Reflux system failure

Reflux System Failure The quantity of reflux used in fractionation systems determines the amount of vapor generation and the consequent pressure differential through the condenser system. If a reflux system fails, lower pressures through the condensers and the vessel may result in higher pressure rise in the system as a whole. 

Reflux System Failure The quantity of reflux used in fractionation... 

Cooling system failure could occur due to failure of pumps or controls supplying cooling media to the reactor vessel jacket, coils, or overhead reflux condensers. Piping to or from the condensers could become plugged or any of the heat exchange surfaces could become excessively fouled. 

Generally, electricity is purchased from a local supplier. Power is required to generate motor drives for lighting and for general uses. Note tliat, in tlie typical distillation operation illustrated in Fig. 16.3.3, a power outage for the entire system would cause a less severe effect tlian failure of a reflux pump alone. 

Reflux Failure (a) At top of distillation column, capacity is total overhead vapor [10], (b) when source of heat is in feed stream, capacity is vapor quantity calculated in immediate feed zone [3], (c) when reboilers supply heat to system, capacity is feed plus reboil vapors [3]. Each situation must be examined carefully. 

In addition to the basic control loops, all processes have instrumentation that (1) sounds alarms to alert the operator to any abnormal or unsafe condition, and (2) shuts down the process if unsafe conditions are detected or equipment fails. For example, if a compressor motor overloads and the electrical control system on the motor shuts down the motor, the rest of the process will usually have to be shut down immediately. This type of instrumentation is called an interlock. It either shuts a control valve completely or drives the control valve wide open. Other examples of conditions that can interlock a process down include failure of a feed or reflux pump, detection of high pressure or temperature in a vessel, and indication of high or low liquid level in a tank or column base. Interlocks are usually achieved by pressure, mechanical, or electrical switches. They can be included in the computer software in a computer control system, but they are usually hard-wired for reliability and redundancy. 

Several attempts to prepare typical alcohol derivatives of [Cr(HO-A)2]-were unsuccessful these include Schotten-Bauman conditions (acetyl chloride in chloroform shaken with cold, aqueous alkaline solution of complex), refluxing in acetyl chloride, in glacial acetic acid, acetyl chloride in hot dimethyl formamide, acetyl chloride and pyridine in acetonitrile, and acetic anhydride in acetonitrile. The failure of refluxing acetyl chloride to effect acetylation brings to mind the work of Keller and Edwards (4) the acetyl chloride system is completely heterogeneous and consequently not conducive to reaction. However, even the homogeneous reaction of [Cr(HO-A)2] — with acetyl chloride in acetonitrile failed to give a measurable quantity of the diester. 

This is a very particular application where we have the potential for overpressure due to loss of overhead condensing or reflux failure. In the event the cooling medium in the condenser is lost, additional vapour may be present at the top of the column. This additional vapour may require pressure relief. In a typical distillation system, a cooling failure also results in a loss of reflux within a short period of time (typically about 15 minutes). API RP 521 states that the required relief rates before and after loss of reflux should be considered. The Berwanger audit method encompassed both of these calculations, as it was not intuitive, which case would require the larger required relief rate. 

The design of a flare system includes the sizing of safety and relief valves, inlet and discharge piping, and flare header. All these should be adequately sized to prevent overpressuring of equipment in case of operational failure, such as fire, inlet or outlet blockage, reflux failure, power failure or instrument failure. 

The vapor product system in Fig. 9.3 is one example of a system that favors location C (60, 369). If the relief valve is upstream of the condenser (location A), the air condenser will be inert-blanketed and become completely ineffective during a power failure. A relief valve on the reflux drum (location C) will meiximize condensation. Location C also offers easier access to maintenance and a shorter blowdown line. 

Control modifications The control system can be modified to avoid opening the heat input control valve in case of a failure. Two incidents have been reported (414) in which the column automatic control system opened the heat input control valve during a coolant failure. In each case, the relief requirement was substantially greater than it would have been had the heat input controller stayed at its initial opening. In both cases, heat input was controlled by column AP (Fig. 9.4). Problems may also occur when heat input is controlled by bottom or reflux drum level. 

Check valve failure Blocked discharge Control valve failure Thermal expansion of liquid Heat exchanger tube rupture Reflux failure and overhead system Loss of reboiler heat Venting of storage tank Failure of individual motor Accidental closure of valve... 

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