Level transmitters

Compensation of the measured value for other process conditions, such as compensating the output of a capacitance level transmitter for variations in process temperature. 

Figure 4.2 shows an example HTA for the task of isolating a level transmitter for maintenance. Redescribing operations into more detailed plans and suboperations should only be undertaken where necessary, otherwise a great deal of time and effort is wasted. Since the description is hierarchical the analyst can either leave the description in general terms or take it to greater levels of detail, as required by the analysis. 

FIGURE 4.2. HTA Diagram of Isolating a Level Transmitter for Maintenance... 

Feedback is information in a closed-loop control system about the condition of a process variable. This variable is compared with a desired condition to produce the proper control action on the process. Information is continually "fed back" to the control circuit in response to control action. In the previous example, the actual storage tank water level, sensed by the level transmitter, is feedback to the level controller. This feedback is compared with a desired level to produce the required control action that will position the level control as needed to maintain the desired level. Figure 3 shows this relationship. 

In the water tank level control system in the example above, the level transmitter measures the level within the tank. The level transmitter sends a signal representing the tank level to the level control device, where it is compared to a desired tank level. The level control device then computes how far to open the supply valve to correct any difference between actual and desired tank levels. 

As a final example, suppose we are controlling the base level in a distillation column with the bottoms product flow rate. The valve would be AO because we want it to fail shut (we don t want to lose base level in an emergency). The level transmitter signal increases if the level increases. If the level goes up, we want the bottoms flow rate to increase. Therefore the base level controller should be increase-increase (direct acting). 

Three vertical cylindrical tanka (10 feet high, 10 feet diameter) are used in a process. Two tanks are process tanks and are level controlled by manipulating outflows using proportional-only level controllers (PB 100). Level transmitter Spans are 10 feet. Control valves are linear, 50 percent open at the normal liquid rate of 1000 gpm, air-to-open, constant pressure drop. These two process tanks are 50 percent full at the normal liquid rale of 1000 gpm. 

In level control applications, this override controller can be a simple fixed-gain relay which acts like a proportional controller. The gain of the controller shown in Fig. 8.4n is five. It would be zeroed so that as the level transmitter dropped from 20 to 0 percent of full scale, the output of the relay would drop from 100 to 0 percent of scale. This means that under normal conditions when the level is above 20 percent, the output of the relay will be at 100 percent. This will be higher than the signal from the temperature controller, so the low selector... 

The liquid flow rate from a vertical cylindrical tank, 10 feet in diameter, is flow-controlled. The liquid flow into the tank is manipulated to control liquid level in the tank. The control valve on the inQow stream has linear installed characteristics and can pass 1000 gpm when wide open. The level transmitter has a span of 6 feet of liquid. A proportional controller is uaed with a gain of 2. Liquid density is constant. 

The tank is vertical and cylindrical with a cross-sectional area of 25 ft and a 2 ft level transmitter span. 

There is a first-order dynamic lag of t minutes between a change in the signal to the steam valve and vapor boilup. The low base-level override controller pinches the reboiler steam valve over the lower 25 percent of the level transmitter span. 

Sensor—Field measurement system (instrumentation) capable of detecting the condition of a process (for example, pressure transmitters level transmitters, and toxic gas detectors). 

Vega Controls Ltd. Technical Bulletins 2.14763 and 2.I472S (August 1992). Ultrasonic and radar based level transmitters. 

Figure 7. Flow and instrumentation diagram for high temperature, balanced pressure bench-scale unit. LR, level recorder LT, level transmitter TR, temperature recorder FR, pressure recorder FR, flow recording RD, densitometer PC, pressure controller TC, temperature controller GM, gas meter F, filter VSD, variable speed drive PCV, pressure regulator CV, control valve SV, solenoid valve S, gamma ray source.

Laser Level Transmitters These are designed for bulk solids, slurries, and opaque liquids. A laser near the vessel top fires a short pulse of light down to the surface of the process liquid, where it reflects back to a detector at the vessel top. A timing circuit measures the elapsed time and calculates the fluid depth. Lasers are attractive because lasers have no false echoes and can be directed through tight spaces. 

On hard-to-handle services, such as the fluidized-bed level measurement in combustion processes, there is little choice but to use radiation gauges. On slurry and sludge services, d/p units with extended diaphragms eliminate the dead-ended cavity and bring the sensing diaphragm flush with the inner surface of the tank. Other level transmitters that can be considered for hard-to-handle services include the capacitance/RF, laser, radar, sonic, and TDR types. 

To improve the accuracy of HTG systems, a third transmitter located at a fixed distance above the bottom transmitter can be used for density compensation (Figure 3.109). These tank expert packages, in addition to level, can also calculate mass, density, and volume on the basis of measurements from three or more d/p cells and one temperature transmitter. Most manufacturers offer optional digital communication and the ability for remote adjustments of suppression and linearization. "Smart" level transmitters can convert the level readings of spherical or cylindrical tanks into actual volume percentage readings (Figure 3.113). 

The standard ranges of displacement level transmitters are from 36 to 150 cm (14 to 60 in.), whereas special units are available up to 18 m (60 ft). Their error... 

Radar level transmitters and gauges use electromagnetic waves, typically in the microwave bands to make a continuous liquid and some solid level measurements. The radar sensor is mounted on the top of the vessel and is aimed down, perpendicular to the liquid surface. Most tank-farm gauges are operated on the FMCW principle (Figure 3.121). Other gauges and transmitters, particularly the lowest-cost units, are operated on the pulse principle. Both principles are fundamentally based on the time of flight from the sensor to the level of the surface to be measured. In the FMCW method, this time of flight is tracked on a carrier wave in the pulse method, it is the echo return. 

LC Level controller LDV Let down valve LHV Lower heating value LH2 or LH2 Liquid hydrogen LN2 or LN2 Liquid nitrogen LNG Liquefied natural gas LSH High level switch LP Low pressure LT Level transmitter... 

Starch slurry make-down systems are designed to prepare a uniform suspension, using batch or continuous systems. In modem paper mills, automated batch or continuous systems are used. Water content and bulk density will affect the flow of dry starch.90 In the batch slurry system, weighed increments of starch are drawn from the silo and periodically added to a measured quantity of water in the make-up tank. The suspension is screened and automatically pumped to a larger storage tank on demand by a level transmitter. Continuous make-down of starch slurry is accomplished by simultaneously feeding starch and water at controlled rates into the slurry make-down tank. The dry starch is metered with a volumetric screw feeder. The slurry is screened before dispersion by batch or continuous cooking. 

Figure 6.187 Sensor of a continuous level transmitter for liquids. A metallic rod in a tank forms a capacitance together with the (metallic) tank walls. A liquid with a relative dielectric constant er > 1 (for values of sr see Table 6.43) increases this capacitance proportional to the liquid level. The transmitter may be either integrated into the sensor head or located separately.

Materials of construction for wetted portions of the instrumentation must be selected using similar considerations to those used for the equipment itself. In addition, plastic diaphragm seals are available to cover and protect the pressure-sensing faces of immersed level transmitters. This allows the use of stainless steel in apph-cations where, if unprotected, the corrosion rate would be unacceptably high on the thin face of the instrument. A diaphragm seal is recommended for pressure instruments in the presence of suspended or entrained solids. 

Vaporizer floods, liquid to reactor fit high-level alarm on LICl with automatic pump shutdown. Add independent level transmitter and alarm LT2. 

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