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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 the German 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 via the 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 reactor power, but the maximum possible volume expansion of the coolant in response to temperature increases. In contrast to this, the main steam safety valves on the secondary circuit of the steam generators have the task of drawing off the steam as a function of the maximum possible reactor power.

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

Installation

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

Each of the pressurizer safety valves has a number of pilot valves. The pilot valve consists of a spring-loaded impulse valve and a check valve. Each pilot valve has a pressure impulse line and a control line. The pressure impulse and control lines are fitted on an inclined pipe in order to prevent condensation accumulating. Each pilot valve can be blocked for repair purposes. The related stop valves are locked by valve locks 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 valve configuration, i.e., for the main valve and its pilot valves. In this case there is no need for any control line piping. The pressure chambers to be relieved are as small as possible. The dead times during pressurization and depressurization are therefore kept small. Faulty assembly at the construction site and mixing up connections is not possible.

Picture 2 shows the control diagram of the compact arrangement of a Russian pressurizer system. The main valve VS99 is controlled by two steam pilot valves VS66. ETF2 angle-type stop valves are located in the control line between the main valve 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 device 111. Device 112 - a position indicator - signals that the supplementary loading has been applied or removed. The EA pressure switches give the instruction to apply or remove the supplementary loading. The pressure sampling lines to these pressure switches can be blocked so that the switching points can be checked via separate testing lines.

A third control line leads to solenoid valve VCA1. This control line can also be isolated with a stop valve. This stop valve is motorised so that the control line can be shut in case the solenoid valve failes to close properly. The solenoid valve is controlled via pressure switch EB. The pressure switch corresponds to pressure switch EA in both structure and installation.

The following hypothetical failure has been devised as a design basis accident for the pressurizer 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 valve responds when the maximum allowable working pressure (calculation pressure) is exceeded. The set pressure of the second safety valve is between the set pressure of the first safety valve and the maximum permissible pressure.

The highest system pressure occurs at the discharge nozzle of the reactor coolant pumps. The safety valves are located on the pressurizer where the system pressure is lower. When defining the set pressures, it is therefore necessary to include the pressure difference between the reactor coolant pump and the safety valve for a design basis accident. The pressure difference is determined primarily by the pressure loss in the reactor pressure vessel and in the volume compensation line. The volume compensation line through which the entire mass flow passes into the pressurizer when the volume of the coolant expands, is dimensioned so that the pressure loss is kept 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 corresponds to the design pressure, does not respond even in the event of the maximum possible accumulation of design faults being reached (hypothetical design basis accident for the pressurizer safety valves). Only the additional hypothetical assumption that both the 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 cause the second pressurizer safety valve to respond.

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

Function

Pressurizer safety valve VS99 (picture 5) is a controlled safety valve which operates on the relief principle. The working pressure is applied to the valve disc and when this pressure 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 the set pressure set for steam pilot valve VS66 (picture 6), the impulse valve opens at once after a slight pressure rise. The steam flows through the exhaust body, strikes the baffle plate and causes it to rise. The check disc in the pilot valve is lifted and the control line opened. The steam flows out of the main-valve control chambers which are to be relieved. The pressure decreases in the control chambers and pressure differences arise at the actuating piston. These generate actuating forces in the opening direction which cause the valve to open.

The opening motion of the valve disc is damped, the volume of steam being trapped above 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 this refill bore and the size of the other refill cross-sections is such that the entire volume of medium flowing into the relief chambers can be safely released through the control line. A sufficient pressure difference thereby remains at the actuating piston in the safety 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 valve drops to the closing pressure, the impulse valve closes in a single lift. As the baffle plate 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 in the control chambers of the safety system which had been relieved earlier. Closing forces result from the weight of the valve disc and the pressure differences above and below the valve disc caused by the flow. The safety valve closes.

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

− extremely high sealing force

− very accurate reproducibility of the set pressure and small working pressure differences

− all parts important for correct functioning are at the working temperature, i.e., no thermoshock

− 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 their respective set pressures, they are fitted with magnetic supplementary loadings which are 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 valve stem. The circuit operates according to the closed-circuit principle; if the magnet's current supply is interrupted, a spring lifts the anchor from the valve stem. However, removing the supplementary loading does not result in the impulse valve actively opening since the spring force can still act in the closing direction. The magnetic attachment is fitted with a lead seal to prevent the supplementary loading being changed.

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 the dome of the pressurizer relief tank; a blow-off line links it to the steam chamber of the pressurizer. 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 the event of the assumed accident combinations, it will be possible to prevent the first pressurizer 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 pilot valve for the pressurizer blow-off valve.

When the system is operating normally, the blow-off valve and the pilot valve are closed. 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 for closing, but in reverse order.

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

The stop valve is an angle-type valve with a low flow resistance. The valve has a flat seat. The disc is connected to the stem so that it can be moved slightly; the stem does not rotate. Expansions of the stem due to heat are absorbed by cup springs on the drive head. When they are in the open position, the back seat gasket provides an additional seal to the outside. The open and closed positions of the valve can be indicated on an external pointer located on the straight-way version by fitting two terminal contacts.

Pressurizer Spraying

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

The task of the pressurizer spraying system is to reduce the coolant pressure when required. Its design is such that the reactor coolant system does not reach the pressure 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 closed in accordance with the coolant pressure. During normal operation, the spray valves are closed. A spray valve has a small borehole in the valve disc so as to be able to hold the spray lines and the volume compensation line at a given temperature with a continuous flow rate.

The solenoid-controlled spray valve works on the working current principle. When required, current flows through the solenoid coil. The resultant magnetic force moves the 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 holds the 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 valve position "open" or "closed".

The solenoid drive is designed to meet the requirements for components which do not suffer loss of coolant in the event of an accident.

Pressurizer Safety Relief Valve VS 99 Compact Design

PICTURE 1 8

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Circuit Diagramm for Control System VS99 of a Russian Pressurized Water Reactor

PICTURE 2 9

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Circiut Diagram for Control System VS99 of a German Pressurized Water Reactor

PICTURE 3 10

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Pressurizer-Safety Valve VS99 Space-Saving Installation

PICTURE 4 11

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Pilot controlled Safety Relief Valve VS99

PICTURE 5 12

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Steam Pilot Valve VS66 . 5

PICTURE 6 13

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Solenoid Valve VCA

PICTURE 7 14

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Stop Valve EBS

PICTURE 8 15

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Spray Valve

PICTURE 9 16

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