Vacuum systems system power failure

Uncontrollable backstreaming can occur if there is a total system power failure that controls your vacuum system. If power in your building should go out and all mechanical pumping ceases, the efficiency of the traps and baffles to maintain a barrier in a static system will be tested. If someone is around at the time, close all stopcocks between each pump from any other and the stopcocks between the pumping system and the vacuum system. 

In addition to the vacuum valves, which perform solely an isolation function (fully open - fully closed position), special valves are needed for special functions. Typical are variable leak valves, which cover the leakage range from 10" ° cm /s (NTP) up to 1.6 10 cm /s (NTP). These valves are usually motor driven and suitable for remote control and when they are connected to a pressure gauge, the process pressures can be set and maintained. Other special valves fulfill safety functions, such as rapid, automatic cut-off of diffusion pumps or vacuum systems in the event of a power failure. For example, SECUVAC valves belong to this group. In the event of a power failure, they cut off the vacuum system from the pumping system and vent the forevacuum system. The vacuum system is enabled only after a certain minimum pressure (about 200 mbar) has been attained once the power has been restored. 

Orifice, ballast (vacuum technology) An opening that conhnuously allows gas from the outside to bleed into the forehne of a pumping system. This prevents suck back in the case of a power failure. By using dry air into the orifice, moist air is diluted to the point that water vapor is not condensed by compression in the mechanical pump. 

Vacuum pump in MER (Provides system vacuum for chemical processing.) Shutdown due to loss of electrical power or electrical failure Onsite or offsite electrical or mechanical malfunction Control and monitoring system indications and periodic maintenance inspections Redundant pumps Loss of all vacuum for controlling noble gas and halogen releases and for moving process fluids. 

Reactor pressure increase Several events may cause this e.g., inadvertent closure of one turbine control valve, pressure regulator downscale failure, generator load rejection, turbine trip MSIV closure, loss of condenser vacuum, loss of nonemergency AC power to station auxiliaries, loss of feedwater etc. All these have been analysed. Features are included in the instrumentation and control systems or redundancies to maintain reactor pressure through a combination of component automatic responses or operator actions, depending on the identified cause. 

Generic Safety Issue (GSI) A-09 in NUREG-0933 (Reference 1), addresses the issue of assuring that the reactor can attain safe shutdown after incurring an anticipated transient with a failure of the Reactor Trip System (RTS). An ATWS is an expected operational transient (such as a loss of feedwater, loss of condenser vacuum, or loss of offsite power to the reactor) which is accompanied by a failure of the RTS to shut down the reactor. 

In a BWR, an isolation coohng system for the reactor core should be provided as a standby source of cooling water to provide a capability for feedwater supply whenever the main feedwater system is isolated from the reactor pressure vessel. Abnormal events that could cause such an isolation include the inadvertent isolation of the main steam lines, the loss of condenser vacuum, the failure of a pressure regulator, the loss of feedwater and the loss of off-site power. 

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