FIELD OF THE INVENTION
This invention generally relates to a mechanical over pressure protecting device for steam service in high pressure, high temperature pressure vessel, venting to atmosphere. More particularly the invention relates to a spring loaded over pressure protecting device for high pressure, high temperature steam boilers.
BACKGROUND OF THE INVENTION
One of the critical parameters of high-pressure steam generators is the System Pressure and temperature. Though the 'System Pressure' is normally kept under a close control, it may however vary beyond safe limits due to sudden load changes, blocking of outlets, changes in feed water flow, variations in heat input to furnace failure of control instruments and interlocks. Under these circumferences, the over pressure protecting device of the stearh generator play an important role for the safety of operating personnel and the capital equipment.
The basic function of an over pressure protecting device is to release pressure at a predetermined value or "set pressure," of the fluid acting on one side of the reciprocal valve member or closing element such that the valve member reaches a full open position quickly, and closes or reseats after the pressure reaches to a safe level. The set pressure is adjusted by preloading of the coil spring acting on the other side of the valve closing element. The amount of preloading of the spring necessary to obtain a desired set pressure is dependent on the rate of the

spring used. In the spring loaded over pressure protecting device, the closing element cannot be fully seated until the fluid pressure has gone below the set pressure at which the valve opens.
Generally accepted criteria are that the valve should open fully when the system or boiler pressure is within five percent above the set pressure and that it should reseat when the system pressure is within four percent below the set pressure. The difference between the set pressure and the pressure at which the valve is again closed is referred to as "blow down" and is measured as a percentage of the set pressure.
To govern the boilers and pressure vessels, industrial codes including past data in the field are used to provide safety for operation and working of the boilers and pressure vessels. The manufacturers and users of these ranges of products take care of the code rules for selection, design, manufacturer, testing and operation. Thus, an over pressure protecting device needs to comply with the Indian Boiler Regulations and ASME Sec-I, Rules for construction of Power Boiler.
OBJECTS OF THE INVENTION
It is therefore an object of the invention is to propose a spring loaded over pressure protecting device for high pressure, high temperature steam boilers.
Another object of the invention is to propose a spring loaded over pressure protecting device for high pressure, high temperature steam boilers in which the base material for the device components are selected from dissimilar materials.

A still another object of the invention is to propose a spring loaded over pressure protecting device for high pressure, high temperature steam boilers, device for high capacity, high temperature and high pressure steam in boiler, in which the base rib must withstand the moment acting on the body.
A further object of the invention is to propose a spring loaded over pressure protecting device for high pressure, high temperature steam boilers boiler, which should be hard facing on a seating surface.
SUMMARY OF THE INVENTION
Accordingly, there is provided a spring loaded over pressure protecting device for high pressure, high temperature steam boilers, device for high capacity, high temperature and high pressure steam in boiler The over pressure protecting device is actuated by the static pressure of upstream of the valve and characterized by rapid full opening or pop action. The device operates during closing on a back pressure principle, that is, the force of trapped steam on the upper side of the holder is utilized to assist a spring of the device in forcing the closing element to be back and seated onto its seat.
The body of the over pressure protecting device comprising inlet and outlet, a valve seat and a valve member which is spring loaded in a closing direction of the valve seat. The valve body has to withstand the forces acting due to flow and the moment due to discharge of steam to atmosphere.

The over pressure protecting device is constructed for high pressure and high temperature up to 730°C. The intended applications are Separator, Super Heater and Hot Re-Heater applications. The materials used in over pressure protecting device are base stainless steel, seat Inconel Alloy and inlet neck Inconel alloy, which are to be integrated by dissimilar material welding.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWING
Figure 1 - fragmentary, cross-sectional view of an over pressure protecting device according to the invention.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION
As shown in Figure 1, the base (02) of the valve is integrated with its inlet neck (01) and seat (03). A closing element (06) connected to a stem (15) and a holder (08) of a tandem guiding element (10) is connected with the base and a cover (11). A spring (13) is connected at the other end of the stem (15). In figure 1, 100 percent lift is attained by proper location of upper and lower adjusting rings (07, 05) respectively. When the full lift is attained, a stopper (09) rests against the Cover (11) to eliminate hunting, thus adding stability to the valve. When the valve discharges in an open position, the steam is bled into a first chamber (A) through two bleed holes in the roof of the holder (08). Similarly, a collar (14) rises to a fixed position above a floating washer (12). The area between the floating washer (12) and the stem (15) is thereby increased by maintaining a difference in the two diameters provided on the collar (14).

Under this condition, steam in the first chamber (A) enters into a second chamber (B) through a secondary area formed by the floating washer (12) and the collar (14) on the stem (15), then through an orifice in the cover (11) and escapes to atmosphere through a pipe discharge connection.
When closing, the stem (15) and the collar (14) is adjusted so that it moves down into the floating washer (12), thereby effectively reducing the escape of steam from the second chamber. The resulting momentary pressure building-up in the chamber, at a rate controlled by the orifice, produces a downward thrust in the direction of spring loading. The combined thrust of the pressure and spring loading results in a positive and precise closing. Cushioning of the closing is controlled by the lower adjusting ring (05).

WE CLAIM :
1. An over pressure protecting device for high capacity, high temperature and high pressure steam boilers, comprising:
- a body (02) integrated with an inlet neck (01) and a seat (03),
- a closing element (06), connected to a first end of a stem (15) and, to a holder (08) in a tandem guiding element (10), the assembly being connected to the body (02) including a cover (11), a second end of the stem (15) connected to a spring (13);
- one each first and second chamber (A, B) for entry and exhaust of steam, the first chamber (A) receiving steam from the holder (08), the second chamber (B) receiving steam from the first chamber (A) through a secondary area formed by a floating washer (12) and collar (14) formed on the stem (15);
- one each upper and lower adjusting rings (07, 05) allows attaining a 100 percent lift of the stem (15), the stem lift being restricted upon attaining said lift by a stopper (09) resting against the cover (11);

- the collar (14) during discharge by the valve is enabled to rise to a fixed position above the floating washer (12),
wherein the collar (14) is constructed to have two differential diameters to allow an increase an area between the floating washer (12) and the stem (15), wherein the movement of the collar (14) above and below the floating washer (12) resulting a downward thrust in a direction of the spring loading leading to a positive and precise closing of the valve device, wherein the body (02), the inlet neck (01), and the seat (03) are respectively formed of dissimilar materials, and wherein the base, inlet neck, and seat are formed as a single integrated component.
2. The device as claimed in claim 1, wherein the base (01) comprises ribs (01A) to minimize the effect on valve performance due to moment during steam discharge in the valve.
3. The device as claimed in claim 1, wherein the seating surface is hard faced to withstand the erosion due to high velocity flow.
4. The device as claimed in claim 1, comprising at least lower adjusting ring (05) to allow a cushioning for a controlled closing of the valve.

5. The device as claimed in claim 1, wherein the body (02), the inlet neck (01) and the seat (03) are respectively formed of SA 351 - CF8M, SB 564-N 06617, and alloy UNS N07740 materials.
6. The device as claimed in claim 1, wherein the base inlet neck and seat are welded together in a welding process of dissimilar material.