Combustible Gas Alarm System Warns of Presence of Explosive Concentrations

USE of propane as a fuel for motor buses has created new problems in storing and handling this highly volatile compound which is a gas in normal atmosphere and a liquid upder pressure. The successul and increasing utilization of LP as gas for household fuels has demonstrated that under proper safeguards it is no more dangerous than any other gas. However, storing the large quantities required for fueling buses and the frequent handling of the propane do pose a different problem.

In recognition of this, the San Antonio Transit Company, San Antonio, Texas, recently built new storage and maintenance facilities for its fleet of propane-fueled buses, designed for maximum safety to minimize fire and explosion hazards.

The transit company engaged the Hudson Engineeering Company, Houston, Texas, to assist in designing the new plant. The Fire Prevention and Engineering Bureau of Texas and the Health and Safety Division of the U. S. Bureau of Mines participated as consultants. Through their combined efforts, the bus company’s new storage and maintenance facilities are said to be a “model” for this type of operation.

One of the chief problems of such an installation concerned a method for ac-

curately detecting and reliably warning of combustible concentrations of propane in the atmosphere around storage tanks, piping, valves, fueling station, and repair pits. This problem was solved by the installation of multi-point combustible gas alarm systems developed by Mine Safety Appliances Company, Pittsburgh. Two separate systems are used—one to protect the outdoor storage and loading and unloading facilities and the other to serve the repair zone inside the modern steel and glass maintenance building which has a floor area of some 45,000 sq. ft.

These two combustible gas alarm systems continuously sample the atmosphere from eight different locations, analyze the samples and instantly warn when the gas-air mixtures reaches 40 per cent of the lower explosive limit. This enables personnel at the terminal to take immediate remedial action.

Four sampling lines run to each alarm panel. The system that protects the storage unloading area and fueling rack has one line running 240 ft. from the instrument to the six 5,000-gallon propane storage tanks. Tanks, valves and piping are inside a two and one-half foot high concrete fire wall. Another sampling head is located in the covered trench that contains fuel lines from the storage tanks and the fueling rack. This line is arranged to detect any leaks in the transmission piping. There is another line drawing samples from the propane unloading dock about 75 ft. from the instrument panel. The fourth line serves the bus-filling station.

About 300 ft. west of the filling station, which is an outdoor canopy-covered area, is the service building and warehouse. The second four-point gas alarm samples air from repair pits in this building, where, it was explained, some propane might conceivably escape through leaks in carburetor lines or fittings, and pressure relief valves on the tanks. Fuel in tanks filled outdoors on a cold day might be liberated through pressure relief valves if the buses are moved inside and warmed up. Temperature rise causes the propane gas to expand. Because propane is so volatile and the fuel systems are under pressure, contamination of the atmosphere would occur faster than with fuels of lower volatility. For this reason, the automatic alarm system is designed to provide a warning long before the air becomes dangerously contaminated.

In cities where the weather is cold during the winter months, the need for a positive, instantaneous gas detection and alarm system is even greater than in warmer climates, due to the gas expansion problem.

The outdoor filling station at San Antonio is equipped with a water fog system that is manually operated from points close to the fueling rack. Water fog systems also are provided for the unloading dock and storage tanks. The terminal itself covers several acres. The area is uncrowded which is conducive to good housekeeping, an essential part of any well-balanced safety program. Buses are stored in the open.

General view of the outdoor bus parking area at San Antonio Transit Company's new terminal. In the center are the six 500-gallon propane tanks for storage of the liquefied petroleum gas used for fueling the buses. This parking area handles more than 300 buses.Transport truck unloading station is protected by water fog sprinkler shown mounted on the reinforced concrete buffer. The station is several hundred feet from the storage tanks. At the left may be seen one of the air sampling tubes for the combustible gas alarm. The mechanical filter of the tube is circled. Trucks are grounded to prevent static sparks during unloading.To detect immediately any propane leaks in piping, pumps or valves at the storage tanks, one air sampling intake is located there. It is shown circled in the center of the photo in the trench. Propane gas is heavier than air and sampling points are located in low spots.

The combustible gas alarm systems, which can be adapted for use with nearly all known combustible gases, are explosion-proof so they can be mounted in the hazardous area. In operation, air samples are drawn into the instrument and passed over one of two heated sensitized platform filaments which form a balanced electrical circuit. One filament, known as a compensator, is sealed in a cell containing uncontaminated air. Any combustibles present in the air sample are burned on contact with the other filament, known as the detector. This raises its temperature and its electrical resistance, unbalancing the circuit.

The degree to which this circuit is unbalanced is directly proportionate to the concentration of combustibles in the sample. This is measured by an indicating contact-making meter, which is designed so that when the needle reaches a predetermined point, a magnetic contact is closed to energize the alarm signal circuit. At the San Antonio installation, the alarm consists of a red light flash on the instrument panel, and a horn sounding. The instrument also can be arranged to automatically operate ventilation systems or to actuate any other mechanical equipment, including sprinklers.

Air samples are constantly drawn through the lines to the instrument by means of a turbo compressor. Indicator lights on the panel of the instrument show from which area the contaminated sample was taken.

Throughout the entire bus terminal, every other precaution is taken to safeguard against fires and explosions. All electrical equipment is explosion-proof and installed in accordance with Article 500 of the National Electric Code, regulations of the State of Texas and recommendations of the National Board of Fire Underwriters. The color identification code of the American Standards Association also is followed.

No posts to display