20 Nov, 2013  |  Written by  |  under Flame Detection

FGD_PUB_201211_Petroleum_Review_Minimising_False_AlarmsHi everyone. I’m Jonathan Saint with Net Safety and I want to talk about ways to prevent false alarms with flame detectors. I wrote on this topic in a recent edition of Petroleum Review. If you’d like to see the whole article, click HERE.

Most flame detectors function using optical systems like ultraviolet (UV) and infrared (IR) spectroscopy. Almost all flames produce heat, carbon dioxide, carbon monoxide, water, carbon and other products of combustion, which emit visible and measureable UV and IR radiation. These spectral emission “by-products” are what flame detectors sense to quickly and accurately determine the presence of a fire. These same emissions from non-flame sources cause nuisance false alarms and plant shutdowns.

Today’s optical flame detector options include single wavelengths of UV and IR, integrated UV/IR sensors, and more advanced units that offer triple wavelength IR sensors. The performance and advantages of each of these systems vary a lot.

A huge consideration in the selection of a flame detector is the potential for false alarms. False alarms are generally not the result of an issue with an instrument but rather its response to non-flame radiation sources that fall within their field of view. There are two basic types – natural and man-made. Natural sources include rain, lightning and sunlight while man-made source examples are artificial light sources, welding, and radiation from heaters and machinery. And falling into these two primary sources are four primary types: solar-blind UV; window contaminates; non-modulated IR; and modulated IR sources.

With non-modulated sources of radiation, the energy is constant over time or varies at an extremely slow rate. Examples of these are IR energy emitted from heaters, lamps and heat from the sun. Additionally, there’s a small amount of IR radiation emitted from all objects which is constantly present in any detector’s field of view. To overcome this, the majority of flame detectors available on the market today are designed to only detect modulated IR radiation sources – a key characteristic of flames. With modulated sources, characterized by varied and sporadic energy or as a combination of non-modulated sources, identification can be very challenging. Examples of these false sources are heated emissions, moving lights, signals or combinations of non-modulating sources being altered by objects moving back and forth in front of them in between the source and the sensor (vehicles, personnel, or fan blades for example). This is overcome by the use of multi-bands which can distinguish on the IR spectrum between flames and other sources of radiation.

While UV detectors work well in sunlight, other factors in outdoor applications may negatively affect them. UV sensors are designed to monitor solar-blind UV, the band of UV energy that is blocked by the ozone layer in the upper atmosphere. Powerful sources of this energy wavelength are commonly produced in industrial settings by halogen lamps, arc welding and even lightning. Additional bands can be employed, or combined UV/IR detectors will overcome almost all of these sources of inference.

Finally, window contamination will negatively affect the detector’s performance and can cause the instrument to go into fault mode. Attenuating energy sources will hit or deposit on the window face of the detector as well as accumulate on external reflectors used for automated visual integrity testing. Water droplets, condensation, snow and ice are powerful absorbers of IR energy that can be delivered in random scales and intensity and are a well-known source to trigger false alarms or faults when combined with modulated energy sources like direct sunlight. UV radiation is also easily absorbed by a range of oils, smoke, carbon and specific gases. Engineers need to be aware of the presence of vapors such as hydrogen sulphide, benzene, ammonia, ethanol, acetone and others when selecting a flame detector for their application.

There’s no perfect flame detection system for every application – all have challenges. But understanding the type of fire to be detected, the environmental conditions surrounding the installation, and the required performance makes the choice of flame detection technology a much more manageable decision. A detection solution that allows for field sensitivity and time delay settings will help mitigate the more challenging false alarm sources by allowing users to fine-tune their instruments in situ for optimal performance.

What type of flame detection system do you use? Do you experience any problems with false alarms?

Hi, I’m Dwain Waguespack, senior account director at Rosemount Analytical. One of the most widely used building blocks of the petrochemical industry is ethylene, which is used to make such common chemicals as polyethylene, polystyrene, and alpha-olefins. A typical ethylene plant also makes a number of other important building block chemicals such as propylene, butadiene, and an aromatics-rich pyrolysis gasoline. In the ethylene plant fractionation train in a refinery, the use of process gas chromatographs is critical for improving process operations.

The typical ethylene plant is divided into two basic sections: the cracking furnaces (hot side) and the fractionation train (cold side). The fractionation train takes the effluent from the furnaces and separates it into the wide range of chemical products. The separation of the furnace effluent into various products is done through a series of fractionator towers that selectively separate one chemical group at a time. The effluent stream moves from one fractionator tower to the next with the remainder being sold as a pyrolysis gasoline.

Improving Unit Performance with Process Gas Chromatographs
With the large number of chemical separations being performed in the ethylene plant fractionation train, process gas chromatographs play an important role in maintaining efficient operation. The first process gas chromatograph (AX #1 in Figure 1) monitors the overhead streams of the demethanizer to minimize the loss of ethylene into this stream. Two more process gas chromatographs (AX #2 and #3 in Figure 1) monitor the separation of this stream into the hydrogen-rich tail gas stream and the methane-rich tail gas.

image001A fourth process gas chromatograph (AX #4 in Figure 1) monitors the bottom streams of the demethanizer to minimize the light gases such as C1 and CO2. Any gases that get to this point ultimately end up in the final ethylene product stream as impurities so the amount needs to be controlled. This is done by controlling the C1 to C2 = ratio. The next series of chromatographs are used to control the purity of the ethylene product. This starts with the measurement (AX #5 in Figure 1) of the deethanizer overhead to minimize the amount of C3 in the stream while still maximizing the recovery of the C2 =. To monitor the removal of the acetylene, one or more gas chromatographs (AX #6 in Figure 1) measure the effluent from the acetylene reactors to insure that the acetylene levels will meet final ethylene product specifications. At the C2 splitter, a gas chromatograph (AX #7 in Figure 1) monitors the ethylene product stream for impurities while another gas chromatograph (AX #8 in Figure 1) measures the bottom streams to minimize any loss of ethylene in the ethane that is being recycled.

A similar series of chromatographs are used to control the purity of the propylene product starting with the measurement (AX #9 in Figure 1) of the stream leaving the bottom of the deethanizer. Any C2 components present would end up in the propylene product stream so they need to be controlled by maintaining the optimum C2 to C3 = ratio. The depropanizer overhead stream is monitored (AX #10 in Figure 1) for C4s to minimize their presence in the propylene product. To monitor the removal of MA and PD, measure at the exit of the MA/PD reactor (AX #11 in Figure 1). Finally, two gas chromatographs monitor the propylene product for purity (AX #12 in Figure 1) and the propane stream (AX#13 in Figure 1) to minimize the loss of propylene.

The final series of chromatographs monitor the final separations of the furnace effluent beginning with the measurement of the depropanizer bottom streams (AX #14 in Figure 1). This analyzer monitors the C3 to C4 = ratio to control the C3 impurity levels in the mixed olefins product stream. Then two more analyzers monitor the purity of the mixed olefins (AX #15 in Figure 1) and minimize the loss of C4 olefins in the gasoline stream (AX #16 in Figure 1).

Process gas chromatographs are critical for maintaining efficient operation throughout the chemical separation processes in the ethylene plant fractionation train.

How do you use process gas chromatographs in your plant?