Pig signaller or Pig detector is a device installed on a pipeline or pig trap assembly to detect movement of pig. It consists of a mechanism which detects the movement of pig.
Types of pig signallers
There are two types of pig signallers depending on the detecting mechanism:
- The intrusive type.
- The non-intrusive type.
COURTESY: GD ENGINEERING
The intrusive type are mechanically operated and are generally specified as bi-directional. They have mechanically operated signal flags which can be reset and are suitable for mounting on both vertical and horizontal pipelines. The indicating device, 'Flag', is normally set in the horizontal position for "pig not passed" and vertical for "pig passed" condition with a manual latch reset to the horizontal position.
Parts of Pig Signaller
A pig signaller or pig detector comprises of the following parts:
- A trigger which penetrates the bore of the pipe and moves when touched by the passing pig.
- A mechanism which transfers the movement of the trigger within the pipeline to the flag or to an electric circuit on the outside.
- The flag or electrical indicator which is visible and mounted external to the pipeline.
- A pressure containment which allows installation of trigger mechanism and which also acts as an isolation.
Requirements of Trigger mechanism
The trigger is one of the important components of the pig detector. Its design shall be such that it neither obstructs the passage of pig nor does it cause any damage to the pig. The trigger design shall be robust to withstand any damage from the pig movement.
Installation of Pig Detectors
Pig signallers should be suitable for mounting in vertical and horizontal section of pipelines. It is important to install the trigger in the correct orientation. The axis about which the trigger rotates should be perpendicular to the pipeline axis. In pipelines where uninterrupted operation is critical, pig signaller with an integral ball valve is used. The ball valve together with a retriever kit facilitates removal of the trigger mechanism while the pipeline is still operating. Retrievable type of pig detectors allow maintenance and/or replacement of the detector without shutting down the pipeline.
Pig signallers from manufacturers such as GD Engineering are designed for installation through a minimum bore of 39mm.
Following video from iNPIPE Products shows how the pig detector flag operates on the movement of pig.
When the red flag is in the horizontal or down position, it is an indication that the pig or scraper has not passed the pig detector mechanism. When the scaper passes the trigger, the flag moves to the vertical position indicating that the scaper has passed the detection mechanism. The trigger is normally set to trip in both directions (bi-directional). However, it can be procured to set in one direction (uni-directional). The pig signaller is reset by pushing the red flag down from vertical position to horizontal position. There are different types of detectors available in the market. The above description of the mechanism is for general illustration on how a pig signaller works.
Materials for Pig Detectors
The pig detector base should be compatible with the pipeline material to which it will be welded. For pig signallers used in sour service the elastomeric materials for ring seals shall resist explosive decompression and shall be suitable for the long-term exposure to the transported fluid at the design pressure and temperature conditions.
While ordering pig signallers the thickness of the pipeline should be provided to the Vendor. Though the trigger mechanism can extended or retracted within the pipeline the amount of adjustment is limited to about 25mm.
For buried pipelines, the pig signaller must be procured with an extended body. Pig signallers upto extensions of 2.2m are commercially available.
Pig Detector Purchase Specification
The following information or parameters should be included for purchasing a pig signaller or pig detector:
- Category of fluid in pipeline as per ISO 13623 and Fluid detail
- Sour service requirement
- Hydrogen Induced Cracking (HIC) test, when specified
- Pipe material & grade and wall thickness or Schedule
- Maximum Design (Operating) Pressure and Temperature
- Pressure connection detail and rating
- Hazardous area classification
- Horizontal or Vertical Installation
- Nozzle length.
- Mating flange of Pig Signaller
- Hot tapping, if required.
- Location - Onshore or Offshore and buried or aboveground shall be as specified.
Information to be requested from Suppliers or Manufacturers
Vendor shall be requested to complete the data sheet issued or issue his standard data sheet complete with the following technical information and submit with the bids:
- Design Pressure
- Design Temperature
- NACE & HIC Test certification confirmation (if applicable for sour service applications)
- Enclosure certification details
- Outside diameter, mm
- Assembly detailed drawing with the following dimensions:
- Pig Detector Signaller housing length
- Actuated Pig Detector / Signaller housing length
- Pig Detector / Signaller housing plus Isolation valve length
- Bill of Material and Materials information including materials for non-metallic (gaskets, seals etc.) materials
- Pressure rating