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Marine/Ocean engineering other topics FAQ
Marine LPG-petrol-gas Detector
FAQ: Marine explosive fuel vapor detectors
Posted: 6 Nov 02
Marine LPG-petrol-gas Detectors - Facts and Figures
by Bert van der Berg, founder, Cruzpro, New Zealand
(submitted by www.Exomarine.com)
Liquefied Petroleum Gas (LPG) and gasoline vapors are heavier than air and will settle in the lowest areas of your boat - usually the bilges. At concentrations as little as one part in 70 to air, they can form a lethal mixture that will explode when ignited. A pint of vaporized gasoline has about the same explosive power of a half stick of dynamite! If you thought that fire was the worst enemy, just think what a violent explosion could do to your boat and the people aboard. For this reason gas vapor detectors are mandatory equipment for many commercial boats and recommended on boats with enclosed spaces that carry LPG or gasoline.
Use of a bilge blower does not insure that explosive levels of gas are automatically removed. Fuel leaks can create new concentrations as fast as the blower clears them. Since bilge blowers are usually only exercised before engine start they offer little protection at other times. On the other hand, just installing a gas detector does not automatically mean you're safe from on-board explosions. Gas detectors have their limitations and knowing how gas detectors work will enable you to make an educated decision as to whether to buy one, how much to trust it and how to maintain it.
Marine gas detectors usually cost under $200.00 and can provide an early warning before gas concentrations reach the Lower Explosive Level (LEL). Most such units are sensitive enough to warn you when gas concentrations reach 20% of the concentration required for an explosion and a few can detect gas concentrations at levels as low as 10% of LEL . The CruzPro GD20 even has three selectable sensitivities (10, 20 and 30% L.E.L.). They are available for both 12 and 24 volt systems and some models work on both voltages. The better units have remote sensors that can be placed in the areas where gas vapors accumulate and enable you to monitor several areas by supporting more than one sensor. All units have built-in self test for the internal electronics which usually also automatically tests the sensor(s) and connecting wires.
For LPG users, look for units that will automatically shut off your electrically operated gas solenoid valve. A side benefit of this feature is that they usually enable you to turn your LPG solenoid valve ON/OFF manually as well. If your boat is unattended for long stretches of time, look for a unit that provides for an external alarm. External alarms are usually louder, can more easily be heard above engine noise, attract more attention, and some external marine alarms will flash your spreader lights or marine strobe.
The best gas detector won't work as designed if not properly used and maintained. It's important to understand that gas detector sensitivity varies with the type of gas, temperature, humidity, age of the detector, presence of contaminating "poisons" and other variables. For example, LPG is manufactured as a mixture of gases such as propene, propane, butene, butane, ethane and ethylene as well as a volatile mercaptan, an odorant added as a safety precaution. Each of these gasses have their own LEL and your LPG can be formulated with any mix of these gasses. The resultant LEL can vary as much as 60%. Most gas detectors use porous catalytic "hot bead" combustible sensors whose response varies with molecule size and heat value of the gas. These hot bead sensors are electrically heated and safely "burn" the hydrocarbon molecules that are absorbed by the porous surface. In the presence of combustible vapors, the temperature of the hot bead rises and a buil-in temperature sensor provides an electrical signal to the electronics.
These sensors can be affected by humidity and their sensitivity can be temporarily or permanently damaged by the presence of aerosols from paints, silicone vapors, sulphur, lead and gasses such as hydrogen sulfide (a contaminant of LPG), halogen, and sulphur dioxide. If the detector is allowed to be immersed in water at any time, it will be immediately and irreparably damaged. Don't place the detector element too low in the bilge. If you use solvents or paint near the detectors, be sure to cover them with tape or a condom to prevent sensor poisoning. Leave yourself a note to remove the protection so you don't forget.
Gas sensors are often tested by squirting a little gas from a gas cigarette litghter into or near the detector. A word of caution - use the gas sparingly because you can litterally burn the detector, causing permanent damage. The results of such a test will not indicate whether the sensor is working properly - only that its not "dead".
The built-in self test will likely show up open or shorted sensors and broken wires but does not actually test if the sensor is working properly or check the sensitivity. A sensor that has been poisoned or destroyed .may not show up in the built-in self test. The only way to insure that your gas detector is functioning correctly is to subject the sensor to a calibrated concentration of gas - not an easy or cheap item to obtain. To be sure that your gas detector is working properly you should have it checked at least once per year (at the start of the boating season would be a good time). The cost to have someone come to your boat and check it with a calibrated gas is approximately $75.00 per visit. It may be cheaper to replace the detector element.
When I informed my insurance company that I had installed a gas detector and bilge alarm, they voluntarily rebated me 5% of the cost of my boat insurance. You might inquire to see if your insurance company will reduce your premiums for installing a gas vapour detector
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