Pump down circuit

A low-pressure switch can also be used in conjunction with a thermostat and a solenoid valve in the pump-down circuit. In this method of control, the thermostat does not stop the compressor but de-energizes the liquid line solenoid valve to stop the supply of refrigerant to the evaporator. The compressor continues to run and pumps down the evaporator until stopped by the low-pressure switch. When the thermostat again calls for cooling, it opens the solenoid valve, liquid enters the evaporator and the low-pressure switch will close again to restart the compressor. This method is used to ensure that the evaporator is kept clear of liquid when the plant is off. If there is any leak at the solenoid valve, it will cause the... 

Many systems are shut down for periods of the year, either for process closure or if not required in winter. The advice of the supplier should be sought as to the correct procedure. In the case of refrigerant circuits, it is advisable to pump down into the receiver or condenser to minimize leakage losses. Water towers should be drained in winter in this climate, if not in use, and the tank heater disconnected. 

Early in the program, critical components (e.g.,the turbomolecular pump) and circuit boards were tested for their ability to survive neutron and gamma irradiation rates and doses similar to those that would be received from exposure to the detonation of a tactical nuclear device. All components were powered up at the start of the gamma irradiation tests but not during the neutron irradiation tests. Circuit boards were protected by circumvention circuits that powered down critical circuits in 10 to lOOps upon detecting radiation. All components survived the nuclear radiation tests. This unusual performance was noted with positive commendations by the staff at the White Sands Missile Range, where the tests were performed. Tests of the fully integrated CBMS II system, installed in a reconnaissance vehicle, will be conducted in the future. 

A pressure switch located so as to sense falling pressure at the earliest moment should be used to activate the standby pump. The switch should be so connected to the system as to permit testing the startup circuit without shutting down the compressor. Figure 8-8 shows four piping arrangements. The figure at b would be the recommended method. 

Provision must be made in the wiring to maintain power to the standby pump driver. This will prevent power interruption due to the pressure switch contacts opening when lubricant pressure is restored. Cycling of the standby pump will occur if the circuit is not maintained. A reset will have to be furnished to permit shutting down the standby pump when the main pump is back on stream. 

It would be possible to remove all of the heat by pumping cold reflux from the distillate drum to the top of the tower and thus eliminate the cost of the pumparound circuit. Where more than one sidestream is withdrawn, however, it is usually economical to withdraw part of the heat in a pumparoimd reflux system farther down the tower. The following economic factors affect the choice ... 

The opportunity is taken, while filling for testing, to ensure that the circuits can be filled without airlocks. Air vents at high points of the circuit may be automatic or manual. While the pipes are full, pumps should be run if possible to dislodge any dirt before draining down and cleaning the strainers. If a lot of dirt is found, the pipework should be filled again and reflushed. In any case, the pumps should be run at the earliest opportunity and the strainers cleaned out. 

In some scrub and strip circuits, the crud is mainly composed of silica, as well as inorganic sulfates. Also, if poor pH control is used in the uranium stripping circuits with ammonium sulfate, then uranium is a major constituent [33,46]. Such crud may be treated with dilute sulfuric acid, and recirculation through a pump results in the crud breaking down. There is evidence, in at least a few uranium circuits, that the presence of humic acids may be a possible cause of the crud problem [34,47]. Lignin appears to be another cause of crud formation [33,46]. Humic acids contained in the feed solution have also been implicated in the formation of waxy cruds in plants extracting uranium from phosphoric acid. 

Since excitation of the alternators of the pump drive sets, compound type, can be interrupted by circuit breakers RCP 03 JA and straps had been installed so that these circuit breakers could open as soon as the 6 6 kV supply of RCP 01 MO tripped out, the system for progressive coast-down of the primary coolant pumps was not operational (common mode defect)... 

The reflector is moved upward by the hydraulic pump during start up. During power operation, the reflector is held by the hydraulic system and gradually moves up for bum-up compensation at a constant speed of Imm/day without any speed control system. To attain this very slow speed, a reduction mechanism composed of paradox planetary gears is installed. The technical reliability of the gears has been demonstrated elsewhere. However, a spare set of gears is installed in the 4S in case of trouble. To shut down the reflector, the scram valve is opened in the hydraulic circuit. When the reflector lowers Im, the core reaches the subcritical cold shutdown state. The length of the downward movement of the reflector is determined by the capacity of the hydraulic cylinder. It cannot move otherwise. 

Under these conditions, natural circulation between the cold pool, the core and the riser pipe will be established through the two hot-cold interfaces along an always-open natural circulation path. The pool of cold, borated water will then enter into the core and will shut the reactor down and remove the decay heat. In a certain sense, PIUS safety is based on the use of an essentially unstable coohng circuit, which needs active pump action to ensure stability during normal operation in off-normal conditions, the system automatically switches to its stable condition which also is a safe shutdown condition. 

One hour after the start of the accident, at 05 00, the four primary water recirculation pumps started to strongly vibrate and had to be shut down. The vibration was indicative of the presence of steam in the circuit and therefore of a scarcity of water. 

---