Flow control downstream

Many misconceptions exist about cascade control loops and their purpose. For example, many engineers specify a level-flow cascade for every level control situation. However, if the level controller is tightly tuned, the out-flow bounces around as does the level, regardless of whether the level controller output goes direcdy to a valve or to the setpoint of a flow controller. The secondary controller does not, in itself, smooth the outflow. In fact, the flow controller may actually cause control difficulties because it adds another time constant to the primary control loop, makes the proper functioning of the primary control loop dependent on two process variables rather than one, and requites two properly tuned controllers rather than one to function properly. However, as pointed out previously, the flow controller compensates for the effect of the upstream and downstream pressure variations and, in that respect, improves the performance of the primary control loop. Therefore, such a level-flow cascade may often be justified, but not for the smoothing of out-flow. 

The above devices are flow intrusive, subjected to wear and the splitter itself cannot control changing downstream conditions. A potentially more direct and efficient approach is to monitor the change in flow conditions downstream of a splitter (Barnes and Mumane, 1995) and employ active splitters (Selves et al., 1995) to control the split ratio of air and hence, material. Some of the active splitters being investigated and developed by Selves et al., 1995 include ... 

Are vaporizers provided with automatic gas line shutoff valve, downstream pressure-reducing valve, gas flow control valve, temperature control system and interlocks to shut down gas flow on low vaporizer temperature, and appropriate alarms in a continuously manned control room ... 

Pressure oscillations with RMS value up to 10 kPa in two models of lean-burn gas turbine combustors, with heat release around 100 kW, have been actively controlled by the oscillation of fuel flow. The flames were stabilized behind an annular ring and a step in one arrangement, and downstream of an expansion and aided by swirl in the other. Control was sensitive to the location of addition of oscillated fuel. Oscillations in the annular flow were attenuated by 12 dB for an overall equivalence ratio of 0.7 by the oscillation of fuel in the core flow and comprising 10% of the total fuel flow, but negligibly for equivalence ratios greater than 0.75. Oscillation of less than 4% of the total fuel in the annulus flow led to attenuation by 6 dB for all values of equivalence ratio considered. In the swirling flow, control was more effective with oscillations imposed on the flow of fuel in a central axial jet than in the main flow, and oscillations were ameliorated by 10 dB for equivalence ratio up to 0.75, above which the flame moved downstream so that the effectiveness of the actuator declined. The amelioration of pressure oscillations resulted in an increase in NOj, emissions by between 5% and 15%. 

Figure 9. Configuration of the DS-IC system A, clean air input B, mass-flow controller C, permeation device chamber D and H, vents E, needle valve-rotameter F, needle valve G, mass-flow meter I, diffusion scrubber Jy scrubber liquid reservoir K, needle valve-rotameter L, suction pump M, injection valve Ny peristaltic pump O, eluent flow F, downstream chromatographic components and Q, sample loop. (Reproduced from reference 96.

The next step is to calculate the pressure loss to the control valve entrance. The designer has located the flow control valve as close as practical to the stabilizer feed entrance. This is a good design location for the control valve. Why The pressure drop across the control valve results in a two-phase flow, vapor and liquid flowing into the stabilizer feed tray. Thus, the downstream flow is a two-phase flow, and this problem will be finished later, in the section on two-phase flow. 

R-V Reflux flow controls distillate composition. Heat input controls bottoms composition. By default, the inventory controls use distillate flowrate to hold reflux drum level and bottoms flowrate to control base level. This control structure (in its single-end control version) is probably the most widely used. The liquid and vapor flowrates in the column are what really affect product compositions, so direct manipulation of these variables makes sense. One of the strengths of this system is that it usually handles feed composition changes quite well. It also permits the two products to be sent to downstream processes on proportional-only level control so that plantwide flow smoothing can be achieved. 

In the next two sections, vre select two different control objectives and develop two different control structures, In the first case, we assume that the flowrate of the product stream B leaving the base of the stripper is set by a downstream customer. Hence it is flow-controlled, and the setpoint of the flow controller is a load disturbance to the process. In the second case, we assume that the fresh feed stream is set by an upstream process. So it is flow-controlled, and the setpoint of the flow controller is a load disturbance to the process. These two cases demonstrate that different control objectives produce different control strategies. 

Step 1. We are assuming in this section that the product stream from the bottom of the stripper is set on the demand of a downstream user. The bottoms stream from the stripper is flow-controlled and so we set the position of the control valve, XMV(8), on this stream (B). The rest of the liquid level controls must be chosen to accommodate this first-priority choice. Note that we could put a flow controller on this stream if necessary, but this was not done in the simulations described later. The quality specification is that component G in the product should not vary more than 5 mol %. 

In this section we illustrate how the control scheme is modified if a different control criterion is specified. Suppose business objectives dictate that the organic product from the decanter must be an on-demand stream, i.e., a downstream unit or customer sets the desired flowrate of this stream and the plant must immediately supply the requested flowrate of organic product. In this situation the organic product flow-rate will be flow controlled, with the setpoint of the flow controller coming from the downstream consumer. A similar case was considered in Chap. 8 with the Eastman process. 

In a classic paper, Pillsbury (1981) described how recycling of salts via irrigation and agricultural return flow controls the salinity of downstream rivers in arid zones. Once the natural salt balance is disturbed and salts begin to accumulate, either in the unsaturated zone or in drainage waters, the salinity increases. The salinity of the Colorado River is derived from a century of activity that includes upstream diversion of freshwater, massive irrigation, evapotranspiration and salt accumulation in the soil, and return of saline drainage back to the river (Pillsbury, 1981). Similarly, the rise of the salinity in the Nile Delta has been attributed to a disturbance of the natural salts balance after the construction of Assuan dam and the reduction of the natural outflow of salts from the Nile River to... 

Flow control is probably the most common control loop in most processes. Typically, a liquid or gas flow rate is maintained in a pipe by a throttling valve downstream of the measurement, as shown in Figure 3.4(A). Locating the valve upstream of the measurement is not recommended because many measurement problems can arise. 

A vaporizer is typically used in a process to provide a vapor feed to downstream equipment. In that case, it may be desirable to set the flow of vapor directly with the setpoint of a flow control loop. Then the heat input is adjusted for pressure control and the liquid level is maintained by adjusting the inlet flow, in a reverse material balance manner, as shown in Figure 3.13(A). If heat transfer limits throughput, then both the vapor valve and the steam valve will operate fully open and the pressure will droop to an equilibrium point, where heat transfer equals the flow through the downstream equipment. 

A typical pSFC instrument, at first glance, is designed like an HPLC system. The major differences are encountered at the pump, the column oven, and downstream of the column. pSFC is best carried out using pumps in a flow-control mode. A regulator mounted downstream of the column and ultraviolet-visible detector (UV) controls the pressure drop in the chro-... 

The test procedure consisted of initially placing a single tablet of sodium sulfite, then 2, 4, 16, and 28 tablets of sodium sulfite in the water flow path 10-ft downstream of the flow control valve and meter. The flow rates ranged from 100 up to 500-1- gpm. Samples were collected 150 ft downstream of the tablets. As shown in Fig. 3, one tablet effectively reduced the chlorine residual of the flow it came in to contact with, to below 0.1 mg/L for 45 min at a flow rate of 100 gpm. The tablet was not fully consumed, but became ineffective after approx 1 h and the chlorine concentration rose again. 

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