Process pressure measurement

Process pressure-measuring devices may be divided into three groups ... 

Foxboro s Model 823 transmitter uses a taut wire stretched between a measuring diaphragm and a restraining element. The differential process pressure across the measuring diaphragm increases the tension on the wire, thus changing the wire s natural frequency when it is excited by an electromagnet. This vibration (1800—3000 H2) is picked up inductively in an oscillator circuit which feeds a frequency-to-current converter to get a 4—20 m A d-c output. 

Industrial and Control Instruments. Mercury is used in many industrial and medical instmments to measure or control reactions and equipment functions, including thermometers, manometers (flow meters), barometers and other pressure-sensing devices, gauges, valves, seals, and navigational devices (see Pressure measurements Process control Temperature measurement). Whereas mercury fever thermometers are being replaced by... 

In other designs, a diffused siUcon sensor is mounted in a meter body that is designed to permit caUbration, convenient installation in pressure systems and electrical circuits, protection against overload, protection from weather, isolation from corrosive or conductive process fluids, and in some cases to meet standards requirements, eg, of Factory Mutual. A typical process pressure meter body is shown in Figure 10. Pressure measurement from 0—746 Pa (0—3 in. H2O) to 0—69 MPa (0—10,000 psi) is available for process temperatures in the range —40 to 125°C. Differential pressure- and absolute pressure-measuring meter bodies are also available. As transmitters, the output of these devices is typically 4—20 m A dc with 25-V-dc supply voltage. 

Fig. 7. Instmment components of a control loop, where A = process measurement devices, in this case, pressure measurement B = transducer ...

Pressure. Most pressure measurements are based on the concept of translating the process pressure into a physical movement of a diaphragm, bellows, or a Bourdon element. For electronic transmission, these basic elements are coupled with an electronic device for transforming a physical movement associated with the element into an electronic signal proportional to the process pressure, eg, a strain gauge or a linear differential variable transformer (LDVT). 

Piezoelectric Transducers Certain ciystals produce a potential difference between their surfaces when stressed in appropriate directions. Piezoelectric pressure transducers generate a potential difference proportional to a pressure-generated stress. Because of the extremely high electrical impedance of piezoelectric crystals at low frequency, these transducers are usually not suitable for measurement of static process pressures. 

Liquid Column Density may be determined by measuring the gauge pressure at the base of a fixed-height hquid column open to the atmosphere. If the process system is closed, then a differential pressure measurement is made between the bottom of the fixed height liquid column and the vapor over the column. If vapor space is not always present, the differential-pressure measurement is made between the bottom and top of a fixed-height column with the top measurement being made at a point below the liquid surface. 

An example of a pneumatic PI controller is shown in Fig. 8-64 7. This controller has two stages of pneumatic amphfication and a Bourdon tube input element that measures process pressure. The Bourdon tube element is a flattened tube that has been formed into a cui ve so that changes in pressure inside the tube cause vertical motions to occur at the ungrounded end. This motion is transferred to the left end of the beam, as shown. 

Temperature is the hardest parameter to control in a fractionation system. It exhibits high process and measurement lag. Temperature can also be ambivalent as a measure of composition. Pressure changes are reflected quickly up and down the column. Temperature changes are not. It is typical to provide three-mode controllers for all temperature applications. 

Mass loss determinations refer to the total change resulting from reactant decomposition and usually include contributions from a mixture of product compounds, some of which would normally be condensed under conditions used for accumulatory pressure measurements. Such information concerned with the overall process is, however, often usefully supplemented by evolved gas analyses (EGA) using appropriate analytical methods. Sestak [130] has made a detailed investigation of the effects of size and shape of reactant container on decomposition kinetics and has recommended that the sample be spread as a thin layer on the surfaces of a multiple plate holder. The catalytic activity of platinum as a reactant support may modify [131] the apparent kinetic behaviour. 

Such systems have the experimental advantage that kinetic data may be obtained by gravimetric or evolved gas pressure measurements. However, these data must be interpreted with care, since gas release is not necessarily concurrent with the solid—solid interaction but may, in principle, be a distinct rate process under independent kinetic control and occur either before or after reaction between the solids. Possible mechanisms to be considered, therefore, include the following. 

HPLC is extremely useful in monitoring and optimizing industrial processes. Conventional process monitors measure only bulk properties, such as the temperature and pressure of a reactor, while HPLC permits continuous realtime monitoring of consumption of starting materials, product composition, and impurity profile. There are a number of new initiatives relevant to HPLC for process monitoring, including sample preparation, automation, miniaturization, and specialized detectors. 

Test run the curves 2 in Fig. 1.71 are taken from the test run, as shown in Fig. 1.63, but with pressure control 0.36 mbar (total pressure measured with Capacitron). The ice temperature has been -22 °C (constant) for 3 h and DR reached 0.05 %/h after 10 h. Secondary drying could have been started much more early, thus shortening the drying process. 

Chase [2.32] presents an alternative method to monitor and control the freeze drying process by measuring the flow of nitrogen to keep the operation control pressure, pc, constant. The Mass Flow Controller (FMC) consists of a proportional valve, an integral flow meter and a capacitance manometer (CA). The CA measures the total pressure in the plant, the valve opens, if the pressure gets below the preset value and vice versa. The flow of... 

Merika [3.51] emphasized from his 17 years of experience with the quality control of freeze dried transplants the importance of sterility and residual moisture control as the decisive characteristics. Furthermore, the leak tightness of the storage containers was constantly controlled. Merika did not measure the product temperature during drying, but controled the process by measuring water vapor pressure and temperatures of the shelves and the condenser. The residual moisture content after 2 years of storage must be below 5 %. All products were sterilized by gamma radiation. 

In the following table the important process steps (Proc. no.), the process description (Process quantity) (measure), the related target data (Target data) and their tolerances (tolerances) are listed and compared with average data measured in three runs (Ave. act. data) and the minimum and maximum data measured in the three runs (min./max.). The last two data have to be taken from the protocols and to be listed. In the last column the identification number of the runs, in which the two extreme data are measured is listed (Ident. no.) The last two column are not given, with the exception of proc. nr. 1.1 as an example- The table is a proposal of how the comparison could be made. The list may not be complete in all possible cases and is concentrated on the time-, pressure- and temperature data. Other methods may be preferred to make the ability of the equipment transparent. 

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