Process design differential pressures

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. 

Process-variable feedback for the controller is achieved by one of two methods. The process variable can (I) be measured and transmitted to the controller by using a separate measurement transmitter with a 0.2-I.0-bar (3-15-psi pneumatic output, or (2) be sensed directly by the controller, which contains the measurement sensor within its enclosure. Controllers with integral sensing elements are available that sense pressure, differential pressure, temperature, and level. Some controller designs have the set point adjustment knob in the controller, making set point adjustment a local and manual operation. Other types receive a set point from a remotely located pneumatic source, such as a manual air set regulator or another controller, to achieve set point adjustment. There are versions of the pneumatic controller that support the useful one-, two-, and three-mode combinations of proportional, integral, and derivative actions. Other options include auto/manual transfer stations, antireset windup circuitry, on/off control, and process-variable and set point indicators. 

Homogenisation processes now extend up to 1500 bar differential pressures. As the materials to be homogenized exhibit varying properties with respect to viscosity, corrosiveness and abrasiveness the high pressure components, such as homogenising pumps and valves, need very careful design and choice of materials. 

Hydrodesulfurization catalysts are normally used as extrudates or as porous pellets, but the particle size and pore geometry have an important influence on process design-especially for the heavier feedstocks. The reaction rates of hydro-desulfurization catalysts are limited by the diffusion of the reactants into, and the products out of, the catalyst pore systems. Thus, as the catalyst particle size is decreased, the rate of desulfurization is increased (Figure 5-9) (Frost and (Nottingham, 1971) but the pressure differential across the catalyst bed also diminishes and a balance must be reached between reaction rate and pressure drop across the bed. 

Once processes have been established, the functions required to implement the processes shall be defined, e.g., equipment sterilization, environmental control such as temperature, differential pressure, particulate control, etc. It is at this stage that we must define the function performance criteria that will provide the basis for design, testing, operation, maintenance, and ultimately continuous improvement. 

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