GB2180629A - Safety valve - Google Patents

Abstract

An overflow safety valve for steam comprises a valve member (7) which is acted on by steam from below and is connected to a perforated cylinder (9) provided downstream of an associated valve seat (6). Above the valve member (7), a valve spindle (8) carrying the member (7) is surrounded by a guide element (13),which is displaceable axially against the force of a spring (19) and provided with a flow passage, preferably formed by longitudinal bores (16). One end of the flow passage is connected with a water feed (15) provided above the valve seat (6),whilst the other end is closable by way of water injection valve means (17). <IMAGE>

Description

SPECIFICATION Safety valve The present invention relates to a safety valve.

In conventional power stations, an overflow station, which also serves as safety equipment, has recently been introduced as a connection from a superheater exit art a high pressure part of a steam generator to an intermediate superheater entry. This overflow station has to fulfil several tasks. Thus, the steam pressure has to be reduced in regulated mannerfrom the high pressure rail to the medium pressure rail ofthe boiler plant. This partially relieved steam is to be cooled down to the temperature at the intermediate superheater entry. Finally, exceeding of the licenced boiler pressure is to be prevented.

Steam transformation valves are known (DE-PS 19 45 035, DE-PS 25 44 970), which serve forthe reduction in pressure and temperature of steam.

These valves have a throttle device, constructed as a parabolic or perforated cone, forthe steam. At the same time, a spraywaterduct is provided, which is connected with injection bores arranged in several planes one abovethe other. In that case, the throttle device and the injection bores are so functionally interlinked that the cross-section ofthe injection bores, which is freed by the setting ofthe throttle device, is matched to the freed steam-regulating cross-section. The known transformation valves do not include a valve cone loaded by pressure from below as required and employed in the case ofsafety valves.

Fortemperature reduction through water injection, it is ofsignificancethatthe sprayed in water is distributed as homogeneously as possible in the steam flow. This is achieved if the water, atomised asfinelyas possible, is injected at high speed. In order to reduce wear in the injection system, the injection speed is limited to 60 to 70 metres per second. This corresponds to a pressure difference between injected water and steam of about 20 bars. This pressure difference shall if possible be maintained for all operational states.

A requirement exists for, for example, a safety valve in which a reduction in pressure and temperature is possible at the same time and the injection speed ofthe water is largely constant for all operational states.

According to the present invention there is provided a safety valve comprising a housing provided with a steam inlet, a steam outlet, a water inlet and a valve seat, and a movable valve element which comprises an axially movable spindle, a valve body connected to the spindle and co-operable with the valve seat to open and close a flow path between the steam inlet and steam outlet, a perforated cylinderconnectedtothevalve body and arranged downstream of the valve seat with respect to flow along the path from the steam inlettothesteam outlet, and a guide element arranged aroundthe spindletoguidethevalve element in the housing and provided with a flow passage extending in the axial direction ofthespindle and communicating atone end with the water inlet and at the other end with water injection valve means, the guide element being displaceable axially ofthe spindle againsta resilient bias to open said valve means.

In an overflowsafety valve embodying the invention, a valve cone with a downstream perforated cylinder can determine the pressure reduction in the steam flow byway of its setting. The resilient bias can be provided by a spring so biassed that the guide element just opens the valve means, such as by lifting off a respective valve seat, on loading with spray water after reaching a predeterminable difference pressure between steam and injected water. The width of an annularflowgap at that seat can set itself in accordance with the supplied water quantity. In this manner, almost constant high injection speeds are possible,which ensure good intermixing of the water and steam.

An embodiment of the present invention will now be more particularly described by way of example with reference to the accompanying drawing, the single figure of which is a longitudinal section of a valve embodying the invention, wherein the closed valve setting is shown in the lefthand half of the figure and the open valve setting in the righthand half of the figure.

Referring now to the drawing,there is shown an overflow safety valve which is arranged in the steam duct of the high pressure part of a boiler plant. The valve comprises a housing 1, which is closed by a cover 3 fastened with the aid of screws 2. The housing 1 has a steam entry stub pipe 4 and a steam exit stub pipe 5. The housing 1 receives a valve seat 6, which is freed or closed by a valve cone 7. Thevalve cone is fastened at an axially resettable spindle 8, which is guided through the cover3. Acting on the spindle 8 is a force which is exerted by a hydraulic system and directed against the pressure of steam present underneath the valve cone 7.

The valve cone 7 is connected downsteam ofthe valve seat 6 with a perforated cylinder 9. The perforated cylinder is guided in a cylinder 10, which is firmly connected with the housing 1. The perforated cylinder 9 is provided with passage bores 11, which are arranged in several planes one above the other and distributed around the circumference of the cylinder 9. The valve seat 6 is freed on raising ofthe spindle 8 out ofthe closed setting (lefthand half ofthe drawing) into the open setting (righthand half ofthe drawing) and increasingly more bores 11 are uncovered by the cylinder 10. Thereby, a path for steam is freed, the steam pressure being relieved in two stages during theflowthrough the annulargap at the valve seat 6 and the bores 11 of the cylinder9.

For further pressure reduction in a third stage, the cylinder 10 is provided with passage bores 12 in the regionfacingthecover3.

The spindle 8 is surrounded abovethe valve cone 7 byacylindrical guideelementl3,which is axially displaceable and guided in a bore ofthe cover3.Two stub pipes 15, which are connected to a spray water feed duct (not shown), open into an annular space 14, enclosing the upper part ofthe guide element 13, of the cover 3. The guide element 13 is penetrated by longitudinal bores 16 parallellyto the spindle 8. The bores 16 are connected atone end, byway ofthe annular space 14 and the stub pipes 15, with the spray water feed duct. The lower ends of the bores 16 are closable by a valve means 17. The valve means 17 is formed by two seating surfaces disposed opposite each other.The one seating surface is situated on the upper side ofthe valve cone 7 and the other seating surface on the underside ofthe guide element 13. In that case, the diameter d2 ofthe valve means 17 is greaterthan the diameter d1 of the bore in the cover 3, in which the guide element 13 is slidably retained.

A ring 18, which is provided with a cylindrical, downwardly pointing skirt, is threaded onto the spindle 8 above the guide element 13. A packetof plate springs 19 is arranged between the ring 18 and the upper end face of the guide element 1 3.The springs 19 are biassed sothatthe guide element 13 is pressed againstthe seating surface on the cone7 under spring force.

If a command for water injection is issued by the temperature-regulating circuit ofthe boiler plant, then initially the regulating valve in the spray water feed duct opens. The water is then conducted through the stub pipes 1 5,theannularspace 14and the longitudinal bores 16 in the guide element 13 into the space between the guide element 13 and the valve cone 7. The bias ofthe springs 19 is so set by means of the ring 18 that a pressure difference of about20bars,which loadstheannularsurface bounded bythediametersdl andd2,justliftsthe guide element 13 off the seating surface of the cone 7. The annularflow gap atthe valve means sets itself in accordance with the supplied quantity of water, for which purpose the difference pressure can rise to about 24 bars.

Claims (6)

1. A safety valve comprising a housing provided with a steam inlet, a steam outlet, a water inlet and a valve seat, and a movable valve element which comprises an axially movable spindle, a valve body connected to the spindle and co-operable with the valve seat to open and close a flow path between the steam inlet and steam outlet, a perforated cylinder connected to the valve body and arranged downstream of the valve seat with respect to flow along the pathfrom the steam inlettothesteam outlet, and a guideelementarrangedaroundthe spindleto guidethe valve element in the housing and provided with a flow passage extending in the axial direction ofthe spindle and communicating atone end with thewater inlet and atthe other end with water injection valve means, the guide element being displaceable axially ofthe spindle againsta resilient bias to open said valve means.

2. Avalve as claimed in claim 1, said valve means being provided byan end portion oftheguide element and by a respective valve seat, co-operable with the guide element end portion, on the valve body.

3. Avalve as claimed in eitherclaim 1 orclaim 2, the resilient bias being provided by a plurality of plate springs arranged on the spindle to act between an end ofthe guide element and abutment means fixed on the spindle.

4. Avalve as claimed in anyoneofthepreceding claims, wherein the guide element is guided in a bore in the housing.

5. Avalveasclaimed inclaim2,whereinthe guide element has an annular pressure surface portion adjacent said end portion,the pressure surface portion being arranged to be so loadable with the pressure of water supplied to the flow passage that the valve is displaceable, by an amount proportional to such pressure, in a direction of opening of said valve means.

6. A safety valve substantially as hereinbefore described with reference to the accompanying drawing.