Safety relief valve function

The safety relief valve is used for liquid and gas service. Safety relief valves function as relief valves for liquids and as safety valves for gases. 

Eliminate turned-down vents from safety relief valves, i.e., upside down U. Possible accidental ignition of releases from such vents wiU likely result in flame impingement on the top external surface of the tank, above the internal wetted surface. BLEXT Some means to handle rainwater from a desirable upward vertical vent have been listed by Bodurtha (ibid., April 1988). Moreover, a safety relief valve must function properly when required and must be sized properly to help prevent an explosion. 

Rupture Disks A rupture disk is a non-reclosing device designed to function by the bursting of a pressure-retaining disk. This assembly consists of a thin, circular membrane usually made of metal, plastic, or graphite that is firmly clamped in a disk holder. When the process reaches the bursting pressure of the disk, the disk ruptures and releases the pressure. Rupture disks can be installed alone or in combination with other types of devices. Once blown, rupture disks do not reseat thus, the entire contents of the upstream process equipment will be vented. Rupture disks of nonfragmented type are commonly used in series (upstream) with a safety relief valve to prevent corrosive... 

For the scenarios that used a safety relief valve as an IPL, the risk reduction required to eliminate the risk gap was achieved without the need of any instrumented function. For these cases, considering an initiating event frequency of 10 per year, a permissive logic IPL (implemented in the BPCS) with a RRF of 10 and a safety relief valve with a RRF of 100, the tolerable frequency of 10 per year was achieved with no need of a SIR... 

The RPV is provided with a pressure relief system which consists of 12 safety (relief) valves connected evenly onto the four steam lines, with blowdown pipes leading down into the condensation pool. The safety (relief) valves are own medium operated valves, each being controlled by two pilot valves, one pressure activated and one electrically controlled this means that actuation can be initiated in a controlled way by pressure monitoring equipment, to avoid over pressurization or to achieve depressurization. In addition, control valves are provided downstream two of the safety valves, in order to enable proper pressure control of the reactor also in the event of isolation (loss of the turbine condenser function). 

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. 

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 ... 

Selected safety/relief valves are associated with the automatic depressurization of the primary system under assumed LOCA conditions. These valves have two independent logic channels powered from different power sources, either of which can initiate depressurization. Valves open automatically and remain open imtil the pressiue falls to a preset closure pressure. These valves open automatically upon signals of high drywell pressure and low reactor water level and confirmation of one LPCI function of the RHR system or LPCS system running. Initiation signals need not be simultaneous. The valves remain open until the primary system pressure is reduced to a point where the LPCI function of the RHR system and/or the LPCS system can adequately cool the core. The initiation of automatic depressurization is delayed from 90 to 120 s to allow the operator to terminate the initiation should the HPCS system initiation and acceptable reactor vessel level have been confirmed. 

In the unlikely event that the RHR shutdown suction line is unavailable during reactor shutdown to cool reactor water and during the period when the LPCI function of the RHR system and/or the LPCS system pumps are injecting water into the reactor vessel, safety/relief valves used for automatic depressurization can be used to pass water from the reactor vessel to the suppression pool via valve discharge lines. For this to occur, the reactor vessel floods to a level above the vessel main steam line nozzles, selected safety/relief valves are opened from the control room to pass reactor water to the suppression pool. 

A pressure relief function is used to control large pressure transients. This system will operate safety/relief valves following closure of the main steam isolation valves or the sudden closure of the turbine admission or stop valves and failure of the turbine bypass system to relieve the excess pressure. For this fimction, the safety/relief valves discharge steam from the steam lines inside the drywell to the suppression chamber. Each safety/ relief valve is operated from its own overpressure signal for the relief fimction, and by direct spring action for the safety function. 

The safety function of BWR containment to contain radioactive materials and to cool the residual heat in the core would be degraded unless the integrity of the suppression is maintained. Accordingly, the containment should be designed carefully to withstand loads during the actuation of a safety/relief valve and a LOCA and to maintain the integrity during normal and abnormal conditions. 

A specific inspection frequency shall be estabhshed for each safety relief valve. The inspection frequency will be a function of inspection history, relief system design, process conditions, operating experience, on-stream service capabilities, economics, and regulatory mandates. 

For the discharge of coolant, safety relief valves (SRVs) are prepared in case of a turbine trip without bypass or MS IV closure. The SRVs also have the function of... 

Relief valve function Safety valve function ... 

In the analysis of CR ejection at hot standby , the inserted reactivity is set as 2.8%dk/k. The peak fuel enthalpy is below 150 cal/g. The increase in the pressure is more extensive compared to the full power case. The active initiation of the SRVs (in their relief valve function) is not credited. The SRVs are assumed to open passively as the safety valve function. The peak pressure is 27.2 MPa. 

In BWRs, the auxiliary feedwater system is usually termed the reactor core isolation cooling system. This system is used to maintain the water level in the reactor vessel in the event of a loss of feedwater in hot shutdown conditions (in such an event residual heat is removed from the reactor core by means of the release of steam through safety relief valves to a suppression pool). Another function of this system is to supply the necessary inventory of reactor coolant in the event of a small loss of coolant during normal operation. 

New systems or processes may also need to be qualified from an operational safety perspective. This is particularly relevant in the case of chemical synthesis involving exothermic reactions. Critical safety aspects are usually identified using hazard operability or HAZOP assessments and studies. For example, a HAZOP analysis of an exothermic reaction vessel would involve consideration of the consequence of failure of the motors for mixers or circulation pumps for cooling water. Thus, the qualification of such a system would involve checks and assessment to ensure that the system/process can be operated safely and that pressure relief valves or other emergency measures are adequate and functional. 

LOPA is a semi-quantitative tool for analyzing and assessing risk. This method includes simplified methods to characterize the consequences and estimate the frequencies. Various layers of protection are added to a process, for example, to lower the frequency of the undesired consequences. The protection layers may include inherently safer concepts the basic process control system safety instrumented functions passive devices, such as dikes or blast walls active devices, such as relief valves and human intervention. This concept of layers of protection is illustrated in Figure 11-16. The combined effects of the protection layers and the consequences are then compared against some risk tolerance criteria. 

It is common on chemical plant to install safety devices such as trips and relief valves which protect the plant in the event of a malfunction of control systems or human error. Unfortunately, these devices can (and do) fail occasionally. The problem is that the failures cannot be seen until they are tested or until they are called upon to act (a plant may operate perfectly normally even though, say, a pressure relief valve is faulty, because under normal conditions the valve is never activated). It is thus necessary to test safety devices periodically to ensure they are functioning. 

As stated above, the hazard and risk assessment and allocation may be concurrent activities or allocation may in some circumstances take place prior to hazard and risk assessment. Decisions on the allocation of safety functions to safety layers are often taken on the basis of what has been found to be practicable by the user organization. Established industry good practice should also be taken Into account. Decisions will then be taken on the safety instrumented systems, assuming credit for the other safety layers. For example, where relief valves have been installed and these have been designed and installed according to industry codes, it may then be decided that these are adequate on their own to achieve adequate risk reduction. Safety instrumented systems would then only limit pressure where size or performance of the relief valve(s) was insufficient for the application or release to the atmosphere is to be prevented. 

The ADS is an active safety grade system, whose functions are to depressurize the RPV to assist the PBIS to inject CV water to the RPV in a high pressure accident condition and to prevent the over pressure of the RPV. The ADS consists of 4 trains, each of which has two relief valves. 

Valves. Valves control the way the gas is used, stopped, or directed. They must function under a variety and range of temperatures. Control or proportional valves in a process system are power-operated mechanisms able to alter fluid flow. A pneumatic valve actuator adjusts valve position by making the air pressure either linear or rotary motion. Ball valves provide the shutoff capability. Gas valves are specialized to control the flow of another medium, such as natural gas. A pressure relief valve is a self-actuated safety valve that relieves pressure. A butterfly valve controls the flow of air or a gas through a circular disk or vane by turning the valve s pivot axis at right angles to the directing flow in the pipe. 

Figure 3 shows a typical function. The objective is to assess the SIL requmemoit of the instrument over-pressure trip function (in tiie minology of BS lEC 61511, a safety instrumented function , or SIF, in emented by a safety instrumented system , or SIS). One issue which arises immediately, when applying a typical riric graph in a case such as this, is how to account for the relief valve, which also protects the vessel from over-pressure. This is a common situation - a SIF backed up mechanical protection. Tlie options are ... 

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. 

More precisely safety and relief valves, when used to perform safety functions such as mitigation of steam generator tube rupture accident or cold overpressure protection, may have to fulfill their function in water flow conditions. However, such conditions were not always anticipated at the original design stage. Moreover, using safety and relief valves not qualified for water flow to limit the RPV pressure in water solid conditions could lead to a loss of coolant accident. 

Decay heat removal by feeding the steam generators and relieving at the SG valves is the only safe procedure to cool the fuel on the secondary side. The inadequate design of the SG valves question the safety function in the long term in case of failure of all relief valves (BRU-A) which is a BDBA. 

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