Fast loop

C. PRESSURE LOOPS. Pressure loops vary from very tight, fast loops (almost like flow control) to slow averaging loops (almost like level control). An example of a fast pressure loop is the case of a valve throttling the flow of vapor from a vessel, as shown in Fig. 7.15o. The valve has a direct handle on pressure, and... 

The functional groups almost exclusively involved in NIRS are those involving the hydrogen atom C-H, N-H, O-H (see Figure 5.1). These groups are the overtones and combinations of their fundamental frequencies in the mid-infrared and produce absorption bands of useful intensity in the NIR. Because the absorptivi-ties of vibrational overtone and combination bands are so much weaker, in NIRS the spectra of condensed phase, physically thick samples, can be measured without sample dilution or the need to resort to difficult short-path length sampling techniques. Thus conventional sample preparation is redundant, and fortunately so, because most PAT applications require direct measurement of the sample " either in situ, or after extraction of the sample from the process in a fast loop or bypass. 

Extractive Fast Loop Sample Conditioning Systems... 

This is based on a sample fast loop with a sample take-off probe in the main process line, and a retnm to either a pnmp-snction or ambient pressnre sample recovery system (Fignre 5.22). The fast loop provides a filtered slip-stream to the analyzer, along with flow control and monitoring (low-flow alarm), and freqnently some form of sample antograb facility for sample capture. The slipstream flow to the internal analyzer sample flow cell is relatively low volnme and allows for a very high degree of sample temperature control (typically... 

Where physical stream switching is acceptable, this arrangement allows for multiple sampling fast loops to feed a single near-infrared analyzer, with stream switching controlled by the analyzer. The analyzer will most likely need to be hazardous area comphant, but may be either located in a dedicated analyzer shelter, or may be field-mounted. 

This is a development of the above where a fiber-optic linked hqnid sample transmission cell is integrated with the sample fast loop cabinet (Figures 5.24 and 5.25). There can be multiple sample streams, take-offs and fast loops, each with its own separate fiber-optic transmission cell. The analyzer can either be local with short fiber-optic runs to the sampling cabinet(s), or remote, where a safe area location for the analyzer module may be feasible, but at the cost of longer, potentially less stable fiber-optic runs. This system avoids physical stream switching. 

Figure 3.26 Extractive sample conditioning fast-loop. Reprinted with kind permission of ABB Analytical and Advanced Solutions.

Adding a cascade slave to a fast loop can destabilize the primary if most of the process dynamics (time lags) are within the secondary loop. The most common example of this is using a valve positioner in a flow-control loop. The... 

In the case of fast loops (fast flow, liquid pressure, small-volume gas pressure), positioners are likely to degrade loop response and cause limit cycling, because the positioner (a cascade slave) is not faster than the speed at which its set point (the control signal) can change. A controlled process is considered "fast" if its period of oscillation is less than three times that of the positioned valve. 

Since high-purity products are usually produced only in situations where the separation is relatively easy, most of these columns have fairly large temperature gradients. Therefore it is possible to use tem-perature/composition cascade control systems. The secondary temperature controller serves as a fast loop to detect disturbances quickly and hold the temperature profile in the column. The primary composition... 

The sample loop The sample (fast) loop for off-line analyzers must bring a representative sample to the analyzer in the shortest acceptable time so that, accounting for the analysis time itself and the output response time of any corrective systems, the system response time is achieved. 

Steam turbines are most often used in processes to provide power to compressors or electric generators as shown in Figure 3.28. Multistage turbines may also admit or extract steam between stages. Turbine speed is a fast loop, controlled by manipulating the supply steam valve or valves, as often there will be a rack of parallel steam valves supplied as part of the turbine system. 

It is really a large-signal response issue. The biggest impact on the undershoot/overshoot observed in this case comes simply from the amount of bulk capacitance (and stored energy) present at the output. Because in the initial instant of sudden application of load, the bulk capacitor (Cqut) alone provides the energy being demanded by the load, until the loop finally kicks in to prop up the falling rail. Note that it takes several cycles for the current to ramp up to the new required level in the inductor. So small inductances tend to help in quick recovery and help achieve a fast loop response (of course provided they don t create full-blown instability in the process). 

Fig. 10.15 Example of a basic fast loop Parker-Hannifin Intraflow system.

The control structures identified by the controllability analysis are tested by full nonlinear simulation. Here the first task consists of implementing and tuning of controllers. The use of prescribed local control structures, or setting perfect control for fast loops simplifies this task and preserves the plantwide character of the analysis. 

With this method, known as a fast-loop samphng system, the sample is rapidly transported to and from the sample conditioning system, with a small bypass fraction drawn into the sample manifold. 

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