Valve normally closed

Hydrogen purge valve Cut-off valve, normally closed, 24VDC, NPT 1/8... 

The second of two valves normally closed during normal reactor operation in system piping which does not penetrate primary reactor containment,... 

Normally open valve Normally close valve (ISA) (ISA)... 

Valve normally open I Valve normally closed... 

Valve, normally closed (NC) A valve that is closed when there is no actuating force. 

Scope of supply On plant On control panel In control panel PC interlock Valve normally closed Valve normally open... 

Opening a manual valve. Manual valves which are normally closed to isolate two or more pieces of equipment or process streams can be inadvertently opened, causing the release of a high-pressure stream or resulting in vacuum conditions. Other effects may include the development of critical flows, flashing of liquids, or the generation of a runaway chemical reaction. 

Accumulation of Noncondensihles - Noncondensibles do not accumulate under normal conditions since they are released with the process vapor streams. However, with certain piping configurations, it is possible for noncondensibles to accumulate to the point that a condenser is "blocked". Such a condition could occur if an automatic vent control valve failed closed for a period of time. This effect is equal to a total loss of coolant, and thus need not be considered separately. 

Water flowed from the relief pipe (20 ft elevation) to tank E610. It first flowed through the jumper to the process pipe to the normally closed blow-down valve which was open (possibly inadvertently left open or failed to seat in the attempt to pressurize the tank). About 119 gallons of water flowed through this open blow-down valve, through another, normally open, isolation valve to enter tank E610, where it reacted with MIC. 

Valve Arrangement of Auxiliary Feedwater System and Main Steam System during Shutdown - the auxiliary feedwater system has six MO Vs in the flow path to the steam generators and the main steam nonreturn valves are normally closed during shutdown. These valves depend on offsite power and are difficult to manually open. 

In all these cases, once the leak starts, particularly if it ignites, it is usually impossible to approach the normal hand-isolation valves to close them. Emergency isolation valves are discussed in detail in Reference 3, and the following incidents show how useful they can be. They can be operated electrically, pneumatically, or in some cases, hydraulically. 

AS—Air Supply BD—Blowdown BF—Blind Flange CBD—Continuous Blowdown CD—Closed Drain CH-O—Chain Operated CSO—Car Seal Open CSC—Car Seal Closed DC—Drain Connection EBD—Emerg. Blowdown Valve ESD—Emerg. Shutdown FC—Fail Closed FO—Fail Open HC—Hose Connection IBD—Intermittent Blowdown LO—Lock Open ML—Manual Loading NC—Normally Closed NO—Normally Open OD—Open Drain... 

Valve 29 in line 14 is a quick opening bypass valve which is normally closed. However, in case of an emergency shutdown, this valve can be instantly opened to shut off the supply of alcohol to the reaction zone and return the alcohol stream via line 31 to alcohol supply tank 11. Such a quick opening by-pass valve normally is not employed in the nitrating acid line, since in case of an emergency shutdown, nitrating acid is employed to sweep out the tubular reactor... 

Directly actuated valves weight or spring-loaded valves that open at a predetermined pressure, and which normally close after the pressure has been relieved. The system pressure provides the motive power to operate the valve. 

A special double block and bleed system, shown in Figure 12-3, is added to every feed line to a reactor. During normal operating conditions, the block valves are open and the bleed line is closed. When the feed pump is shut off, the block valves are closed and the bleed line is open. 

Normally, isolation valves should be designed to fail closed. However, depending on the process and operating conditions, the valve failing closed may create more problems and the failure position should be reviewed by process personnel. Examples of where the valve should fail open include on ... 

Relief Valve A relief valve is used in pumps, reactors, and equipment which create positive pressure. These valves are usually connected between the pump outlet and a vent or a large-volume accumulation vessel. The valve is normally closed, but will open whenever the system pressure exceeds the desired maximum operating pressure. 

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