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STHE Overpressure Protection

Colin Deddis, Senior Process Engineer, EPT22 March 2010

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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

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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.”

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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.”

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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

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Response Times of Relief Devices

• Bruce Ewan, University of Sheffield

Relief device

Relief diameter

(in)4mm orifice 8mm orifice 15mm orifice

Relief pressure (bar)

Open tube 39 38 37 0

Graphite disc 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

Summary of test conditions and test numbers – phase 1

05

1015

2025

3035

4045

0 0.002 0.004 0.006 0.008 0.01 0.012

Test 49p 1.9ms Rupture

K4 Pressure Time (seconds)

Pressure (bar)

1. 10

Sheffield Model - Re-Calibrated after Test 39p

Max = 40.87

Min = 0

4” graphite disc. Rupture time = 1.9 msHigh pressure test

2” spring loaded RV. 110% open capacity in 6 ms

Low pressure test

-10

0

10

20

30

40

50

0 0.002 0.004 0.006 0.008 0.01 0.012

Test 59p 10% Capacity in 6ms

K4 Pressure Time (seconds)

Pressure (bar)

6. 8

Sheffield Model - Taken from Test 51p

Max = 27.38

Min = 0

-20

0

20

40

60

80

100

120

0 0.002 0.004 0.006 0.008 0.01 0.012

Test 57p 10% Capacity in 4ms

K4 Pressure Time (seconds)

Pressure (bar)

5. 8

Sheffield Model - Taken from Test 51p

Max = 100.75

Min = 0

2” spring loaded RV. 110% open capacity in 4 ms

High pressure test

-5

0

5

10

15

20

25

30

0 0.002 0.004 0.006 0.008 0.01 0.012

Test 66p 10% Capacity in 4ms

K4 Pressure Time (seconds)

Pressure (bar)

8. 8

Sheffield Model - Taken from Test 51p

Max = 22.11

Min = 0

2” pilot operated RV. 110% open capacity in 4 ms

Low pressure test

-20

0

20

40

60

80

100

0 0.002 0.004 0.006 0.008 0.01 0.012

Standard

Test 64p 10% Capacity in 2.5ms

K4 Pressure Time (seconds)

Pressure (bar)

7. 8

Sheffield Model - Taken from Test 51p

Max = 84.07

Min = 0

2” pilot operated RV. 110% open capacity in 2.5 ms

High pressure test

Summary of test conditions – phase 2

Relief Device Size

Driver

Pressure

(barg)

4mm

Orifice

8mm

Orifice

15mm

Orifice

Safety Valve (SRV)

2 H 3 100 Test no. 2 Test no. 1 Test no. 3

4 L 6 100 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

Graphite Disc3 in 20 Test no. 18 Test no. 16 Test no. 15

4 in 20 Test no. 13 Test no. 12 Test no. 11

4” relief valve4L6 safety relief valve

0

5

10

15

20

25

30

35

0 0.002 0.004 0.006 0.008 0.01 0.012

Standard Test

Test Vent 23

K4 to Device Pressure Time (seconds)

Pressure (barg)

3. 15

Sheffield Shock Tube

Max = 27.34

Min = .45

4L6 safety. 110% open capacity in 10 ms

Low pressure test

0

20

40

60

80

100

0 0.002 0.004 0.006 0.008 0.01 0.012

Standard Test

Test Vent 25

K4 to Device Pressure Time (seconds)

Pressure (barg)

1. 15

Sheffield Shock Tube

Max = 91.39

Min = .18

4L6 safety. 110% open capacity in 4 ms

High pressure test

SRV, RV and Graphite Disc at High Pressure

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Dynamic Analysis of Tube Rupture

• Ian Wyatt, Atkins

Dynamic Modelling of Tube RuptureIan 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

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

Liquid

LiquidGas

Gas

Liquid

LiquidGas

Gas

Gas

GasLiquid

Liquid

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

Liquid

LiquidGas

Gas

Flare Header

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

Liquid

LiquidGas

Gas

Flare Header

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

Liquid

Flare Header

Liquid

Pressure

Liquid

LiquidGas

Gas

Flare Header

Liquid

LiquidGas

Gas

Flare Header

Liquid

LiquidGas

Gas

Flare Header

Liquid

LiquidGas

Gas

Flare Header

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

Liquid

LiquidGas

Gas

Flare Header

Phase IV – Gas Discharge

• Gas from rupture passes out of system

• Pseudo steady state depending on gas supply

• Usually not modelled

Liquid

LiquidGas

Gas

Flare Header

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

0%

100%

200%

300%

400%

500%

600%

700%

800%

0% 100% 200% 300% 400% 500% 600% 700% 800% 900% 1000%

%HP/LP Design Pressure

%P

eak/

LP

Des

ign

Pre

ssu

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Results LP Short Term Design Limit "2/3rds Rule" Max HP

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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?1. IP Guidelines for the Design and Sae Operation of Shell & Tube Heat Exchangers to Withstand the Impact of Tube Failure, Aug 20002. Estimate based on incidents known to BP

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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

STHE Overpressure Protection – the “problem”

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Incident #1 – liquid overfill

Relief Header

Flare Knockout Drum

Flare

• 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

PSHH

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225 psig

50 psig

100 psig

225 psig

Note: The top disc impacted bottom disc causing it to also rupture

80 psig Burst

80 p

sig

Bur

st

Incident #2 – excessive backpressure

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Incident #2 ctd.

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Any other incidents……?

???

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Design & Operational Issues

• HSE Safety alert 01/2008 Steve Murray, HSE

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

gas SWS

SWR

HP Flare Drum

Heat Exch.

HSE Safety Alert 01/2008

PAH

gas

SWS

SWR

Heat Exch.LAH ESD

ESDV

Closed drain

HP Flare Drum

LP flare drum

Over-board

ESDV

HSE Safety Alert 01/2008

PAH

gas

SWS

SWR

Heat Exch.LAH ESD

ESDV

Closed drain

HP Flare Drum

LP flare drum

Over-board

ESDV

disc failure

no alarmtell-tail blocked?

water enters drum

does not trip

seawater pumps

not tight shut-off

fills fills

overfills

no level >LAH

press = 4 barg

(no alarm)

closed

What happened?liquid @+40m

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

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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.

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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.

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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.

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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?

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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.

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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.

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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.

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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.

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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