Protection systems depressurization

Ideally most oil or gas incidents will be controlled by the process shut down systems (ESD, depressurization, drainage, etc.) and hopeful the fire protection systems (fireproofing, water deluge, etc ), will not be required. However these primary fire defense systems may not be able to control fire incidents if previous explosions have previously occurred. Before any consideration of fire suppression efforts, explosion effects must first be analyzed to determine the extent of protection necessary. Most major fire incidents associated with hydrocarbon process incidents are preceded by explosion incident. 

Overpressure protection system and depressurization system 4 - Emergency core cooling system... 

Manifold barriers confine the radioactivity to the 1) ceramic fuel pellet 2) clad 3) cooling water, as demonstrated by the TMI-2 accident 4) primary cooling loop 5) containment and 6) separation from the public by siting. Further protection is provided by engineered safety systems pressurizers, depressurization, low pressure injection, high pressure injection and residmil heat removal systems. 

FIGURE 31.8 Typical interior suction point. (Adapted from U.S. EPA, Sub-Slab Depressurization for Low-Permeability Fill Material—Design and Installation of a Home Radon Reduction System, EPA/625/6-91/029, U.S. Environmental Protection Agency, Washington, DC, July 1991.)... 

Process system emergency safety features (i.e. ESD, isolation, depressurization and blowdown) should be considered the prime safeguards for loss prevention over fire protection measures (i.e. fireproofing or barriers, firewater systems, manual fire fighting). 

Depressurization and Blowdown Capabilities - A mathematical calculation of the system sizing and amount of time needed to obtain gas depressurization or liquid blowdown according to the company s philosophy of plant protection and industry standards (i.e., API RP 521). 

Survivability of Safety Systems - An estimation of the ability for safety systems to maintain integrity from the effects of explosions and fires. (Safety systems may include ESD, depressurization, fire protection - active and passive, communication, emergency power, evacuation mechanism, etc.). 

Guide for Pressure-Relieving and Depressuring Systems (API, 1997) and Guideline for Protection of Pressurized Systems Exposed to Fire (Scanpower, 2002). 

Bare pressure vessels in a process unit protected by vapor depressuring usually do not require protection by fixed water spray systems. 

NFPA 57 recommends that LNG dispensers be protected from collisions with vehicles and that they incorporate an emergency shut-down system. The maximum delivery pressure of the dispenser shall not exceed the operating pressure of the LNG tanks on the vehicle being refueled, and the LNG delivery hoses must have a shut-off valve at the end and a breakaway valve in the event a vehicle drives off with the refueling hoses attached. Bleed and vent valve must be incorporated into the dispensing lines to allow them to be drained and depressurized prior to disconnection if necessary. 

A transfer vessel is a device that receives the contents of another vessel for emergency or nonemergency purposes. It can be as simple as a vacuum truck or as complex as a hard-piped, dedicated system. For liquids, the system typically consists of a container or containment system located below the protected vessel where gravity will promote a rapid transfer. In the few instances where a transfer vessel is used with gases, it assists in the depressurization of a process. In other instances, it may consist of a spare vessel capable of accepting the contents of a nearby vessel (in case of fire or leak) so that the damaged vessel s entire contents are not destroyed or released (Lees, 1980). In this case, a pump may be used to make the transfer between vessels. 

Fusible links are made of low melting point materials designed to vent pneumatic systems as the fire melts the link. The depressurization can open fire deluge valves. Fusible links are very reliable, but they do require that the fire be well under way before they work, whereas otho- detectors, such as fire eyes, act more quickly. Depressurization of a fusible loop is considered to be a confirmed detection of a fire, and will automatically initiate appropriate shutdowns and activate fire protection equipment. 

The line has an equivalent flow area corresponding to a circular opening of 150 mm diameter. It ensures a quick chain reaction shutdown by void formation in the core and a reliable depressurization (that is protected from the effect of partial plugging) down to pressures lower than 1 MPa, even without any other primary water cooling system, in a time of minutes. 

The primary functions of the Nuclear Boiler System (NBS) are (1) to deliver steam from the RPV to Ae turbine main steam systan (TMSS), (2) to deliver feedwater from the condensate and feedwater system (C FS) to the RPV, (3) to provide overpressure protection of the RCPB, (4) to provide automatic depressurization of the RPV in the event of a LOCA where the RPV does not depressurize rapidly, and (5) with the exception of monitoring the neutron flux, to provide the instrumentation necessary for monitoring conditions in the RPV such as RPV pressure, metal temperature, and water level instrumentation. 

The nuclear pressure relief system consists of safety/relief valves (SRVs) located on the MSLs between the RPV and the inboard MSIV. There are four SRVs per MSL. The SRVs provide three main protection functions overpressure safety, overpressure relief, and depressurization operation, which is discussed below separately. 

Implemented System Pressure and Inventory Pressurizer provides principal control method. The Liquid Relief Valves provide Control System overpressure protection together with reactor power stepback In accidents with LOCA crash cooling by opening the Main Steam Safety Valves depressurizes the secondaiy circuit, which cools and depressurizes the primary circuit and helps ECC injection for small breaks... 

The safety/relief valves are dual-function valves discharging directly to the pressure suppression pool. The safety function provides protection against overpressure of the reactor primary system. The relief function provides power-actuated valve opening to depressurize the reactor primary system. The valves are sized to accommodate the most severe of the following two pressurization transient cases determined by analysis ... 

PubI 521 Guide for Pressure-Relieving and Depressuring Systems PubI 2004 Inspection for Fire Protection PubI 2007 Safe Maintenance Practices in Refineries PubI 2015 Cleaning Petroleum Storage Tanks... 

Diversion of flow safely All the above systems discussed are part of electronic system requiring power. As a next layer of protection there are mechanical devices (requiring no power to meet power failure situations) to divert the flow safely. Safety and relief valves are used to in case to depressurize when SIS fails to take care (e.g., say due to control power failure). Pressure relief valve diverts the fluid for safe passage. These relief valves are spring force to close so that when pressure is below setting it is closed. Many cases rupture discs are used but in that case system needs to be closed to attain the disc. To a certain extent quartz bulb in sprinkler system does the same function. 

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