HF02

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INTRODUCTION

HF02 is intended to be used as a sensor in alarm systems in case of possible high thermal radiation exposure. A typical application is close to flares. A common assumption is that the heat flux level for a safe environment for personnel must not exceed 5 kW/m² (or 1500 BTU/hr ft²), or otherwise an alarm is activated.

A typical alarm system will include multiple sensors, each aimed at the main source of radiation.

The sensor in HF02 will measure the heat flux to the black (fully absorbing) surface of the sensor itself. It thus provides a reasonable measure of the worst-case effective exposure for personnel and equipment from the particular direction that the sensor aims at, at the location of the sensor.

As the detector is essentially open (no windows) the measurement is influenced by wind in much the same way as any exposed surface; increased wind reduces the effective exposure and leads to reduced measured flux.

The rain- and radiation screen act to locally reduce the effect of wind, preventing the false indication of a safe situation.

Rain on the detector can also lead to cooling and a reduction of the measured flux. This again could lead to a false indication of a safe situation. The rain screen will in most cases prevent this from happening.

To perform a representative measurement, the sensor temperature should be as close as possible to ambient temperature. A secondary purpose of the radiation screen (in combination with cooling fins) is to reduce heating of the sensor.

It can be debated if compensation or correction of the measurement for the influences of solar radiation is necessary. As the maximum solar radiation level is about 1.3 kW/m², it is certainly significant. It is however not part of the source that is usually monitored.

On the one hand the safety is determined by the total heat flux that personnel or material is exposed to. On the other hand the determination of the contribution of solar radiation is virtually impossible because the solar source is moving. Solar radiation might therefore have its maximum in a direction that is not seen by the detector.

The typical solution is to accept that solar radiation is part of the measurement in case that it is within the sensor’s field of view, and to neglect it if it is not within the sensor’s field of view.

Using HF02 is easy. For readout one needs an accurate voltmeter that works in the millivolt range. To calculate the heat flux, the voltage must be divided by the sensitivity; a constant that is supplied with each individual instrument.

Temperature is monitored as a redundant safety precaution. In case the sensor goes above a certain temperature level, say 70 °C, it is recommended to take action.