header
Gas
Liquid
Courtesy of Ian Wyatt - Atkins
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
Courtesy of Ian Wyatt - Atkins
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
Courtesy of Ian Wyatt - Atkins
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
Courtesy of Ian Wyatt - Atkins
Changes in Industry Guidance (API521/BS EN ISO
23251)
• 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 added:
− “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.”
The views expressed in this paper are those of the individual authors / presenters and are not intended to represent the view s or position of BP on the matters
discussed
Changes in Industry Guidance (API521/BS EN ISO
23251)
• Tube rupture design basis added:
− “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.”
The views expressed in this paper are those of the individual authors / presenters and are not intended to represent the view s or position of BP on the matters
discussed
Current Industry Design Practice
• 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 selection etc.
The views expressed in this paper are those of the individual authors / presenters and are not intended to represent the view s or position of BP on the matters
discussed
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
SRV, RV and Graphite Disc at High Pressure
Example of Pressure Transients in Shell
(HP gas at 180 barg in tubes; cooling water at 5.4 barg; PSV opening in 50 msecs)
Pressure in shell local to break at tubesheet
Peak pressures due to reflected waves
Phase 2
Phase 3
Shell design
pressure = 14 barg
Tube rupture occurs at 0.01 s
The views expressed in this paper are those of the individual authors / presenters and are not intended to represent the view s or position of BP on the matters
discussed
“Typical” HP/LP shell & tube heat exchanger design
(compressor recycle cooler for offshore service)
Relief to flare
Shutdown
valve
(designed for slug flow)
Gas inlet
Bursting disks
in parallel
Water outlet
Segmental baffles
(no tube in window)
Water inlet
Gas outlet
Check valve
Possible shutdown
valve
Adapted from IP Guidelines for the Design and Sae Operation of Shell & Tube Heat Exchangers to Withstand the Impact of Tube Failure, Aug 2000
The views expressed in this paper are those of the individual authors / presenters and are not intended to represent the view s or position of BP on the matters
discussed
STHE Overpressure Protection
• Increased use of bursting disks to protect STHEs over past 10 to 15
years
• Estimated frequency of tube failure
− 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
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 in upstream oil and gas industry known to BP
The views expressed in this paper are those of the individual authors / presenters and are not intended to represent the view s or position of BP on the matters
discussed
STHE Overpressure Protection
Two major hazards associated with bursting disk failures:
• Impairment of relief system – liquid inflow & overfill
• Incident escalation - reverse rupture may lead to
uncontrolled hydrocarbon release from relief system
The views expressed in this paper are those of the individual authors / presenters and are not intended to represent the view s or position of BP on the matters
discussed
Incident #1 – liquid overfill
Flare
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
The views expressed in this paper are those of the individual authors / presenters and are not intended to represent the view s or position of BP on the matters
discussed
Health and Safety
Executive
Bursting disc failure: flare
system impairment
Stephen Murray
HSE Inspector, Offshore Division
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
Hazard #2 – excessive backpressure
80 psig backpressure
50 psig
100 psig
225 psig
225 psig
The views expressed in this paper are those of the individual authors / presenters and are not intended to represent the view s or position of BP on the matters
discussed
Design & Operational Issues
• Bursting disks when utilised for overpressure protection of
STHEs
− Once opened, they maintain an open flow path from the
process/utility system to the relief system.
− A sufficient margin (~30%) needs to be maintained
between operating and set pressure to avoid opening in
absence of a tube failure. 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.
The views expressed in this paper are those of the individual authors / presenters and are not intended to represent the view s or position of BP on the matters
discussed
Design Requirements
• Selection of relief route should consider:
− Multiphase – high velocity liquid slugs
− HP or LP flare system (high pressure gas under relief
conditions but large liquid volumes under a failure case)
− segregation from other relief routes to avoid mitigate
reverse rupture
• HAZOP required to identify potential failure modes and
consequences.
• Additional detection and safeguards required for failure
cases.
The views expressed in this paper are those of the individual authors / presenters and are not intended to represent the view s or position of BP on the matters
discussed
Gaps in current industry guidance
• Broader design requirements associated with bursting disks
and interface with relief systems not addressed
• No industry guidance on a pressure ratio at which relief
valves are 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.
• No acceptance criteria available for short duration transient
overpressures
The views expressed in this paper are those of the individual authors / presenters and are not intended to represent the view s or position of BP on the matters
discussed
Aims of Energy Institute JIP
• Eliminate or mitigate hazards associated with overpressure
protection of STHEs
• Develop revised set of design guidelines for overpressure
protection of STHEs principally to address:
− Heat exchanger design.
− Relief device selection.
The views expressed in this paper are those of the individual authors / presenters and are not intended to represent the view s or position of BP on the matters
discussed
Possible Scope of Energy Institute JIP
Heat Exchanger Design (1)
• 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.
The views expressed in this paper are those of the individual authors / presenters and are not intended to represent the view s or position of BP on the matters
discussed
Possible Scope of Energy Institute JIP
Heat Exchanger Design (2)
• 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.
The views expressed in this paper are those of the individual authors / presenters and are not intended to represent the view s or position of BP on the matters
discussed
Possible Scope of Energy Institute JIP
Relief Device Selection
• Develop a rule-set for relief device selection to accommodate the
tube rupture case
− 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.
The views expressed in this paper are those of the individual authors / presenters and are not intended to represent the view s or position of BP on the matters
discussed
JIP Proposal Summary
• Total cost estimate ~£330k
• Fee structure
− £30k – operating companies and safety regulators
− £15k - other participants (design houses, consultancies
etc.)
− in-kind support from relief valve manufacturers, software
providers etc.
• 18 months commencing 3Q2011
The views expressed in this paper are those of the individual authors / presenters and are not intended to represent the view s or position of BP on the matters
discussed
Energy Institute JIP Next Steps
• Details on EI website:
− www.energyinst.org/sthe
• Kick off meeting 19 July 2011, Energy Institute, London
The views expressed in this paper are those of the individual authors / presenters and are not intended to represent the view s or position of BP on the matters
discussed