US3211135A - Steam generator unit control system - Google Patents

Description

Oct. 112, 1965 A. S. GRIMES ETAL STEAM GENERATOR U NIT CONTROL SYSTEM Filed March 7, 1963 United States Patent STEAM GENERATOR UNIT CQNTROL SYSTEM Arthur S. Grimes, 143 Wicks Road, Commack, N.Y.;

Robert S. Hunter, 69 Helen Sh, Fauwood, N.J.; and

John A. Tillinghast, 102 Tauglewylde Road, Bronxville, NY.

Filed Mar. 7, 1963, Ser. No. 263,657 9 Claims. (Cl. 122406) This invention relates to the art of steam power plants having forced circulation steam generators, and, more particularly, to high pressure steam generators of the type commonly referred to in the industry as oncethrough generators. The invention pertains, in one of its more specific aspects, to a pressure breakdown control system for a once-through high pressure steam generator unit, which system constitutes a distinct improvement and affords a number of important advantages as compared to conventional pressure breakdown systems.

A once-through steam generating unit may be defined herein as meaning a forced circulation steam generator and associated equipment, including flow circuitry, in

which all of the feed water entering the unit, when the unit is operating at its rated capacity, makes a single pass through the flow circuitry and discharges through the superheater steam outlet of the unit as superheated steam.

While water and steam derived therefrom are the commonly used and preferred fluid mediums in generators of the character indicated, it is recognized that other specific fluid mediums may be employed in lieu thereof. Accordingly, the terms water and steam, appearing in this description and in the appended claims, are respectively understood to be the equivalent of other suit able liquids and vapors of such liquids.

In a once-through steam generator, resort must be had to a pressure breakdown system which is placed in active service during its start-up operation. The startup operation may be considered as comprising successive first and second procedural phases. The first phase occurs immediately prior to firing the boiler of the unit and consists of passing water at high pressure through the flow circuitry to prevent overheating when the boiler is fired. The second phase occurs during the initial firing period of the boiler. During this period, the temperature and specific volume of the water admitted to the unit increase progressively until the fluid emerging from the flow circuitry consists of steam having a sufficiently high temperature that it can be safely introduced into a turbine or other steam utilizing device which communicates with the flow circuitry. The pressure of the water and steam exiting from the steam generator unit, during the referred-to first and second phases, must necessarily be reduced substantially before being returned to and reused in the unit-hence the need for a pressure breakdown system which serves the function of effecting requisite pressure reduction.

A pressure breakdown system must have sufficient capacity to effectively handle a large percent of the full load fluid flow for which the steam generator unit is designed. It is required to successively accomplish desired pressure reduction of high pressure water during the first phase and the same or higher pressure steam during the second phase. The duration of the start-up operation may be quite lengthy, in some instances as long as several days, depending primarily on the purity of the water entering the steam generator unit.

It has heretofore generally been the practice to employ, in once-through steam generator units, pressure breakdown systems which use and rely on pressure reducing valves to reduce the high water and steam pressure developed during start-up operations to levels at 3,Zil,l35" Patented Oet. 12, 1965 which the fluids may be properly reintroduced into the unit. Experience has demonstrated that such systems are objectionable for a number of important reasons. For one thing, the extremely high fluid velocities through the ports and diffusing sections of the pressure reducing valves create highly erosive conditions, especially during the first phase. This necessitates the use of special, expensive, trim materials in order to achieve even reasonable periods of satisfactory operation before incurring repairs or replacements. Also, the pressure reducing valves, inasmuch as they must operate at large pressure differentials, require powerful and costly mechanical operators.

The fluid discharged from the pressure reducing valves is usually transmitted into a separation tank where any steam, which may be present in the fluid, is separated from the water content. The purpose of this procedure is to minimize disturbances and heat loss in reintroducing the fluid at proper reduced pressure to the heat cycle of the unit. In order to prevent damage to the separation tank, a target is usually positioned in the discharge line of the pressure reducing valve to avoid subjecting the tank to high velocity fluid jets. As in the case of the pressure reducing valves, such targets are also subject to rapid erosion.

Another object to pressure breakdown systems using pressure reducing valves to effect requisite pressure reduction is that they frequently exhibit large amplitude vibrations when in service. Such vibrations, unless adequately dampened by employing suitable and oftentimes expensive sway braces and/or shock absorbers, may result in costly damage to the system. The vibrations are further objectionable as they are the cause of sharp and disturbing sound levels. Sound levels greater than decibels have been produced by such vibrations in the past.

The above discussed objections and inherent disadvantages of conventional pressure breakdown systems are summarized as follows:

(1) Rapid erosion of pressure reducing valves and associaed equipment, requiring frequent and expensive repairs or replacements;

(2) High manufacturing, installation and maintenance costs;

(3) Severe vibrations which may result in damage to the equipment unless properly dampened by costly auxiliary equipment; and

(4) Disturbing noise conditions.

The enumerated objections and disadvantages of conventional pressure breakdown systems are completely obviated or effectively minimized by utilizing the system of this invention, as will be evident to persons trained in the art from the ensuing discussion and the detailed description appearing further along herein. The pressure breakdown system of our present invention comprises an arrangement of one or more pressure reducing tubes that define relatively long flow paths. Each tube establishes communication between the normal flow circuitry of a steam generating unit and a steam-Water separation tank, from whence the fluid is recycled through the unit. The individual tubes may be connected directly to the fluid flow circuitry and/or a header branching from the flow circuitry at corresponding spaced points downstream of a selected steam generating section of the unit. Ac-

' oordingly, the tubes may be considered as being arranged in parallel flow relationship. The term parallel, as used herein, is to be interpreted in its broadest sense rather than in a geometric sense. Each tube is provided with a shut-off valve to discontinue fluid flow therethrough, as required. While shut-off valves are essential in placing the system into and out of active service, they are not subjected to large fluid pressure differentials and high velocity fluid flow. Therefore, such valves may be of any suitable, relatively small, standard type.

In our system, requisite pressure reduction is effected at least during the above-mentioned first phase, by fluid friction occasioned by the flow within each pressure reducing tube; and, as a consequence, the pressure or energy reduction is spread over the entire length of these tubes. This results in a smooth and gradual transition of the fluid from a high pressure condition to a desired low pressure condition, regardless of whether the fluid flowing through the tubes consists only of water or steam or a mixture of water and steam. The smooth transition of the fluid from a high pressure condition to a low pressure condition successfully eliminates the objectionable vibrations and noises created in conventional pressure breakdown systems. Moreover, the erosion problems and difficulties inherent in conventional systems are effectively minimized. Any erosion which may occur in our system by repeated use over a long time period may be readily rectified at total costs, involving parts, labor and time, which are very small in comparison to corresponding costs in repairing and replacing usual pressure reduc ing valves and targets of the types discussed above.

In designing a once-through steam generating unit according to this invention, the selection of the number, internal diameter and length of the pressure reducing tubes to be employed depends on and must be considered in relation to certain factors, including the physical requirements of a particular installation, the desired operating characteristics of the tubes and the economics of the installation. The number, internal diameter and length of the tubes are inter-dependent variables and are interrelated to the enumerated factors.

Generally speaking and for the purpose of arriving at an economical design, the overall length of the pressure reducing tubes should be such that the tubes will effect a relatively high pressure drop per unit length. Extensive experimentation with water and steam has demonstrated that average pressure drops up to and in excess of 25 pounds per square inch per lineal foot of the tubes do not create the referred-to objectionable vibrations or noise levels.

The velocity of steam or other compressible fluid flowing through our pressure reducing tubes is limited to its sonic velocity within these tubes. The sonic velocity is dependent on certain factors, namely the specific fluid employed and the temperature and pressure of such fluid. The maximum velocity when handling water or other liquid is uncertain. We have successfully operated the pressure reducing tubes with water at average velocities up to 250 feet per second without experiencing vibrations or noise and without incurring appreciable erosion in either the pressure reducing tubes or associated equipment. It will be appreciated from this that water may be passed through the pressure reducing tubes at substantial velocities without encountering the difliculties and objec tions discussed earlier herein. Once the the overall tube length has been chosen it will be appreciated that the selected internal diameter of such tubes when considered in relation tothe characteristics of the fluid flowing, i.e., temperature, pressure, etc., and the condition of the interior surface of the tubing, i.e., its degree of smoothness or roughness, determines the fluid velocity, the incremental friction loss per unit tube length and the fluid flow rate.

The number of such tubes to be used becomes a matter of choice and economics and should be such as to provide reasonable step-wise control of the fluid flow rate as individual tubes are opened or closed sequentially, without utilizing too great a number of tubes or excessively long tubes to accomplish the required pressure reduction.

As the fluid flowing through the steam generator passes from its liquid state (Water) to the vapor state (steam), the capacity of individual pressure reducing tubes is reduced due to the greater specific volume of the vapor.

This condition may be satisfied either by providing additional tubes to afford the required .added capacity, when needed, or by providing one or more relatively small pressure reducing valves in parallel with the pressure reducing tubes. If pressure reducing valves are used, they should be operated only when the fluid is essentially all steam to avoid vibrations and noises which occur when handling cold fluid.

It is to be understood that the term tubes appearing in this description and in the claims is not to be limited to conventional pipes or conduits but includes various forms of one-piece or multi-piece devices having suitable passage to permit fluid flow therethrough for the indicated purposes. One such device may consist of a suitable, elongated, solid article having a plurality of relatively long, small diameter bores formed therein to allow parallel flow of the fluid.

It is the primary object of this invention to provide a once-through steam generating unit with an improved pressure breakdown system or means which affords important advantages as compared to conventional pressure breakdown means.

Another object of the invention is to provide a pressure breakdown system or means which minimizes or eliminates rapid erosion, objectionable vibrations and disturbing noise conditions which are inherent in the operation of present day systems that employ pressure reducing valves to effect necessary fluid pressure reduction.

A further object of the invention is to provide a pressure breakdown system or means wherein requisite pressure reduction is effected partly or entirely by fluid friction along the inner surface of at least one elongated small diameter tube.

A still further object of the invention is the provision of a pressure breakdown system or means of the character indicated that is simple in design; that is sturdy and durable in construction; that is reasonable in manufacturing, installation and maintenance costs; and that is capable of performing its intended functions in an eificient and dependable manner.

The enumerated objects and advantages of the invention will be comprehended bypersons trained in the art from the following detailed description and the accompanying drawing which respectively describe and illustrate an arrangement of apparatus embodying the invention.

The drawing is a diagrammatic representation of a once-through steam generator unit according to this invention.

Referring to the drawing, a once-through steam generator, which is generally indicated by the numeral 1, may be of any suitable construction known to the art and is adapted to be fired in the usual manner by an appropriate firing means (not shown). The generator includes a first fluid flow section 2 and a second fluid flow section 3 which are series connected, as shown. Interposed between flow sections 2 and 3 is a flow control valve 4. A feed pump 5 forces water under selected high pressure through flow section 2 wherein the water is converted into steam which passes through valve 4 and flow section 3 wherein it is converted into superheated steam. When the unit is operating at its rated capacity, the superheated steam is transmitted by a line 6, having a flow control valve 7, to a steam turbine 8 wherein the energy supplied by the superheated steam is utilized. Spent steam is transmitted by a line 9 to a condenser 10. A return line 12 delivers condensate from the condenser to pump 5 for reuse in the unit.

The illustrated steam generator unit includes a pressure breakdown system or means 13 which was discussed earlier herein and which will now be described. The pressure breakdown means comprises a first tubular header 14 which is equipped with a flow control valve 15 and communicates with the outlet end of flow section 2 at a point 16 which is upstream of control valve 4, a second tubular header 17 which is equipped with a flow control valve 18 and communicates with a steam-water separator 19, and a plurality of earlier described elongated, small diameter pressure reducing tubes 20 which permit parallel flow from header 14 to header 17, as allowed by corresponding shut-off valves 21. The illustrated construction also includes a conduit 22 having a pressure reducing valve 23 and establishing communication between the headers. By locating control valve 4 downstream of point 16, flow section 3 and turbine 8 may be by-passed during a start-up operation.

A line 24, having a flow control valve 25, is utilized to transmit steam from separator 19 to condenser 10 while a similar line 26, having a flow control valve 27, is utilized to transmit water from the separator to the condenser. It will be understood that lines 24 and 26 need not communicate directly with condenser 10, but may, if desired, communicate with another suitable receptacle (not shown) in series with return line 12.

The illustrated steam generator unit is equipped with a second pressure breakdown system or means 28, certain parts of which are the same as or similar to parts of pressure breakdown means 13, in construction and function. Such parts are identified by corresponding primed numerals and consist of headers 14' and 17, flow control valves and 18', pressure reducing tubes 20', shutoff valves 21', conduit 22' and pressure reducing valve 23'. Header 14 communicates with the flow circuitry at a point. 29 which is intermediate the outlet of flow section 3 and turbine 8. By locating control valve 7 downstream of point 29, turbine 8 may be isolated from the remainder of the flow circuitry during a start-up operation.

During the previously mentioned first phase of a startup operation, control valve 4 is closed and water at high pressure is passed through flow section 2 and is delivered to and through one or more of selected tubes 20. The pressure of the water is reduced, as described earlier, in the course of its passage through tubes 20. The water at reduced pressure is transmitted by header 17 to separator 19 and thence by line 26 to condenser 10. Water accumulated in the condenser is returned to pump 5 by line 12. Pressure reducing valve 23 is maintained in closed position during the first phase.

On completion of the first phase and initial firing of the generator, the second phase of the startup operation commences. During this phase, the water admitted to flow section 2 increases progressively in temperature and specific volume until the fluid at point 16 consists essentially of steam. The pressure breakdown means 13 is operated as discussed above so long as the fluid exiting from flow section 2 contains water. When such fluid consists of steam only, pressure reducing valve 23 may be opened and one or more shut-off valves may, if desired, be closed. The second phase of the start-up operation is, in effect, completed when the unit begins actual generation of steam in flow section 2, at its designed operating pressure. At this juncture, it is now possible to direct steam through the superheater, i.e. flow section 3, and thence to the steam turbine. This is accomplished by closing, in sequence, the pressure reducing tubes, thereby effectively directing the steam to and through the superheater.

Valve 4 may be opened during the second phase of the start-up operation and control valve 7 closed. This allows part of the steam generated in flow section 2 to pass through pressure breakdown means 13, and the remainder of such steam to be superheated in flow section 3 and passed through pressure breakdown means 28. The amount of fluid passing through each pressure breakdown means depends on the particular pressure reducing tubes which are in active service. If desired, all of the pressure reduction may be handled by means 28 alone during the second phase of the start-up operation. Alternatively, pressure breakdown means 28 may be utilized alone or with means 13 during the first phase of the start-up operation.

Pressure reducing valve 23 may be placed in active service alone or with one or more of the pressure reducing tubes when the fluid in the circuitry consists of steam.

One design of a once-through stream generating unit according to this invention utilizes a steam generator 1, employing 14 in. tubing, at least between flow section 2 and 3, and a 17 in. line 6 from the generator to the turbine. In this design, the various headers, 14, 17, 14' and 17' and the conduits 22 and 22' have a diameter of approximately 8 in. The several pressure reducing tubes 20 and 20' are approximately 2 in. in diameter. Each pressure breakdown means 13 and 28 is provided with a total of seven (7) pressure reducing tubes, each of which is approximately feet in length. The herein indicated diameters are outside diameters. The inside diameters are dependent on the particular design operating conditions.

It will be evident from the consideration of this description and the drawing that the illustrated arrangement of apparatus may be used in various ways to accomplish the objects of this invention.

From the foregoing, it is believed that the construction, operation, objects and advantages of our present invention will be readily comprehended by persons skilled in the art, without further description. It: is to be clearly understood, however, that various changes in the construction described above and illustrated in the drawing may be made without departing from the scope of the invention, is being intended that all matter contained in the description or shown in the drawing shall be interpreted as illustrative only and not in a limiting sense.

We claim:

1. In combination with a one-through steam generating unit comprising pump means, a steam utilizing device, fluid flow circuitry establishing communication between the pump means and the steam utilizing device and including steam generating means having series connected first and second fluid flow sections, the pump means being connected to and adapted to transmit water at high pressure through the first flow section wherein it is adapted to be converted into steam which is transmitted through the second fiow section wherein it is adapted to be converted into superheated steam, and a receptacle for condensate from the steam utilizing device and communicating with the pump means, the improvement comprising pressure breakdown means for transmitting, during a start-up operation of the steam generating unit, fluid from a region intermediate the inlet of the first flow section and the steam utilizing device to the receptacle and for effecting reduction in pressure of the fluid from a start-up operating value in the flow circuitry to a predetermined relatively low value for admission to the receptacle, said pressure breakdown means comprising at least one elongated tube that defines a fluid flow passage having a transverse area that is substantially smaller than that of the flow circuitry at the region the pressure breakdown means is connected to the flow circuitry, said reduction in pressure being effected substantially entirely by friction of the fluid within the tube in the course of its flow therethrough, and valve means for controlling flow of the fluid in the tube without effecting substantial pressure drop of the fluid.

2. Apparatus according to claim 1 wherein the tube communicates with the flow circuitry at a. point between the first and second flow sections.

3. Apparatus according to claim 1 wherein the tube communicates with the flow circuitry at a. point between the second flow section and the steam utilizing device.

4. Apparatus according to claim 1 wherein the pressure breakdown means also comprises a plurality of the tubes, one end of each tube communicating with the flow circuitry, means establishing communication between the other end of each tube and the receptacle, and valve means for controlling flow of fluid in each tube without effecting substantial drop in pressure of the fluid.

5. Apparatus according to claim 1 wherein the pressure breakdown means also comprises a plurality of the tubes, a header establishing communication between the flow circuitry and one end of each tube, means establishing communication between the other end of each tube and the receptacle, and valve means for controlling flow of fluid in each tube without effecting substantial drop in pressure of the fluid.

6. Apparatus according to claim 1 wherein the pressure breakdown means also comprises steam-water separator means communicating with the receptacle, a plurality of the tubes, one end of each tube communicating with the flow circuitry, a header establishing communica tion between the other end of each tube and the separator means, and valve means for controlling flow of fluid in each tube without effecting substantial drop in pressure of the fluid.

7. Apparatus according to claim 1 wherein the pressure breakdown means comprises steam-water separator means communicating with the receptacle, a plurality of the tubes, a first header establishing communication between the flow circuitry and one end of each tube, a

8 second header establishing communication between the other end of each tube and the separator means, and valve means for controlling flow of fluid in each tube without effecting substantial drop in pressure of the fluid.

8. Apparatus according to claim 7 wherein the first header communicates with the flow circuitry at a point between the first and second flow sections.

9. Apparatus according to claim 7 wherein the first header communicates with the flow circuitry at a point between the second flow section and the steam utilizing device.

References Cited by the Examiner UNITED STATES PATENTS 734,868 7/03 Hill 18146 2,080,574 5/37 McCoy 122379 X 2,620,969 12/52 Stephens 181-47 X 2,989,038 6/61 Schwarz.

3,009,325 11/61 Pirsh. 3,019,774 2/62 Beyerlein.

PERCY L. PATRICK, Primary Examiner.

KENNETH W. SPRAGUE, Examiner.

Claims (1)

1. IN COMBINATION WITH A ONE-THROUGH STEAM GENERATING UNIT COMPRISING PUMPS MEANS, A STEAM UTILIZING DEVICE, FLUID FLOW CIRCUITRY ESTABLISHING COMMUNICATION BETWEEN THE PUMP MEANS AND THE STEAM UTILIZING DEVICE AND INCLUDING STEAM GENERATING MEANS HAVING SERIES CONNECTED FIRST AND SECOND FLUID FLOW SECTIONS, THE PUMP MEANS BEING CONNECTED TO AND ADAPTED TO TRANSMIT WATER AT HIGH PRESSURE THROUGH THE FIRST FLOW SECTION WHEREIN IT IS ADAPTED TO BE CONVERTED INTO STEAM WHICH IS TRANSMITTED THROUGH THE SECOND FLOW SECTION WHEREIN IT IS ADAPTED TO BE CONVERTED INTO SUPERHEATED STEAM, AND A RECEPTACLE FOR CONDENSER FROM THE STEAM UTILIZING DEVICE AND COMMUNICATING WITH THE PUMP MEANS, THE IMPROVEMENT COMPRISING PRESSURE BREAKDOWN MEANS FOR TRANSMITTING, DURING A START-UP OPERATION OF THE STEAM GENERATING UNIT, FLUID FROM A REGION INTERMEDIATE THE INLET OF THE FIRST FLOW SECTION AND THE STEAM UTILIZING DEVICE TO THE RECEPTACLE AND FOR EFFECTING REDUCTION IN PRESSURE OF THE FLUID FROM A START-UP OPERATING VALUE IN THE FLOW CIRCUITRY TO A PREDETERMINED RELATIVELY LOW VALUE FOR ADMISSION TO THE RECEPTACLE, SAID PRESSURE BREAKDOWN MEANS COMPRISING AT LEAST ONE ELONGATED TUBE THAT DEFINES A FLUID FLOW PASSAGE HAVING A TRANSVERSE AREA THAT IS SUBSTANTIALLY SMALLER THAN THAT OF THE FLOW CIRCUITRY AT THE REGION THE PRESSURE BREAKDOWN MEANS IS CONNECTED TO THE FLOW CIRCUITRY, SAID REDUCTION IN PRESSURE BEING EFFECTED SUBSTANTIALLY ENTIRELY BY FRICTION OF THE FLUID WITHIN THE TUBE IN THE COURSE OF ITS FLOW THERETHROUGH, AND VALVE MEANS FOR CONTROLLING FLOW IN THE FLUID IN THE TUBE WITHOUT EFFECTING SUBSTANTIAL PRESSURE DROP OF THE FLUID.

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