The Bomb in the Basement

Gas powered plant in enclosed plantrooms

IntroductionNOTE - This presentation is only for lighter than air natural gas

(methane) – not LPG!

Building Code requirements• Compliance documents• Functional objectives• Overseas codes and standardsFailure modesDesign guidance and recommendations

Current NZ requirementsCompliance documents1. Enclosed in a fire rated room2. External access 3. Any internal door must have a

smoke lobby and heat detector4. Floor at or above ground level5. At least one side is on an external

wall

Hazardous atmosphere fires / explosions are outside C/VM2.

NZBC compliance docs allows mechanical or natural ventilation

NZBC requirementsNZBC classesC2.3 Fixed appliances ..so that there is a low probability of explosive or hazardous conditions occurring within any spaces..

C3.9 Buildings must be designed with regard to the likelihood and consequence of failure of any fire safety system.

Fire Service access and operations

Overseas building codes allow internal gas plant, or make no mention of it. A risk assessment may be required in some cases.

• NFPA 52 (National Gas Code) • NFPA 101 and IFC model codes (USA)• Approved Document B (UK)• Institute of Gas Engineers and Managers SR/25 and UP 16

Risks and consequences• Risk of explosion – not just fire• How do you clear the room if it is full of

gas, especially with no external access?

7 dead, 50 injured in Harlem gas explosion, destroys 2 buildings (2014)

2 dead in house gas explosion (September 2015 – UK)“Boiler explosion

rattles California school” 2014

Design approachAir and ignition sources cannot be controlledSuppression won’t work for explosions - control the room so it is considered to be non – hazardous- Objective is zoning to be “Zone 2 – negligible extent”

Most guidance on this is by the petrochemical industry and not completely relevant to building services.

Design guides include:• IGEM SR/25 2nd edition – Hazardous area classification of Natural Gas installation• IGEM UP/16 – Design of Natural Gas installations on industrial and commercial

premises with respect to hazardous area classification and preparation of risk assessments.

• AS NZS 60071-10-1 (2010)• A more generic risk assessment approach such as AS/NZS 31000 and HB436:2013

From King - 1998

Flame failure detector

Auto gas shut down valve

Manual gas shut down valve

Pressure regulator

Fault assessment

References

1- Flamegurad rtech sheet

2 -Bentley Nevada tech manual pg 134 - worst case = 0.927

3 -from OGP risk assessment data directory4 - estimatedFrom HB of smoke control for smoke control fans

Pfail (gas leak from fitting) 9.95E-05Pfil (gas leak) - mech not indep 9.95E-04

Conditional on gas detection

working unless running at full capacity all the

time

• Failure is taken as >30% LEL with an annual occurrence of less than 1*10-4 / year (approx. 1 hour per year) (Cox etal).

• Need to assess operating frequency / duration for exposure risk• “Failure on demand” assessment using fault tree, HAZOP etc

Simplified burner fault tree -failure on demand

IGEM SR25 assessment information required

1. Get information on appliances, number of fittings (bends, elbows, joints, length of pipe, regulators, valves, and flexible connectors) to calculate primary and secondary gas releases

2. Does the appliance have flame failure detection?3. What is the gas demand and pressure? 3. Energy use of equipment? How many? Are they independent?3. Frequency of formal inspection? Will people be there at other times?

• Check if proposed ventilation is “more than adequate”, or “adequate”?• Check if there are specific requirements for the installation e.g. congested areas

where gas can build up? Local zones of non compliance?• Will the gas detection activate in time for a leak?• What details are required for improving safety, fire fighting?

Fault assessment

SR/25 lists 3 types of gas releases

Classification

Secondary - Gas is released not through normal operation, and likely to be infrequent and short duration and due to catastrophic failure. E.g. pipe joins, flanges, hole<0.25mm

Primary Expected periodically during normal operation. E.g. burners, some regulators

Continuous Gas is released as an everyday part of the process – eg gas bottle filling. This cannot be Zone 2 NE

SR25 assessment methodology

Number of primary releasesCount number of primary sources (burners)SR25 gives the expected number of simultaneous releases being:

1 burner = 1 source2 burners = 2 sources3 burners = 3 sources.4-5 burners = 4 sources

Flow rate = sum maximum flow from each primary source- Gas consumption from manufacturer’s data sheet- Or calculate from kW rating and calorific value of gas.

- Refer to SR/25 A7.1.1 for other primary sources.

SR25 assessment methodology

Number of secondary releases – pipes and joinersCount number and type of fittingsSecondary gas flow rate = sum of (inspection interval)*(failure frequency per year)*(number of devices)

ItemFrequency of failures per year (fi)

Maximum time between inspections T max (months)

Flexible pipes and bellows in adverse conditions (vibration 0.01 2

Pin hole fail of regulator diaphragm 0.005 4

Rising stem valve stem 0.005 4Rotating stem valve stem 0.001 4

Screwed union/ single ferrule compression joint 0.001 6

Screwed fitting - sealed 0.0008 6Flange, flexible pipes and bellowed - normal conditions 0.0005 6Twin ferrule compression joints 0.00005 6

Ventilation rates

1. Check for room mixing especially with mechanical ventilationAir volume for good mixing Qmix >0.15 (A.Ve)0.5

Ve = volume of enclosure (limit <100m3)A = area of grilles (m2)

2. Adequate ventilation Qreq = 90.G.p

3. “More than adequate” ventilation Qreq = 225.G.p

Gas release G = sum of primary and secondary gas flow ratesP = gas density

“Adequate” ventilation is a minimum and “more than adequate “ may be required for some installations

Ventilation rates

CHECK CONGESTED AND CONSTRICTED AREAS AND ZONING AROUND LEAK POINTS- Will you need local exhaust at tight points around the room?- Check where within 1m of a wall or other equipment- Affects mechanical layout with local exhaust points- Refer to SR/25 for requirements.

Comparison with NZBC- SR/25 exhaust rates are much higher than NZBC G4 or NZS5601

—6.5. G4 compliance may not be adequate if you are following SR/25 in full

- In the order of 2.5-3 times higher for SR/25- Opening sizes for wall grills are also much higher than G4

Ventilation rates

E.G – 5 Rinnai HD250 condensing boilers in a 47m3 room250MJ/hr each (69kW) with atmospheric burners

NZBC G4 – 7.2m3/hr per kW = 690 l/s=0 .69m3/sNZS5601 – 0.5 l/s per MJ/hr = 621 l/sIGEM SR/25 – min 1.6m3/s for mixing and 1.9 m3/s exhaust..

Which is right? NZBC G4 complies with NZBC for ventilation...

SR25 assessment methodology

Gas detection reaction timeWill the gas detector react in time given the expected simultaneous release rate?

• T90 time is time for gas detector time to react with a step change in gas concentration to 90% trigger level

• Information is in manufacturer's data sheet – typically around 30s.

• Check T90 time from gas detector • Gas release rate into room volume to LEL level

assuming good mixing.

SR25 assessment methodologyOther items• Manual shut off valve in an accessible location (not just in gas filled room) –

required by NZS5601• Fire Service

• Ventilation fan control, shutoff, hazard warnings.• How do you clear the room of gas? NZS5601 requires an explosion proof

(remote motor or listed as suitable Zone 1) fan• Ventilation airflow is important

- Exhaust at high level – above all pipework with supply at low level- Air flow should be diagonal across the room for good mixing (supply low level

one wall , exhaust other side)• Fan airflow interlock (similar to smoke exhaust fans) – if fan is not operating, gas is

isolated – also required by NZBC• Fire alarm shutdown is required by NZS5601 -6.5• Gas detection is a must - and required by NZBC

• Location and correct installation is important• Will you need more than 1?

• Compliance schedule requirements• Maintenance and inspection methods needs to be clearly detailed• Define the frequency of inspection – especially for remote rooms.

References • Cox, Lees, Ang – classification of hazardous locations. IChemE. 1990 • IGEM SR25 2nd edition – Hazardous area classification of natural gas

installations. Institute of gas engineers and managers - August 2013• IGEM UP/16 – Design of natural gas installations on industrial and commercial

premises with respect to hazardous area classifications and preparation of risk assessment, Institute of gas engineers and managers – 2011

• AS NZS 60079-10-1: 2009 – explosive atmospheres – classification of areas• Area classifications of natural gas installations. - R C Santon and M J Ivings -

2009• Safety in the Process Industries 2nd ed – Ralph King 1998• Modelling Ignition Probabilities within the Framework of Quantitative Risk

Assessments. Pesce, Paci at al. Chemical Engineering Transactions, vol 26, 2012

• OGP risk assessment data directory of system reliabilities - Report No. 434 – A1. International Association of Oil and Gas Producers. March 2010