Week 5 - Level Part 2 (3) PDF

Weight Method As we discussed before, we can use weight to calculate the level for solids and liquids. This is considered an indirect method . Mechanical Springs Springs are located under each leg of a vessel, or the vessel is suspended by them, and the springs are connected to an indicator device....

Weight Method As we discussed before, we can use weight to calculate the level for solids and liquids. This is considered an indirect method . Mechanical Springs Springs are located under each leg of a vessel, or the vessel is suspended by them, and the springs are connected to an indicator device. Bathroom scales are and example. 1 Weight Method Instead of Mechanical Springs we can use Load Cells 2 Single point Load cell • A load cell is a type of transducer, specifically a force transducer. They convert a force such as tension, compression, pressure, or torque into an electrical signal that can be measured and standardized. As the force applied to the load cell increases, the electrical signal changes proportionally. 3 Weight Method Load Cells The load cells are located either at the base of the tank, under each leg, or above the tank with the tank suspended from them. They are very accurate. 4 Weight Method Load Cells • Tare weight is the empty vessel weight. • Measured weight is the total weight of the vessel and its contents. • Can be used for liquid or solid materials • Indirect, non-invasive, continuous measurement 5 Formulas Content Wt(lb) = Measured Wt (lb) – Tare Wt (lb) Content Weight (lbs) Volume (cubic feet) = Density (lbs/ft3) Volume (cubic feet) Level (feet) = Surface Area (square feet) Area=πr2 6 Question 1 A cylindrical tank has an area of 10 ft2 and a height of 8 ft. It has a tare weight of 1500 lbs. Wheat flour has a density of 37 lbs/ft3. 1. What is the height (level) of wheat flour in the tank if the tank weighs a total of 3500lbs? 2. What would the current signal be from the transmitter? 7 Question 1 What is the approximate height of the Wheat flour in the tank? Area = 10 ft2 Content Wt (lb) = Measured Wt (lb) – Tare Wt (lb) Content weight = 3500lb – 1500 lb= 2000 lb 8 Question 1 Volume (cubic feet) = V= 2000 lb 37 lbs/ft3 Content Weight (lbs) Density (lbs/ft3) V = 54.05 ft3 9 Question 1 1. Level (feet) = 54.05 ft3 L= 10 ft2 2. O/P signal = Volume (cubic feet) Surface Area (square feet) L = 5.405 ft 5.405𝑓𝑡 −0 𝑓𝑡 ( )16 mA 8𝑓𝑡 + 4mA = 14.81 mA 10 Question 2 If we used Mercury instead of wheat, with a density of 848 lb/ft3, what would its height be? 11 Question 2 What is the approximate height of the mercury in the tank? V = 2000lbs/848 lb/ft3 V = 2.36 ft3 L = 2.36 ft3/10 ft2 L = 0.23 ft yes that is about ¼ of a foot (3”)! 12 Rotational Suppression Method Paddle Wheel Detector • Typically used granular or powered materials • Paddle rotates until materials restricts rotation • Direct, invasive, point measurement 13 Nuclear Level Measurement • This type of level measurement is based on the weakening of gamma radiation as it penetrates materials. The radioactive isotope is installed in a container, also referred to as shielding, which emits the radiation only in one direction. • The source container and the transmitter used to detect the radiation are mounted on opposite sides of a vessel or pipe. • The higher the level of the process in the vessel the lower the intensity of the radiation received. • For example, at 50% of the full range level, only half of the detector receives the radiation. • Why and where would we use nuclear measurement? 14 Nuclear Level Measurement 15 Electronic Sensors Ultrasonic Sensors Ultrasonic sound waves are transmitted, and the time required for the return echo is measured. The length of time determines the distance from the transceiver. No moving parts promote long life. Like sonar used for underwater depth detection but sound wave are above the human hearing range. Indirect, non-invasive, continuous measurement 16 Electronic Sensors Radar Sensors • Similar in principle to ultrasonic, but uses high frequency radio waves instead of sound • Ideal for applications with poor conditions such as foamy materials • Indirect, non-invasive, continuous measurement 17 Electronic Sensors Conductive Probes • Rods are suspended in an electrically conductive material and a circuit path will form between the rods • Several rods can be used for multiple point levels • Direct, invasive, point measurement. 18 Electronic Sensors Capacitive Probes • A rod is suspended in non-conductive material to be measured and the capacitance between the rod and the vessel wall is measured. • As the material level rises, a decrease in capacitance will be measured. • Indirect, invasive, continuous measurement. 19 Differential Pressure Method Differential Pressure Level Detector • Same as a hydrostatic pressure system but used with a sealed and pressurized tank. • Differential pressure transmitter has two pressure ports: wet leg (high pressure, bottom of vessel) and a dry leg (low pressure, top of tank) 20 Differential Pressure Method 21 DP Transmitter with Manifold • D 22 Calibrating DB (in service) 23 Druck Pressure Calibrator 24 Differential Pressure Method 25 Differential Pressure Method Differential Pressure Level Detector • Measures the difference in pressure between the top of a pressurized tank, and the head pressure in the bottom of the tank. • Placement of the transmitter is critical to the calibration of the transmitter. • Indirect, invasive, continuous measurement 26 Differential Pressure Formulas Differential Pressure = ΔP ΔP= High Pressure - Low Pressure Pressure = 0.433 x H x SG 27 Differential Pressure Method Question: A pressurized tank 30‘ tall containing water has a wet leg pressure of 20psi and a dry leg pressure of 8psi. The weight density of the Hi-Quality water is 60.4 lb/ft3, what would the water level be in feet? 28 Differential Pressure Method Solution: ΔP = Hl – LO = 20-8 = 12 psi SG= 𝑥 𝑑𝑒𝑛𝑠𝑖𝑡𝑦 62.4 = 60.4 62.4 ΔP =k*h*sg H= 12 0.433 𝑥 0.968 =28.63ft 29 Differential Pressure Method Same question BUT we have the level but not the dry leg pressure. A pressurized tank 30‘ tall containing water shows a wet leg pressure of 20psi with a level of 28.63 feet. If the weight density of the water is 60.4 lbs/ft3, what would the dry leg pressure be?? 30 Differential Pressure Method Solution: sg = 60.4 62.4 =1 ΔP =k*h*sg ΔP = 0.433 x 28.63ft x 0.968 = 12 psi ΔP = Hl – LO LO = 20psi – 12psi = 8psi 31 Bubbler System The Bubbler System is one of the oldest types of level measurement. It is used for liquid and fine powders. It uses a dip tube with the open end located near the bottom of a vessel. Supply air pressure is adjusted to provide airflow into the tube and to a pressure transmitter. The supply is adjusted so that it will deliver a pressure that is slightly higher than the highest hydrostatic pressure that will be measured. 32 Bubbler System • Changes in level provide changes in head pressure, thus a change in backpressure in the bubbler supply. • As the level increases the backpressure will increase, letting less airflow out the bottom of the tube. That means more pressure is applied to the D/P transmitter. • It is important to remember that 27.72” water is 1 psi. We see that even a ½ psi change can represent over 13.85” of level change. This is why we use a regulator or a constant differential pressure relay. The relay is essentially a very accurate regulator. 33 Constant Differential Pressure Relay 34 Bubbler system Indirect, invasive, continuous measurement • Advantages: • Not affected by foaming, conductivity, solids. • Reliable because only part in contact with process is a tube. Self clean, • Can be used for many different types of processes, such as slurries, corrosive, acidic or radioactive. • Cost effective and relatively easy to configure • Disadvantages: • Essential to maintain a constant supply pressure. • Hole in dip tube. • Turbulent process will affect reading 35