Gas Monitor Diagnostics - Are They of Any Use?

Introduction

Millions of gas detectors are used all over the globe for monitoring the presence of toxic, explosive or otherwise harmful gases and vapors, as well as those that monitor Oxygen levels. Gas Monitors can be classified into three types as under.

1) Toxic Gas detectors
2) Explosive Gas Detectors
3) Oxygen detectors

Gas detectors and gas monitors mean more or less the same things, so we will use both the terms interchangeably in this article.

We can use gas detectors to detect regular gases, as well as vapors that are generated by volatile liquids. Oxygen gas monitors are generally used to check if a confined space (for example like the inside of a large process vessel) contains enough Oxygen, to sustain a person who may be present in that area. In applications where Nitrogen blanketing is used, to make the vapor space above an inflammable liquid inert, an Oxygen gas detector can tell us if the vapor space is really inert or not.

All the above types of gas detectors are widely used in oil & gas industries, gas compressor units, gas turbines, power plants, hydrocarbon processing plants, cargo ships that carry inflammable materials, petrochemical plants, mines, coal processing units, natural gas filling stations and so on. In fact, one only reads about the rare accident that happens when a gas leaks, however, many such incidents are averted each year, due to the preventive monitoring of gas leaks by these gas detectors, which are never reported.

Need for diagnostics

Why do we require diagnostic features in gas detectors? There are many reasons, some of which we will list below:

a) Gas Monitors even today, are not as sophisticated as other instruments, like those for measuring pressure and temperature; performance variations are wide across the board, especially the ones that use catalytic combustion, or electrochemical sensors. Repeatability is an issue.

b) To make sure that the gas monitor is functioning properly, we have to test these instruments more frequently, than other process instruments. Hence, getting a calibration gas and then testing each monitor with this calibration gas, consumes a lot of resources. In today’s manpower deficit plants, this is an issue. On the other hand, if we do not test them regularly we won’t be sure that they will work when needed. The solution is to have inbuilt diagnostics, that can help reveal hidden faults (instead of being revealed only in the next calibration cycle).

How do the diagnostics work?

What kind of diagnostics could be provided? We can classify them into two types, one are diagnostics that cover the electronics part and the second are those that cover the sensor part.

Electronics diagnostics are similar to the ones that are present in other process instruments, like in smart transmitters that use HART, FF, etc for communication. The same diagnostics can be used in gas monitors for the electronics part.

For the sensors, some gas monitors come with an inbuilt gas generator system, that generates precise quantities of the same gas, that is sought to be detected. It means that a small chemical plant exists on-board the gas detector, which produces a little of the same gas, which is to be detected by the sensor.

Such gas generator type devices are available for toxic gases like Chlorine, Hydrogen Sulfide, Hydrogen Cyanide, Hydrogen Chloride and many more toxic gases. This testing provides the ultimate in diagnostics and self testing, as the sensor is tested every 24 hours. Hence the reliability of such gas monitors is extremely high. Theoretically, these diagnostics can be duplicated by manual checking, but it is practically impossible, as we would have to employ a technician who will be armed with a gas calibration bottle and has to test all gas detectors in the plant, every 24 hours. Even if there are just 24 detectors in the plant, it would mean that he will have to check each one continuously without a break, for all the 24 hours in a day.

Thus, an automated system is far superior to a manual test, however these gas detectors are costlier, as compared to the normal electrochemical type. Hence, these are generally available only for highly toxic gases, where reliability needs to be very high.

For other types of gas detectors, that operate on the catalytic combustion type or the electrochemical type, mostly the tests are confined to the electronics part only. Other tests on sensors are supposed to be done manually, so they are not included in the diagnostics.

Similar diagnostics are also available for Infrared type gas detectors. In this case, no gas is generated for checking (as many of these detectors are designed to monitor not one single gas, but a variety of hydrocarbon gases and vapors). The functioning of the infrared cell can be easily checked by the detector electronics, as the design generally uses an IR reference compartment and a sampling compartment. The reference compartment sensitivity, can be thus checked by closing the sampling compartment and checking the difference in the output signal.

Thus we can say that full diagnostic coverage is available for IR types of gas monitors too.

Costs of having the diagnostics

Having a full fledged diagnostic system, is much more costlier than having a system with diagnostics only for the electronic parts, or none at all. The need for diagnostics arises when the application demands a high level of reliability and a high Safety Integrity Level. The amount of faults that can be covered by the diagnostics is called as Diagnostic Coverage, the higher the coverage, the more reliable the system is. We have seen that both high and low diagnostic coverage systems are available, it depends on the budget and the goal that is to be achieved.

Conclusion

If one has to ensure a high level of reliability, then it is better to have a better diagnostic coverage for all the gas monitors installed in the plant. If however, the budget is limited, then having a gas detector without diagnostics is of course better, than not having one at all. Hence, a trade off has to be made between the increased availability, reliability and accuracy of the gas detector, with extensive diagnostics against the cost of investment that is needed to achieve it. Depending on the evaluation, one can have full diagnostic coverage of all the gas monitors, or none at all, or something in between (like having some gas detectors for critical locations with diagnostics and others without diagnostics). It all depends on what is the goal of having the gas monitor in the plant.