Pressure flows

For liquid chromatography, a sample of the mixture solution is injected through a loop injector which allows a quantity of the solution to be placed in a small tubular loop at atmospheric pressure. By manipulating a valve, the high-pressure flow of solvent to the column is diverted through the loop, carrying the sample with it (Figure 35.5). 

Head-Area Meters. The Bernoulli principle, the basis of closed-pipe differential-pressure flow measurement, can also be appHed to open-channel Hquid flows. When an obstmction is placed in an open channel, the flowing Hquid backs up and, by means of the Bernoulli equation, the flow rate can be shown to be proportional to the head, the exact relationship being a function of the obstmction shape. 

The four process control parameters are temperature, pressure, flow, and level. Modem process level detection systems are varied and ubiquitous in modem chemical plants there are thousands of processes requiring Hquid level indication and Hquid level control. From accumulators to wet wells, the need for level devices is based on the need for plant efficiency, safety, quaUty control, and data logging. Unfortunately, no single level measurement technology works rehably on all chemical plant appHcations. This fact has spawned a broad selection of level indication and control device technologies, each of which operates successfully on specific appHcations. 

The pressure flow is independent of screw speed, except as the latter affects the viscosity of the melt. 

The net flow, of an extmder is simply the difference between drag and pressure flows ... 

Process Measurements. The most commonly measured process variables are pressures, flows, levels, and temperatures (see Flow LffiASURELffiNT Liquid-levell asurel nt PressureLffiASURELffiNT Temperaturel asurel nt). When appropriate, other physical properties, chemical properties, and chemical compositions are also measured. The selection of the proper instmmentation for a particular appHcation is dependent on factors such as the type and nature of the fluid or soHd involved relevant process conditions rangeabiHty, accuracy, and repeatabiHty requited response time installed cost and maintainabiHty and reHabiHty. Various handbooks are available that can assist in selecting sensors (qv) for particular appHcations (14—16). 

Flow. The principal types of flow rate sensors are differential pressure, electromagnetic, vortex, and turbine. Of these, the first is the most popular. Orifice plates and Venturi-type flow tubes are the most popular differential pressure flow rate sensors. In these, the pressure differential measured across the sensor is proportional to the square of the volumetric flow rate. 

The relative abundance of neutral SiH and H2 species have been measured as a function of power, pressure, flow rate, and dilution. For low power levels, eg, 5 W, up to 50% of the SiH gas is dissociated and the percentage increases to 80% for a power of 50 W. The decomposition of SiH gas proceeds more readily with lower flow rates. These observations, coupled with infrared (ir) measurements performed on the films, suggest that deposition under conditions in which the silane gas is not entirely decomposed leads to a majority of SiH units, whereas those deposited under conditions in which silane is strongly dissociated contain a majority of dihydride units leading to a deterioration of the semiconductor. Also, when the dwell time of SiH in the plasma region increases, the resultant film exhibits a pronounced peak at 2090 cm from the ir spectra corresponding to S1H2 inclusion. 

FIG. 8-85 Pressure, flow, and power for a throttling process using (1) a control valve and a constant speed pump and (2) an adjustable speed pump. 

Chile [Prog. Aerosp. Sc7, 16, 147-223 (1975)] reviews the use of the pitot tube and allied pressure probes for impact pressure, static pressure, dynamic pressure, flow direction and local velocity, sldn friction, and flow measurements. 

In this analysis F will be constant but it could be described more accurately as a function of parameters influencing heat transfer in the condenser (temperature, pressure, flow rate, fluid thermodynamical, and thermophysical characteristics. . . ). 

Another instance in which the constant-temperature method is used involves the direc t application of experimental KcO values obtained at the desired conditions of inlet temperatures, operating pressure, flow rates, and feed-stream compositions. The assumption here is that, regardless of any temperature profiles that may exist within the actu tower, the procedure of working the problem in reverse will yield a correct result. One should be cautious about extrapolating such data veiy far from the original basis and be carebil to use compatible equilibrium data. 

Pressure. Flow 0t(AP/pd - Only source of energy is from fluid being atomized. Simplicity and low cost. Limited tolerance for solids uncertain spray with high-viscosity liquids susceptible to erosion. Need for special designs (e.g., hypass) to achieve turndown. 

Consider Inherently Safer Design alternatives (e.g., to withstand maximum upset conditions—temperature, pressure, flow)... 

Source Pollutant Scrubber pressure loss and water pressure Flow rate... 

Nozzle arrangements for various applications vary considerably. For subcritical flow measurement at the outlet end, where nozzle differential pressure p is less than the barometric pressure, flow should be measured with impact tubes and manometers as shown in Figure 20-3. 

Full documented formal test runs at design and normal conditions. All instruments need to be calibrated for the runs and the documentation should include more than just the primary process variables of temperature, pressure, flow, and composition... 

The best anti-surge control is the simplest and most basic that will do the job. The most obvious parameter is minimum-flow measurement, or if there is a relatively steep pressure-flow characteristic, the differentia pressure may be used. The latter parameter allows for a much faster response system, as flow measurement response is generally slow however, the speed of response need only be fast enough to accept expected transients. One major problem with the conventional methods of measurement and control is the need to move the set point for initiation of the control signal away from the exact surge point to allow some safety factor for control response time and other parameters not directly included... 

In order to avoid the need to measure velocity head, the loop piping must be sized to have a velocity pressure less than 5% of the static pressure. Flow conditions at the required overload capacity should be checked for critical pressure drop to ensure that valves are adequately sized. For ease of control, the loop gas cooler is usually placed downstream of the discharge throttle valve. Care should be taken to check that choke flow will not occur in the cooler tubes. Another cause of concern is cooler heat capacity and/or cooling water approach temperature. A check of these items, especially with regard to expected ambient condi-... 

Fig. 4.20 Pressure-Flow-rate characteristics fora spool-valve.

Inventory Reduce inventory of chemicals Continuous operation may be preferable to batch Low residence time contacting equipment may be better than cheaper alternatives etc. Monitor temperature, pressure flow, composition, freedom from contamination and other appropriate properties of all streams where relevant. Consider automatic control... 

Instrument failure, pressure, flow, temperature, level or a reaetion parameter, e.g. eoneentration. Failure of instrument air or eleetrieity. 

Can the plant pressure or the inventory of process materials, or both, be reduced effectively and safely in a major emergency Are the operating parameter limits which require remedial action, known and measured, e.g. temperature, pressure, flow, concentration ... 

Temperature, pressure, flow rate, and ozone concentration of the ozonecontaining gas being discharged from all the ozone generators. This is the only effective method by which ozone dosage and the ozone production capacity of the ozone generator can be determined. 

As discussed in the previous section, it is convenient to consider the output from the extruder as consisting of three components - drag flow, pressure flow and leakage. The derivation of the equation for output assumes that in the metering zone the melt has a constant viscosity and its flow is isothermal in a wide shallow channel. These conditions are most likely to be approached in the metering zone. 

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