Pig Signaller
Once a pig has been inserted into
a pipe system its position cannot be visually confirmed directly. A system is
therefore required to indicate a pig’s position, either continuously or
intermittently at pre-determined points in the pipeline. Continuous detection
is known as ‘pig tracking’ whilst intermittent detection is referred to as pig signalling.
Each method involves the use of specialized equipment which, in most instances,
must be incorporated into the pig or piping system before the pig is launched.
Pig tracking requires an active
transmitting source to be attached to the pig, with various technologies being
used to provide that source - electro-magnetic, acoustic and radioactive. Pig
signalling, however, falls into two categories: intrusive and nonintrusive. Non-intrusive
signallers are predominantly electro-magnetic and, along with pig tracking
techniques, are covered more fully elsewhere.
Intrusive pig signallers are, by
definition, static, mechanically actuated and provide a momentary indication of
a pig’s presence at a specific point in the pipeline. Because the actuating
mechanism is a mechanical trigger that intrudes into the pipeline, a signaller
must also incorporate a satisfactory means of retaining the pressure within the
pipeline.
Every signaller must incorporate a mechanism that will
provide a positive indication that a pig has passed. This is normally done in
one of two ways:
- Mechanical/Visual (a local flagged indicator)
- Electrical (magnetically linked proximity switch provides an electrical signal to a controller)
Pig signallers are robust and
designed to be installed for long periods. They are particularly suitable for
inclusion in a pig trap system at points along the pipeline where the course of
the pig must be confirmed (e.g. wyes, diverters, tees) and for providing a
warning of approach at receiver and booster stations.
There are two types of Pig Signaller
- Intrusive
type pig signaller
- Non-intrusive type pig signaller
Intrusive Pig Signaller - Operating Principles
When a pig passes beneath a
correctly installed signaller the pig discs push the protruding bi-directional
trigger forward in the direction of the pig travel. The trigger is hinged at
two points which converts the angular motion gained from contact with the pig
into axial motion withdrawing a spring-loaded, permanent magnet holder down through
the cap. At rest, the proximity of the magnet at the top of the cap retains
the mechanical, spring-loaded flag and/or the contacts of an externally mounted
proximity switch.
When the magnet is withdrawn, the
magnetic flux influencing the components diminishes, the flag springs into an
upright position and/or the proximity switch contact is broken. (This can be
detected by a linked controller). Immediately after the pig has passed, the
spring in the cap returns the magnet and the linked trigger to their respective
resting positions ready to detect the next pig – from whichever direction it
arrives. The flag of an MV/MVE must be manually reset. Proximity switches,
however, will be reset at the control panel
Non-intrusive type pig signaller
Non-intrusive type pig
signaller is quick and Easy for
installation on the pipe. There is no hot-tapping required non-intrusive type
pig signaller. Non-intrusive type pig signaller can be installed in minutes by
any field technician. Non-intrusive means less maintenance and corrosion, and
no possibilities of pipeline leaks.
