Pressurizer Safety Relief and Spray Valves

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Pressurizer Safety Relief

and Spray Valves

(Volume Compensator Protection)


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Pressurizer safety valves are part of the main coolant pressurizer system.

Design, construction and control of the safety valves fulfil the design criteria of theGerman TRD regulations (TRD = Technical Rules for Steam Generating Installations)and can be adjusted to each international standard.

Marginal Conditions of the System

With pressurized water reactors, the energy produced in the reactor is drawn off viathe steam generators. The pressurizer (volume compensator) compensates for the

coolant volume expansions occurring during operation and during accidents.Consequently, the design criteria for the pressurizer safety valves is not the reactorpower, but the maximum possible volume expansion of the coolant in response totemperature increases. In contrast to this, the main steam safety valves on thesecondary circuit of the steam generators have the task of drawing off the steam as afunction of the maximum possible reactor power.

For this reason, the design of the pressurizer safety valves takes into account thecombination of failures which will produce the maximum possible pressure andtemperature rise on the secondary and primary sides and, consequently, in themaximum blow-off rates required for the pressurizer safety valves.

Installation

The pressurizer safety valves and their pilot valves are connected with the pressurizersteam chamber.

Each of the pressurizer safety valves has a number of pilot valves. The pilot valveconsists of a spring-loaded impulse valve and a check valve. Each pilot valve has apressure impulse line and a control line. The pressure impulse and control lines arefitted on an inclined pipe in order to prevent condensation accumulating. Each pilotvalve can be blocked for repair purposes. The related stop valves are locked by valvelocks in accordance with the appropriate regulations so that each of the pressurizer

safety valves can always be operated by at least one functioning pilot valve.

A compact arrangement (picture 1) is recommended for the entire safety valveconfiguration, i.e., for the main valve and its pilot valves. In this case there is no needfor any control line piping. The pressure chambers to be relieved are as small aspossible. The dead times during pressurization and depressurization are thereforekept small. Faulty assembly at the construction site and mixing up connections is notpossible.

Picture 2 shows the control diagram of the compact arrangement of a Russianpressurizer system. The main valve VS99 is controlled by two steam pilot valvesVS66. ETF2 angle-type stop valves are located in the control line between the mainvalve and these pilot valves. These stop valves allow a control line to be shut off


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temporarily. The steam pilot valves are fitted with a supplementary loading device111. Device 112 - a position indicator - signals that the supplementary loading hasbeen applied or removed. The EA pressure switches give the instruction to apply orremove the supplementary loading. The pressure sampling lines to these pressureswitches can be blocked so that the switching points can be checked via separatetesting lines.

A third control line leads to solenoid valve VCA1. This control line can also be isolatedwith a stop valve. This stop valve is motorised so that the control line can be shut incase the solenoid valve failes to close properly. The solenoid valve is controlled viapressure switch EB. The pressure switch corresponds to pressure switch EA in bothstructure and installation.

The following hypothetical failure has been devised as a design basis accident for thepressurizer safety valves:

turbine trip from full load

suspected failure of main steam bypass equipment

suspected failure of control rod feeding circuit

suspected failure of coolant pressure regulator

suspected failure of coolant temperature regulator

suspected failure of coolant pressure limitation

However:

Response of main steam safety valves

Reactor trip either by first shutdown criteria,"Coolant pressure too high", or second shutdown criteria,"Main steam pressure too high"

The set pressures of the pressurizer safety valves are defined so that a safety valveresponds when the maximum allowable working pressure (calculation pressure) isexceeded. The set pressure of the second safety valve is between the set pressure ofthe first safety valve and the maximum permissible pressure.

The highest system pressure occurs at the discharge nozzle of the reactor coolantpumps. The safety valves are located on the pressurizer where the system pressure islower. When defining the set pressures, it is therefore necessary to include thepressure difference between the reactor coolant pump and the safety valve for adesign basis accident. The pressure difference is determined primarily by the pressureloss in the reactor pressure vessel and in the volume compensation line. The volumecompensation line through which the entire mass flow passes into the pressurizerwhen the volume of the coolant expands, is dimensioned so that the pressure loss iskept low, approximately 5 - 6 bar. This means that the set pressure of the first

pressurizer safety valve is this much lower than the design pressure of the reactor


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cooler system. The second pressurizer safety valve, which set pressure correspondsto the design pressure, does not respond even in the event of the maximum possibleaccumulation of design faults being reached (hypothetical design basis accident forthe pressurizer safety valves). Only the additional hypothetical assumption that boththe first and second criteria for the reactor trip fail, or the assumption of the ATWS(Anticipated Transiants Without Scram) accident (failure of the reactor trip) can causethe second pressurizer safety valve to respond.

Picture 3 shows the circuit diagram for the safety system of a pressurised waterreactor of German design and picture 4 the compact arrangement.

Function

Pressurizer safety valve VS99 (picture 5) is a controlled safety valve which operateson the relief principle. The working pressure is applied to the valve disc and when thispressure increases the sealing force on the valve seat also rises.

The safety valve can only follow the instructions of the pilot valves:

When the pilot valves open, the safety valve also opens, when the pilot valves close,the safety valve also closes.

During operating conditions, all chambers of the safety valve including the control line

have the same working pressure. If the working pressure in the system exceeds theset pressure set for steam pilot valve VS66 (picture 6), the impulse valve opens atonce after a slight pressure rise. The steam flows through the exhaust body, strikesthe baffle plate and causes it to rise. The check disc in the pilot valve is lifted and thecontrol line opened. The steam flows out of the main-valve control chambers whichare to be relieved. The pressure decreases in the control chambers and pressuredifferences arise at the actuating piston. These generate actuating forces in theopening direction which cause the valve to open.

The opening motion of the valve disc is damped, the volume of steam being trappedabove the piston and compressed by the further lift movement.

When the valve disc is in the open position, a refill bore is exposed. The size of thisrefill bore and the size of the other refill cross-sections is such that the entire volumeof medium flowing into the relief chambers can be safely released through the controlline. A sufficient pressure difference thereby remains at the actuating piston in thesafety valve and the valve disc is held securely open.

If the pressure in the system or in the pressure sampling line to the steam pilot valvedrops to the closing pressure, the impulse valve closes in a single lift. As the baffleplate falls, the check disc also closes, once again blocking the control line.

The closing pressure difference can be set with the throttle.


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If the pilot valves block the control line again, the pressure is equalised once again inthe control chambers of the safety system which had been relieved earlier. Closingforces result from the weight of the valve disc and the pressure differences above andbelow the valve disc caused by the flow. The safety valve closes.

The medium-operated, controlled safety valve VS99 has a number of very significantfeatures:

extremely high sealing force

very accurate reproducibility of the set pressure and small working pressuredifferences

all parts important for correct functioning are at the working temperature, i.e., nothermoshock

there is only one moving part in the main valve, i.e., the disc piston

simple disassembly through the body opening at the top

Supplementary loading

In order to provide the impulse valves with a high sealing force until they reach theirrespective set pressures, they are fitted with magnetic supplementary loadings whichare removed when the respective set pressure is reached. During this process, a

direct current magnet applies an additional sealing force to the valve disc by the valvestem. The circuit operates according to the closed-circuit principle; if the magnet'scurrent supply is interrupted, a spring lifts the anchor from the valve stem. However,removing the supplementary loading does not result in the impulse valve activelyopening since the spring force can still act in the closing direction. The magneticattachment is fitted with a lead seal to prevent the supplementary loading beingchanged.

In order to establish a defined response behaviour for the pressurizer safety valves,the magnetic supplementary loadings on the impulse valves are removed separately(in stages).

Blow-off Device for Pressure Limitation in Reactor and Reactor Cooling

Systems

In addition to the pressurizer safety valves, a pressurizer blow-off valve is fitted to thedome of the pressurizer relief tank; a blow-off line links it to the steam chamber of thepressurizer. A stop valve is installed upstream of the blow-off valve.

The set pressure and the outflow of the pressurizer blow-off valve are such that, in theevent of the assumed accident combinations, it will be possible to prevent the firstpressurizer safety valve responding when the RESA (reactor trip) is triggered by the

first criterion "Coolant overpressure too high".


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The design of the blow-off valve corresponds to that of the pressurizer safety valves.In contrast to the pressurizer safety valves, a solenoid valve (picture 7) is used as pilotvalve for the pressurizer blow-off valve.

When the system is operating normally, the blow-off valve and the pilot valve areclosed. When requested, the control line is opened by the solenoid pilot valve,thereby relieving the upper piston chamber of the pressurizer blow-off valve.

The pressurizer blow-off valve opens. The sequence of operations is the same forclosing, but in reverse order.

A stop valve (picture 8) with electrical actuator is installed upstream of the pressurizerblow-off valve. It is always open while the system is in operation. It is closed only ifexcessive leakage occurs during normal operation or if the blow-off valve defectivelyrefuses to close after a response.

The stop valve is an angle-type valve with a low flow resistance. The valve has a flatseat. The disc is connected to the stem so that it can be moved slightly; the stem doesnot rotate. Expansions of the stem due to heat are absorbed by cup springs on thedrive head. When they are in the open position, the back seat gasket provides anadditional seal to the outside. The open and closed positions of the valve can beindicated on an external pointer located on the straight-way version by fitting twoterminal contacts.

Pressurizer Spraying

The spray system is also part of the main coolant pressurizer system. It consists ofspray lines, spray valves and spray boxes with spray nozzles. The main spray linesbranch from the cold lines of the reactor coolant pipings and split into several spraylines before reaching the pressurizer.

The task of the pressurizer spraying system is to reduce the coolant pressure whenrequired. Its design is such that the reactor coolant system does not reach thepressure limit value at which the pressure blow-off valve would be actuated. The

solenoid-controlled spray valves (picture 9) (open/closed valves) are opened or closedin accordance with the coolant pressure. During normal operation, the spray valvesare closed. A spray valve has a small borehole in the valve disc so as to be able tohold the spray lines and the volume compensation line at a given temperature with acontinuous flow rate.

The solenoid-controlled spray valve works on the working current principle. Whenrequired, current flows through the solenoid coil. The resultant magnetic force movesthe anchor and, via the stem, lifts the disc from the seat.

The power supply is interrupted when the preset closing pressure is reached. The

magnetic force is reduced. The valve is closed by the flow forces acting on the disc,


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together with the weight forces and the spring force. The integrated spring also holdsthe valve closed against a higher pressure in the outlet (up to 2 bar).

An inductive position indicator is directly linked to the valve stem and signals the valveposition "open" or "closed".

The solenoid drive is designed to meet the requirements for components which do notsuffer loss of coolant in the event of an accident.


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Pressurizer Safety Relief Valve VS 99Compact Design

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Circuit Diagramm for Control System VS99of a Russian Pressurized Water Reactor

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Circiut Diagram for Control System VS99of a German Pressurized Water Reactor

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Pressurizer-Safety Valve VS99Space-Saving Installation

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Pilot controlled Safety Relief Valve VS99

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Steam Pilot Valve VS66 . 5

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Solenoid Valve VCA

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Stop Valve EBS

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Spray Valve

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Pressurizer Safety Relief and Spray Valves

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