EP0105311A1

EP0105311A1 - Improved desuperheater

Info

Publication number: EP0105311A1
Application number: EP83901246A
Authority: EP
Inventors: Kevin G. Schoonover
Original assignee: EUR-CONTROL USA Inc
Current assignee: EUR-CONTROL USA Inc
Priority date: 1982-04-02
Filing date: 1983-03-29
Publication date: 1984-04-18
Also published as: WO1983003365A1

Abstract

Un désurchauffeur (1) comprend une chambre d'admission d'eau à haute pression (20) fermée par un corps d'ajutage (18) ayant un alésage de piston central s'étendant axialement (21) se terminant par une courte section cylindrique de refoulement (24) de diamètre réduit. Un piston (26) coulisse dans l'alésage (21) et une série de passages borgnes (31) entourant l'alésage (21) coopèrent avec des orifices obliques et inclinés axialement vers le bas (32) pour définir des orifices ou lumières (30, 33) débouchant dans l'alésage (21) du piston à des niveaux espacés axialement. Le corps d'ajutage (18) est adapté et fixé à une section du corps d'admission d'eau (16) qui ferme les ouvertures d'eau des orifices (32). Les orifices (30, 33) sont répartis à différents niveaux de manière à obtenir une distribution exponentielle de leurs surfaces.A desuperheater (1) comprises a high pressure water inlet chamber (20) closed by a nozzle body (18) having an axially extending central piston bore (21) ending in a short cylindrical section discharge (24) of reduced diameter. A piston (26) slides in the bore (21) and a series of blind passages (31) surrounding the bore (21) cooperate with oblique orifices axially inclined downward (32) to define orifices or lights ( 30, 33) opening into the bore (21) of the piston at axially spaced levels. The nozzle body (18) is adapted and fixed to a section of the water intake body (16) which closes the water openings of the orifices (32). The orifices (30, 33) are distributed at different levels so as to obtain an exponential distribution of their surfaces.

Description

IMPROVED DESUPERHEATER

This invention relates to devices for spraying a fluid into a gas to obtain and/or maintain a specific thermal condition. This is commonly applied in the desuperheating of steam in which case water is sprayed into a steam flow stream to reduce the temperature or degree of superheat.

Steam generators and boilers are designed to produce steam at a particular temperature and pressure. Systems and processes utilizing steam as a source of mechanical and/or thermal energy normally require steam conditions somewhat different from those generated by the boilers for efficient operation.

Normally the steam being provided by the boiler is supplied to the process with superheat in excess of what would optimally be required. To obtain and maintain the desired temperature conditions, it is usual to cool the superheater inserted into the steam line which injects cooling water into the flow stream. To effectively control the temperature of the steam, the desuperheater will be required to perform several functions simultaneously. One of the most important functions to be performed by the desuperheater is the precise control of the amount of spray water admitted to the steam flow. Obviously, too much or too little cooling water will not provide adequate control of steam temperature.

Another important function is that the cooling water be injected into the steam line in a form which provides for efficient evaporation in the steam. If the cooling water does not evaporate quickly it will collect at the bottom of the steam line, reduce available surface area, and evaporate in a more or less uncontrolled manner, thus making precise control of steam temperature almost impossible.

Finally, the desuperheater must function to provide that the cooling water be distributed in the steam line in a generally uniform pattern so that the temperature of the steam is reduced uniformly throughout.

BRIEF SUMMARY OF THE INVENTION

The object of the invention is to provide an easily and exactly controllable device for precisely regulating the amount of cooling water injected into a steam flow stream.

It is another object of this invention to provide a device that injects the cooling water into the steam in a form which will allow for easy and rapid evaporation of the water by the steam.

It is another object of this invention to provide a device that is capable of delivering the cooling water in a generally uniform manner throughout the flow stream cross-section.

Another object of this invention is to provide an improved form of physical structure and arrangement of components for providing a desuperheater having the characteristics of this invention.

A further object of this invention is to provide a desuperheater which readily accommodates for providing a quantity of cooling water discharge which is nonlinearly related to the movement of the control member.

Briefly, the above objects are accomplished by providing a desuperheater arrangement in the form of a body providing a large chamber into which an inlet for water under high pressure is admitted, the chamber having a cylindrical end portion within which is located a nozzle body fitted into the end thereof. The nozzle body is provided with a central bore of enlarged diameter which is spherically reduced to a discharge opening of predetermined size. Clustered around the central bore are a series of circumferentially spaced blind passages which are in direct communication with the high pressure chamber and a series of ports are drilled from the outside of the nozzle body to intersect the various passages and to incline at angles about 45° with respect to the axis of the bore and to be contained within planes which are at about 15° inclination from the respective planes containing the axis of the bore and the axis of the passageway in question. The passageways are grouped about the bore at such a distance that a 15° inclination causes the ports to intersect the bore non-tangentially. The ports are drilled from the outside of the body at prescribed intervals axially of the nozzle body bore so that the intersection points of these bores with the cylindrical surface of the bore, in conjunction with the number of ports of any given level and the areas thereof affect the aforementioned non-linearity.

BRIEF DESCRIPTION OF THE DRAWING FIGURES Figure 1 is a perspective view of a steam line and associated therewith is a desuperheater according to the present invention;

Figure 2 is an enlarged transverse section taken substantially along the plane of section line 2-2 in Figure 1;

Figure 3 is a longitudinal section taken generally along the plane of section line 3-3 in Figure 2;

Figure 4 is an end view of a nozzle body according to a specific embodiment of this invention;

Figure 5 is an enlarged sectional view taken through a portion of the desuperheater and indicating the axially downward inclination of the ports; Figure 6 is a longitudinal section taken through a nozzle body of the specific embodiment prior to hold drilling to provide the ports; and

Figure 6a is a table indicating the disposition of the ports.

DETAILED DESCRIPTION OF THE INVENTION

In Figure 1, a desuperheater assembly according to this invention is indicated generally by the reference character 1 and will be seen to include an inlet body section 2 provided with a flange 3 by means of which it is connected to the flange 4 of the nozzle section 5 which, at its nozzle end, is welded as at 6 to the stream line 7. The direction of steam flow is indicated by the arrows 8. The inlet section 2 is provided with an inlet spud 9 terminating in the flange 10 by means of which it is connected to the flange 11 of a water inlet pipe 12 which leads to a source of high pressure water. The inlet section 2 is also provided with a packing gland 15 which sealingly but slidably engages the end of a piston rod 13 and an actuator assembly 14 is provided to axially position the piston rod 13 in the manner set forth hereinafter. The entire assembly 1 is disposed at an angle to the steam line 7 as shown in order to aid in the equal distribution of the injected water uniformly into the flowing stream.

As shown in Figures 2 and 3, the nozzle body section 5 includes the tubular body member 16 having a cylindrical inner wall 17 within which the cylindrical outer wall surface of the nozzle body 18 snugly fits. Preferably, the surface 17 is formed as a counterbore to the slightly reduced diameter inner wall 19 of the inlet chamber 20 formed within the sections 2 and 5 and which is closed at one end by the packing gland 15 and the piston rod 13 passing through it and at the other end by the nozzle body 18. The nozzle body is provided with a central bore 21 extending inwardly from the rear face 22 of the nozzle body 18 and passing completely axially through the nozzle body to open at the front face 23 of the nozzle body through a short, reduced diameter cylindrical section 24, the bore 21 being blended into the cylindrical section 24 by the hemispherical section 25. The piston rod 13 terminates in an enlarged piston portion 26 having a circumferentially extending crown 27 and a bulbous tip 28 provided with a cylindrical axial extension 29. In the fully seated position of the piston 26, the bulbous portion 28 seats on the hemispherical section 25 at the cylindrical discharge opening 24 thereby fully to seal off the discharge opening and, in this position, the crown 27 is below the level of the lowest port opening 30.

Clustered around the bore 21 are a series of axial passageways 31 and, as is shown more clearly in Figure 5, each port 32 is drilled from the outside of the nozzle body 18 at an angle of inclination of about 45° with respect to the axis of the passage 31 and of the bore 21 to intersect both the passage 31 and the bore 21 to provide a discharge port 33 at the intersection between the port 32 and the bore 21. In addition to this axial downward inclination, each port 32 is skewed slightly with respect to a plane containing the axis of the passageway 31 and the axis of the bore 21. Specifically, the skewing is at an angle of approximately 15° with respect to this plane, as will be seen more clearly in Figure 2. However, it should be noted that the diameter of the circle upon which the passages 31 lie is so related to the diameter of the bore 21 that each port intersects the bore 21 at the aforesaid angle of 15° with respect to the radial direction.

The reason for this is that the ports 32 must intersect the bore 21 cleanly to provide well defined discharge ports 33 and to provide no abrupt changes in flow direction, at the discharge port 33, which would be the case if the ports were disposed tangentially.

The passages 31 are of relatively large diameters compared with the cumulative diameters of the ports 32 which intersect them so that, in effect the passageways 31 effect a continuation of the large high pressure chamber 20 so as to eliminate any substantial pressure drop within the passages 31 under high flow conditions, as will be clearly evident hereinafter.

The inlet water is provided at a fixed high pressure and of course the discharge characteristics of the assembly, irrespective of how many of the discharge ports 33 are exposed below the crown 27 will depend upon the differential pressure between this inlet water in the chamber 20 and the pressure in the steam line 7. To appreciate this, it will be understood that the steam flow 8 in the steam line 7 is variable, dependent upon the amount of steam being consumed in a given instant by the process or equipment which is supplied. Even at minimum steam flow conditions, line velocities must be relatively high, e.g., greater than about 25 feet per second. In the industry in general, a desuperheater is normally constructed so that the discharge water flow rate is directly proportional to the piston travel. That is, the maximum flow requirement is calculated and the ports associated with the piston 26 are simply equally distribured along the path of piston travel so that the result is propor tional discharge volume over the stroke range of the piston. Thus, the change in water discharge flow rate observed with respect to piston travel will be equal throughout the total stroke of the piston but since the steam flow rate which requires the variable water discharge rate will diminish also in a generally linear fashion between its maximum and minimum flow conditions, it is difficult properly to adjust the piston position particularly at the lower end of the steam flow range. That is to say, with an equal distribution of the water discharge ports such that for a unit X of piston travel, the change in port area is constant (say 10% water flow change), a piston movement of X when the water discharge rate is at 90% will change the water discharge rate by 10/90 or 11.1%. On the other hand, this same piston movement when the water discharge rate is at 15% will change the water discharge rate by 10/15 or 66.7%. Thus, the normal construction of a desuperheater renders the adjustment at the lowest end of the water discharge flow very sensitive to piston movement.

The present invention achieves much more precise control at low water flow rates. Preferably, the ports are distributed axially along the bore 21 such that a unit of movement of the piston will produce a greater percentage change of water discharge flow at the high end of the water flow range than the change produced by this source unit of piston movement at low water flow rates. That is, if the water flow is already at 90% of maximum, the unit x of piston movement should provide a water flow rate percentage change y such that y/90 is greater than y'/N (where y' is the water flow rate percentage change caused by the unit of piston movement when the water flow rate is N% of maximum, N<90) . Preferably, again, the relationship is logarithmic. Specifically, it is preferred that the following equation be at least approximated:

(1) where %Ap is the percentage of port area which is exposed below the piston, S is the stroke of the piston, s is the position of the piston relative to the closed position, In is the base of natural logarithms, and k and R are constants such that when s = S , equation (1) will equal 100 (i.e., 100% of the port areas is exposed). By inspection, then, kR must equal 100 and equation (1) is not applicable when s = 0. The term R will be called "rangeability" and has the significance that it determines the slope characteristics of the exponential curve when s is plotted as abscissa and %Ap is plotted as ordinate.

The rangeability R typically may range between 15 and 50, with lower values giving the most graduality of %Ap per unit of piston movement at the lower end of the %Ap range.

Another consideration which comes into play is the determination of the size of the nozzle body bore 24 in relation to the total area of the ports 32. For a given maximum water flow Q_C and pressure differential Δ P (water pressure minus steam pressure), the following equation applies;

where C_T is the flow coefficient for the desuperheater and G is the specific gravity of the fluid being discharged. For water at 60ºF, G = 1 so that equation (2) reduces to:

(3)

With Q and ΔP known, C_T is determined and values of C_P and C_N are then determined by assigning the desired ΔP' which is to be due to the ports and the remainder Δ P" which is to be due to the nozzle opening 24, noting that ΔP' +ΔP" =ΔP in equation (3). Typically, one chooses ∆P' = .4ΔP so that Δp" = .6∆p. For the known Q_C , these values ofΔP' and∆P" are substituted into equation (3) to determine C_P and C_N respectively.

Then, equations (4) and (5) :

(4) with C_P being the flow coefficient of the ports

(5) with C_N being the flow coefficient of the nozzle are used to calculate the total port area A_P and the nozzle opening A_N. A standard drill size is selected which provides the nozzle opening 24 of the closest area to A_N.

A_P is used in equation (1) , as shown in Figure 6A. In the specific example shown, S = 2 inches and the rangeability R = 25 was chosen. At the level A it will be appreciated that all of the ports except half the areas of ports at level A are uncovered or exposed. Also , s/S = (2-.098)/2

= .951. Thus, from equation (1), %A_P = 85.4 For this particular desuperheater, A_P was determined to be .4875 in² , so that %A_P equal (A_P - A_A/2 ) /A_P where A_A is the area of the ports at level A. Five holes formed by a 3/16" drill provide an area of .138 in². Thus, the actual %A_P provided by these five holes equal 83.8% which is reasonably close to the calculated value of 85.4%. At the next level, B, the %A_P equals 100 (A_P - A_A - A_B/2)/A_P. The calculated %A_P is 65.8%. A_A has already been determined equal to .138 in so that if A_B is formed by three standard 5/32" holes, A_B = .0576 and the actual %A_P at level B is 65.7%. This agrees almost exactly with the calculated value of %A_P (s/S = .87 at level B) .

This process is continued for the remainder of the levels used. It will be understood, of course, that other and different level selections may be used, both fewer and greater in number to those indicated in Figure 6A. For the particular selections shown, it should be noted that the ports at levels A and B would overlap if formed through the same passages 31. Thus, eight passages are grouped around the bore 21.

In addition to the simplicity of the structural assembly, it will be appreciated that it is a relatively simple matter to bore the port holes. They are blocked off at the outer surface of the nozzle body by the inner surface 17 of the body section 16.

The nozzle body 18 is simply formed as a cylinder with an enlarged end 34 and the bevel 35. It fits snugly within the end of the section 16 as shown in Figure 3 and may be brazed as at 36 to secure it.

The combination of the downward and skewed inclinations of the ports 32 creates a downwardly swirling flow ahead of the piston 26. The bulbous end 28 of the piston aids in properly directing the water so that it issues from the short cylindrical passage 24 as a conical curtain as disclosed in the Gustafsson patent mentioned above. This materially aids good atomization by the flowing steam and excellent distribution of the atomized water droplets. Consequently, the steam is uniformly desuperheated to control temperature uniformly. The exponential distribution of the ports assures that for low steam flow conditions and consequent low water discharge rate, piston movement very slowly changes the discharge rate. This allows very precise control of the desuperheating effect under those conditions where it is normally very difficult to do so.

It will be appreciated that the principles of this invention may be applied to fluids in general and are not limited to the desuperheating of steam.

Claims

1. An assembly suitable for desuperheating a flowing stream of steam by introducing controlled quantities of water uniformly thereinto, which comprises: an inlet body section defining a high pressure chamber and having a nozzle body closing one end thereof, said nozzle body having a cylindrical outer face, a rear face exposed to said high pressure chamber and a front face adapted to direct a conical curtain of water into a steam line, said nozzle body having a piston bore therethrough from its rear to its front faces, in which the bore has an enlarged section extending from the rear face substantially to the front face, said enlarged section terminating in a hemispherical surface merging with a short cylinderical discharge bore section opening onto said front face, a piston axially movable snugly within said enlarged section of the piston bore, said nozzle body having a plurality of blind passages parallel to and surrounding said piston bore and extending inwardly from said rear face whereby said passages provide an extension of said high pressure chamber, a plurality of port bores angled downwardly from the outer surface of said nozzle body each to intersect one of said passages and the enlarged section of the piston bore at a region thereof which is located between a radial position and. a tangential position, said inlet body section snugly receiving said cylindrical outer face of the nozzle body whereby to block off the outer openings of said port bores and at least one of said port bores being disposed at different axial positions relative to said enlarged section whereby axial movement of said piston varies the port opening area.

2. The assembly as defined in claim 1 wherein the downward inclination of said port bores is about 45°.

3. The assembly as defined in claim 2 wherein the skew angle of said port bores is about 15° as measured between a plane passing through the axis of a passage and the axis of said enlarged section of the piston bore and a plane passing through the axis of the passage and extending axially of the port bore.

4. The assembly as defined in any one of claims 1-3 wherein said port holes intersect said enlarged section of the piston bore at different levels such that, for each level, %A_P = ke ^{(lnR) s/S}, where k and R are constants such that kR = 100, s is the position of the piston from the bottom or closed position of its stroke, S is the total stroke of the piston and %A_P is the percentage of port area exposed by the piston at the level under consideration.

5. The assembly as defined in claim 4 wherein R ranges between 15-50.

6. The assembly as defined in claim 5 wherein R equals 25.

7. An assembly suitable for desuperheating a flowing stream of steam by introducing controlled quantities of water uniformly therein, which comprises: an inlet body section defining a high pressure chamber and having a nozzle body closing one end thereof, said nozzle body having a piston bore therethrough terminating in a reduced diameter section, a piston axially slidable in said piston bore, a plurality of port bores communicating with said piston bore at axially spaced levels thereof for discharging water ahead of said piston, said port holes substantially satisfying the equation %A_P = ke^(lnR)s/S where k and R are constants such that kR = 100 ,s is the position of the piston from the bottom or closed position of its stroke, S is the total stroke of the piston and %A_P is the percentage of port area exposed below the piston at the level under consideration.

8. The assembly as defined in claim 7 wherein R ranges between 15-50.

9. The assembly as defined in claim 8 wherein R equals 25.

10. An assembly suitable for desuperheating a flowing stream of steam by introducing controlled quantities of water uniformly therein, which comprises: an inlet body section defining a high pressure chamber and having a nozzle body closing one end thereof, said nozzle body having a piston bore therethrough terminating in a reduced diameter section, said flowing stream of steam being in communication with said reduced diameter section through a nozzle opening, a piston axially slidable in said piston bore, a plurality of port bores communicating with said piston bore at axially spaced levels thereof for discharging water ahead of said piston, said port bores having an equal percentage flow characteristics so as to have an exponential discharge of volume of water through said nozzle opening over the stroke range of said piston.