FIELD OF THE INVENTION
This invention relates to an over pressure protecting device for steam service in
high capacity, high pressure super critical boilers, vented to atmosphere.
BACKGROUND OF THE INVENTION
This invention relates to safety valves, and more specifically to spring loaded
safety relief valves for high pressure systems vented to atmosphere.
The critical parameter of a modern high-pressure steam generator is the System
Pressure. Though the 'System Pressure' is kept under a close control, it may
however vary beyond 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 etc., Under these circumferences the pressure
relieving devices of the steam generator play an important role for the safety of
operating personnel and the capital equipment.
The basic function of a pressure relieving device (safety valve) is to release
pressure at a predetermined value or "set pressure," of the fluid acting on one
side of the reciprocal valve member or disc, 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
disc. 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 safety
valves, it is usual that the disc will not be fully seated until the fluid pressure has
gone somewhat below the set pressure at which the valve beings to open.

Generally accepted criteria are that the valve should open fully when the system
or boiler pressure is within three percent above the set pressure and that it
should reseat when the system pressure is within four percent below the set
pressure. In spring loaded safety valves it is unusual that the disc will not be fully
seated until the fluid pressure has gone somewhat below the set pressure at
which the valve begins to open. The difference between 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 boilers and pressure vessels industrial codes based on Past experience
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. This pressure relieving device (safety valve) needs to comply the IBR
and ASME Sec-I, Power Boilers codes.
OBJECTS OF THE INVENTION
It is therefore an object of the invention is to propose a pressure relieving device
for high capacity, high temperature and high pressure steam in super critical
boiler.
Another object of the invention is to propose a pressure relieving device for high
capacity, high temperature and high pressure steam in super critical boiler, 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 pressure relieving device for
high capacity, high temperature and high pressure steam in super critical boiler,
in which a welding technique for dissimilar materials is applied.
A further object of the invention is to propose a pressure relieving device for high
capacity, high temperature and high pressure steam in super critical boiler, which
reduces re-work, fabrication time and cost of the device during manufacturing.
SUMMARY OF THE INVENTION
According to the invention, the pressure relieving device is actuated by the static
pressure upstream of the valve 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 disc holder is utilized to assist a spring
of the device in forcing the disc back down onto its seat.
The safety valve designed for pressure of 350-400 Kg/Cm2g and temperature of
600-650°C. The intended applications are Separator, Super Heater and Hot Re-
Heater applications. The materials used in safety valve base are A217-C12A, seat
bushing A182-F347H and inlet neck A182-F91, which are to be integrated by
dissimilar material welding. The base A217-C12A material is difficult to rework in
case of blow holes in casting and rework due to dissimilar material welding.
Hence, we propose base material of A351-CF8M, which is suitable for the above
designed parameters.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWING
Figure 1 - fragmentary, cross-sectional view of a Pressure Relieving Device
(Safety Valve) according to the invention.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE
INVENTION
As shown in Figure 1, the base (02) of the valve device is integrated with its inlet
neck (01) and seat bushing (03). A closing element Disc (06) connected to a
spindle (15) and a disc holder (08), in a tandem guide (10) is connected with the
base and a cover plate (11). A spring (13) is connected at the other tend of the
spindle (15). In figure 1, 100 percent lift is attained by proper location of upper
and lower adjusting rings (01, 05) respectively. When full lift is attained, a lift
stop (09) rests against the Cover-plate (11) to eliminate hunting, thus adding
stability to the valve. When the valve discharges in an open position, steam is
bled into a first chamber (A) through two bleed holes in the roof of the disc
holder (08). Similarly, a spindle overlap collar (14) rises to a fixed position above
a floating washer (12). The area between the floating washer (12) and the
spindle (15) is thereby increased by maintaining a difference in the two
diameters provided on the overlap 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 overlap collar (14) on the spindle (15), then through an orifice in the cover
plate (11) and escapes to atmosphere through a pipe discharge connection.

When closing, the spindle (15) overlap 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. A pressure relieving device for high capacity, high temperature and high
pressure steam in super critical boiler, comprising:
- a base (02) integrated with an inlet neck (01) and a seat bushing
(03),
- a closing element disc (06), connected to a first end of a spindle
(15) and, a disc holder (08) in a tandem guide (10), the assembly
being connected to the base (02) including a cover plate (11), a
second end of the spindle (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 disc holder
(08), the second chamber (B) receiving steam from the first
chamber (A) through a secondary area formed by a floating washer
(12) and a spindle overlap collar (14) formed on the spindle (15);
- one each upper and lower adjusting rings (07, 05) allows attaining
a 100 percent lift of the spindle (15), the spindle lift being
restricted upon attaining said lift by a lift stop (09) resting against
the cover plate (11);
- the spindle overlap collar (14) during discharge by the valve is
enabled to rise to a fixed position above the floating washer (12),
the spindle overlap collar (14) further being enabled during closing
of the valve spindle to move down to the floating washer (12),

wherein the overlap collar (14) is constructed to have two differential
diameters to allow an increase an area between the floating washer
(12) and the spindle (15), wherein the movement of the overlap 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 base (02), the inlet
neck (01), and the seat bushing (03) are respectively formed with
A351-CF8M, A182-F91, and A182-F347H materials, and wherein the
base, inlet neck, and seat bushing are formed as a single integrated
component through a welding process of dissimilar materials.
2. The device as claimed in claim 1, wherein the disc holder (08) is
configured with at least two bleed holes for bleeding steam into the first
chamber (A).
3. The device as claimed in claim 1, wherein the cover plate (11) is provided
with at least one orifice to allow escape of steam to atmosphere.
4. The device as claimed in claim 1, wherein the lower adjusting ring (05) is
constructed to allow a cushioning for a controlled closing of the valve.
5. Manufacturing process simplification by selecting base material as A351-
CF8M, with seat bushing A182-F347H & inlet neck A182-F91, which is
suitable for the intended design parameters, which have lesser rework
and lead time.

ABSTRACT

The invention relates to a pressure relieving device for high capacity, high temperature
and high pressure steam in super critical boiler, comprising a base (02) integrated with
an inlet neck (01) and a seat bushing (03), a closing element disc (06), connected to a
first end of a spindle (15) and, a disc holder (08) in a tandem guide (10), the assembly
being connected to the base (02) including a cover plate (11), a second end of the
spindle (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 disc holder
(08), the second chamber (B) receiving steam from the first chamber (A) through a
secondary area formed by a floating washer (12) and a spindle overlap collar (14)
formed on the spindle (15); one each upper and lower adjusting rings (07, 05) allows
attaining a 100 percent lift of the spindle (15), the spindle lift being restricted upon
attaining said lift by a lift stop (09) resting against the cover plate (11); the spindle
overlap collar (14) during discharge by the valve is enabled to rise to a fixed position
above the floating washer (12), the spindle overlap collar (14) further being enabled
during closing of the valve spindle to move down to the floating washer (12), wherein
the overlap collar (14) is constructed to have two differential diameters to allow an
increase an area between the floating washer (12) and the spindle (15), wherein the
movement of the overlap 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 base (02), the inlet neck (01), and the seat
bushing (03) are respectively formed with A351-CF8M, A182-F91, and A182-F347H
materials, and wherein the base, inlet neck, and seat bushing are formed as a single
integrated component through a welding process of dissimilar materials.