Piping problem control system

When tronbleshooting a more complex control system, it is advisable to start by comparing the existing control loops with those from the piping and instrnmentation diagram (P ID). The current problem may be the resnlt of inappropriate modifications to the original control configuration. 

The location of the control valve (if any), the control system, length of the vertical leg downstream of the high point, and actual piping configuration also affect the quantity and direction of the vapor flow, and therefore the performance of arrangements b, c, and e. Attention to these details, and judicious use of nonreturn valves, can mitigate the above problems. 

It has been required to take into account the feedback of experience related to the mechanical problem of isolated safety injection lines when small leaks through the isolation valves may occur ( Farley-Tihange problem responsible for several small breaks on the safety injection piping). Implementation of on-line temperature and pressure measurements will allow to identify any small leak through the isolation valve to the primary circuit and the dedicated corrective actions can be taken to evacuate the leak to the Chemical and Volume Control System. ... 

December 15, 2005, a Dutch dairy firm had to recall packets of chocolate milk after children turned sick from drinking the milk. The problem was caused by traces of cleaning substance left after cleaning the pipes in the factory. With an early warning system the problem could have been detected before the packets were actually sold and consumed. A proactive control system could even have prevented using the pipes that were not properly cleaned. 

Just prior to the incident the pipe-laying operation had been stopped. Operators reported a system failure and that the hydraulic power had been lost. Such an occurrence was not particularly unusual and, in line with company procedures, this was investigated immediately. A team of technicians led by the chief electrician tried without success to resolve the problems. After these attempts, a more in-depth analysis was made. It was decided, on the basis of input from the system diagnostics, to perform a memory reset. Following this the system appeared to be running correctly. This was the first time that a full memory reset was requested by the internal diagnostics of the control system during a project operational phase. 

Check valves are required in the piping system at any point where backflow of gas after a shutdown has the ability to restart the compressor, running it backwards or, for that matter, even in the normal direction. Reverse rotation is totally bad, as many components of the various compressor types are not designed for reverse rotation, and there is some possibility, generally remote, that the compressor could reach a destructive over speed. Forward rotation is bad primarily because the intent was to stop the compressor, and it is now operating out of control. This is a problem, particularly if the shutdown was caused by a compressor failure indication, and the need to stop was to prevent further damage. In this mode, it is unlikely that the compressor can attain an overspeed condition. An application with a high potential for backflow is the parallel operation of two or more compressors. 

In the majority of chemical processes heat is either given out or absorbed, and fluids must often be either heated or cooled in a wide range of plant, such as furnaces, evaporators, distillation units, dryers, and reaction vessels where one of the major problems is that of transferring heat at the desired rate. In addition, it may be necessary to prevent the loss of heat from a hot vessel or pipe system. The control of the flow of heat at the desired rate forms one of the most important areas of chemical engineering. Provided that a temperature difference exists between two parts of a system, heat transfer will take place in one or more of three different ways. 

While we laud the virtue of dynamic modeling, we will not duphcate the introduction of basic conservation equations. It is important to recognize that all of the processes that we want to control, e.g. bioieactor, distillation column, flow rate in a pipe, a drag delivery system, etc., are what we have learned in other engineering classes. The so-called model equations are conservation equations in heat, mass, and momentum. We need force balance in mechanical devices, and in electrical engineering, we consider circuits analysis. The difference between what we now use in control and what we are more accustomed to is that control problems are transient in nature. Accordingly, we include the time derivative (also called accumulation) term in our balance (model) equations. 

Simpler plants are friendlier than complex plants because they provide fewer opportunities for error and because they contain less equipment that can cause problems. Often, the reason for complexity in a plant is the need to add equipment and automation to control the hazards. Simplification reduces the opportunities for errors and misoperation. For example, (1) piping systems can be designed to minimize leaks or failures, (2) transfer systems can be designed to minimize the potential for leaks, (3) process steps and units can be separated to prevent the domino effect, (4) fail-safe valves can be added, (5) equipment and controls can be placed in a logical order, and (6) the status of the process can be made visible and clear at all times. 

Many control problems can be better solved with a diaphragm controller. The function of the diaphragm controller (see Fig. 3.27) can be easily derived from that of a diaphragm vacuum gauge the blunt end of a tube or pipe is either closed off by means of an elastic rubber diaphragm (for reference pressure > process pressure) or released (for reference pressure < process pressure) so that in the latter case, a connection is established between the process side and the vacuum pump. This elegant and more or less automatic regulation system has excellent control characteristics (see Fig. 3.28). 

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