US2799291A - Safety valve - Google Patents

Description

' July 16, 195% J. ORR EI'AL 2,799,291

SAFETY VALVE Filed Jan. 25, 1954 v 3 Sheets-Sheet 1 JAMES ORR BOHDAN HA WRYLYSH YN July 16,1957 9 J. ORR ETAL I 2,799,291

v SAFETY VALVE Y Filed Jab. 25,1954 3 sheets sheet 2 ln v nto JAMES C BOHDAN HAWRYLY'SHYN Attys July.16, 1957 N J. ORR Erm- 2,799,

SAFETY VALVE Filed Jan. 25, 1954 s shets-sheet a LOAD LBS. IN EXCESS OF POPP INC LOAD LIFT INCHES FIG.S"

5 w lnvznto'rs JAMES ORR BOHDA/V n4 wnrLrsur/v At'ys United States Patent 9 SAFETY VALVE James Orr and Bohdan Hawrylyshyn, Toronto, Ontario, Canada, assignors to The James Morrison Brass Mfg. Co. Limited, Toronto, Ontario, Canada, a corporation Application January 25, 1954, Serial No. 405,875

8 Claims. (Cl. 137-478) This invention relates to a safety valve of the type having a throat for the emergency escape of fluid that is 'normally closed by a disk and also having what is commonly called a huddling chamber that controls the escape of fluid past the valve when the valve blows off.

It is desirable that valves of this type should blow down or close quickly when the pressure in the vessel to which they are mounted has dropped below the emergency level for which the valve is set to open or blow off. A high lift is also desirable to permit the rapid escape of fluid in an emergency. 9

To achieve a rapid blow-down combined with a high lift, we have found that it is desirable to have what is known as the load life curve for the valve approximate the spring line for the spring that normally retains the valve in a closed position and with a relatively long flat shape reaching a maximum load value at a relatively high lift. The geometry of the huddling chamber affects the load lift curve. More particularly there are two restrictions to the flow of fluid through the huddling chamber as the disk lifts, that have a marked effect on the load lift curve. One is directly proportional to the lift of the valve and its area is defined by the valve seat and the under surface of the disk. The other restriction is the one having its area defined by the inner vertical wall of the huddling chamber and the edge of the disk. We have discovered that the manner in which the ratio of the area of the first restriction to the area of the second restriction varies with the valve lift affects the manner in which the load exerted by the escaping fluid forces-varies with the lift.

In fact, the two curves are similar and the latter is dependent on the former. It is therefore possible to design a valve with a long sloping fiat load lift curve that approximates the spring line and reaches its maximum load point at a high lift. When this relationship is achieved in a valve, the inner surface of the adjuseting ring has a smoothly cupped shape.

Our invention will be clearly understood after reference to the following detailed specification read in conjunction with the drawings.

In the drawings:

Figure 1 is a cross-sectional view of a valve constructed according to our invention.

Figure 2 is a view along the line 22 of Figure 1.

Figure 3 is a view along the line 33 of Figure 1.

Figure 4 is a partial view of our valve in cross-section illustrating the disk in a raised position.

Figure 5 is a graphical illustration of the operation a valve.

Referring to the drawings, the letter A generally indicates a valve according to our invention. It has a casing 10 threaded at the bottom as at 11 for mounting in a boiler steam chest or the like upon which it is to be used. There is provided a throat 12 having a seat 13 at its exit end upon which the disk 14 normally sits under the pressure of the compression spring 15.

mum lift of the valve for that drum pressure.

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5 valve 14 to form therewith what is commonly called a huddling chamber.

We should perhaps mention that the disk 14 has fins 19 which extend into the throat 12 for the purpose of guiding it in operation, and that it is connected atits upper end to the valve spindle 20 as at 21. The compression spring 15 is compressed between the washers 22 and 23, washer 22 resting against the shoulder 24 on the valve spindle and washer 23 resting against the end of compression screw 25 which is rigidly mounted in the casing. The

15 valve can be manually lifted by turning the lever 26 in an upward direction whereby to lift the'spindle and thus the diskagainst the compression of spring 15. It is thought that the construction of the valve spindle and its mounting in the casing in general are well known and since they do 20 not form an essential part of this invention, and since the drawings are thought to sufliciently describe these parts to a person skilled in the art, further reference will not be specifically made to them.

The invention concerns improvements in the design of the huddling chamber, whereby it is possible to achieve a load lift curve for a pressure close to the blow off pressure for the spring employed that is relatively flat and that can be made to lie quite close to the spring line curve.

To explain our invention we are going to refer to the theoretical graph illustrated in Figure 5 of the drawings. It is a graph for-a one-inch valve having a blow off pressure of fifty pounds per square inch. In that figure, the numeral 30 indicates the spring line of the compression spring 15. It shows the manner in which the force of the spring increases as the disk 14 is raised and the spring compressed. It is a straight line, and it will be seen that for this example the spring first yields when a pressure of fifty pounds per square inch is. reached. The scale is calibrated in pounds above the set blow off pressure, that is, fifty pounds perv square inch.

Numeral 31 refers to a load lift curve for the valve at the pressure of fifty pounds per square inch, that is,

the pressure that disk 14 first lifts or blows off. It repcreases to a maximum and then decreases.

, Theforce .tending to lift or drive the valve in an upward direction at any given moment is represented by the difference between the spring line and the load lift curve. Where the two cross, as at 32, you have maxi- At this point the load exerted on the underside of the disk is equal to the strength of the spring.

If the valve were to be lifted with a drum pressure other than fifty pounds per square inch, the load lift curve would be substantially parallel to the load lift curve 31 and the load lift curves for all pressures would be a series of substantially parallel curved lines.

It will be apparent that when the valve is lifted to its maximum height that fluid or, in most cases, steam from the vessel to which the valve is mounted will escape and the pressure in 'the vessel will drop. As the pressure in the vessel drops, a point will be reached where the load on the underside of the valve due to the fluid pressure will be less than the downward force of the spring 15 at all lifts. At this point, the valve closes or blows down. The pressure at which blowdown will occur is the pressure for which the load lift curve is tangential to the spring line 30. Such a curve has been partially indicated at 33. It is desirable to have a blowdown pressure that is close to the blowofi pressure and this consideration will make obvious the reason for the desirability of a relatively flat load lift curve that approaches the spring line. If the load lift curve 31 were to have a shape something like the dotted line curve 34, blowdown would not occur until the load had reached a value substantially lower than the load at blowoff and at a drum pressure of almost zero. This curve is a load lift curve for a much reduced drum pressure as seen from the point where the curve intersects the vertical axis of the graph.

We have discovered that the load lift curve for a safety valve can be modified by design of the huddling chamber, and more particularly we have discovered that by cupping the vertical wall 18 thereof, which .is part of the adjusting ring, according to a predetermined manner that we can achieve a valve in which the load lift curve is long and fiat and relatively close to the spring line. We have found that we can achieve such a result by appropriate design of the huddling chamber.

The force or load on the underside of the valve is comprised of the static pressure force due to the pressure in the vessel on which the valve is mounted against that portion of the valve that overlies the throat 12 when it is seated, plus the static force due to the same pressure on that portion of the valve that becomes exposed thereto when the valve is 1i ted on the seat (this latter portion will comprise an annular area bounded by the edge of the disk and the outer extremity of the first mentioned area), plus a third force which we will call the impact force, due to the rushing steam as it leaves the valve.

The total force is controlled by the desi n of the huddl ng chamber. There are two restrictions in the huddlinq chamber that are critical. The first restriction is the restriction between the seat of the valve 13 and the underside of the disk 14. We have indicated this restriction by the numeral 36 and will call it An. The second restriction is the one between the side wall 18 of the adiusting ring 16 and the outer edge of the disk 14. We have indicated this restriction by the numeral 37. and will refer to it as A7. It will be noted that the valve in question has the marginal portion on the underside cut away, as at 38. This really locates the portion of the valve that forms the restriction 37 with respect to the wall 18.

To design a valve according to our invention, we work from curves similar to the curves illustrated in Figure 5,

selecting the desired lift (which should approximate oneeighth of the seat diameter in inches).

From experience, a ratio of the final load to the initial or pop load, is established and from the desired lift and this r tio a spring rate may be determined and the spring line 30 drawn in. The desired load lift curve is then drawn in. intersecting the spring line at the chosen lift. It should be relatively fiat and close to the spring line.

A curve of the ratio of the area at 36 An to the area at 37 A VQIVlHFZ with the lift should be substantially co-- incident with this lead lift curve and, therefore, it remains to determine the scale for this curve (values of An over A corresponding to pounds of load). We have found by experiment that for a valve having a throat diameter of one inch and a pop pressure or blowofi pressure of fifty pounds per square inch, that the ratio Aa to A should equal one, at a load of fifteen pounds on the load lift curve. For valves of other throat dimensions and blowoff pressures, the ratio will equal one at various loads on the load lift curve. The ratio of area at 36 Aa to the area at 37 Af for a given throat diameter should be one at approximately pounds on the load lift curve. This is on the assumption that the large diameter of the disk equals between /5 and 1.5 times the throat diameter. If other disk diameters are used, a difierent scale for the ratio of the Au to A curve would be determined. Generally speaking, if disk diameter is below /i of the throat diameter, the ratio of Aa to A) would equal one at a load greater than indicated above. If disk diameter is above 1.5 throat diameter, the ratio would be smaller.

Aa to At would, of course, have a value of zero at no load.

For increments of lift on the Au to A) curve, we solve for the area at 37. This is a calculation that can be readily made because for any given lift the area at 36 can be calculated (it varies directly as the lift and equals 1r throat diameterXlift). Knowing the area at 36 and the ratio of the two areas Aa to A from the curve, the area :at 37 can readily .be calculated.

This is .done for a .number of points and .from the area at v37 the distance between the disk 14 and the wall 18 is calculated. The result of the calculations is a series of diameters for the wall .18 for corresponding .lifts. The wall 18 is then formed to fulfill each of these conditions for each of these lifts. On making these calculations for the one inch diameter fifty pounds per square inch valve, we have found that the wall 18 is substantially an arc of a circle having its centre line on the centre line of the adjusting ring and a radius approximately equal to .8 the seat diameter .and the height of the centre point about the bottom of the wall to the adjusting ring equals .265 t-he seat diameter.

In some cases, for some valves, it turns out to have slight irregularities, but by taking a mean of the wall so generated and making it smooth, we find that we achieve a very much improved valve with an early blowdown and a high lift.

Cupper wall 18 is formed on adjusting ring 16 which threads over the throat and can be varied to achieve a certain amount of flexibility in sensitivity of operation. For example, springs may vary in rate as much as 10%. If a stronger spring were used than the ideal, to achieve the same lift one would raise the adjusting ring thereby decreasing A slightly and increasing the load on the under side of the disk.

The 10% variation provided for in the formula in column 3 accounts for the permissible variation in spring rates.

The formula in column 3 [can be written in a more general manner as follows:

q l'oad in excess of popping load 50 A f 15 Xblow ofi" pressure -(throat diameter) What we claim as our invention is:

1. In a safety value, a casing having a throat through which a fluid can escape in an emergency, a valve seat formed at said throat, a disk adapted to seat on said valve seat, resilient means loaded to normally maintain said valve in a closed position on said seat, a member having a wall extending upwardly from the exit of said throat and in spaced relation to the edge of said disk whereby to form in cooperation with the edge of said disk a 'hnddling chamber open at the top, said disk when raised from said seat defining two restrictions, the area of the first one being defined by the under surface of said disk and said valve .seat, the area of the second one being defined by said wall of said member and the outer perimeter of .said disk, said wall being cupped, the variation of ratio of said first area to said second area with respect to lift of the disk being substantially the same as the variation of theload tending to open said disk with respect to disk lift .for the blowolf pressure of the valve.

2. .A safety valve as claimed in claim 1 in which said upwardly extending wall is cupped .in the shape of the e f a i e- 3. A safety valve as claimed in claim 1, in which the ratio of said first area to said second area equals load in excess of popping ad 50 popping pressureX (throat diameter) load in excess of popping load 50 15 X popping pressure X (throat diameter) where the outside disk diameter equals between \/2 and 1.5 times the throat diameter.

5. In a safety valve, a casing having a throat through which a fluid can escape in an emergency, a valve seat formed at said throat, a disk adapted to seat on said valve seat, resilient means loaded to normally maintain said valve in a closed position on said seat, a ring member having a wall extending upwardly from the exit of said throat and in predetermined spaced relation to the edge of said valve, whereby to form in cooperation with the edge of said valve a huddling chamber open at the top, said valve when raised from said seat defining two restrictions, the area of the first one being defined by the under surface of said disk and said valve seat, the area of the second one being defined by said wall of said ring member and the outer perimeter of said disk,-said wall being cupped, said wall being in predetermined spaced relation to the edge of said valve as aforesaid, in a manner that the variation of ratio of said first area to said second area with respect to lift of said disk is substantially the same as the variation of the load tending to open said disk with respect to disk lift.

6. A safety valve as claimed in claim 5 in which said upwardly extending wall is cupped in the shape of the arc of a circle.

' load in pounds in excess of popping load 50 )i 10% 7. A safety valve as claimed in claim 5, in which the ratio of said first area to said second area equals 15 X popping pressure (throat diameter) 8. A method of designing a safety valve having a casing with a throat through which fluid can escape in an emergency, a valve seat formed at said throat, a disk adapted to seat on said valve seat, resilient means loaded to normally maintain said valve in a closed position on said seat, a member having a wall extendingupwardly from the exit of said throat and in spaced relation to the edge of said disk whereby to form in cooperation With the edge of said disk a huddling chamber open at the top, said disk when raised from said seat defining two restrictions, the area of the first one being defined by the under surface of said disk and said valve seat, the area of the second one being defined by said wall of said member and the outer perimeter of said disk; comprising the steps of: selecting the maximum lift, selecting a spring line for said resilient means, selecting a load lift curve, solving for said first and second areas for increments of lift on the assumption the ratio of said first area to said second area varies with lift in the same manner as the load tending to raise said disk varies with lift.

References Cited in the file of this patent UNITED STATES PATENTS 1,772,004 Hopkins Aug. 5, 1930 1,925,323 Hopkins Sept. 5, 1933 1,949,150 Eplett Feb. 27, 1934 2,547,862 Gilmore Apr. 3, 1951 2,631,605 Tobis Mar. 17, 1953 FOREIGN PATENTS 127,512 Australia Nov. 28, 1946

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