1. STHE Overpressure Protection
Colin Deddis, Senior Process Engineer, EPT
22 March 2010

2. 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

3. Changes in 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:
“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 kPa (approx 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.”

4. Changes in Guidance – API521/BS EN ISO 23251
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.”

5. Current Practice
API521/BS EN ISO 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

6. Response Times of Relief Devices
Bruce Ewan, University of Sheffield

8. Summary of test conditions and test numbers – phase 1
Relief device 
Relief diameter (in)
4mm orifice
8mm orifice
15mm orifice
Relief pressure (bar)
Open tube 39 38 37
Graphite disc 4 51 50 49 10 6 55 54 53
Stainless steel disc 41 42 40 15
(reversed dome) 8 48 47 46
2” Spring loaded pop action RV - 59 58 57
2” Bellows RV 62 61 60
2” Pilot operated RV 66 65 64

9. High pressure test
4” graphite disc. Rupture time = 1.9 ms

10. Low pressure test
2” spring loaded RV. 110% open capacity in 6 ms

11. High pressure test
2” spring loaded RV. 110% open capacity in 4 ms

12. Low pressure test
2” pilot operated RV. 110% open capacity in 4 ms

13. High pressure test
2” pilot operated RV. 110% open capacity in 2.5 ms

14. Summary of test conditions – phase 2
Relief Device
Size
Driver
Pressure
(barg)
4mm
Orifice
8mm
15mm
Safety Valve (SRV)
2 H 3
100
Test no. 2
Test no. 1
Test no. 3
4 L 6
Test no. 23
Test no. 24
Test no. 25
Relief Valve
4 in
Test no. 21
Test no. 20
Test no. 19
Stainless Steel Disc
3 in
Test no. 6
Test no. 5
Test no. 4
Test no. 7
Test no. 8
Test no. 9
Graphite Disc 20
Test no. 18
Test no. 16
Test no. 15
Test no. 13
Test no. 12
Test no. 11

15. 4L6 safety relief valve
4” relief valve

16. Low pressure test
4L6 safety. 110% open capacity in 10 ms

17. High pressure test
4L6 safety. 110% open capacity in 4 ms

18. SRV, RV and Graphite Disc at High Pressure

19. Dynamic Analysis of Tube Rupture
Ian Wyatt, Atkins

20. Dynamic Modelling of Tube Rupture
Ian Wyatt - Atkins
JIP on Bursting Disks for Shell & Tube Exchangers – 1st Stakeholders Meeting

21. API-521/BS EN ISO – 5.19
API-521.BS EN ISO 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 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 ...

22. 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

23. 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

24. Phase I – Percussive Shock
Rapid rupture creates percussive shock wave
Extremely short lived <0.1ms
Shell does not ‘feel’ the pressure spikes
Not Model

25. 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

26. 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

27. Phase III – Liquid Discharge
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

28. Phase IV – Gas Discharge
Gas from rupture passes out of system
Pseudo steady state depending on gas supply
Usually not modelled

29. 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

30. STHE Overpressure Protection – the “problem”
Increased use of bursting disks to protect STHEs over past 10 to 15 years
Estimated frequency of guillotine tube rupture
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?
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

31. STHE Overpressure Protection – the “problem”
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

32. Incident #1 – liquid overfill
Relief Header
Flare Knockout Drum
Flare
PSHH
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

33. Incident #2 – excessive backpressure
80 psig Burst
80 psig Burst
50 psig
100 psig
225 psig
225 psig
Note: The top disc impacted bottom disc causing it to also rupture

34. Incident #2 ctd.

35. Any other incidents……?
???

36. Design & Operational Issues
HSE Safety alert 01/2008 Steve Murray, HSE

37. Bursting disc failure: flare system impairment
Stephen Murray
HSE Inspector, Offshore Division

38. HSE Safety Alert 01/2008 Alerts: to advise industry of incidents
Alerts:
to advise industry of incidents
enable lessons to be learned
industry takes appropriate action to avoid similar incidents

39. HSE Safety Alert 01/2008
SWR
HP Flare Drum
Heat Exch.
SWS
gas

40. HSE Safety Alert 01/2008 SWR HP Flare Drum Heat Exch. LP flare drum
PAH
LAH
ESD
HP Flare Drum
Heat Exch.
LP flare drum
gas
ESDV
Closed drain
SWS
ESDV
Over-board

41. does not trip seawater pumps
HSE Safety Alert 01/2008
What happened?
press =
4 barg
(no alarm)
disc failure
does not trip seawater pumps
water enters drum
tell-tail blocked?
no level >LAH
SWR
PAH
no alarm
overfills
LAH
ESD
HP Flare Drum
Heat Exch.
LP flare drum
gas
ESDV
not tight shut-off
fills
Closed drain
fills
SWS
ESDV
Over-board
closed

42. Summary HSE Safety Alert 01/2008
uncontrolled flow of seawater into flare system
several hours to identify source
flaring event may have lead to serious gas release

43. Lessons HSE Safety Alert 01/2008
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

44. Legal requirements HSE Safety Alert 01/2008
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

45. Bursting disc failure: flare system impairment
Stephen Murray
HSE Inspector, OSD

46. Design & Operational Issues
Bursting disks 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%) 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.

47. Design & Operational Issues
Bursting disks 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%) 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.

48. Design & Operational Issues
Selection of relief route
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.

49. Gaps in current guidance
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?

50. Aims of 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.

51. 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.

52. 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.

53. 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.

54. Shopping list – issues captured in Stakeholders’ meeting
Deliverable – software
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