A combustible gas monitoring system in severe environment such as sewage processing plants for a long period of time has been developed. The concept and design of the novel gas sensing system is introduced in this page.
It has been a grave concern for big cities like Tokyo that spilled combustible liquid, such as gasoline, caused by traffic accidents in highways may flow into the cityfs sewage system, and this may result in abrupt increased risk of explosion in the sewage disposal facilities. In order to cope with such a disaster swiftly, a development of the combustible gas monitoring system has initiated.
(1)Degradation of sensor performance in the severe environment
Hydrogen sulfide and other gases that degrade gas sensorfs performance are often contained in the sewage atmosphere. Keeping sensorfs integrity for a long time is a tough challenge. Conventional catalytic sensors could not be used for this reason.
(2)Difficulty in gas sampling
Combustible gases are usually heavier than air; therefore, the gases are always stays near the water surface. In order to promptly detect combustible gases flown in, it is desirable to sample gases near the surface. However, there are two difficult challenges to achieve the goal:
- The surface level changes all the time due to rain fall and other human factors.
- Sampling tubes easily clog due to various contaminants in the sewage water.
THE DEVELOPMENT OF THE FLOAT GAS SENSOR
(1)Utilization of the optical VOC Sensor (IER method)
Principle of the VOC sensor (IER method)
A thin film VOC (Volatile Organic Compounds) sensor developed by O.S.P. Inc. is used in the system. The polymer thin film selectively absorbs organic compounds and slightly changes its thickness; the displacement is then optically detected to measure gas concentrations. Long time durability under the sewage atmosphere has been confirmed.
(2)Float Gas Sampling System
A plastic float, which is suspended by a stainless wire, is utilized in order to sample gases near the water surface. A curled sampling tube is attached to the float. The wire is spooled out from a tensional reel so that the wire will not touch the water regardless of the surface level. This is very important because accumulation of debris on the wire and tube will eventually hampers smooth movement of mechanical parts. This system has been also tested for a long time at a sewage facility. There was no problem even when experiencing extraordinary water flow due to a typhoon passing through the Tokyo area at the time.
(3)Can be used in other gas applications
Even though sewage facilities can be well prepared for the explosion risk by combustible liquids using this system, the sampling system can also be used in other gas monitoring applications as well.
Hard approach and soft approach
We found engineering projects in sewage facilities are much more challenging than those in other fields. Anything existing in cities will eventually flow in to the terminal processing facilities, and the processing system has to accept all of them. A very high reliability of the system is required. You might think that an easy solution to achieve the goal is to filter those before processing, but unfortunately, it won’t work. The filter will be clogged up by lots of debris in sewage water almost instantly. We chose rather “soft approach” than the hard one to tackle the complex problem. The sensor system swinging back and forth in the sewage water may look fragile, but it does work even in floodwater.
This system was developed in collaboration with Bureau of Sewage Tokyo Metropolitan Government, Tokyo Metropolitan Sewerage Service Corporation, O.S.P. Inc, Sakaguchi-giken, Co., and Tokyo Gas Engineering Co.,Ltd.