Reverse flow from drains

This has often caused flammable liquids to turn up in some unexpected places. For example, construction had to be carried out next to a compound of small tanks. Sparks would fall onto the compound. Therefore all flammable liquids were removed from the tanks while the construction took place. Nevertheless a small fire occurred in the compound. 

Water v. as being drained from a tank on another part of the plant. The v. ater flow was too great for the capacity of the drains, so the water backed up into the compound of small tanks, taking some light oil with it. This oil was ignited by welding sparks. 

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One of the most notable features of the synchronous buck topology is that on decreasing the load, it does not enter discontinuous conduction mode as a diode-based (conventional) regulator would. That is because, unlike a bjt, the current can reverse its direction in a mosfet (i.e. it can flow from drain to source or from source to drain). So the inductor current at any given moment can become negative (flowing away from the load) — and therefore continuous conduction mode is maintained — even if the load current drops to zero (nothing connected across the output terminals of the converter) (see Chapter 1). 

In practice, each transistor in the full-wave or half-wave case is shunted by a fast recovery diode placed in the reverse direction from drain to source. The diode provides a path for current to flow when the load R is better represented by a complex impedance, Z = R . jX. A reactive component in the load causes out-of-phase currents to want to flow backward through any off transistor. Without the diode, the output capacitance of an off transistor will develop a spike of voltage due to the out-of-phase current charging the capacitance, and permanent damage will result. The diodes must be fast enough to conduct or not conduct dictated by the switching phase. Ordinary 60-Hz diodes imitate a resistor at 100 kHz. 

Connections to equipment are typically 50 mm and 80 mm for process vessels and exchangers, according to the size of the equipment. Each connection includes an accessible block valve. Double block valves are provided if required. A check valve should be included if overpressure or other hazard could result from reverse flow during simultaneous drainage from more than one vessel. Individual connections from the equipment are made into the top of the drain header. 

The operation of the NMOS transistor shown schematically in Figure 12 can be considered in the light of the previous discussion of a MOS capacitor. When no voltage is applied to the gate, the source and drain electrodes correspond to p-n junctions connected through the p region therefore only a small reverse current can flow from source to drain. On the... 

The most common process used for this is depth filtration through a bed of sand or similar material charged in a vertical vessel. The incoming water flows from top to bottom. To improve the efficiency of such filters, two or more layers of media with various particle sizes are used. Coarse and less dense material such as anthracite is located at the top of the bed, whereas finer and denser particles of sand are placed at the bottom. Such multimedia filters can remove most particles larger than 10-20 pm. Periodically the filter bed is back-washed by reversing the flow direction (from bottom to top) and by increasing the flow rate. During backwash, the captured particles are removed and sent to drain, whereas heavier particles of the filter bed remain in the vessel and settle back at the end of the cycle. 

Reverse flow of vapor from the column into the reflux line would interfere with liquid downflow, condenser action, and may cause hammering if the reflux is subcooled. A seal loop (Fig. 5.1a to c) is almost always used to avoid reverse flow. The low point in the seal loop needs a drain (normally closed). 

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