This invention relates generally to pipeline monitoring systems and more particularly concerns a sensor for detecting passage of an object, such as a pipeline pig, through a pipeline.
Various means have been developed for detecting the passage of a pig through a pipeline. “Dumb pigs” or “smart pigs” may be detected by intrusive mechanical devices such as threaded adapters with spring-loaded shafts. The shaft has an exposed end with a spring loaded-lever or flag and an opposing end that extends into the interior space of the pipe. Intrusive detection devices, however, require making a hole or hot-tapping into an active piping, an often costly and inconvenient process for a pipeline operator. As a result, non-intrusive detection devices, which are fully located outside of the pipeline and do not require additional hot-tapping or welding, are often preferred by the pipeline operator.
There are two main types of non-intrusive detection devices: acoustic/ultrasonic detectors, which detect a change in sound, and electromagnetic detectors, which detect a change in ambient magnetic field. Passive acoustic detectors can detect a change in sound caused by an object travelling through a pipeline but cannot easily distinguish between this sound change and that caused by a surrounding noise such as a pump or motor vehicle. Active acoustic detectors can eliminate this problem by transmitting an ultrasonic signal, but these devices are costly, require a high level of power and, because of the power requirements, limit or prevent battery-power options.
Electromagnetic detectors often use one or more coils to detect a change in magnetic flux over time. A change in the ambient magnetic field inducts a voltage in the coil or coils proportional to the change of the magnetic field over time. As a result, a slow travelling ferromagnetic object may not generate enough voltage in the coils to generate a detection event.
Magnetometers—which determine a change in magnetic flux by measuring the instantaneous flux over time—are not object-speed dependent. Magnetometers, therefore, can detect any object causing a change in the electromagnetic field regardless of object speed. Magnetometers, however, can be subject to false alarms. Therefore, appropriate methods must be used for noise cancellation, signal processing, and shielding of ambient magnetic fields.