Draining tank control

Where the MU water supply to cast-iron boilers does not precisely keep up with steam generation demands, the water level can quickly decrease and the problems become even more acute. Conversely, where MU does precisely keep up with steaming rates and is supplied to a common condensate receiver-FW tank via automatic level control, the tank can easily overfill when condensate finally drains back under on-off operating conditions. This gives rise to a loss of valuable hot, treated water from the system and the start of another chain of cause and effect problems. 

Stabilization of the system including repositioning of valves, switches, and controls, draining of tanks, fire fighting activities... 

At about 120 min (2 h), a conference phone call began between the control-room technical superintendent and (at their homes) the station superintendent, the vice president of generation, and the Babcock Wilcox site representative. The conference call lasted 38 min. Conferees realized that something was abnormal since the reactor coolant pumps were off yet they were unable to get a steam bubble in the pressurizer. The blown out rupture disk on the reactor coolant drain tank and the water on the reactor building floor did not seem surprising, since this had happened before. The condition of the block valve upstream of the PORV was questioned. It was reported to be shut, but it was not. The conferees decided to restart a reactor coolant pump, and all officials planned to report to the control room. 

At 192 min (3 h 12 min) the PORV block valve was reopened in an attempt to control reactor coolant pressure. Opening the valve resulted in an increase in the valve outlet temperature, a limited pressure spike in the reactor coolant drain tank (rupture disk had previously burst at 15 min), an increase in reactor building pressure, and a pathway by which hydrogen radionuclides from the damaged core could reach the reactor building. 

Shown in Figure 2.1 is a vessel known as a pressurizer (it is the half-fuU vessel to the left of the right-hand steam generator). The pressurizer is a part of the primary water circuit, and plays a critical role in controlling the pressure and temperature in that circuit. If the reactor temperature rises, then the volume of water increases and the level in the presstuizer increases (and vice versa). If the pressure becomes too high, the Pilot Operated Relief Valve (PORV) opens and steam is vented to the drain tank. 

Several plants employ cooled-belt flakers. These consist of flexible steel belts, ca 1-m wide and up to 50-m long, that have short mbber skirting at the edges. Molten pitch flows from a thermostatically controlled tank over a weir to give a flat thin sheet on the belt, which is cooled from below by water sprays. At the end of the belt, the solid pitch is broken up by rotating tines. The pitch flakes are drained and transported to a covered storage silo by belt conveyor, during which time the surface moisture evaporates. 

In some locations, it is necessary to drain the tank frequently to clear other contaminants. With careful control, this can be used as the necessary bleed-off. 

Processes and equipment should be designed to reduce the chances of mis-operation by providing tight control systems, alarms and interlocks. Sample points, process equipment drains, and pumps should be sited so that any leaks flow into the plant effluent collection system, not directly to sewers. Hold-up systems, tanks and ponds, should be provided to retain spills for treatment. Flanged joints should be kept to the minimum needed for the assembly and maintenance of equipment. 

If we use a controller with positive gain (+KC), the controller output increases as the liquid level drops. We can only reduce the flow if we use an air-to-close valve (-Kv). In the case of a power outage, the valve will stay open. This fail-open valve can drain the entire tank, an event that we may not want to happen. 

The object to be coated is dipped into the tank full of coating and pulled out, and the excess coating drains back into the dip tank. To minimize the thickness differential the rate of withdrawal is controlled. 

The simplest type of ultrafiltration system is a batch unit, shown in Figure 6.17. In such a unit, a limited volume of feed solution is circulated through a module at a high flow rate. The process continues until the required separation is achieved, after which the concentrate solution is drained from the feed tank, and the unit is ready to treat a second batch of solution. Batch processes are particularly suited to the small-scale operations common in the biotechnology and pharmaceutical industries. Such systems can be adapted to continuous use but this requires automatic controls, which are expensive and can be unreliable. 

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