Comments Off on Is an LEL sensor dependable in an H2S LEL environment?
Question: One of our Canadian friends in the Industrial Hygiene community sent us the following question:
A colleague of mine was talking the other day about the presence of H2S in crude oil. In July, 2013 we had a terrible rail accident involving crude oil that occurred in the town of Lac-Mégantic in Quebec. I have heard no mention of H2S in official reports about the accident. Nevertheless, some unofficial data is indicating that H2S may have been present in very high concentrations, perhaps even reaching the flammable limit of 4%. You have mentioned that H2S is a poison to catalytic combustible LEL sensors. Am I correct in guessing the LEL sensor would not be dependable in an H2S LEL environment? Also, if H2S is damaging to the sensor, what concentration of H2S can the LEL sensor handle, and for how long?
This is a great question. I think the answer will make a good post for our gas detection blog on the company website ( www.goodforgas.com ).
It is true that H2S is a potentially flammable gas. The LEL concentration for H2S is only 4.0%. H2S can easily reach 100% LEL in many confined space and oil industry settings, especially when the situation involves “sour gas” or “sour crude” that contains high concentrations of sulfur.
Exposure to high concentrations of H2S can inhibit or poison catalytic LEL sensors. To keep this from happening, most LEL sensors, especially the ones used in portable instruments, have an internal filter to remove the H2S before it can reach and damage the active bead in the sensor. This is why you can use multi-component calibration gas that includes both H2S and LEL gas to calibrate the combustible gas sensor without causing damage to the sensor.
Catalytic LEL sensors detect gas by catalytically oxidizing or “burning” the gas on an active bead or “pellistor” located within the sensor. The sensor contains two coils of fine platinum wire which are coated with a ceramic or porous alumina material to form beads. The beads are wired into opposing arms of a balanced Wheatstone Bridge electrical circuit. The “active” bead is treated with a platinum or palladium-based catalyst that facilitates the oxidation of combustible gas on the bead. A “reference” bead in the circuit that has not been treated with catalyst provides a comparison value. As oxidation occurs the active bead is heated to a higher temperature. Since heating due to oxidation of the combustible gas only occurs on the active bead, the difference in temperature between the two beads is proportional to the concentration of gas in the area where the sensor is located. Because the two beads are strung on opposite arms of the Wheatstone Bridge circuit, the difference in temperature between the beads is registered by the instrument as a change in electrical resistance.
The LEL sensor depends on the activity of the catalyst to detect gas. If the catalyst is harmed by exposure to sensor poisons or inhibitors, the sensor’s ability to detect gas is affected. In the case of sensor inhibitors the effects may be reversible, and the sensor may recover over time (at least to an extent). In the case of virulent LEL sensor poisons like tetraethyl lead and vapors that contain silicone, the damage is rapid and irreversible. H2S functions both as a poison and as an inhibitor. The sensor may recover some of its lost sensitivity over time, but once the bead has been exposed to H2S, there will be permanent harm.
The internal filter used to remove H2S has a capacity on the order of 20,000 ppm minutes. Each time you expose the LEL sensor to gas that contains H2S you use up a little of the remaining capacity of the filter. For most instrument users the capacity of the filter is sufficient to protect the LEL sensor for the entire expected life of the sensor. Bump testing the instrument before each day’s use has very little effect on shortening the life of the LEL sensor.
During the bump test the sensors are exposed to known concentration test gas. The concentrations used in the test gas need to be high enough to activate the alarms. In Canada, CSA takes the requirements a step further. Because LEL sensors can be poisoned or damaged by exposure to sensor poisons like H2S, it’s not enough simply to activate the LEL alarms. To pass the test CSA requires that the reading of the LEL sensor is between “minus 0% and plus 20%” of the concentration of gas applied. In other words, if you use 50% LEL gas to test the combustible sensor, to pass the CSA version test the readings must stabilize between 50% LEL and 60% LEL.
Typical “four gas” multi-gas atmospheric monitors include sensors used to measure O2, LEL, CO and H2S. The gas typically used to perform bump checks on these meters is a multi-component mixture that is designed to test all of the sensors at the same time. The gas (depending on the manufacturer) usually includes 20 ppm or 25 ppm H2S.
It typically takes less than 30 seconds to perform a manual bump test. Thirty-seconds of exposure to 20 ppm H2S is the same thing as “10 ppm minutes” of exposure. Since the capacity of the filter is 20,000 ppm minutes, it takes around 2,000 bump tests to saturate the filter. Even if you bump test the instrument every day, five days a week, 50 weeks per year, it will still take over 8 years before you begin to cause damage to the LEL sensor due to the H2S in the calibration gas.
Docking stations are quite a bit faster, and expose the sensors to less gas, which is one of the reasons gas detector manufacturers encourage their use. They save money as well as time, improve the accuracy of the test results, and simplify documentation.
By definition, “sour” natural gas contains at least 4 ppm H2S. However, the concentration can be much higher. The gas from one well in Canada is known to contain 90% hydrogen sulfide, while other wells are documented have H2S in the tens of percent range. Some oil industry customers have specialized procedures that can expose workers (while wearing SCBA and other protective equipment) to H2S concentrations that can reach several thousand ppm. Even a short period of use in high concentration H2S can rapidly saturate the filter, and damage the LEL sensor. Twenty minutes of continuous exposure to 1000 ppm H2S would be enough to saturate the filter. Once the filter is saturated, the active bead can be damaged very rapidly by further exposure. This is one of the reasons it is so important to perform a bump test to verify the performance of the LEL sensor before each day’s use.
We normally expect (and warrant) GfG catalytic LEL sensors to last for at least three years. However, some of our sour gas customers need to replace LEL sensors on a yearly basis.
It’s hard to predict in advance when (or if) the internal filter is close to saturation. While damage to the bead is electronically detectable, saturation of the filter is not.
The important thing is to verify that the readings of the LEL sensor are accurate by testing or calibrating the sensor before each day’s use! Sensors that fail a daily bump test need to be calibrated or replaced before further use.
Thanks again for a great question!