Occasionally it is necessary to make measurements in hydrogen/oxygen mixtures. The increased use of fuel cells and the use of hydrogen as a vehicle fuel have increased the frequency with which this requirement occurs. The requirement occurs usually within process vessels since when the mixture is released the problem becomes a mixture of hydrogen/oxygen /air which is a slightly different problem. Conventional intrinsically safe equipment certified to the IEC standard is considered to be adequately safe under normal atmospheric conditions. The ATEX guidelines on the 94/9/EC directive state ‘A product within a potentially explosive mixture without the presence of air is not in the scope of the directive’. It follows therefore that that a safety analysis of an electrical installation in a hydrogen/oxygen mixture must be based on a risk analysis using the best available information. The conventional IS certification ensures a satisfactory level of construction but additional consideration must be given to the levels of voltage, current and power used. This note proposes an approach, which the author considers, achieves an acceptable level of safety.
Most of the available data is quite old, and this note relies heavily on an SMRE paper P6 published in 1974. [I would appreciate copies of any recent data relating to hydrogen/oxygen mixtures that anyone has and if necessary I can then amend this note.] The IEC apparatus standard IEC60079-11 includes some oxygen-hydrogen test mixtures, which simulate the 1.5 safety factor for the various gas groups. For example the IIC mixture is 60% hydrogen and 40% oxygen. The most readily ignited mixture of hydrogen and oxygen is said to be 33% hydrogen and 67% oxygen and quite how this compares with the sensitivity of the IIC test mixture is not clear from the available evidence. The ignition energy of the most sensitive mixture, when tested using a break-flash tester appears to be approximately 11mJ with a threshold voltage of 10V. In the literature there are very low figures for capacitive derived ignition energy [2mJ] but these are obtained using high voltages and very special electrodes, which are not relevant to intrinsically safe circuits.
A practical approach is that where possible the circuit voltage should be kept below the threshold voltage, since this appears to be well established and also to limit the inductive energy. Figures of 6V and 6mJ would achieve an acceptable level of safety and are not impractical. The 6mJ has a factor of safety of three on the 18mJ permitted in ‘ia’ circuits. [40mJ with a 2.25 factor of safety]. The permitted inductance or L/R ratio in a hydrogen/oxygen mixture can therefore be derived by dividing the figures for IIC by three. Frequently the L/R ratio is the predominant factor. At 6V capacative energy is not a problem but some large capacitors have a significant self-inductance, which can create a problem. Setting a limit of 10mF is reasonable and in line with current practice.
There is very little information on the effect of oxygen on the ignition temperature of hydrogen [5600C]. Probably the ignition temperature is much lower and the use of T4 [1350C] apparatus seems a prudent and practicable solution. The 1,3W small component relaxation for T4 apparatus is possibly not applicable to a hydrogen/oxygen mixture hence a reduction by a factor of two seems a reasonable precautionary measure.
The proposed limits are therefore an ‘ia’ IIC system with T4 apparatus, a circuit voltage of 6V, an inductance or L/R ratio derived by dividing the IIC figures by three and a capacitance of 10mF. Where the small component relaxation is used for temperature classification then the matched power should be reduced by a factor of two. With the factors of safety implicit in these values it seems acceptable to use ‘simple apparatus’ with the usual parameters.
There are commercially available galvanic isolator interfaces for the majority of the sensors used in this type of Zone 0. For example suitable interfaces are available for switches, proximity detectors, thermocouples and RTDs. Some forms of oxygen content monitors also meet these criteria.
The range of shunt diode safety barriers, which satisfy the 6V requirement, is limited. The usual solution is the two channel 3V 10W barrier which when combined with ‘simple apparatus’ would have its IIC cable parameters of 100mF, 130mH and 69mH/W reduced to 10mF, 43mH and 23mH/W. The matched power of 450mW is acceptable and the permitted L/R ratio makes the installation a practical possibility.
Many of the field mounted transmitters which would not normally be mounted directly in this atmosphere but having sensors in the Zone 0 have sensor input parameters which satisfy the proposed requirements of this note.
Using conventional intrinsically safe instrumentation in a hydrogen/oxygen atmosphere with the proposed limitations can be considered to be acceptably safe. The resultant installation is not intrinsically safe in accordance with the IEC standards but achieves a comparable level of safety.
The proposed approach is arguably the only acceptable approach using the conventional methods of explosion protection.