Flow control upstream

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. 

Now it appears that the value of may be estimated by using the loss coefficient K determined at choking provided K is not too small. This is unlikely since in most valves effective flow control occurs at very small throat area when the valve is in the 10-30% open range. The loss coefficient is determined from the pressure loss across the valve and the velocity in the upstream pipe at choking. 

Disturbance, such as upstream pressure, which specifically leads to changes in the fuel gas flow rate is now drawn to be part of the secondary flow control loop. (A disturbance such as change in the process stream inlet temperature, which is not part of the secondary loop, would still be drawn in its usual location as in Section 5.2 on page 5-7.)... 

It is quite important not to operate a turbine-driven pump by throttling the steam flow to the turbine. Let s assume that the operators have set the turbine speed at 3500 rpm, by adjusting the steam inlet gate valve upstream of a malfunctioning governor. Suddenly, the discharge flow-control valve cuts back, and the pump s flow decreases from 2000 to 1200 GPM. The pump speed will then increase, because fewer pounds of liquid are being pumped, and less horsepower is required to spin the pump. 

Thus, some load variables can be eliminated by the application of guideline (i) to Fig. 7.9, e.g. it is relatively easy to maintain the feed quality by installing a preheater with the outlet temperature controlled as shown, and the feed flow can be fixed by using a simple flow control loop (as long as intermediate storage is provided between the distillation plant and any plant upstream which supplies the feed to the column). 

A. Flow Control without Feedback. Plow can be controlled by means of a needle valve if the pressure drop across the valve is constant. The pressure on the upstream side often can be held constant with a single- or two-stage mechanical diaphragm regulator (Section 10.1. B). If the stream of gas does not experience a variable constriction after the needle valve, the above combination provides a simple and convenient means of providing a steady flow. Often an arrangement such as this is used in conjunction with a rotameter or electronic mass flow meter (Fig. 7.14). 

The S scenarios. These are steep slope cases. They may be analyzed in much the same fashion as the M scenarios, having due regard for downstream control in the case of upper-stage flow and upstream control for lower-stage flow. Thus a dam or an obstruction on a steep slope produces an Si scenario, which approaches the horizontal asymptotically but cannot so approach the uniform depth line, which lies below the critical depth. Therefore this curve must be preceded by a hydraulic jump. The S2 scenario shows accelerated lower-stage flow, smoothly... 

A typical control scheme for a distillation column is shown in Fig. 19. Flow controllers (FCs) regulate the flow rates of the feed and overhead products. Each flow rate is measured by a device such as an orifice plate placed upstream... 

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. In this section we alter the control objective relating to production rate. Instead of flow controlling the product stream from the bottom of the stripper, we assume that an upstream process sets the flow of the Fr,c stream and the process must take whatever amount is fed into it. Most of the steps in the design procedure are the same as the previous section, but the control of liquid levels is now in the direction of flow. The product quality criterion is the same. 

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. 

The P7T ratio is a process variable or parameter to be affixed by the operator. Furthermore, it can be assumed that Pl and Py are set by back-pressure controllers on gas streams L and V. The feed rate Emay be increased or reduced by a valve in the line (e.g., by a flow controller), where the upstream feed pressure is sufficiently high. Ordinarily, it would be set at a constant rate, at a fixed reject pressure Pj. ... 

However, these latter two controls on effluent flow and upstream river flow also may affect the downstream transport through the velocity. A reduction in concentration at the outfall due to increased dilution from low flow augmentation may result in an increase in the concentration downstream because of an increased velocity. A reverse situation may occur if the flow is reduced. The concentration profile also depends on the decay rate. Thus, a final general control point is to ... 

A cross section of the sensor element shows the heating zone where the temperature is controlled to be about 180 K above ambient temperature. Without air flow the temperature is identical upstream and downstream of the diaphragm. With air flow the upstream side of the diaphragm is cooled. Temperature resistors upstream and downstream of the diaphragm detect the resulting temperature difference, which is used as an airflow signal... 

The feed is introduced into the external circulating fines destruction stream to provide sufficient liquid head to suppress flashing, which would spawn nuclei and sabotage the fines reducing effort. The feed is controlled by an independent flow control loop from an upstream surge tank. The surge tank prevents upstream flow variations from forcing oscillations in the residence time of the crystallizer. A second and more critical reason for... 

Secondly, candidates for manipulated inputs can be found. When they are flow rates connecting BFS s, the choice affect the control of both upstream and downstream. As an example, it was proposed to keep reactant recycle on flow control and to change the setpoint of this loop when production changes are required. This implies that the inventory control of the upstream unit is in direction opposite to flow, while the inventory control of the downstream unit is in the direction of flow. Manipulated flows should be chosen with care to avoid over-specification with respect to plant mass balance. The set-points of the BFS s form another category of plantwide manipulated variables. Examples are reaction conversion, separation performance, etc. 

The discussions and diagrams in this chapter exclude manipulation of column feed. This manipulation seldom belongs to the column control system. Feed is usually manipulated as part of an upstream control system (e.g., to regulate bottom level of an upstream column), or is maintained constant by a flow controller. One exception... 

Until now, it was assumed that feed is controlled to satisfy some control criterion upstream of the column or is on flow control. While this is usually the case, there are instances when it is beneficial to incorporate the feed into the column control system. A typical example is when it is desired to maintain a constant bottom flow rate to a downstream unit (e.g., a reactor or a furnace), and it is impractical to smooth out bottom fluctuations by one of the common control schemes or by adding surge capacity at the hot bottom temperature. 

The physical system on which the flow control scheme is implemented can be subdivided into three subsystems [8] (a) flow sensors, which characterize the upstream coherent structures, (b) actuators, which modify the wall boundary conditions experienced by the approaching coherent structures, and (c) control sensors, which establishes the coherent structures evolution between upstream and downstream locations and the effect of actuations. 

Some means is usually provided to control the carrier gas flow. In a packed column chromatograph, this is usually accomplished with a differential pneumatic flow control valve placed in the gas line upstream of the injection port. 

A. Operation of Saturators. As shown in Fig. 11.1, neutralized depleted brine is fed to the saturatorfs). The total rate of flow is regulated in the upstream equipment, and no automatic flow control can be applied in the saturation system itself. The feed brine should be at pH 8 or 9, but an upset in the neutralization system could allow the pH to drop well into the acidic range. Although Monel is the metal normally used for instruments in alkaline brine applications, Hastelloy C has the advantage of a greater resistance to attack by acids. 

Flow from the reservoir tank is achieved by pressurizing the tank with ambient temperature helium immediately prior to the calibration test. Flow control of the pressurization gas is achieved using a regulator sensing the line pressure immediately upstream of the flowmeter being calibrated. In this manner, the line pressure is maintained within of the set pressure. 

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