Transportation lag

Dead time. Probably the best example of a measurement device that exhibits pure dead time is the chromatograph, because the analysis is not available for some time after a sample is injected. Additional dead time results from the transportation lag within the sample... 

The dead time function is also called the time delay, transport lag, translated, or time shift function (Fig. 2.3). It is defined such that an original function f(t) is "shifted" in time to, and no matter what f(t) is, its value is set to zero for t < to- This time delay function can be written as ... 

Many physio-chemical processes involve a time delay between the input and output. This delay may be due to the time required for a slow chemical sensor to respond, or for a fluid to travel down a pipe. A time delay is also called dead time or transport lag. In controller design, the output will not contain the most current information, and systems with dead time can be difficult to control. 

For the sake of illustration, let s presume that the temperature transmitter has a built-in amplifier which allows us to have a measurement gain of Km = 5 mV/°C. Let s also presume that there is no transport lag, and the thermocouple response is rapid. The measurement transfer function in this case is simply... 

In a chemical plant, time delay is usually a result of transport lag in pipe flow. If the flow rate is fairly constant, the use of the Smith predictor is acceptable. If the flow rate varies for whatever reasons, this compensation method will not be effective. 

The size and complexity of the N-reactor plant and the limited amount of computing equipment that was available necessitated a judicious use of simplifying assumptions. For instance, primary coolant temperature transport lags were lumped into two groups, one each for the hot and cold loop legs thermodynamic effects in the secondary system condensate headers and surge... 

Primary coolant thermal model with lumped transport and mixing lags (the transport lag was varied as a function of the flow rate). 

Delay time, transportation lag, or deadtime is frequently encountered in chemical engineering systems since we did not earn our reputation as underpaid plumbers for nothing ... 

This type of lag is encountered frequently in flow systems and may also be termed dead time or transportation lag. The presence of much DV lag in any control loop or configuration can lead to instability in the control action03 (see Fig. 7.49). 

A first-order system coupled with dead time (transportation lag) is a good model for many process systems. The dead time (L or td) is the time that has to elapse before the output first starts to respond to a change in the input. The effect of a change in steam rate on the water temperature at the end of the pipe will depend not only on the resistance and capacitance effects in the tank but will also be influenced by the length of time necessary for the water to be transported through the pipe. The effect of dead... 

Dead time is also called transportation lag, because it is the time required for fresh heat transfer fluid to displace the contents of the exchanger and its associated piping. The dead time is the worst enemy of control, because until it has expired, a change in the heat transfer fluid flow (or temperature) will not even begin to have an observable effect. For a heat exchanger, the dead time is usually between 1 and 30 seconds. When the equipment is correctly designed, the dead time is much less than the time constant. 

Whenever a sample has to be brought to an analyzer, a transportation delay and a potential for interference with the integrity of the sample are inevitable. If an automatic controller maintains the measured composition, the transportation lag can seriously deteriorate the closed-loop control stability of the loop. An even more serious consequence of the use of sampling systems is the potential for interference with the integrity of the sample. This can occur due to filtration, condensation, leakage, evaporation, and so on, and these operations cannot only delay, but also change information and measurement. 

Bypass-stream transport is a method for maintaining high sample transport velocity to minimize transportation lag. This method is used when samples are vaporized at the sampling tap and no facilities exist for returning the vapor to the process. If the sample bypass is piped to a drain or vent, this will not only waste the process material but might also pollute the environment. Therefore, the use of a fast bypass-return loop is preferred. After selecting the appropriate sample transport method, the sample time lag should be calculated and used in the tuning of the analyzer controller. 

If the material to be removed is dust, the self-cleaning bypass filter with automatic blowback constitutes a potential solution, whereas in other instances, cyclone separators should be considered. In the former device (Figure 3.5), the process stream enters tangentially to provide a swirling action, and the cleaned sample is taken near the center. Transportation lag can be kept to less than 1 minute, and the unit is applicable to both gas and liquid samples. This type of centrifuge can also separate streams by gravity into their aqueous and organic constituents. 

Compute the distance-velocity lag. The time in minutes needed for the thermal element to detect a change in temperature in the storage tank is the distance-velocity lag, which is also called the transportation lag or dead time. For this process, the distance-velocity lag d is the ratio of the quantity of water in the pipe between the tank and the thermal bulb, that is, 15 lb (6.8 kg), and the rate of flow of water out of the tank, that is, 25 lb/min (0.114 kg/s), or d = 15/25 = 0.667 min. 

Process dead time refers to the delay in time before the process starts responding to a disturbance in an input variable. It is sometimes referred to as transportation lag or time delay . Dead time or delays can also be encountered in measurement sensors such as thermocouples, pressure transducers, and in transmission of information from one point to another. In these cases, it is referred to as measurement lag. 

The oversimplified picture given above is contrary to our physical experience, which dictates that whenever an input variable of a system changes, there is a time interval (short or long) during which no effect is observed on the outputs of the system. This time interval is called dead time, or transportation lag, or pure delay, or distance-velocity lag. 

Time-Delay Compensation, Time delays are a common occurrence in the process industries because of the presence of recycle loops, fluid-flow transport lags, and dead time in composition analysis. In crystallization processes, certain techniques for CSD analysis and concentration measurement involve significant time delays. The presence of a time delay (0) in a process severely limits the performance of a conventional PID control system, reducing the stability margin of the closed-loop control system. Consequently, the controller gain must be reduced below that used for a process without delay. Thus, the response of the closed-loop system will be sluggish compared to that of the system with no time delay. 

Delay time, transportation lag, or deadtime is frequently encountered in chemical engineering systems. Suppose a process stream is flowing through a pipe in essentially plug flow and that it takes D minutes for an individual element of fluid to flow from the entrance to the exit of the pipe. Then the pipe represents a deadtime element. 

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