Gases contained in a sanitary sewer contain not only a health risk but a safety concern. A sewer can contain toxic levels of ammonia, methane, carbon monoxide and hydrogen sulfide. All present health risks when inhaled. Methane and hydrogen sulfide are both extremely combustible gases. An inadvertent spark can cause not only a fire but an explosion. The concentration level of gas in a closed system such as a sanitary sewer is exponentially higher than an open system.
For a technician to work safely in a sanitary sewer line, the air must be ventilated. Merely removing manhole covers is not sufficient ventilation. A motor driven ventilator is utilized for this purpose.
Proper manhole ventilation is not not performed by placing the machine at the nearest manhole to the worker. Rather, the ventilator is placed at the next manhole that is upstream or downstream from the worker. This method serves three purposes. First, placement at the next manhole provides the worker easy ingress and egress to the sewer. Second, the motor is further away from the worker, providing a quieter work area. Third, the ventilator draws in fresh air through cross ventilation.
The ventilator will be manufactured to fit snugly over the manhole and provided with a foam rubber seal under the cowling. A small engine, like those used to operate lawn mowers is attached to the cowling. It is used to rotate the impeller or fan blades. Where a highly explosive environment is present, the ventilator can be fitted with an electric motor. The rubber seal also minimizes vibrations. Quality ventilators utilize aluminum fan blades since plastic blades can be easily damaged or broken. Aluminum is also used instead of steel to avoid rust and corrosion.
The motor must be powerful enough to draw several thousand cubic feet from the sewer per minute. This rate of flow ensures the work area of the sewer is properly ventilated in seconds and prevents a build up of gases from upstream areas of the sewer line. Finally, the ventilator can serve a dual purpose by acting as a line stringer. A quality ventilation machine can string a line between manholes in under twenty seconds.