Abstract:
The present invention is directed to a desuperheater spraying valve assembly. More particularly, the invention provides a new and improved valve assembly, whereby cooling liquid may be controllably injected into a gas or a liquid stream, typically steam, to selectively maintain the gas or liquid at a predetermined temperature level. In a possible embodiment of the present invention, a class V valve assembly is disclosed. It comprises an actuator coupled to a plug with a valve seat in the proximity of said plug. The plug is affixed to an actuator rod which transverses the body of the valve assembly. A spray tube may be affixed to said valve seat and at least one spray nozzle is affixed to a to the spray tube. The plug and said valve seat are conical in shape as to when said plug is inserted into said valve, they form a seal.

Description:
BACKGROUND OF THE INVENTION 
     The present invention is directed to a spray desuperheater apparatus and method. More particularly, the present invention is directed to an improved desuperheater valve assembly, which may have a plurality of nozzles affixed to the spray assembly. The aforementioned assembly allows for the controlled injection of a cooling liquid into a superheated fluid to selectively reduce the temperature of the superheated fluid and maintain the downstream flow at a predetermined temperature level. 
     Superheated fluid is fluid that is at a temperature that is higher than the boiling point of the fluid. Many industrial applications operate most efficiently using a fluid that is saturated or only slightly superheated, but many fluid generators tend to produce a fluid that is excessively superheated. Excessive temperatures (superheat) may damage system components, adversely affect the efficiency of operation or quality of the product being manufactured. The process of injecting a controlled amount of coolant to reduce the temperature of the superheated medium to a specific and precise temperature is defined as desuperheating. Also superheated steam excessive thermal energy can cause damage to the utilizing devices or processes, making it necessary to rigidly control and maintain the steam temperature. Steam desuperheating refers to the process of reducing and controlling the temperature of the superheated steam by introduction of cooling water into the steam flow. 
     A common method to desuperheat steam is by spraying or injecting a cooling liquid into the flow of superheated steam while it is passing through a steam pipe or the like. Once the cooling liquid is sprayed into the superheated steam flow, water droplets are formed. The droplets rapidly mix with the superheated steam and evaporates, drawing thermal energy away from the steam and thus regulating its temperature. Droplet sizes and spray pattern are among the main parameters, which determine desuperheating efficiency. Thus it is important to utilize a spray nozzle that allows for the injection of a cooling fluid into the processed steam or any superheated fluid the smallest available droplets with optimum spray patterns and additional mixing control capability. 
     Because the amount of superheat varies with the amount of steam production the most accurate regulation can be made by sensing the temperature of the steam at a point downstream where the injected water has been completely vaporized and heated so that equilibrium conditions have been reached. Conventionally this would require a spray nozzle that is optimized for a very narrow coolant flow rate, and if the flow rate is varied outside of certain parameters, the resulting spray pattern may not give prompt enough heat transfer to allow an equilibrium condition to be sensed and proper steam conditions be attained. However, spring-loaded nozzles allow to eliminate this problem. 
     An important aspect of desuperheating steam or any gas, is a valve&#39;s ability to control the flow of cooling liquid being injected into the superheated steam or any gas. The main control element of a valve is a trim, which usually comprised of three components, a plug, cage, and a seat. In a plug throttling trim types, the plug has a contour on its lower end, below the seat. The contoured portion provides varying flow area with lift, thus regulating the flow. The flow characteristic of the trim varies according to the profile of the plug. Thus, having control valve as a separate unit adds installation and maintenance cost for a desuperheater user. 
     Due to the service locations, there are more strict requirements for leak tightness for desuperheater coolant control valves in the closed position. If the valve were to leak when the desuperheating was not needed, the unrequited cooling of the steam may cause damage or impede the performance of equipment using the steam. The ANSI/FCI 70-2 (American National Standards Institute/Fluid Controls Institute) standard establishes a series of seat leakage classes for control valves. Class V is usually specified for the desuperheater coolant control valves. It is the most restrictive leakage class for metal-to-metal trim design. 
     A power positioning actuator is required for kipping the valve fully closed and moving the valve closure membrane, plug, to any position in response to a signal of a temperature control system. The flow over the seat with unbalanced plug design utilizes the valve inlet pressure to assist closure of the valve, which significantly reduces required force from the actuator to provide tight seat leakage shutoff. 
     To effect movement of the aforementioned plug, which is located inside a valve, there needs to be a transfer of motion from a power positioning actuator to the valve&#39;s plug through the body of the valve. The valve may contain liquid at a high pressure and the having a hole in the valve&#39;s body to transfer motion to the plug via a stem may enable leaking at the place where the stem goes through the body of the valve. One way to prevent leaks is to employ a stuffing box. A stuffing box may be a gland seal or the like, used to seal the reciprocating stem from the fluid. 
     Additionally, it would desirable to have a replaceable seat ring with a modular design which would permit multiple configurations which can be assembled with a minimum number of unique components. This will increase profit margins, reduce the end user part replacement costs for repair and maintenance activities. It would also be desirable to employ more durable materials to increase the service life and provide a wider range of capacity. 
     Thus, it would be desirable to integrate the cooling liquid control valve into the desuperheater apparatus to reduce costs by eliminating the need of a separate cooling liquid control valve and its corresponding conduit assemblies. It would also be desirable to restrict the coolant leakage to meet the ANSI/FCI 70-2 class V classification characteristics. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to a spray desuperheater. More particularly, the invention provides a new and improved spray tube assembly which may having a plurality of nozzles, whereby liquid coolant may be controllably injected into a gas or a superheated fluid, typically steam, to selectively maintain the fluid at a predetermined temperature level. 
     In a possible embodiment of the present invention, a Mechanical Atomizing Desuperheater with Class V Shutoff (MADV) introduces liquid coolant into a superheated medium. The MADV is a probe style direct contact heat exchanger that combines a flow control element (trim) and a spray tube with spring-loaded injection nozzles. The flow control element modulates the coolant flow using a top guided plug-throttling trim, which is configured to provide the required flow control characteristics. The trim is comprised of two components, a top guided plug and a seat ring. The plug has a contour on its lower end, below the seat. The contoured portion of the plug defines flow characteristic, by varying flow area relative to the plug position. The plug is guided by replaceable guide insert that is press-fitted into the upper part of the MADV body. Throttling occurs between the contoured portion of the plug and the seat ring bore. The seat ring is mounted at the top of the spray tube. 
     Prior art Multi-nozzle spray desuperheaters and spring assisted multi-nozzle desuperheaters both have an internal flow control element, which is located at the highest temperature zone of the desuperheater. The resulting high thermal stresses and different thermal expansions of the internal parts create the potential for accelerated operational failures and/or premature wear of internal parts. The present invention resolves this problem by locating the trim outside of highest temperature zone. A series of injection nozzles are fitted to the end of the spray tube to complete the assembly and this assembly is inserted through the header connection. In addition, current embodiments have the MADV&#39;s nozzles also have a higher capacity and more robust design than the nozzles used in the multi-nozzle spray and spring assisted multi-nozzle desuperheaters. The replaceable seat ring and modular design of the MADV permits multiple configurations to be assembled with a minimum number of unique components to increase margins, reduce end user part replacement cost for maintenance activities, enable the use of more durable materials to increase the service life, and provide a wider range of capacity. 
     Each injection nozzle is an assembly that consists of a spring-loaded plunger that is located in a cylindrical housing with a central cylindrical cavity having a fluid inlet and a fluid outlet. The injection nozzle also has a plunger inserted into the fluid outlet of the housing so that the plunger and the housing form a fluid path. A plunger stop is attached to the housing at the fluid inlet. A spring retainer is attached to the plunger. At least one spring is disposed between the housing and spring retainer. 
     Additionally, the replaceable seat ring and modular design of the MADV permits multiple configurations to be assembled with a minimum number of unique components which will increase margins, reduce end user part replacement cost for maintenance activities, enable the use of more durable materials to increase the service life, and provide a wider range of capacity. 
     A method for desuperheating steam in a steam pipe utilizing a Seat Leakage Class V valve assembly is disclosed as another possible embodiment. The method comprises detecting a temperature of steam in a steam pipe and sending the temperature to a controller. The controller then sends a control signal to a valve actuator coupled to a plug inside a valve. The valve actuator positions said plug proximate to a valve seat inside the valve to increase or decrease a flow rate of a cooling traversing through said valve. The cooling liquid is then sprayed with a spray nozzle into a steam pipe to perform desuperheating. 
     In an additional possible embodiment, a class V desuperheater system is disclosed. The system comprises a means for detecting for a temperature of steam in a conduit carrying a liquid or a gas and a means to send said temperature to a controller. A means to control sends a control signal to a valve actuator coupled to a plug inside a valve, which positions said plug proximate to a valve seat inside said valve to increase or decrease a flow rate of a coolant traversing through said valve. A spraying means to spray said liquid coolant with a spray nozzle into said gas or liquid conduit to perform desuperheating. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view of a steam line incorporating of a prior art desuperheater apparatus. 
         FIG. 2  illustrates a cross sectional view of an embodiment of a desuperheater apparatus in accordance with the present invention. 
         FIG. 2   a  illustrates may be an example of the composition on the injection nozzle of a desuperheater apparatus in accordance with the present invention. 
         FIG. 3 . is an enlarged view of the desuperheater apparatus of the present invention in the closed position. 
         FIG. 4  is a side view of a plug employed in the valve of a desuperheater apparatus of the present invention. 
         FIG. 5  is a cross sectional view showing a valve in the closed position in a desuperheater apparatus of the present invention. 
         FIG. 6  is a cross sectional view showing an example of a stuffing box utilized in an embodiment of the present invention. 
         FIG. 7  is a graph illustrating three types of flow control characteristics of the present invention during operation. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The many features and advantages of the invention are apparent from the detailed specification, and thus, it is intended by the appended claims to cover all such features and advantages of the invention which fall within the true spirit and scope of the invention. Further, since numerous modifications and variations will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and operation illustrated and described, and accordingly, all suitable modifications and equivalents may be resorted to, falling within the scope of the invention. 
     There has thus been outlined, rather broadly, certain embodiments of the invention in order that the detailed description herein may be better understood, and in order that the present contribution to the art may be better appreciated. There are, of course, additional embodiments of the invention that will be described below and which will form the subject matter of the claims appended hereto. 
     In this respect, before explaining at least one embodiment of the present invention in detail, it is to be understood that the invention is not limited in its application to the details of construction and to the arrangements of the components set forth in the following description or illustrated in the drawings. The invention is capable of embodiments in addition to those described and of being practiced and carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein, as well as the abstract, are for the purpose of description and should not be regarded as limiting. 
     As such, those skilled in the art will appreciate that the conception upon which this disclosure is based may readily be utilized as a basis for the designing of other structures, methods and systems for carrying out the several purposes of the present invention. It is important, therefore, that the claims be regarded as including such equivalent constructions insofar as they do not depart from the spirit and scope of the present invention. 
     Desuperheaters are commonly used to cool steam or vaporized water, but the present invention may be deployed in piping or conduits carrying gases. Referring now to the drawings in greater detail and specifically to  FIG. 1 , a schematic arrangement of a prior art superdeheater. As illustrated, on the inlet a portion of a gas or liquid conduit  26 , a flanged mounting fitting  20  is provided on the sidewall and a injection nozzle  30  is mounted inside the gas or liquid conduit  26  and is supplied with cooling liquid. The cooling liquid is under pressure and is supplied with said cooling liquid from a liquid supply line  17 . Also, depicted is a control valve assembly  15  which operates to control the flow of liquid to the injection nozzle  30 . The injection nozzle  30  controls the flow of cooling liquid which is atomized and injected into the liquid coolant line  9 . The cooling liquid flow is a function in response to a temperature sensor  19 , mounted in the gas or liquid conduit  26  spaced a distance downstream from the injection nozzle  30 . The temperature sensor  19  functions to send a temperature measurement signals to a temperature controller  11 , which evaluates the temperature against a desired set point and sends a corrective signal to the control valve assembly  15  to either restrict or increase the flow of the cooling liquid. Preferably, the distance between the temperature sensor  19  and the injection nozzle  30  is distance sufficient to allow the steam to come to equilibrium after the liquid has been injected in the stream to ensure a true measurement of the steams thermal energy. Thus, the control valve assembly  15 , operates to vary the water flow in the liquid coolant line  9 , which in turn modulates the amount of liquid sprayed into the gas or liquid conduit  26 . 
     In the present embodiments, the control valve assembly  15  is incorporated into the body of the Desuperheater, as show in  FIGS. 2-6 . Continuing with  FIGS. 2-4 , each depicts a cross sectional close-up view of an embodiment of the present invention. More specifically, turning to  FIG. 2 , a gas or liquid conduit  26  incorporating the Mechanical Atomizing Desuperheater (MADV) apparatus unit  10 , in accordance with an embodiment of the present invention is illustrated.  FIG. 2  depicts the gas or liquid conduit  26 , wherein a flanged mounting fitting  20  is mounted on the sidewall of the gas or liquid conduit  26 . As illustrated, the main body  12  of the MADV apparatus unit  10  is attached to the flanged mounting fitting  20  with via mechanical connection, for example, mounting bolts  14 . An attached liquid coolant inlet  24  is a flange type connector. A first end of a spray tube  21  is fitted at the outlet of the main body  12 . The second end of the spray tube  21  has a spray unit  28  affixed to it. The spray unit  28  has a at least one injection nozzle  30  for atomizing the liquid coolant into the gas or liquid conduit  26 . 
     In an embodiment of the present invention, the main body  12 , can be constructed from varying materials including carbon steel, wherein the main alloying constituent is carbon. The main body  12  may be cast, forged or machined in the desired dimensions and geometry. Also, depending upon the pressure and temperature requirements where the valve will be deployed, the choice of material can vary in composition of alternative carbon steel formulations, such as WC6, WC12, WC12A or any other suitable metal, alloy or plastic. 
     Turning now to  FIG. 2   a , may be an example of the composition on the injection nozzle  30  and it may comprise an assembly comprising of a spring-loaded plunger  31  that is retained in a cylindrical housing  32  having a central cylindrical cavity. As illustrated, each nozzle  30  has a fluid inlet  33  and a fluid outlet  34 . The injection nozzle  30  also has a plunger inserted into the fluid outlet of the housing so that the plunger and the housing form a fluid path. Each injection nozzle  30  further includes a plunger stop which is attached to the housing at the fluid inlet. A spring retainer is attached to the plunger and functions to close the injection nozzle when no fluid under pressure is present. 
     Now referring to  FIG. 4 , a cross sectional view of plug which be deployed in accordance with one non-limiting embodiment is illustrated. The plug  18  is attached to a valve stem  37   b  and is held in place with a pin  37   c . The plug  18  has a mostly conical shape. As depicted in FIG.  2 , the other side of the valve stem  37   b  is attached to an actuator control rod  37   a . Referring back to  FIG. 2 , the other side of the actuator control rod  37   a  is connected to a pneumatic valve actuator  36 . The pneumatic valve actuator  36  moves the plug  18  to a various position based upon control instructions provided by the temperature controller  11 . The aforementioned translation and positioning of the plug is accomplished via the linkage assembly that is formed between the actuator control rod  37   a  and the valve stem  37   b . The plug  18  is positioned above and inside a valve seat  16 . The shape of the valve seat  16  is also mostly conical in shape having its minor frustrum near the valve&#39;s outlet and its major frustrum inside the body of the valve. The plug  18  and valve seat  16  can be any shape as long as they are similar. For example, the plug may be a block and the valve seat may be an corresponding square, and when the block is “seated” on the valve seat, the flow through the valve is restricted. 
     In one preferred embodiment, the plug  18  may be made out of soft annealed type 420 Stainless Steel. This alloy provides both outstanding corrosion resistance and exceptional wear resistance. It is also known as cutlery grade martensitic stainless steel. The valve seat  16  can also be made out of this material. Alternatively, the plug  18  and the valve seat  16 , may be manufactured from other materials, such as carbon steel, Stellite (cobalt-chromium alloys), brass, beryllium-copper or any heat durable corrosion resistant metal or alloy. 
     As illustrated in  FIG. 4 , the valve seat  16  is fitted next to the spray tube  21 . Also as illustrated, C-ring  16   a  is positioned between the seat ring  16  and the spray tube  21  to ensure a leak proof seal. The C-ring is essentially a metal O-ring with an open side. In contrast to a O-ring, the C-ring requires approximately one-half of the load of an O-ring with little change in leak rate over its compression. This results in joints with less sensitivity to flange displacement, while retaining the required pressure and temperature characteristics of the O-ring. The C-ring key may be constructed from varying materials, however, one preferred embodiment, for example, one preferred material is a high-duty alloy such as Alloy  718 , which is a nickel-chromium-iron-molybdenum with the addition of niobium to permit age-hardening. 
     As previously discussed the plug  18  modulates the flow of coolant through the valve assembly by changing the positioning of the valve stem  37   b . One embodiment of the MADV apparatus unit  10  encompassed by the present invention utilizes a pneumatically operated control valve actuator, however, electric, hydraulic, and manual actuators may also be utilized. Returning to  FIG. 2 , the pneumatic valve actuator  36  is a spring-and-diaphragm pneumatic actuator type and is depicted in accordance with an embodiment of the present invention. This type of pneumatic valve actuator  36  is oftentimes preferred due to its dependability and simplicity of design. Electric and electro-hydraulic actuators can be utilized with the present invention, for example, in environments where an no air supply source is not available, or having low ambient temperatures which could freeze condensed water in pneumatic actuators, or where unusually large stem forces are needed. 
     The plug  18  position relative to the valve seat  16  permits the actuator to modulates the coolant flow. For example, when the plug  18  is seated upon the valve seat  16 , the coolant flow through the MADV apparatus unit  10  is nil. Additionally, as the plug  18  is moved away from the seat ring  16 , the cooling liquid will start to flow. The shape of the plug  18  and the shape of the valve seat  16  are chosen to provide certain coolant flow characteristics. For example, the amount travel or stroke of the plug  18  can be directly proportional to the flow of the cooling liquid flowing to the spray tube  21 . If the plug  18  of the MADV apparatus unit  10  is opened to three-quarters of the plug&#39;s travel distance (from a fully closed position), the flow rate through the MADV apparatus unit  10  will likely be 75% of the valve being fully opened. 
     Turning now to  FIG. 7  is a graph illustrating three algebraic flow characteristics based upon the plug&#39;s  18  and valve seat&#39;s  16  geometries and dimensions. The percent of travel of the plug  18  is referenced on the x-axis, while the percent Cv is referenced on the y-axis. Cv is a measure for the flow of a valve and is defined as the volume of flow per unit of time. Cv may be calculated in U.S. gallons per minute (g.p.m.) of water at a temperature of 60° Fahrenheit with a pressure drop across the valve of 1 pound per square inch (p.s.i.) It can be calculated by formula (1)
 
 Cv =g.p.m./√{square root over (fully_open_pressure_drop)}  formula (1)
 
     A linear plot “L” illustrates a possible linear relationship between the valve&#39;s flow to plug&#39;s  18  position within the valve. A modified parabolic plot “M” illustrates a possible modified parabolic relationship of the valve&#39;s flow to plug&#39;s  18  position within the valve and an equal percentage plot “E,” illustrates a possible equal percentage relationship between the valve&#39;s flow to the plug&#39;s  18  position within the valve. 
     During operation of the MADV apparatus unit  10 , while in the full open position, the flow of the cooling liquid is at a maximum rate. As the valve is transitioned to the closed position, the plug  18  is translated towards the valve seat  16 , decreasing the flow rate of the cooling liquid. Once the plug  18  is fully seated into the seat ring  16 , the liquid coolant flow completely terminates and the valve is closed. As illustrated in  FIG. 5 , the top of the valve seat  18  has a seat ring shoulder  18   a  that compliments plug shoulder  19   a . Consequently, when the valve is in the closed position (seated) position the seat ring shoulder  18   a  and the plug shoulder  19   a  are in contact one another producing a tight seal. 
     In the above described closed position, the desuperheater application of the present invention provides minimal or no leakage. For example, to obtain class V status, as defined by ANSI/FCI classification, the leakage permitted is limited to 5×10 ml per minute per inch of orifice diameter per p.s.i. differential. Moreover, the valve must operate with a pressure differential of 50 p.s.i. at 125° Fahrenheit. 
     Referring to  FIG. 6 , when operating the MADV apparatus unit  10 , there is a need to transfer longitudinal motion from the pneumatic valve actuator  36  to the valve&#39;s plug  18 . However, due to the above-described design of the MADV apparatus unit  10 , this transfer is carried out through the body of the valve containing a liquid at a pressure. Accordingly, embodiments of the present invention may utilize a stuffing box to prevent the likelihood of leakage of the cooling fluid between sliding parts of the valve stem  37   b . A stuffing box may be a gland seal or the like, used to seal a rotating or reciprocating shaft against a fluid. 
       FIG. 6  illustrates a live loaded stuffing box in accordance with an embodiment of the present invention is depicted. As illustrated, the stuffing box is affixed to the main body  12 . The stuffing box  62  includes an extended length guide bushing  71  which provides better guidance for the valve stem  37   b . The stuffing box  62  further includes a first layer of braided graphite packing  70   a , followed by three layers of spring graphite packing  70   b  and an additional layer of braided graphite packing  70   a , disposed above the quite bushing  71 . A compression ring  68  holds the aforementioned packing layers in place. While a compression ring positioner  66  retains the compression ring  68  in place. A first washer  64  is placed above the compression ring positioner  66  which has a bolt  63  in its center. On the other side of the first washer  64 , six shoulder spring discs  65  are placed forming the live load three concave shaped module. A second washer  64  is placed on top of the six shoulder spring discs  65  and the assembly is held in place with bolt  63 . 
     As illustrated in  FIG. 6 , the a first layer and last layer of braided graphite packing  70   a  along with the three layers of spring graphite packing  70   b  are compressed against the main body  12  and the valve stem  37   b  forming a water tight seal. This picking and resulting seal allow the valve stem  37   b  to move up and down. Other packing schemas may also be employed without taking away from the spirit of the invention. For example, the spring type packing material me be substituted with PTFE or any other low friction durable material. 
     During operation, the stuffing box may be exposed to extreme temperature variations and vibration forces. After time, the bolts holding the packing together would eventually come loose and effect the performance of the packing. A first solution would have a technician perform periodic inspection and adjust the bolts back into a predetermined torque value. An alternative to prevent the bolts from coming loose in the first place and is achieved through live loading stuffing box. Live loading is achieved through use of the six shoulder spring discs  65  which help absorb the vibrations and compensate for the temperature variations. The present invention may be utilized without live loading, but as stated above, it would require periodic maintenance.