Optical flame detection and field of view

Many flame detector manufacturers look to develop products with unique specifiable features that customers value. As part of the product development cycle, engineering specifications are developed to make it challenging for other manufacturers to match the specification. As with most things in life, a balance needs to be found between universal parameters that are needed for basic functionality whilst establishing a set of ‘device specific’ characteristics that other manufacturers find difficult to match.

If we consider optical flame detection, the ‘lock out’ specification balances the needs of a detector to integrate seamlessly with a wide range of industrial control systems and/or fire panels whilst defining a set of performance credentials that meets the detection needs for the application under consideration. The basics of operation normally require flame detectors to be powered from 24Vdc and to provide a 4–20mA (or more correctly termed a 0–20mA) analogue output and/or fire and fault volt free relay contacts for interfacing purposes. Other features that make the detector more ‘intelligent’ like Highway Addressable Remote Transducer Protocol (HART) or Modbus RTU are also becoming commonplace in hazardous process industries. These protocols are termed open source, whereas some manufacturers use proprietary protocols. These too can provide valuable information from the detector to the control system, but it’s important to understand that the user is locked into that one supplier when using a detector having a proprietary protocol.

Turning now to device-specific characteristics, we need to consider the target application. What will burn and what fire size do we need to detect? Today FM 3260 and EN-54-10 are international standards for the performance of optical flame detectors. According to manufacturer’s published test data to these standards, triple IR flame detectors generally tend to be more sensitive (80m)¹ than UV-IR flame detectors (30m)², to a one-foot square (0.1m²) n-heptane fire when used on the highest sensitivity setting the detector can employ. This response data can form part of the specification. One of the other main characteristics that can be specified is the detector field of view, which is also sometimes called the cone of vision.

What is the field of view of an optical flame detector?

As the name implies, the field of view (FOV) of an optical flame detector defines the unobstructed area of coverage a flame detector has for a given fuel.

The greatest sensitivity to a fire is seen directly through the centre line to the detector face, this is sometimes called on-axis sensitivity. The detection distance then rolls off the further you move away from the centre line with the shape formed by the field of view generally looking like a teardrop.

FM3260 defines the field of view by stating ‘the detector response shall be at least 50% of the on-axis sensitivity (measured in units of distance) in at least four directions (left, right, up and down)’.

The horizontal field of view of a detector is typically 90° but can vary due to the optomechanical construction of the detector and/or the fuel being burnt. The vertical field of view tends to be smaller than the horizontal one due to obstructions from the reflector plates used by through-the-lens optical tests.

It should be noted that some flame detectors have FOVs that are greater than 90° and whilst on paper this may seem to offer an advantage in terms of coverage, there can be some unforeseen consequences.

A wider FOV must be better, right?

Some manufacturers claim their flame detectors provide a FOV angle up to 120°. At first glance, this characteristic may appear beneficial; however, performance evaluation testing has shown that the actual FOV coverage and detection distance claims of ‘wide-angle’ detectors are often inconsistent. Wide-angle FOV detectors typically utilise lesser-quality optical filters and optical sensors, resulting in reduced detection range and reduced false-alarm rejection capability.

In addition, these detectors are often fuel-specific, meaning the devices are ‘tuned’ for detection of a specific fire type, like n-heptane, but fall short of their claimed capability in response to different fuel fires such as natural gas or methanol.

Looking at the FlameSpec design (IR3) and that of the Det-Tronics X3301 we can see each of IR sensors is tucked behind a bandpass filter selected to the wavelength of interest.

The window is tucked in a precision-machined (or stepped for the X3301) pocket that restricts incident light to 45° either side of the mid-point, thereby giving each sensor a 90° horizontal field of view.

Detector designers do this because IR sensors are affected by something called ‘blue shift’. This is where the transmission properties of a bandpass filter changes depending on the angle of incoming radiation.³

In basic terms, when IR energy enters from angles greater than 45° the sensors see slightly different wavelengths/energy.

This unwanted additional energy can distort the signals received by the detector which may impact the ability of the algorithms to correctly differentiate between fire and false alarm. This means that the response of devices using a FOV of greater than 90° may be compromised.

But not all IR3 detectors with a 90-degree field of view are the same

If we consider an alternative, semi-controlled sensor design, where the manufacturer claims to have a field of view of 90°, we can see incident light to the extreme left of the three sensors is restricted to 45°, but light to the right of the centre point is unrestricted and can be impacted by blue shift.

The only way to be certain that detectors are not affected by blue shift is to ensure light entering each IR sensor is tightly controlled to 90° around the central axis of each sensor.

Additional application points to consider

Practical installations tend to favour placing detectors in the corners of rooms or process modules meaning there is little or no benefit to be gained with detectors having a wider FOV. Another point for consideration is fire zoning, limiting the FOV to 90° makes zoning easier and helps ensure flame detectors from one area are not activated by a fire in an adjacent zone.

Conclusion

This paper has discussed some of the elements that flame detector manufacturers develop into unique specifiable features that customers value. Detector interfacing follows one of two paths: either to be as ‘universal as possible’ so that a device can be integrated into the widest range of control systems or supplied with a proprietary protocol that locks the detector to the alarming system. The other point that is commonly discussed relates to the detector field of view.

Real world comparisons for projects utilising 90° and 120° FOV devices shows little to no cost savings benefit from using a device having 120° FOV. In fact, the costs associated with nuisance alarms and shutdowns initiated by wide-angle detectors can result in significantly higher life cycle costs.

For more information, go to www.fg-detection.com

References

1. FM approved performance data taken from FlameSpec-IR3 product datasheet F120V0010.08 dated, April 2022.

2. FM approved performance data taken from FlameSpec-UV-IR-F product datasheet F160V0010.06 dated, April 2022

3. Patent Number 5995008 Fire Detection Method And Apparatus Using Overlapping Spectral Bands, Detector Electronics Corporation. Dated 30th November 199, now lapsed.