Isolation valve requirements

In accordance with the requirement of NUREG-0737, the design of instrumentation and control systems for the automatic containment isolation valves is such that resetting the isolation signal does not result in the automatic reopening of the valves. Reopening of containment isolation valves requires deliberate operator action to open valves on an individual containment penetration basis (see CESSAR-DC Section 6.2.4.5). 

The PWS consists of six primary coolant loops that circulate moderator to cool the reactor. Each loop consists of a main coolant pump, two parallel heat exchangers, expansion joints, and valves. All branch lines connected to the primary water system up to the first normally closed isolation valve require seismic qualification to assure pressure boundary integrity. This qualification has been demonstrated by various dynamic analyses or by the previously mentioned interim verification program procedure. 

Piping for snuffing steam injection into a heater firebox is required to help put out a fire if a tube mpture occurs. The snuffing steam isolation valve needs to be located at an accessible spot remote from the heater. Also, a remote fuel shutoff valve should be located adjacent to the snuffing steam valve so that both valves can be accessed quickly in case of fire. 

Unhke the control valve, the centrifugal pump has poor or nonexistent shutoff capabihty. A flow check valve or an automated on/off valve may be reqiiired to achieve shutoff reqiiirements. This requirement may be met by automating an existing isolation valve in retrofit apphcations. 

Understanding the positions of sample and other measurement locations within the equipment is also important. The presence or absence of isolation valves needs to be identified. While isolation valves may be too large for effective sampling, their absence will require that pipe fitters add them such that sample valves can be connected. This must be done in advance of any test. If analysts assume that samples are from a liquid stream when they are vapor or that temperature measurements are within a bed instead of outside it, interpretation of results could be corrupted. Analysts should also develop an understanding of control transmitters and stations. The connection between these two may be difficult to identify at this level in fully computer-controlled units. 

Installation of a shut-off valve in the expander exhaust line prevents baekflow during expander isolation (Figure 6-37). Although the need for this valve may not be immediately obvious, it ean be very bene-fieial if the expander is operated for an extended period in the windmilling mode. Often this type of operation is not antieipated during the initial evaluation of valve requirements, but beeomes neeessary at some later date. For example, it is useful when it is deemed prudent... 

The 1997 edition of the API RP 521 extends the two-thirds rule to include the upstream and downstream system. At a minimum, the inlet and outlet piping up to and including isolation valves must be designed for the two-thirds rule to be able to block in the exchanger. If the upstream and downstream equipment is not designed for the two-thirds rule, relief devices may be required on both the inlet and outlet piping to protect the piping and adjaeent equipment. 

Isolation Valves for Pressure Relief Systems - Block valves for maintenance isolation purposes are permissible in pressure relieving systems, provided that they are car sealed open and comply with the requirements of CSO valves. The particular locations where such CSO valves are p>ermitted are ... 

In addition, they are usually constructed without isolation valves on the fuel supply lines. As a result the final connection in the pipework cannot be leak-tested. In practice, it is tested as far as possible at the manufacturer s works but often not leak-tested on-site. Reference 32 reviews the fuel leaks that have occurred, including a major explosion at a CCGT plant in England in 1996 due to the explosion of a leak of naphtha from a pipe joint. One man was seriously injured, and a 600-m chamber was lifted off its foundations. The reference also reviews the precautions that should be taken. They include. selecting a site where noise reduction is not required or can be achieved w ithout enclosure. If enclosure is essential, then a high ventilation rate is needed it is often designed to keep the turbine cool and is far too low to disperse gas leaks. Care must be taken to avoid stagnant pockets. 

It is bad practice (and in some countries illegal) to fit a valve between a vessel and its rupture disc (or relief valve). Tbe valve had been fitted to stop escapes of gas into the plant after the disc had blown and while it was being replaced. A better way, if isolation is required, is to fit two rupture discs, each with its own isolation valve, the valves being interlocked so that one is always open. 

For ESD isolation valves (i.e., EIVs) a fail safe mode is normally defined as fail closed in order to prevent the continued flow of fuel to the incident. Blowdown or depressurization valves would be specified as fail open to allow inventories to be disposed of during an incident. Special circumstances may require the use of a foil steady valve for operational or performance reasons. These applications are usually at isolation valves at components, i.e., individual vessels, pumps, etc., where a backup EIV is provided at the battery limits that is specified as fail closed. The fail safe mode can be defined by the action that is taken when the ESD system is activated. Since the function of the ESD system is to place the facility in its safest mode, by definition the ESD activation mode is the foil safe mode. 

Emergency isolation valves (EIV) should be located based one two principals (1) the amount of isolatable inventory that is desired and (2) protection of the EIV from the affects of external events. EIV valves are normally required to have a firesafe rating (i.e. minimal leakage and operability capability, Ref. Table 14). Valves and their actuating mechanisms should be afforded adequate protection when they are required to be located in an area that has potential to experience explosion and fire incidents. 

A reaction quench is a system where an inhibiting substance (quench solution stored in a separate container vessel) can be quickly and effectively fed into the reactor via a pipe which is protected with appropriate isolation valves. This action is independent of other process actions that may be required. The reaction quench can be manually initiated or automatically when certain process parameters are exceeded. 

An active fire protection system requires some action to occur before it functions per its design intent. This action may be taken by either a person or control system. Examples of active fire protection systems are monitors, water spray systems, foam systems, emergency isolation valves, and ESD systems. 

Isolation valves should be easily accessible under adverse conditions or valve should be remotely operable. The isolation valve should be fire rated and the actuator and power cables should be fire proofed. Twenty minutes of fireproofing is required when the design is not "fail closed." Isolation valves can serve a dual function, such as equipment isolation. The isolation valves should be located as close to the outlet flange of the vessel as possible. 

Remotely operated isolation valves may be installed at the piping connection to the vessel in place of manually operated valves. The advantage of remotely operated isolation valves is that they can be quickly activated when so required. 

A fire water main should be provided to the shore terminus area of the wharf. Hydrants and monitor nozzles should be located so that effective fire water streams can be remotely applied to any berth or loading/unloading manifold from two directions. Where the wharf is of such length that onshore monitors cannot adequately cover the berths, the fire water main should be run onto the wharf to permit the required monitor and fire hose coverage. Monitors located on marine wharves may be remotely operated from onshore and use of elevated monitors is common. The offshore segment of the main can be a dry system. In all cases, isolation valves and fire department connections should be provided at the wharf-to-shore connection (ISGOTT, 1996). 

Nuclear Boiler Assembly. This assembly consists of the equipment and instrumentation necessary to produce, contain, and control the steam required by the turbine-generator. The principal components of the nuclear boiler are (1) reactor vessel and internals—reactor pressure vessel, jet pumps for reactor water circulation, steam separators and dryers, and core support structure (2) reactor water recirculation system—pumps, valves, and piping used in providing and controlling core flow (3) main steam lines—main steam safety and relief valves, piping, and pipe supports from reactor pressure vessel up to and including the isolation valves outside of the primary containment barrier (4) control rod drive system—control rods, control rod drive mechanisms and hydraulic system for insertion and withdrawal of the control rods and (5) nuclear fuel and in-core instrumentation,... 

Isolation A means for pressure-isolating control valves, pumps, and other piping hardware for installation and maintenance is another common application for an on/off valve. In this application, the valve is required to have tight shutoff so that leakage is stopped when the piping system is under repair. As the need to cycle the valve in this application is far less than that of a throttling control valve, the wear characteristics of the valve are less important. Also, because many are required in a plant, the isolation valve needs to be reliable, simple in design, and simple in operation. 

On-column injection into columns of 320 pm or less is more difficult and not easily automated. This usually requires the use of a fused silica needle of sufficiently small outer diameter or a needle capable of entry into the analytical column. The column is usually placed into a specially designed injection port fitted with duck bill or isolation-valve-type septa. A second approach which has shown great success is the use of a precolumn of a wide-bore capillary (530 pm ID) which is connected to the analytical... 

The requirement of the isolation valve actuation time would be determined in the next step considering the results of public radiation exposure assessment and detail valve design. 

Most analyzer sampling systems require a filter with at least one wire mesh strainer (100 mesh or finer) to remove larger particles that might cause plugging. Available filter materials include cellulose, which should only be considered if it does not absorb the components of interest. Sintered metallic filters can remove particles as fine as 2 ym, cellulose filters can remove down to 3 /on, and ceramic or porous metallic elements can trap particles of 13 ym or larger. When the solids content is high, two filters can be installed in parallel with isolation valves on each. Motorized self-cleaning filters are also available for such services. 

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