Confined Space Gas Monitors- Some basic facts

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What is a confined space gas monitor?
A confined space gas monitor is an instrument that allows people to enter and work safely in confined spaces. These confined spaces are commonly found in the process industries. For example a worker who enters an empty petroleum storage tank, is entering a confined space. These confined spaces may have several hazards and a confined space gas monitor helps in reducing these hazards.

Why use one?
For starters, using a confined space gas monitor is absolutely essential to save lives of people who work in such confined spaces. Also by law it is compulsory to use certain protective equipment while working in confined spaces and one of these equipment is a gas monitor.

What does a confined space gas monitor have?
A confined space gas monitor, is any portable kind of gas monitor that can sense at least two gases- Oxygen and Explosive gas. More the gases detected, obviously the better. Also if the confined space is normally used to store or process toxic gases (like Chlorine) then this gas monitor must also include the toxic gas sensor (in this example it would be a Chlorine sensor).
There is an excellent article on basics of toxic gas detectors that you can read here).
Thus a typical 4 gas detector that has one of the sensors as oxygen, can be used as a confined space gas monitor.
The monitor must in addition be lightweight, shock and fall proof, explosion protected (if used in a hazardous area), be small enough to be carried by a worker in his pocket or on a belt clip and have an easily visible display and a loud enough hooter/buzzer.
Some newer ones may also feature a wireless communication to the control room that transmits the readings continuously as well as alarms the control room operators in case of high/low levels of the gases detected.

Calibration
The monitor must be capable of being calibrated easily with calibration gases on site. Further the plant engineer should ensure that at least bump tests of these instruments are regularly done, if not full cycle calibrations, to ensure that these instruments continue to work as desired.

That's all!
For a detailed training course on confined spaces, have a look at the course below

Four gas monitor or PID meter? Which one do you use?

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Many people in the process industry used to use a standard "four gas" monitor for years, until the advent of PID type gas monitors. So this is just an open ended question to you all... what's your favorite meter? To the newbies, a four gas monitor is basically what it says, a portable gas monitor that can measure upto four different types of gases. These can be any mix of four sensors, or a mix of one LEL sensor (for explosive gases and vapors), an Oxygen sensor (for confined space entry) and two toxic gas sensors (say H2S and Chlorine for example). These type of four gas monitors have been used for several years and are workhorses of the plant operations and maintenance teams. However the advent of PID type meters has meant a threat to the domination of the four gas monitor. Essentially a PID meter stands for a gas monitor that uses a PID (Photoionization detector) to measure a wide range of gases. These cover most compounds found in industry such as most volatile organics and toxic gases like H2S. Many PID meters also come with a built in software and systems to automatically do datalogging and calculations of parameters like TWA, etc for regulatory compliance. Calibration is easier too, with just one gas IsoButylene being used.
So tell us what's your favorite meter- PID or four-gas?

How to verify if a gas monitor is really working?

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As you all know the reliability and accuracy of an instrument can only be verified after it is calibrated against a known standard. This is also the case with toxic gas monitors as well as other types of gas detectors.

In case of gas monitors, there are two ways to check the functioning, one is a full fledged calibration where the sensor is exposed to different concentrations of a known air-gas mixture (to calibrate it at various points from minimum to maximum) and the other is to subject it to a "bump" test. A bump test exposes the sensor to just one known concentration of the gas, to verify if it responds.

Now which amongst these two methods should you use?

There are two issues here. First, are you interested in the accuracy of measurement, or merely the functional testing? Accuracy of measurement is important in many applications, where, for example, an oxygen meter is used for confined space entry. A reading that shows 20% when the actual value is say 17% can be disastrous. Here, we require that the instrument always function at it's best accuracy and has to be thoroughly calibrated always. Also a "bump" test is simple, just check the reading in the outside air-it should Oxygen of about 21% (more or less depending on the area and the pollution levels).

However in other cases, the instrument may be used more for detecting the presence of a toxic gas rather than its exact concentration. Suppose a Chlorine gas monitor is installed on the outlet of a vent line, that carries away vent gas away from a confined Chlorine storage area-here the presence of Chlorine must be detected, rather than the exact ppm level. It does not matter whether it shows 2 ppm when the level was 1 ppm. It has to detect the presence (according to its minimum resolution). Here, instead of subjecting the Chlorine gas monitor to a full calibration, it may be simpler to bump test it once a while. As long as it responds to a bump test of the lowest concentration of Chlorine, it's fine, you need not carry out a full fledged 3 or 4 point calibration.

What are the advantages of bump testing?
1. It is simpler than a full fledged calibration
2. Less lecetrolyte is consumed in case of electrochemical sensors-this increases the life of the sensor substantially
3. Cheaper and easier-just one gas bottle per type of gas is needed.

Any other advantages? Please use the comments section below to post.

Gas Monitors-Placement is key to ensuring safety of your Industrial plant

Area monitoring

Many of the fixed type toxic and combustible gas detectors in usage in industry today, are used for area monitoring. What do we mean by area monitoring? We do a strategic placement of these gas detectors in locations that will have the explosive vapor or gas present, in case of a leak. Obviously, we cannot place these gas detectors at all possible places, as this will be very costly, not only in terms of the cost of acquiring these gas detectors and their associated panels, wiring, controls, etc, but also because each gas detector has a maintenance cost associated with it.

Each gas monitor requires periodic calibration with a test gas, replacement of sensor and/or electronics and other maintenance from time to time. We cannot avoid this cost as otherwise the gas monitor will fail to function properly. Just having a gas monitor physically present in an area, may at the most satisfy a factory inspector, or insurance surveyor, it cannot actually detect anything, if not maintained well. Hence, we designate certain areas of our plant or facility as being the most vulnerable to a gas leak. We then pin point the place where the gas is most likely to be present immediately after a leak. We then ensure that vat least we have a gas detector at that point to monitor such a leak.

Placement is the key to safety at a reasonable cost

In the UK, there is a concept known as ALARP (As low as reasonably possible). This indicates that we can spend only so much on safety as to yield a benefit that is more than the cost of the spending. Thus, it is not possible to have swarms of gas detectors all over a plant, as it would cost a fortune to install and maintain them. Maybe the number of such gas detectors would be many more than the number of conventional instruments like pressure or flow transmitters in the plant! Thus strategic placement of gas monitors is the key to achieving safety under an ALARP principle. How does one therefore go about the placement?

Different strokes for different folks

As the saying goes, we employ different types of placement strategies for different types of gas detectors. Therefore, for those gases and vapors that are heavier than air, we select places that are closer to the floor or grade, for lighter gases like Hydrogen, we select higher levels where the gas is most likely to go after a leak. For a gas like Carbon Monoxide which is only slightly heavier than air, one should mount the sensor at approximately 1.6 meters above the ground level for best results.

The next step is to pinpoint potential leakage sources. These could be the outlet of a Pressure Safety relief valve, or flange joints at the inlet or outlet of a large reactor or other process vessel. All such points should be marked on a layout drawing of the plant, before we visit the site. Note that in this case, we are not targeting fugitive emissions, but only places where there can be a large amount of sudden release of a toxic or explosive gas. Other places include the inlet/outlets of blowers, gas turbines, ventilation equipment, etc.

After this study is complete, we can visit the site and actually see if our marked locations are logically correct. If it is an old site, talk to the operators or other plant personnel who can have an idea about the likely points of gas leakage. Once these are ready, we can mark these on drawings.

Next Steps

Next, select the gas detectors based on the amount of coverage for each point/area. If the area is small, a point type gas monitor will do. If the gas is expected to diffuse into the gas detector, nothing else is necessary. If however, it is felt that the gas may not easily diffuse into the detector, we may need additional forced sampling (like small vaccum pumps and collection funnels) to route the samples from the surrounding area into the gas detector. If the area is larger however, we will need an area monitor like an Infrared open path gas detector. These types of detectors cover a large open path (which is the gap between the infrared source and detector) that can cover hundreds of meters of an area. Ultimately the placement will depend on the likelihood of the gas, the type of gas monitors used and the total budget that can be allocated to the project.

Optimization

After the gas detectors have been ordered, installed and placed in the designated spots, the next stage is do a continuous performance monitoring. Are the detectors really detecting gases? Have leaks taken place that were not detected? If so, why? Were these not detected due to wrong placement or the gas detectors themselves were not functioning? Answers to these will enable you to optimize the placement further and ensure that your gas monitors do the thing that they are best at-protecting the plant, people and the environment from undesirable events.

Sam. P. Whittle is a technical expert on gas detectors and gas monitors. He is an engineer with several years of experience in various industries. At present he is on the panel of subject matter experts of Abhisam Software

Article Source: http://EzineArticles.com/?expert=Sam_P._Whittle

Welcome to the Gas Monitors blog!

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Industrial Plants, especially those that manufacture chemicals, process oil & gas or in other ways store or process flammable and/or combustible materials need various kinds of gas detectors to ensure operational safety. This blog post will give a brief overview of the various kinds of gas detectors used and the technologies behind them.

Usage of gas detectors

Gas detectors are mainly used for monitoring the presence (or in some cases, the absence) of gases and vapors. If we classify gas detectors on the basis of the gas being detected, we can classify them into three types as under.

1) Toxic Gas detectors

2) Explosive Gas Detectors

3) Oxygen detectors

Note that we can use the first two types of gas detectors to detect regular gases as well as vapors that are generated by volatile liquids. The third type is generally used to check if a restricted area (also known as a confined space-like the inside of a large storage tank) contains enough Oxygen to sustain a workman 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 detector can tell us if the vapor space is really inert or not. Thus in these cases, we are more interested in knowing about the absence of Oxygen, rather than its presence.

Technologies

There are a few major technologies that are in usage today for detecting gases. They are listed below.

1. Catalytic Combustion type

These detectors utilize actual combustion of the gas that is to be detected, on a hot “bead” that is part of a Wheatstone bridge network of resistors. The presence of the gas causes combustion that alters the resistance and thus can be converted into an electrical signal. This is the most common type of gas detector in usage today. The same technology is used in propane detectors.

2. Electrochemical Type

In this type of gas monitor, the sensor is an electrochemical cell with an electrolyte (which may be in the form of a gel) and two electrodes. The gas that enters this cell undergoes electrolysis and a voltage difference appears across the electrodes, thus generating an electrical signal that can indicate the amount of gas. This technology is used in toxic gas detector, h2s detector and hydrogen gas detector.

3. Infrared Type

These can be point type or continuous (also known as open path). It is well known that each chemical compound absorbs infrared light of a unique band of frequencies. Also the amount is that is absorbed is in proportion to the concentration. This principle is used in infrared type gas detection.

4. Semiconductor type

In this type of gas detector, the sensor is a doped version of Silicon or Germanium, that gets its conductivity altered by the presence of a gas. This principle is used to generate a usable electrical signal.

This is a short overview of the various kinds of gas detectors used in industries and homes today.