Hydrogen Hazard Summary and Preliminary FMECA

and HAZOP

Yury Ivanyushenkov

Elwyn Baynham

Tom Bradshaw

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Hydrogen properties

The potential hazards of liquid hydrogen stem mainly from three important properties:

1. Its extremely low temperature

2. Its very large liquid to gas expansion ratio

3. Its wide range of flammable limits after vapourising to gas

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Hydrogen Hazards

Consequences (hazards) of these properties:

1. => - severe burns can be produced upon contact with the skin; - some materials can become brittle and easily broken

2. => overpressure => - excessive deformation of a vessel; - rupture of the pressure vessel

3. => fire => - injury of personnel - damage to equipment / property

Fire is a primary hydrogen hazard !

4

Hydrogen hazards (2)

Fire is a primary hydrogen hazard !

A fire can result from two scenarios [1]:

(1) - Hydrogen is released, - mixes with an oxidizer, - forms a combustible mixture, - the mixture contacts an ignition source, - and ignition occurs.

(2) - The hydrogen system is contaminated with an oxidizer (as a result of improper purging and/or in leakage of an oxidizer, such as air),

- the hydrogen and the oxidizer form a combustible mixture; - the combustible mixture contacts an ignition source; - and ignition occurs.

[1] Guide for Hydrogen Hazards Analysis on Components and Systems, NASA TP-WSTF-937

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MICE Hydrogen system layout

P P VP Vacuum pumpBursting diskPressure relief valveValve

Pressureregulator

Pressuregauge

18 K Heto Compressorvia Radiation shield

14 K Hefrom Cold box

Liquid level gauge

LH2 Absorber

Vacuum

Vacuum vessel

LHe Heat exchanger

Internal Window

70 K Safety window

H2 Gas bottle

PPFill valve

Metal hydride hydrogen storage unit

(20 m3 capacity)

Vent outside flame arrester

He Purge system

Non-return valve

Vent outsideflame arrester

Vent valve

Vent valve

1.7 bar

2.1 bar

H2 Detector

H2 Detector

P

P

PP

Evacuated vent buffer tank

VP

P

VP

X 2X 2

VP

H2 Detector

Ventilationsystem

Vent outsideflame arrester

Chiller/ heaterunit

Node 1

Node 2

Node 3

Node 4

Node 5

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Preliminary FMECA

No.

Failure mode Effect Criticality Comments

1 Leakage of inner window – detected from hydrogen detectors and or vacuum gauge.

Rise in pressure in interspace between windows. Will cause a heat load on the absorber leading to rapid evolution of hydrogen.

System design should take into account thermal conductance and thermal short circuit.

We need to consider the effect of hydrogen in the region between the windows in the thermal design. Need to know heat load on absorber and hydrogen evolution rate.2 Leakage on outer

windowNo effect except in the event of rupture of the inner window.

  How do we detect leakage on outer window?Can training of rf `cavities puncture the window ?

Failure Mode Effect and Criticality Analysis (FMECA)

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Preliminary FMECA (2)

3 Vacuum failure to inner window

This will increase the heat load to the hydrogen and initiate a rapid boil-off.

  Calculations (ref Bradshaw, Baynham and Green) indicate that the pressure drop in the connecting pipework will not cause a large pressure rise.4 Vacuum failure to

outer windowThis will cause plating of some condensibles on the window as the temperature is fairly low.

  Need to ensure that oxygen does not plate out on the outer window. Need good thermal design and analysis. Note that it is not possible to measure the temperature of the outer window directly.

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Preliminary FMECA (3)

5 Failure of electricity supply

The temperature control of the hydride bed will be lost and the hydride will absorb hydrogen up to its equilibrium point. If there is liquid hydrogen in the system it will be evolved and absorbed by the hydride. The loss of vacuum will accelerate this process.

  Need to ensure that the hydride bed can accommodate the evolution rate else hydrogen will be lost through the vent line. Normally loss of vacuum is gradual. What is the absorbtion rate of the hydride bed ?

6 Failure of chiller/heater unit to hydride

Temperature control of the hydride will be lost and it will absorb up to its equilibrium pressure. This will reduce pressure on the hydrogen and lower its temperature, at which point the flow of helium to the absorber heat exchanger will be reduced.

  Need to ensure that the hydride chosen has a low pressure at room temperature. What is the pressure above the hydride bed at room temperature ?

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Preliminary FMECA (4)

7 Rupture of line between hydride and absorber

Hydrogen will be lost and detected by hydrogen detectors in the vent line.

  The hydrogen detectors are the only indication of system gas loss.

8 Fire in vicinity of hydride

This will cause a rise in temperature of the hydride bed leading to evolution of the hydrogen. The pressure will rise leading to venting as soon as the relief valve operates. The hydrogen will be vented at roof level away from the scene of the fire.

  Is the hydride flammable ?

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Preliminary FMECA (5)

9 Magnet quench – eddy current heating

This will induce eddy currents in the absorber windows and forces on the magnet support structure. There will be an evolution of hydrogen which needs to be absorbed by the hydride.

  The influence of a quench on the absorber has been studied (ref Rochford and Witte) and the eddy current heating is low – enough to boil off a few litres of hydrogen. Need to check that the hydride can absorb the hydrogen quickly enough or that the volumes are such that the system pressure does not exceed the rated value for the inner window.

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Preliminary FMECA (6)

10 Magnet quench – forces on structure

A magnet quench will put large forces on the structure. In the event of mechanical failure the connecting pipework may rupture emptying the hydrogen into the MICE experiment and RF cavities

  There is a large margin of safety in the force calculations and steps need to be taken to make sure that the magnets are not operated past the design limits with hydrogen in the system. Reference Rochford and Baynham.What are the consequences of having hydrogen in the rf cavities ?

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Preliminary HAZOP: Node 1

No

Parameter

Guide word

Cause Consequence Safeguards Recommendations

1 Pressure Lower 1.      Storage unit is leaking. 2.      Pipe is leaking. 3.      Absorber vessel is leaking.

 1-2. Hydrogen is leaking out in the experimental hall. 3. Hydrogen is leaking into vacuum jacket. 

Pressure gauge to trigger an alarm.Hydrogen ventilation system collects and vents hydrogen out. Hydrogen detector to trigger an alarm.

Implement an active pressure gauge.Implement ventilation system. Implement an active hydrogen detector.

2 Pressure Higher 1.      Fill valve is accidentally open or leaking2.      Storage unit is overheated 

1-2. Absorber windows can break. 

 Pressure relief valve to vent outside.Temperature sensor triggers an alarm.

 Implement a pressure relief valve.Implement a temperature monitor.

Node 1: Hydrogen storage unit

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Preliminary HAZOP: Node 2

No

Parameter Guide word

Cause Consequence Safeguards Recommendations

1 Temperature

Lower Too much cooling power from the He cooling system. 

1.      Pressure in the hydrogen system drops.

Pressure gauge to trigger an alarm.Temperature sensor to trigger an alarmAdditional: Liquid hydrogen level meter to trigger an alarm.

Implement both the active pressure gauge and the temperature sensor.Additional:Implement an active liquid level meter.

2 Temperature

Higher 1.      Not enough cooling power from the He cooling system.2.      Power cut. 

1-2. Liquid hydrogen evaporates and LH2 level goes down.1-2. Hydrogen pressure rises.

Temperature sensor to trigger an alarmAdditional: Liquid hydrogen level meter to trigger an alarm.Pressure gauge to trigger an alarm.

Implement both the active pressure gauge and the temperature sensor.Additional : Implement an active liquid level meter.

3 Pressure Lower 1.      Window is leaking or broken.2.      Pipe is leaking.3.      Hydrogen storage unit is leaking.4.      Absorber is over cooled.

1. Hydrogen leaks into vacuum vessel.2-3. Hydrogen is leaking out.4. Pressure in the system drops and air can leak in the system in case if the system seal is broken.

Hydrogen detector to trigger an alarm.Hydrogen ventilation system collects and vents hydrogen out.Temperature sensor to trigger an alarm.

Implement an active hydrogen detector.Implement hydrogen collection and ventilation system. Implement a temperature sensor.   

4 Pressure Higher Temperature is increased. Windows can

break.

Pressure relief valve to dump hydrogen into a buffer tank

Implement a pressure relief valve and a buffer tank.

Node 2: Hydrogen absorber internal vessel with hydrogen windows

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Preliminary HAZOP: Node 3

Node 3: Hydrogen absorber vacuum jacket with safety windows

No

Parameter Guide word

Cause Consequence

Safeguards Recommendations

1 Pressure Higher 1. Hydrogen window broken2. Vacuum pump failure

1. Hydrogen bursts into vacuum jacket2. Vacuum drops.

Pressure relief valve to dump hydrogen into a buffer tank and then to vent it outside.

Implement a buffer tank.

2 Hydrogen concentration

Higher Hydrogen gets in due to window is broken orseal is leaking

An explosive mixture can be formed if there is an air leak in as well.

Active hydrogen sensor detects hydrogen and trigger s an alarm.

Implement an active hydrogen detector.

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Preliminary HAZOP: Node 4

Node 4: Buffer tank

No

Parameter Guide word

Cause Consequence Safeguards Recommendations

1 Pressure Higher 1. Venting path is blocked. 2. Tank is leaking.  3. Vacuum pump failure 

Absorber vacuum jacket windows can break. Buffer tank can’t be used for dumping hydrogen in case of accident with absorber.

Active pressure gauge triggers an alarm. Oxygen sensor triggers an alarm.  Use a spare pump.

Implement an active pressure gauge. Implement an active oxygen sensor.  Keep a spare pump.

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Preliminary HAZOP: Node 5

Node 5: Hydrogen module enclosure

No

Parameter Guide word

Cause Consequence

Safeguards Recommendations

1 Hydrogen concentration

Higher 1. Hydrogen leaks out absorber module

2. Hydrogen leaks out hydrogen pipes

3. Hydrogen leaks out storage unit.

1-3. Explosive oxygen-hydrogen mixture can be formed

Ventilation system to quickly vent hydrogen out.

Hydrogen detector to trigger an alarm and to start a high rate mode for the ventilation system.

Implement ventilation system equipped with hydrogen detector.

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Preliminary HAZOP: Recommendations

Hydrogen storage unit Implement: active pressure gauge temperature monitor

Hydrogen absorber internal vessel with hydrogen windows Implement: active pressure gauge temperature sensor active liquid level meter (additional) Hydrogen absorber vacuum jacket with safety windows Implement: active hydrogen detector

Buffer tank Implement: active pressure gauge  active oxygen sensor

Hydrogen module enclosure Implement: ventilation system equipped with hydrogen detector.