Gas and H2S Detection

Gas and H2S Detection covers fixed point, open path and acoustic detectors that watch for flammable gas and hydrogen sulphide around the well, the mud system and air intakes.

Each head reports to a fire and gas panel or safety PLC. The sensing technology, the Ex certificate and the output signal decide whether a used unit fits your system.

Where fixed gas detection sits on a rig

Gas from the well shows first where returns reach atmosphere. Typical locations are the bell nipple, the flowline and possum belly, the shale shaker room, the mud pit room, the degasser and mud gas separator vents, the well test area and every HVAC air intake. Intake detectors close dampers and trip fans so gas cannot be drawn into the accommodation or engine rooms.

Each detector sends a continuous signal to the fire and gas panel. The panel compares it with alarm levels set in %LEL or ppm and runs the actions in the cause and effect chart. Mount H2S heads low enough to see a heavier-than-air gas, and methane heads near ceilings or vents where it collects.

Sensing technologies

Technology	Detects	Main limitation
Catalytic bead (pellistor)	Most flammable gases, including hydrogen	Needs oxygen; poisoned by silicones and lead; saturates above 100% LEL
Point infrared	Hydrocarbons	Blind to hydrogen; optics foul in mud vapour
Open path infrared	Hydrocarbon clouds across a beam, in LEL·m	Beam blocked by fog, spray or moving equipment
Electrochemical	H2S, CO, O2	Finite cell life; drifts in heat and low humidity
Metal oxide semiconductor	H2S	Needs heater power; cross-sensitive to other gases
Ultrasonic	The sound of a pressurised leak	Measures no concentration; misses low-pressure leaks

Infrared has largely replaced catalytic beads for methane on drill floors because it cannot be poisoned and fails to a fault signal. Catalytic heads still have a place where hydrogen or mixed gases matter.

Key specifications

Parameter	Why it matters	Typical classes or units
Gas and range	Sets the calibration and alarm points	0 to 100% LEL; 0 to 20, 50 or 100 ppm H2S
Response time	Decides how soon the panel acts	T90 in seconds
Ex marking	Decides which zones it may be fitted in	Ex d or Ex ia, IIC, T4, EPL Gb
Output	Decides panel compatibility	4 to 20 mA, HART, Modbus RS-485, relays
Ingress protection	Washdown and spray offshore	IP66 or IP67
Operating temperature	Hot shaker rooms and cold intakes	Stated range in °C and °F

Standards and what they ask of a used head

IEC 60079-29-1 covers performance for flammable gas detectors and IEC 60079-29-4 covers open path units. IEC 60079-29-2 tells you how to select, site, calibrate and maintain them, and it expects a documented calibration and maintenance record for each head. EN 45544 covers toxic gas detectors such as H2S heads. The Ex certificate covers the enclosure only for the configuration listed on it, so a replacement sensor must be the maker's approved part.

Failure modes and inspection

Electrochemical cells dry out or run down. Check the manufacture date, not just the install date.
Catalytic beads lose sensitivity quietly after exposure to silicone sprays or H2S. A bump test shows it before the panel does.
Point IR optics foul with mud vapour and oil mist. Inspect the window and the weather protection cap.
Flameproof joints corrode. Look for pitting on flame paths, missing cover bolts and painted-over threads.
Cable entries lose their seal. Check glands and stopping plugs for certification and fit.

A proper check on a used unit is a full calibration with certified gas, a response time test, an output loop check at 4, 12 and 20 mA, and a fault simulation.

Documentation

Ex certificate and nameplate photos showing the serial number.
Type approval from DNV, ABS or another society if the rig is classed.
Manufacture date of each sensor and the last calibration certificate.
SIL certificate or safety manual where the head forms part of a SIL-rated function.

Integration

Match the head to the panel's input type and fault current before anything else. Then check supply voltage, mounting bracket, sunshade and calibration adapter. Detectors from the same family as your installed heads save a change to the cause and effect chart and to the spare parts list.

Recertification and refurbishment

Recertification of a fixed head means a new sensor, a full calibration and an output check, recorded per serial number. Only the maker or a repairer working to IEC 60079-19 should open a flameproof enclosure for repair. A surplus unused head still needs a fresh sensor if its cell has aged on the shelf.

For personal monitors used alongside fixed heads, see portable gas detectors. For the signals that trip equipment, see alarm and shutdown systems.

FAQ

Frequently asked questions

Do H2S sensors expire?

Yes, electrochemical H2S cells have a finite life that runs from manufacture, whether or not the detector is powered. Makers state the expected life in the manual, often around two to three years for H2S cells, and less in heat and dry air. Check the date code on the cell, not the detector.

Catalytic or infrared, which fixed gas detector suits a drill floor?

Infrared suits most drill floor and shaker room methane duties because it cannot be poisoned and needs no oxygen. Catalytic bead detectors remain the choice where hydrogen or a mix of gases must be detected. Many rigs run both.

What should I check on a used fixed gas detector?

Check the Ex marking and certificate, the sensor manufacture date and the last calibration record first. Then inspect flame paths, cable entries and the optical window. Finish with a full calibration, a response time test and an output loop check.

Can I fit a different maker's sensor in my gas detector?

No, use only the sensor the detector's Ex certificate lists. A non-approved sensor voids the certification and the performance approval to IEC 60079-29-1. It may also change response time and alarm behaviour.

What does LEL·m mean on an open path gas detector?

LEL·m is the reading unit of an open path detector, the gas concentration in %LEL multiplied by the length of the cloud along the beam. A small rich cloud and a long lean cloud can give the same reading, so set alarm levels with that in mind.

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