An Example of How to Make a Sensor Work As a Process Analyzer
Introduction: The heart of any gas monitoring instrument is the sensor. Each sensor type has its own characteristics which define its strengths and limitations. For applications which are beyond the capabilities of a sensor, a sample conditioning system can be incorporated which can make these difficult applications possible. With this in mind, it is sometimes possible to use a simple, inexpensive gas monitor for a given application in place of an expensive analyzer. In this article we will illustrate an example of such an application.
The Application: Many bulk commodities such as grain require fumigation for import or export in order to meet prescribed international quality standards. The port of Melbourne, in Australia, has a silo farm that is designed to handle anywhere from 1,500 to 12,000 tons of different commodities. For grain applications, grain is loaded into a silo, and then methyl bromide gas is used for fumigation. The process requires grain to be submerged in 0.9%, or 9,000 ppm, of methyl bromide for a period of eight hours. To accomplish this, pure methyl bromide is introduced into the silo, and it takes about one to three hours, depending on the system, for the concentration to reach a level of 0.9%. At the end of the fumigation, a few more hours are required to evacuate the gas. A complete cycle typically takes approximately fifteen hours, during which time gas monitors can be used to monitor for proper gas concentration. This data can be documented, and the grain can then be certified as suitable for export. The operator skill required for this system is fairly limited, as the system is typically simple and easy to operate, with minimal maintenance requirements.
Sensor Selection: Because methyl bromide is an organic gas, electrochemical sensors used for detection often perform poorly. On the other hand, solid-state sensors detect methyl bromide quite well, and also offer long-term reliability with low maintenance. However, the solid-state sensors effective measurement range for Methyl Bromide gas is about 1,000 ppm, or 0.1%. The solid-state sensor responds well at this concentration, and exhibits good resolution. However, this concentration is lower than the concentrations used in grain silo applications. Additionally, sensors are generally intended for use in applications in which they receive only periodic exposure to gas. Continuous exposure to gas for fifteen hours will ultimately cause the sensor to deteriorate and change its response characteristics. For these reasons, a sampling system is needed to condition the sample in order for the sensor to perform properly in this application.
Sampling System Considerations: The solid-state sensor has limitations that necessitate the use of a sampling system. The sampling system must dilute the sample as well as take periodic readings. The sample is diluted to less than 1000 ppm by mixing it with air. During mixing, the flow of both the sample and air are controlled by needle valves, and the flow rate is metered. For periodic sampling, a timer controlled 3-way solenoid valve is used to control the amount of time the sensor is exposed to either sample or clean air.
Each of the silos is very large, up 30 meters in diameter, and there are over twenty silos in the facility. Thus, it is not practical to install a measurement system for each silo and instead silos are grouped according to their intended use, with up to six silos in a group.
System Design: Most samples require some type of conditioning, such as filtering or drying, prior to exposure to the sensor. To reduce maintenance requirements, the amount of sample entering the sensor chamber should be as little as possible. The sampling pump that supplies sample to the sensor normally is a low volume type, incorporating small ID tubing for better response time. Sampling distances between the silo and the measurement point are about 100 feet, and therefore a high volume circulation pump should be used to bring the sample close to the instrument. A small amount of pre-conditioned sample is then extracted by the instrument pump and further conditioned before entering the sensor chamber. A two stage sampling system is shown in Figure 1. A corrosion resistant pressure/vacuum AC pump is used to circulate sample in large quantities, which shortens the transport time. This standard laboratory pump is economical and available from most laboratory supply companies. It has free air capacity of 0.6 cubic feet per minute, maximum vacuum at 20 inches of mercury, and maximum pressure at 18 psi. Explosion-proof models are also available.
Sample is extracted from the silo through a dust filter. This dust filter must be large and is installed external to the silo for easy maintenance. Sample is then circulated into another filter and then pumped back to the silo. A small amount of sample is extracted for measurement through a fine particle filter. This pass-through filter bowl is a standard filter used in an air compressor. The line size is one-half inch ID flexible tubing. Parts are standard and readily available.
Figure 2: Sampling System
A 3-way solenoid valve is used to switch the stream between air and sample. Both air and sample are filtered through a hydrophobic filter and metered using a needle valve to adjust the flow rate. The air/sample mixture is adjusted as desired and fed into input port A. Clean air is fed into input port B. A 3-way solenoid valve is controlled by timers in the circuit card, which control the amount of time the sensor is exposed to sample, or alternately air. A small internal pump is on constantly to flow either sample or clean air to the sensor.
Figure 3: Control Circuit
Three timers are used to control the proper functioning of the system. The timers turn the external circulation pump on only as needed, which prolongs the pump life and reduces pump maintenance. The timers function as follows:
1. External pump delay: This is the time delay from the beginning of the cycle before the external pump is activated. During this time, the external pump is off.
2. External pump lead time: This is the amount of time the external pump is ON, before the solenoid valve is switched to let the sample flow to the sensor.
3. Solenoid activated: This is the amount of time the solenoid valve is activated to switch sample to the sensor. This is the only time the sensor is exposed to sample gas. It is the measurement cycle.
There is a micro-switch that activates a relay contact if there is a failure of the flow into the sensor chamber. A calibration switch with an LED indicator allows the user to perform a calibration function.
System Calibration: An International Sensor Technology sensor transmitter with display and 4-20 ma output is used for this application. The transmitter is calibrated with 0.1%, or 1000 ppm, calibration gas, but the display is 1%, or 10,000 ppm, full scale. There is a SPAN adjustment in the sensor transmitter itself, but for the purpose of this application this adjustment is secondary. In the actual system calibration, 9000 ppm calibration gas is used. A switch activates the solenoid valve into the sampling position, and 9000 ppm calibration gas is introduced into the sampling port. Flowmeters control the sample and clean air flow rate, and are adjusted so that the sensor displays a reading of 9000 ppm. Thus, flowmeters are used as the primary adjustment, with the transmitter span adjustment used only as a backup.
Conclusion: The sampling functions described above are incorporated into International Sensor Technologys Model S2K sampling system, which is specifically designed for applications with specific sampling needs. Alternately, these same functions can be programmed in a customers existing DCS. For this type of fumigation application, sample concentration is read four times an hour after the silo reaches its final designated concentration of 9000 ppm. The advantage of non-continuous sampling is the fact that the zero and span points are confirmed on each reading, which increases the users confidence level in that reading (Figure 4 below illustrates a plot). The calibration function can be performed during processing, which further improves the reliability of the system. Data can be plotted and printed out, enabling certificates to be produced which verify the reliability of the procedure.
Another advantage of this system is that it can be used for multiple applications, where different gases are used for different commodities. Accomplishing this simply requires changing the sensor transmitter to match the target gas. In this particular application, for example, monitoring is done for both Methyl Bromide and Phosphine by interchanging sensor transmitters for these two gases.
Fig. 4: Phosphine Plot using Periodic Sampling