STHE Overpressure Protection

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. 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.”

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 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

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

8. Summary of test conditions and test numbers – phase 1

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

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 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 modelcan 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 ...

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

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

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

41. LAH ESDV HSE Safety Alert 01/2008 What happened? press = 4 barg (no alarm) liquid @+40m disc failure does not trip seawater pumps water enters drum tell-tail blocked? no level >LAH SWR no alarm PAH overfills ESD HP Flare Drum Heat Exch. LP flare drum ESDV gas not tight shut-off fills fills Closed drain SWS Over-board closed

42. 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

43. 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

44. 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

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.