1. Field of the Invention
The present invention relates to a device for detecting and signaling the passage of a pig past a point in a pipeline. More specifically, the present invention relates to a device employing a magnetic field to initiate either a visual or an electronic signal to indicate passage of the pig.
2. Description Of The Related Art
In the pipeline industry it is common practice to utilize a movable plug or a pig (hereinafter referred to as a pig) within a pipeline either to travel between and separate different products as they flow consecutively through a single pipeline or to travel through the pipeline in order to service it by cleaning it, mapping it, etc. Whenever a pig is being used, it is necessary to be able to detect its location. One way this is done is to utilize a device which remains stationary relative to the pipeline and is provided with means for detecting and signaling passage of the pig past the device.
Previous devices for detecting and signaling passage of a pig can be categorized as one of two general types. The first type, as exemplified by U.S. Pat. No. 3,109,410 issued on Nov. 5, 1963 to inventor Burton Ver Nooy, employs a trigger mechanism internally disposed within the pipeline which is physically tripped by passage of a pig. The physical movement of the trigger is transmitted by a variety of gears or other types of mechanical means through a sealing mechanism in an enclosure of the device to a signaling device located exterior to the enclosure.
One of the main problems with this type of device is that leakage occurs around the movable element which connects the internal trigger to the external signaling device. This type of device can be problematic because of its sealing method when used on high pressure pipelines and when used on pipelines transporting materials corrosive to the materials used in the sealing mechanism.
The second type of device for detecting and signaling passage of a pig is represented by U.S. Pat. No. 4,638,278 issued on Jan. 20, 1987 to inventor Mark E. Bullock. This second type of device employs a specially designed pig, usually one to which magnets have been attached. This special pig is magnetically detected by a sensing unit located external to the pipeline. The sensing unit normally employs a magnetically triggered switch which can be used to operate a signaling device.
One of the main problems with this type of device is that the pig must be constructed of a magnetically detectable material or must be retrofitted with magnetically detectable material, such as magnets, in order for the sensing unit to detect its passage. Use of magnetically detectable material in a pig adds to the pig's weight and the magnetically detectable material can be chemically degraded by corrosive materials contained within the pipeline.
The present invention addresses these problems by providing a device which combines the reliability of mechanical triggering with the non-intrusiveness of magnetic sensing. Specifically, the present invention utilizes an enclosed and sealed trigger mechanism provided with a polar magnet on its upper end and provided with an actuator on its lower end which extends down into the pipeline. A passing pig engages the actuator, causing the trigger mechanism and the attached polar magnet to move upward where a magnetic field associated with the polar magnet passes through a sealed enclosure housing the trigger mechanism and can either activate a visual signal device or, optionally, can activate a proximity switch.