Hydrogen Sulphide H2S Gas Detection – See How Crowcon Detects H2S Gas Levels
Published 23 Sep 2016
T&D, the specialist distributor of Gas Detection, Detectors & Monitoring Equipment for H2S gas detection.
Crowcon Hydrogen Sulphide H2S Gas Detection
H2S Gas Detection
Watch, listen and learn how Crowcon, H2S gas detection specialists, protect people and plant from the gas risks posed by flammable and toxic gas.
Hydrogen sulphide gas is considered a broad-spectrum poison, meaning that it can poison several different systems in the body, although the nervous and respiratory systems are most severely affected. Besides being highly toxic H2S is a flammable gas – heavier than air and hence tending to accumulate in low-lying areas.
H2S is pungent but rapidly destroys the sense of smell.
H2S gas is toxic at breathable concentrations between 500-1000 ppm, but death is not instantaneous. However, at concentrations of greater than 1000 ppm, H2S is lethal.
Industry practice is to recognise that fatality from H2S exposure can occur over a wide concentration band but at around 500–1000 ppm exposure for a short period, the fatal exposure levels would be significant. 800 ppm is the generally accepted lethal concentration for 50% of an exposed human population for 5 minutes exposure.
- Hydrogen sulphide naturally occurs in crude petroleum and natural gas reservoirs
- H2S gas is toxic at less than 20 parts per million
- Gas becomes flammable at 4.3% of air by volume
- H2S gas is 1.2 times heavier than air so stays low in windless conditions
- Hydrogen sulphide has a highly corrosive nature which increases the chance of a gas leak
- H2S inhibits pellistors so infrared sensors may well provide a better gas detection option
- Personal and fixed gas monitors are needed to ensure worker safety against hydrogen sulphide levels
h2s gAS eXPOSURE – THE EXPOSURE RISKS
- 1,000-2,000 ppm: Loss of consciousness and very possible death
100-1,000 ppm: Serious respiratory, central nervous, and cardiovascular system effects
150-200 ppm: Olfactory fatigue (sense of smell significantly impaired)
100 ppm: Immediately Dangerous to life and health (IDLH concentration)
5-30 ppm: Moderate irritation of the eyes
5-10 ppm: Minor metabolic changes in exercising individuals during short-term exposures
Less than 5 ppm: Metabolic changes observed but not clinically significant
5 ppm: Increase in anxiety symptoms (single exposure)
5 ppm: Start of the dose-response curve (short-term exposure)
0.032-0.02 ppm: Olfactory threshold (begin to smell)
H2S Gas Detection offshore
The UK HSE (Health & Safety Executive) have provided guidance to duty holders for detecting and managing hydrogen sulphide (H2S) hazards in hydrocarbon processing systems. As reservoirs begin to water out, hydrogen sulphide can become an issue when processing returning fluids. The gas is toxic in relatively low concentrations and the risks to the workforce need to be safely addressed once the presence of hydrogen sulphide has been detected.
H2S presence in reservoir streams tends to increase with time and it is important therefore that duty holders ensure that if the concentration levels in modules begin to increase the toxicity status of the areas are revised accordingly. The recommended strategy for fixed and portable detection is based on accepted management practice for confined space entry, and normally accessible process modules.
On an offshore installation, the standards recommend a 3-tier approach to H2S gas detection; similar to the flammable hazardous area classification arrangements:-
- Category 0: Areas where H2S will be present during normal operations – confined spaces, vessels etc.
Fixed H2S detection is not recommended in areas where H2S is known to be present at high concentrations during normal operations. The argument is that if the presence of the toxic gas is known and known to be unacceptably high, there is no reason for installing detection.
Access is controlled by special precautions, confined space entry management, that starts with the removal of H2S and initial entry is by specifically trained personnel with portable H2S detection equipment wearing breathing apparatus (BA) and other protective equipment.
- Category 1: Areas in which H2S may be encountered during normal operations
Entry is only allowed with portable toxic detection equipment and fixed detection is recommended for these areas to maintain a risk history, but should not be used for making safety-related decisions.
- Category 2: Areas which are H2S free in the atmosphere during normal operations, but which may be contaminated by a leak, or equipment malfunction or intrusive activities.
For ‘open access modules’, where H2S is not normally present but could be following a leak, fixed detection is recommended in certain scenarios. This is based on a concentration of 500 ppm H2S in the ‘carrier’ stream of process fluids. If the H2S concentration is below 500 ppm, in the carrier stream, the areas may be classified as “2A” and H2S detection is via the standard hydrocarbon detection instruments. At concentrations greater than 500 ppm, in the carrier stream, the area is designated “2B”, and requires a H2S specific detection system because the hydrocarbon detection systems cannot provide a timely response to the more harmful, higher H2S concentrations levels.
This argument is based on an assumption in the standard (unsubstantiated) that process hydrocarbon carrier gas on release is diluted to 1% by air entrainment. Hence if the hydrocarbon gas contained 500 ppm H2S, the toxic component would be diluted to 5 ppm; the occupational exposure limit (OEL) for the gas. However, there is no supporting argument in the standard that a 100:1 dilution will be achieved if a hydrocarbon gas stream is released to atmosphere.
OFFSHORE HYDROCARBON RELEASES (2001-2008)
The offshore industry employs about 28,000 personnel in the UK involved in a wide range of activities.
Since the Piper Alpha disaster in 1988, health and safety issues concerning offshore platforms have vastly reduced, however, the work practices involved are not risk free and still have the potential to cause considerable loss of life when things go wrong.
A falling oil price, declining reserves and an ageing infrastructure have resulted in increased drilling activity around marginal fields. Operators have looked for new ways in which to cut costs, which could affect the health and safety of the workforce.
In recent years there has been an increase in the number of major and significant hydrocarbon releases on offshore platforms that require investigation. – the following report aims to identify the immediate cause of hydrocarbon leaks and determine if there are discernable reasons for the increasing trends.
H2S HYDROGEN SULPHIDE GAS
CONCENTRATIONS & EFFECTS
- 0.0047 ppm is the recognition threshold of human smell, the concentration at which 50% of humans can detect the characteristic odour of hydrogen sulphide.
- 10-20 ppm is the borderline concentration for eye irritation.
- 50-100 ppm leads to eye damage.
- At 150-250 ppm the olfactory nerve is paralyzed after a few inhalations, and the sense of smell disappears, often together with awareness of danger.
- 320-530 ppm leads to pulmonary edema with the possibility of death.
- 530-1000 ppm causes strong stimulation of the central nervous system and rapid breathing, leading to loss of breathing.
- Concentrations over 1000 ppm cause immediate collapse with loss of breathing, even after inhalation of a single breath.
h2s Hydrogen sulphide is a highly toxic gas
Hydrogen sulphide is a colourless gas that is known by its characteristic rotten egg like odour appearing naturally as a by-product of decomposition. A drawback to trusting the senses (olfactory) for protection against hydrogen sulphide is that prolonged exposure to the gas renders the sense of smell inoperative.
H2S reacts with the enzymes in the blood stream which inhibit cell respiration.
Put simply, high concentrations of hydrogen sulphide can shut off the lungs. Low concentration exposure to the gas can burn the respiratory tract and cause swelling around the eyes.
Detecting Hydrogen Sulphide With Crowcon
The following table provides an overview of the Crowcon Gas Detectors available for detecting hydrogen sulphide – for expert technical support and product selection guidance please call T&D Sales Engineers.
gas detection – h2s ch4 o2 co
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