Control sets

In petrochemical plants, fans are most commonly used ia air-cooled heat exchangers that can be described as overgrown automobile radiators (see HeaT-EXCHANGEtechnology). Process fluid ia the finned tubes is cooled usually by two fans, either forced draft (fans below the bundle) or iaduced draft (fans above the bundles). Normally, one fan is a fixed pitch and one is variable pitch to control the process outlet temperature within a closely controlled set poiat. A temperature iadicating controller (TIC) measures the outlet fluid temperature and controls the variable pitch fan to maintain the set poiat temperature to within a few degrees. 

Thiophene and 3-methylthiophene are Hsted on the TSCA chemical substances inventory. Thiophene is regulated as a hazardous material under OSHA and also regulated under the Clean Air Act, Section 110, 40 CFR 60.489, but there are no exposure limits or controls set for 3-methylthiophene. Both materials are regulated under sections 311/312 of the Superfund Amendments and Reauthorization Act, 1986 (SARA), as materials with an acute health and fire hazard, and under the Resource Conservation and Recovery Act, as ignitable hazardous wastes (DOOl). 

For a PI or PID controller, the integrated deviation—better known as the integrated error IE—is related to the controller settings ... 

FIG. 8-53 The reactor temperature controller sets coolant outlet temperature in cascade, with primary integral feedback taken from the secondary temperature measurement. 

Sample Transport Transport time, the time elapsed between sample withdrawal from the process and its introduction into the analyzer, shoiild be minimized, particiilarly if the analyzer is an automatic analyzer-controller. Any sample-transport time in the analyzer-controller loop must be treated as equivalent to process dead time in determining conventional feedback controller settings or in evaluating controller performance. Reduction in transport time usually means transporting the sample in the vapor state. 

With respect to their response, the discussion should emphasize why these are important anci why they adjust certain control settings. Among the deviations on which analysts should focus the discussion are the high and low alarm settings. Some alarms will require rapid response. Alarms may give insight into equipment-operation boundaries as well as process constraints. 

Process equipment function changes with different steps in process sequence (e.g., same vessel used as feed tank, reactor, crystallizer pump used to pump in/out). Instrumentation and controls not kept in phase with the current process step (e.g., control set points, interlocks etc.). 

Control in the wrong position Control settings do not change Control settings may be purposefully changed for different phases of batches, different batches or similar batches... 

The unit shown on the next page in figure 4.4.1 is a somewhat simplified version of a tested, actual unit. The six gaseous feed components enter through check valves at a pressure regulated to about 4 atm higher pressure then experimental pressure, e.g., 22 atm. Six mass flow controllers set the flows and all but the nitrogen lines are secured with power to open solenoid valves (SV). 

Figure 4-75. Control settings during speed synchronization of turboexpander in FCC service.

Avoid the use of instrumentation that may have low first cost, but is very expensive to operate or maintain. Blind controllers, for example, are completely unsatisfactory for most applications. The author has seen examples of temperature controllers set at the factory, but with no method of readout or calibration. These almost always require retrofitting of additional instrumentation later. Internal level floats on process vessels that require plant... 

Anderson, G. D., Initial Controller Settings to Use at Plant Startup, Chemical Engineering, July 11, 1983, p. 113. 

G. D. Anderson s article recommends initial controller settings for those control loops set on automatic rather than manual for a plant startup. For liquid level, the settings depend upon whether the sensor is a displacer type or differential pressure type, or a surge tank (or other surge) is installed in the process ... 

Control problem Given the system parameters, the control problem is to determine the controller settings for K, T and T. This will be undertaken using the Zeigler-Nichols process reaction method described in Section 4.5.5(a). 

From the R and D values obtained from the process reaction curve, using the Zeigler-Nichols PID controller settings given in Table 4.2... 

Use maintenance records to inspect HVAC equipment and make sure that it is in good operating condition. Identify items of equipment that need to be repaired, adjusted, or replaced. Record control settings and operating schedules for HVAC equipment for comparison to occupancy schedules and current uses of space. 

Data about the plans and routines used by workers in controlling a process can be obtained by means of an "activity analysis," a type of input-output analysis. A chart can be made showing how certain process indicators change over time in response to changes of the control settings. From this chart it is possible to determine the type of process mformation that workers use to carry... 

For tasks that rely on decision-making rather than on fine manipulations, the activity chart can assume a columnar format, with columns recording process information attended and subsequent changes of discrete control settings. 

Action Wrong action on right object Controller set to wrong value Same as above Same as above Introduce check in checklist... 

With another drive (such as a turbine), with control set for constant-speed operations. 

Fire override to open ventilators automatically by means of a connection to the fire-detection system or fireman s switch. This normally overrides all other control settings ... 

The flow controllers are often used to set desired flows for the fresh feed, stripping steam, and dispersion steam. Each flow controller usually has three modes of control manual, auto, and cascade. In manual mode, the operator manually opens or closes a valve to the desired percent opening. In auto mode, the operator enters the desired flow rate as a set-point. In cascade mode, the controller set-point is an input from another controller. 

Normally, the reactor temperature and the stripper level controllers regulate he movement of the regenerated and spent catalyst slide valves, le algorithm of these controllers can drive the valves either fully Of [ or fully closed if the controller set-point is unobtainable. It is ext nely important that a positive and stable pressure differential be mail ined across both the regenerated and spent catalyst slide valves. r safety, a low differential pressure controller overrides the tempera re/level controllers should these valves open too much. The shutdov is usually set at 2 psi (14 Kp). 

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