TAS Tool Box Talk – Boreal Laser Detection Systems

Introduction

This is a brief overview of the Boreal Laser Detection systems – how they work, answers to common safety questions, and what workers will see in the field. This is for site workers’ awareness after the system is in place.

This technote does NOT replace the Terra Applied Systems (TAS) system training for operations and instrumentation personnel, nor is it a replacement for any Boreal manuals.

The Open Path Detection System

The Boreal Laser detection system uses a tuned laser to measure the concentration of a specific gas in the open air. It measures only one gas and only that gas.

The system has a few basic components:

  • Central instrument cabinet – contains the Multichannel Control Unit (MCU) and a central connection point for power and data.
  • The Boreal GasFinder Multichannel Unit (MCU) – supplies the laser for the field equipment, collects data signal from the field equipment, and processes the data to calculate the concentration.
  • Open Path Transceiver – receives laser light from the MCU analyzer, transmits it to the reflector, receives the bounced light from the reflector, and sends a data signal to the MCU.
  • Retro-reflector – bounces the laser light from the transceiver directly back to it. It has a 110V intrinsically safe heater unit inside to keep the window clear.

How it works:

  • Laser light from the MCU is directed through fiber-optic cables to the transceiver.
  • The transceiver transmits the light through the open air to the retro-reflector.
  • The reflected light is collected by the transceiver on a light sensor.
  • This signal from the light sensor is returned through coax cables to the MCU analyzer in the instrument cabinet.
  • The MCU processes the signal and sends the concentration results to the site DCS.

Some things to note about the Boreal Laser System:

  1. These systems are tuned for a specific wavelength on the laser and looks for a specific signal shape for the gas being measured. As a result, it will only detect the specific gas it has been set up for.
  2. This is a line-of-sight based system. It requires the laser beam to travel through the air to the reflector and back to the transceiver. This means it can be blocked by an object that crosses the beam. This will disable the path as long as it is blocked. While we try to position the path out of the vehicle and pedestrian traffic, the laser beam can still be blocked by larger vehicles, scaffolds, ladders, nesting birds, wasp nests, or anything else in the path. Please be aware of the system around your unit and try to be aware of work in the path trajectory. If you must block the path for extended periods of time, please let your Control Room know so they will be aware. The first thing I&E will look for when receiving a Low Light Error is for something in the path trajectory.
  3. The concentration readings indicate total gas in the entire path. They are reported as parts per million * meter (ppm*m). This means that the gas plume can be a broad spread of low concentration or a tight spread of high concentration and still send the same ppm*m reading. Operations and the DCS are aware of this and have set your site’s alarm levels accordingly.
  4.  The Boreal system uses an OSHA Class 3 laser that is invisible to the human eye. As a Class 3, OSHA says you can stare at it for 15 minutes with no harmful effects. But remember: OSHA also recommends against staring directly at a laser for any amount of time. Please avoid staring directly at the source of the visible laser because it will block the beam.
  5. If the path is blocked it will not false alarm. The analyzer will indicate a fault condition that the channel has a low light and is not functioning.
  6. Very heavy rain, heavy fog, or steam plumes can block the paths. This is one of the few possible fault conditions. We try to design to avoid this, but it can happen.
    What You Will See Around the Unit
    There are two main assemblies that you will see around the unit: The Transceiver assembly and the Reflector assembly (see attached drawings).

The Transceiver Assembly is often mistaken for a camera but is, in fact, a dummy head that shoots the laser through the air and then collects it again on a diode. There are two main types being fielded: the BLI Open Path Transceiver (OP3) and the TAS Mid-Distance Probe (MDP). The OP3 is the larger head used for paths greater than 50 m (~160 ft) and mounted inside a stainless steel enclosure with the laser shooting out the front opening. The MDP is smaller and used for paths up to 50 m. This unit is mounted more like a camera without an enclosure. Both Transceiver heads will be connected to a local junction box that houses the fiber and coax splices. Your site may have one or both transceiver types as part of your system. [These are typically mounted on…. ]

The Reflector Assembly is an array of retro-reflector cubes in a stainless steel enclosure. There is an intrinsically safe heater and thermostat inside with 120v run to it to aid in preventing condensation on the window. You will see these assemblies mounted on both existing and new structures around the unit on the opposite end of the path from the transceiver assemblies. There will also be conduits holding the power wires.

The transceiver is aligned on the reflector. If you must work in close proximity to either field assembly, please be careful not to bump or move them. Like with a rifle, a small adjustment on the transceiver alignment can result in a large displacement on the reflector end.

The last component of the system is the integrated cabinet. This can be indoor or outdoor and is most commonly installed in or around an RIE or control building. The specific location and details of the cabinet are different from site to site. Your unit operators can provide more details if you want to know. The only people working in the instrument cabinet should be trained site personnel and TAS representatives.

In Summary

  • The detection system is laser based with an OSHA Class 3 Laser. Okay to look at, but remember OSHA says to avoid doing so.
  • Please avoid blocking the open path for extended periods because it disables that part of the system.
  • When a blocked path can’t be avoided, please let operations or control know.
  • The system will only detect the gas it is designed for.
  • There are 2 main field components, the Transceiver and the Reflector. Be careful not to move or bump them when working nearby.
  • Site Operators and Instrumentation Techs are trained on the system and its maintenance. If you notice something looks off, please let them know and they can check it.