EXAMPLE OF IN-THE-FIELD JOB ASSIGNMENT

You are requested to select and install several sensors for a new process application that involves mixing a liquid with two dry materials to make a polymer additive that is used in making rubber products. This application wll involve a series of three 10,000-gallon tanks that will hold raw materials. The tank that holds the liquid is heated with a steam jacket. A mixing tank is mounted on a rail and it moves under each tank to receive its 200 lbs of each of the dry ingredients and 100 gallons of the liquid ingredients that will be mixed together to form the polymer additive. The mixing tank must stop at exactly the correct position precisely below each tank so that it can receive the proper amount of each ingredient.

Your job is to research the sensors needed for this system and recommend which ones you would use and explain why you made your selection. A temperature sensor and level sensor are needed for the tank that holds the liquid. A pressure sensor is needed to indicate the amount of steam pressure in the steam jacket.

A level sensor is needed on each tank that holds the dry ingredient, which is in powder form. A sensor is needed to measure the amount of the dry ingredient as it's dumped into the mixing tank. A flow meter is needed to measure the liquid ingredient as it's dumped into the mixing tank. Position sensors are needed to ensure the mixing tank is in the proper position before the ingredients are dumped into it. A density sensor is needed to determine when the mixture is the proper consistency.

You should also include all of the pertinent formulas that would help you determine if the sensor you are choosing for each application is the proper size or in the range for the amount of material that is being sensed.

SOLUTION TO IN-THE-FIELD JOB ASSIGNMENT

Each of the tanks requires a level sensor. The tanks that hold dry material should use a level sensor like the paddlewheel sensor, vibrating tines, or the beam breaker if you are trying to measure a single point where the tank is considered full or empty. If a continuous reading is required, a weighing system or a sounding-type sensor that uses a weight and line could be used. A sonic-type sensor could also be used.

The tank that holds the liquid material could use a float, displacer, or conductance sensor to indicate a single point, or it could use a capacitance-type, sonic, or ΔP-type sensor to indicate a continuous level.

The temperature detection system could use thermocouples, RTDs, thermistors, or IC temperature sensors. You would need to match the temperature range of the liquid with each type of sensor (see this figure for thermocouples and this figure for RTDs). Since this is a liquid, you will need to include a thermowell so you can install and remove the sensor for calibration without allowing liquid to leak from the tank.

The temperature of the steam jacket could be determined by a pressure gauge. Since a steam pressure gauge is specified, it could be used to indicate the amount of steam pressure, and the temperature of the steam jacket could be determined from a conversion chart. The type of gauge would normally be a Bourdon tube gauge.

The flow meter to measure the amount of liquid going into the tank should be a fairly accurate type of flow meter similar to the paddlewheel or turbine flow meter. If more accuracy is required, a positive-displacement flow meter like the gear type should be used.

The positioning sensors to ensure the tanks are in the correct position for loading could be as simple as a proximity switch. If more accuracy is required, a ball-screw mechanism could be used for motion, and an encoder or resolver could be used to determine the correct position. A linear potentiometer, LVDT, or magnetostrictive sensor could be used if the amount of distance is not over 6 ft.

The density sensor should be similar to the one shown in this figure and this figure. It could take a sample of the mixture to determine its density. If the density sensor is connected to a closed-loop controller, it could allow more liquid to be added if the mixture is too dry, or more powder to be added if the mixture is too wet.

The formulas for flow are found in Exercises 3, 4, and 5. The formulas for temperature conversion for Fahrenheit and Celsius and a table are shown in this figure.