4 gas monitors-how do they work?

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In this informational post, we will briefly describe how a 4 gas monitor works. The name is self explanatory, the instrument is a gas monitor (also called a 4 gas detector), that can measure 4 different types of gases and is mostly used as a portable instrument. Typically a 4 gas monitor will have four different sensors, out of which one would be a combustible gas detector, the second would be an Oxygen sensor and the remaining two would be toxic gas sensors for gases such as Chlorine or H2S (Hydrogen Sulfide). There are different combinations of sensors available of course, with different manufacturers and every one has several different models. So a typical one may be Oxygen plus three toxic gases (say Chlorine, H2S and CO (carbon Monoxide) Or it could be Oxygen, LEL (any explosive gas), H2S and CO. Advantages: A single portable handheld unit can measure four different kinds of gases, which is very useful, for example if one is working in a confined space. It is very handy and popular as the worker needs to only carry a single unit, strapped to his body to monitor the surroundings. Disadvantages: If the unit fails, then suddenly one has nothing to measure any of the gases! So for example if one had four different monitors and a typical confined space entry job did not necessarily ask for H2s monitoring, then one could theoretically use the LEL and O2 meters separately, but if a 4 gas monitor fails (without a spare one) then one has nothing...The second disadvantage is the calibration status of each sensor may be different and labeling becomes complicated. e.g. if the Oxygen sensor is calibrated but the LEL one is overdue then there has to be some way to mark it on the label (assuming the unit itself does not record its status).

Positioning and Placement of Gas Detectors

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 Perhaps the most difficult task of any project that involves gas monitors and gas detectors is to decide on the placement of the various detectors and monitors. Even with all the new software( and all the associated bells and whistles) available now, it is more of an art than a rigid science.  Some questions that need answering before you start the job are

1. How do you estimate where the leak would likely take place? From flange joints in piping? Or from valve stems/packings? Or from sampling points? Or from reactor agitator shafts (even with all the mechanical seal stuff)?
2. Once leaked, how would the gas flow? How will the molecules move in the air? We can study gas dispersion models of leaks, but they are not accurate predictions. Would the gas flow just as the dispersion model says it should? Or will it flow in some other pattern?
3. How much would reach the detector?
4. Would the detector sense it? (Would the sensor be "alive" or already poisoned to death a long time back?

Only then can we start siting the detectors. This is of course after the technology has been selected (InfraRed or Catalytic Combustion or Semiconductor or other).

 

Hot work permit without a gas test!

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Recently the US Chemical Safety Board released a video animation of the sequence of events at a DuPont facility near Buffalo,NY that had an accident due to hot work being carried out on a storage tank that still had explosive vapors. Surprisingly the workers who carried out the hot work were issued a mandatory "hot work" permit WITHOUT carrying out any gas test ! (A gas test is a test that is carried out to ascertain the presence of an explosive gas before allowing hot work such as welding or cutting inside a plant).This negligence unfortunately caused the accident. It could have been easily prevented had a Gas Monitor been used.

The CSB has posted an animated video that details the incident. watch it below.

HAZOP-What does it mean?

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In this post we will diversify a bit away from our topic of Gas Monitors and go a bit into risk assessment techniques. One of the better known ones is the HAZOP technique.
HAZOP that started out in somewhere around the 1960s in ICI (Imperial Chemical Industries) UK when some some bright spark decided to try it out on a spanking new design of a phenol plant was on the design board.This was a phenol plant that wa sto produce phenol through the Cumene route (Benzene + Propylene-->Cumene-->Phenol)

Well, he opened a can of worms, as it were, because the study found several holes in safety aspects and the technique of HAZOP was born. Initially it was confined only to ICIs internal departments, but later on it spread via conferences and seminar papers to other organizations as well and soon became an established risk assessment and safety technique.

The Bhopal incident that happened two decades later spurred action in the global chemicals and process industries and HAZOP began to be used not only for new plants at the design stage but also for older plants, modifications, shutdowns and turnarounds and so on. The process not only improved Safety but also operational ease.

In the typical HAZOP methodology, a plant or a subsystem of the plant (called a node) is chosen for study. Then all deviations are studied against the original intentions and permutations and combinations give rise to possibilities that can result in accidents. These are then assessed to modify the design or the work flow, so that the process becomes safer. There is much more to this and it requires training in the technique plus a few real life HAZOP studies to really master the technique.

Today one cannot think of not using HAZOP in a process plant, it has become commonplace in many industries. However being subjective, the result of the HAZOP study depends greatly on the people that comprise the team carrying it out and a bad HAZOP from a bad team results in the plant being even more unsafe than had there been no HAZOP at all!
So it is very important that only trained and competent people are in charge of HAZOPs.Hence HAZOP training should be a pre-requisite for anybody wishing to take up the task.