Flow control loop

Continuous in-line measurements and control of the mass material balance in the process, with automatic feedback to the reactants dosing devices (performed either by computerized system or by traditional flow control loops). 

A cascade control system can be designed to handle fuel gas disturbance more effectively (Fig. 10.1). In this case, a secondary loop (also called the slave loop) is used to adjust the regulating valve and thus manipulate the fuel gas flow rate. The temperature controller (the master or primary controller) sends its signal, in terms of the desired flow rate, to the secondary flow control loop—in essence, the signal is the set point of the secondary flow controller (FC). 

This inner flow control loop can respond immediately to fluctuations in the fuel gas flow to ensure... 

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.)... 

A more sophisticated implementation is full metering control (Fig. 10.6). In this case, we send the signals from the fuel gas controller (FC in the fuel gas loop) and the air flow transmitter (FT) to the ratio controller (RC), which takes the desired flow ratio (R) as the set point. This controller calculates the proper air flow rate, which in turn becomes the set point to the air flow controller (FC in the air flow loop). If we take away the secondary flow control loops on both the fuel gas and air flow rates, what we have is called parallel positioning control. In this simpler case, of course, the performance of the furnace is subject to fluctuations in fuel and air supply lines. 

Occasionally an incident occurs that results in a common mode failure. This is a single event that affects a number of pieces of hardware simultaneously. For example, consider several flow control loops similar to Figure 11-4. A common mode failure is the loss of electrical power or a loss of instrument air. A utility failure of this type can cause all the control loops to fail at the same time. The utility is connected to these systems via OR gates. This increases the failure rate substantially. When working with control systems, one needs to deliberately design the systems to minimize common cause failures. 

The common types of control loops are level, flow, temperature, and pressure. The type of controller and the settings used for any one type are sometimes pretty much the same from one application to another. For example, most flow control loops use PI controllers with wide proportional band and fast integral action. 

Derive a dynamic mathematical model of the flooded-condenser system. Calculate the transfer function relating steam flow rate to condensate flow rate. Using a PI controller with tj = 0.1 minute, calculate the closedloop time constant of the steam flow control loop when a closedloop damping coefTident of 0.3 is used. Compare this with the result found in (u). 

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). 

Although we do not necessarily know the relationships involved in the external world, they do exist and determine the values of the relevant load disturbances. Consequently, we can say that the external world removes as many degrees of freedom as the number of disturbances. In Fig. 7.9 we are introducing a flow control loop to keep F constant, and a temperature control loop with a preheater to maintain qF. The feed composition is fixed by a relationship that we do not know—but all the same must exist. The control objectives are achieved by fitting suitable control systems to the plant and we can say that these control systems remove as many degrees of freedom as the number of control objectives in the overall control strategy. 

Substantial effort in modelling and/or experimental measurement is required in order to derive GFFA(s) and GFFB(s). Due to errors in determining the individual transfer functions (GM(s), G 2(s), etc.), to errors in measurement, and to load variables which have not been accounted for in the models, feed-forward compensation can never be perfect, and considerable drifting of the controlled variable(s) can occur. On the other hand, the two variable feed-forward control model expressed by equation 7.165 automatically takes into account any interaction between the reflux and steam flow control loops (see also Section 7.15). 

Fig. 7.114. Arrangement of simple pneumatic flow control loop with narrow-band...

To achieve the flow and pressure precision needed for the more demanding HPLC applications, a number of proprietary electropneumatic pressure and flow control loops have been created to ensure repeatable pump performance. 

Flow control loops are designed to maintain constant retention times by providing constant flow delivery from the pump, commonly by the use of an encoder wheel attached to the main motor drive. The encoder accurately measures the rotation of the motor extremely accurately to parts-of-a-degree of rotation, and it regulates the motor revolutions to as constant a value as possible. In a well-maintained pumping system, a constant motor speed will deliver a constant flow for a given solvent. In modem pumps a flow control loop is often combined with a pressure loop, to provide both... 

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... 

If the ratio calculation is made outside the secondary flow control loop, its setting does not change the dynamics or response of the loop. 

One effective method of keeping the valve gain (GJ perfectly constant is to replace the valve with a linear flow control loop. The limitation of this cascade configuration (in addition to its higher cost) is that if the controlled process is faster than the speed of response of the flow loop, cycling will occur. This is because the slave—in this case, the flow control loop—in any... 

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. 

The dynamics of fhe sensor are quite fast, and the dynamics of the flow control loop are usually fast compared with the dynamics of the process (i.e., percentage level change for change in flow... 

Consider a wastewater neutralization process with a titration curve for the wastewater that exhibits a high gain at neutrality. To control the pH to 1.0 pH units at a setpoint of pH 7, the base flow rate must be metered accurately to within 0.5%. A single-flow control loop with a control valve with a positioner can meet this metering precision. But if the total flow rate of base were to range from 0.1 to 10 GPM, one flow control loop could not meter the base flow rate to within 0.5% at both 0.1 and 10 GPM. 

In chemical processes, flow rate control loops are almost always cascaded with other control loops. Why does this happen [Note Take into account the following two facts (a) The flow rate itself is subject to changes and is regulated by the flow control loop, and (b) flow rates are the most common manipulated variables in chemical processes.]... 

To eliminate conflict (a) we can delete the feedforward flow control loop in the coolant system. To erase conflict (b) we delete the flow control loop in the flash drum. Thus the final control configuration for the overall process is shown in Figure 23.6b. It has no conflicts among the loops and the process is exactly specified. 

These values are only indicative of the sampling periods one should expect for the corresponding loops and simply manifest the fact that flow control loops are faster than level and pressure loops, with temperature loops being the slowest. Thus the faster a loop, the higher its sampling rate. This is in agreement with our discussion in Section 27.1. 

IV.l Consider the flow control loop shown in Figure 13.2a. The following information is also available (1) An orifice plate is used to measure the flow (2) a variable capacitance differential pressure transducer is employed (see Appendix 11 A) to sense and transmit the pressure difference developed around the orifice plate (3) the controller is PI and (4) the control valve is of equal percentage, with the valve flow characteristic curve given by... 

The last equation describes the necessary steady-state decoupler which cancels any effects that the flow control loop might have on the composition control loop. 

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... 

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