Types & Operation of Onboard Gas & Vapor Detectors

^ Gas Operator Checking Leakages – Image Courtesy Zorandim at ShutterStock.com

In the Interest of Onboard Safety

Transporting over 90% of the globally traded goods, ships carry cargo as diverse as chalk and cheese. Between chalk and cheese, ships haul great many flammable and toxic consignments with the latent capacity to blow up and poison the ship, the crew, and the vicinity.

Take the case of Liquefied Natural Gas (LNG) and Liquefied Petroleum Gas (LPG). Progressively tightening environmental norms are making many in international shipping use these clean fuels. But even positive measures come with their own set of challenges.

LPG vapors are heavier than air and settle in the lowest part of the vessel, usually the bilge area. Concentrations as low as one in seventy are explosive. Using bilge blowers does not guarantee safety as vapors may accumulate faster than the blower flushes them away.

Even before ships started burning LPG and LNG, they carried hazardous cargo. And there have been countless catastrophes triggered by such cargo. This is precisely why regulations mandate the use of vapor and gas detectors aboard ships.

Safety Standards

Standards such as IEC, NEC 505 (North America), CENELEC (Europe), and NEC 500 regulate explosion protection measures in potentially hazardous areas.

ATEX 95 lays down European norms for manufacturers of equipment and systems used in toxic or flammable environs. ATEX 137 prescribes European safety regulations for employers and end users dealing with such areas.

Relevant Terminology

Gas is any matter / substance that naturally exists (in the gaseous state) at temperatures above its boiling point. When considering normal pressure and normal operating conditions, a gas is any substance with a boiling point below 200C.

Vapor is gaseous matter below its boiling point. They are the gaseous state of a liquid or a solid created when the temperature and pressure of the solid / liquid changes. Remember, this change of state occurs without boiling of the liquid / solid.

Even harmless, non-reactive gases such as helium, argon, and nitrogen are dangerous because they displace oxygen and promote suffocation. That apart, all gases and vapors can be lethal because people in the vicinity breathe them. Hazards include:

  • Explosion of flammable gases
  • Poisoning by toxic gases
  • Suffocation / Asphyxiation due to oxygen displacement
  • Flammability Increase on account of oxygen enrichment

Lower Explosion Limits (LEL) is the least concentration (in volume%) of a gas or vapor (in air) at which it can trigger a fire or a flash in the presence of a source of ignition. The LEL of all known flammable gases and vapors is between 0.5 and 15 (in volume%).

Flashpoint is the minimum temperature at which a volatile substance vaporizes to a level above its LEL. Gases with low LEL and vapors with low flashpoints are more likely to explode.

LEL Gas Detector Image Courtesy photostock77 at ShutterStock.com

LEL Gas Detector Image Courtesy photostock77 at ShutterStock.com

Flashpoint is a more comprehensive indicator of the explosion potential of a flammable liquid because it is a combined measure of the volatility and LEL of the liquid.

Gas & Vapor Detectors

Detectors set off alarms when the concentration of the target gas (the one that we have to detect) reaches a certain percentage of its LEL, usually 20%. They may also initiate actions to prevent the consequences of the leakage from materializing.

Vapor and Gas Detection Transmitters operate at a standard voltage of 24V and standard currents of 4-20mA. Manufacturers superimpose a ±0.5mA frequency modulated signal on this 4-20mA current to exchange digital information between the transmitter and the controller.

Such superposed signals are the HART signals. These enable remotely located clients to check if the transmitter is functioning properly and initiate preventive maintenance if necessary.

Check cargo tanks and other enclosed spaces for vapor and gas concentration before:

  • entry
  • repairs
  • loading

Before permitting entry into an enclosed space, check it repeatedly with the detector. Allow entry only if you consistently get safe readings. Detectors are most essential, if you:

  • Use LPG aboard ships. In this case, select units that automatically shut off the electrically-operated gas solenoid valve
  • Leave your boat unattended for prolonged durations. In this case, go for detectors that sound loud external alarms audible above the engine noise. If such alarms come with warning lights, so much the better
Gas Leak Detector & Vaporizer Control Panel   Image Courtesy N center at ShutterStock.com

Gas Leak Detector & Vaporizer Control Panel
Image Courtesy N center at ShutterStock.com

Detector sensitivity depends on the:

  • type of gas
  • temperature
  • humidity
  • age of detector
  • presence of impurities such as silicone vapors, lead, aerosols in paints, sulphur, and gases such as halogens, hydrogen sulphide, and sulphur dioxide

Conditions at the location of the target gas can corrode or even destroy the sensor. Material such as silicones, lead, halogenated gases, oils, acids, and water deactivate electrical sensors. This is why detectors sample the gas away from the site before measuring its concentration.

Detectors work well only when you install, position, operate, and maintain the sensors properly. Always place the sensor in leakage prone areas (with sampling measures of course) such as pumps, bellows, valves, shut-off devices, flexible tubes and their connections, and flanges.

Sensors can be:

  • Electromagnetic
  • Catalytic Bead
  • Infrared (IR)
  • Diffusion Controlled
  • Open Path

Electrochemical Sensors utilize the hyper reactivity of toxic gases. The sensor mobilizes electrons when gases hit it. Electrons in motion are current. The amount of current depends on the electron flow. Because operations involve very low currents, these sensors are safe.

Catalytic Bead Sensors detect the rise in temperature of the catalytic material that gets heated up in the presence of flammable gases and vapors. Sinter Disks and Flameproof Enclosures guard such sensors against explosion.

Wires carrying electric current pass through tiny catalytic beads. When the flammable gas passes over them, the beads get heated up and heat the wire. This increases the wire’s electrical resistance. Makers calibrate this change in resistance in terms of gas-vapor concentration.

Because the response of these sensors changes when the size, type, and calorific value of the gas-vapor molecules changes, there is an inherent inaccuracy in detection and concentration measurement.

For example, LPG is a blend of several gases. Different samples of LPG contain different percentages of these constituents. The variation means that the LEL for LPG varies over 60%.

Infrared (IR) Sensors operate on the tendency of certain gases and vapors to absorb radiation. The outgoing infrared intensity is less than the incoming intensity. And, a defined gas concentration always produces the same drop in intensity and therefore the same measuring signal.

Multi Gas Detector Image Courtesy florin opera at ShutterStock.com

Multi Gas Detector Image Courtesy florin opera at ShutterStock.com

Hydrocarbons absorb IR radiation in the wavelength range of 3.3µm to 3.5µm (micrometer). This is a very useful, distinguishable property as hydrocarbons make up most of the flammable gases and vapors.

Condensation, contaminated optics, and issues with the intensity of the source of IR radiation can introduce errors. Proper maintenance and operation overcomes these glitches. IR sensors are increasingly popular and find application even in the process industry.

Diffusion Controlled Sensors use the natural diffusion process of gases to measure concentration. They maintain a concentration difference to pull in the target gas. These are more accurate than electrochemical and catalytic sensors because they do not react with the target gas.

Open Path Detectors are similar to IR Detectors, but with an optical path extended many times over. These are more useful for detection than for concentration measurement.

Personal Hydrogen Sulphide Gas Detector

Image Courtesy photostock77 at ShutterStock.com

These are typically used in natural gas exploration where personnel activate safety measures when they detect the target gas, regardless of its concentration.

Alarms can be:

  • Main Alarms alert you and shut down the gas-vapor system when the gas-vapor concentration reaches the pre-set threshold value

Although effective, shutdowns are expensive and cumbersome, for you have to examine the system and start it again. Good alarm designs never or seldom set off the main alarm

  • Pre Alarm alerts you when the gas concentration reaches a certain fraction of the threshold value. You can initiate precautionary measures to prevent the setting off of the main alarm that triggers a troublesome shutdown
  • Fault Condition Alarm indicates that the detector is inoperative, partly or totally

Flameproof Enclosures are the earliest safety device. Totally mechanical, they allow the explosion to occur inside them but do not allow the violent effects of explosion to move outside.

Design Considerations & General Safety Precautions

Electrical sparks do not ignite even strongly flammable mixtures unless they possess certain minimum energy. And electrical components cannot ignite even a highly inflammable gas unless they get heated up to its ignition temperature.

Use carefully designed electrical circuits that do not:

  • create sparks strong enough to ignite the leaked gas
  • heat up to a temperature that ignites the gas

General precautions:

  • study the maker’s handbook before calibration and use
  • recalibrate the instrument as frequently as the maker recommends
  • be extra careful when setting zero
  • regularly replace limited-life parts such as tubes
  • maintain all parts of the detector correctly
  • ensure steady flow of gas
  • frequently check the power of batteries of portable detectors

Never immerse the detector in water. That will end its life. For the same reason, do not place it too low in the bilge. When testing the sensor by squirting gas from a cigarette lighter, use the gas sparsely. Or you will burn the sensor. This and the built-in safe test are makeshift tests only.

For thorough testing, examine the sensor using calibrated concentration of gas. Although complex and expensive, this is a thorough test. The technician will charge you around $75 for the testing visit. Replacing the sensor might be simpler and cheaper.

Finally

Preventive maintenance may appear redundant. Gas and vapor detectors do come at a cost but avert calamities that can cost many times as much . . . in lives and in dollars. Talking of expenses, your insurer will be happy if you install a detector and even provide a discount!

For more exhaustive details on safety practices aboard ships, visit our blog. And for the best in marine fabrication services, marine pipe fitting, and large scale custom metal fabrication, contact Kemplon Engineering.