APEK - Hydrogen Hazards and Safety With Power Station Alternators

Hydrogen Hazards and Safety With Power Station Alternators

Details: Category: General - Regulatory, Legal, Standards, E&T

1 Scope

This Note briefly presents a real case where safety was not achieved in internal examination of an alternator and three persons were killed in Australia over 20 years ago. Similar potential incidents have been reported since, and this Note aims to diffuse knowledge and understanding for the avoidance of such hazards in the future.

2 Alternator

In power stations alternators are used to generate power and they:

range from a few MW to over 1000 MWE;
have a rotor of up to 100 tonne rotating at 3000 rpm inside a gas tight stator or housing up to 10 m length and 3 m diameter;
have a hydrogen (H₂) atmosphere in large units to reduce power loss due to windage;
can exceed 65 kPa pressure inside; and,
are not classified as pressure equipment but as a pressure machine.

3 Incident

The alternator was purged of hydrogen, test vents were checked, and then the alternator was opened up near one end of the housing. Entry to the equipment was made some time later and a portable electric lamp was used.

A violent H₂/O₂ (air) explosion resulted, killing one man inside and two near the access opening of the alternator.

The subsequent investigation indicated a gradual build-up of hydrogen had resulted in an explosive mixture which was easily ignited by the hot lamp.

4 Causes

Purging with inert gas e.g. nitrogen(N₂) from one end at mid height did not remove all H2 which is lighter than air and was trapped at top spaces away from the test vents;
Gas testing at the bottom soon after opening the alternator had not detected H2 build up at top of the housing.

5 Lessons and Recommendations

Since this major incident, the design of alternators has been changed to include top test vents in all critical areas and much equipment in operation has been modified.

It is recommended that:

Purging is carried out properly to flush all areas of an alternator, this is vital when dealing with explosive, flammable or toxic contents;
Gas testing is done initially in critical areas especially at the top of the equipment for ligher than air gases and the bottom for heavier than air gases;
Gas testing is repeated continuously or at suitable intervals – timing is vital to monitor any build up of gasses;
The correct gas detector must be used – it should be proven at the time and suitable for flammable gas and oxygen. Persons have been killed with lack of oxygen (O2 as well as through excessive N2;
The hydrogen re-fill (and purge) connecting pipe should be held and issued by a nominated senior manager who first satisfies himself that the inspection will be carried out safely.
Low voltage and low temperature light source and non-sparking gear should be used

Knowledge management, informed personnel and JSAs are vital for safety and performance.