US3532453A - Start-up system for once-through boiler - Google Patents

Description

Oct. 6, 1970 A. J.-ZIPAY ETAL START-UP SYSTEM FOR ONCE-THROUGH BOILER Filed July 2, 1968 SUPERHTR.

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IXI-ENTOR. ALBERT 3. ZlPAY JUSTIN EWINKIN ATTOKNEY5 Patented Oct. 6, 1970 3,532,453 START-UP SYSTEM FOR ONCE-THROUGH BOILER Albert J. Zipay, Clifton, and Justin P. Winkin, Fairlawn,

N.J., assignors to Foster Wheeler Corporation, Livingston, N .J a corporation of New York Filed July 2, 1968, Ser. No. 741,964 Int. Cl. F22b 29/06 US. Cl. 122-406 13 Claims ABSTRACT OF THE DISCLOSURE A once-through vapor generator start-up system of the type which includes a generator main flow path having in series heating surfaces, a condenser, and one or more feedwater heaters. A constant pressure start-up bypass line extends between the heating surfaces and the condenser and feedwater heaters, comprising a flash tank and first and second drain connections leading from the flash tank to the feedwater heaters and condenser respectively. A valve in the second connection to the condenser controls the drain flow to the condenser in response to maximum and minimum water levels in the flash tank. At levels intermediate the maximum and minimum levels, the flow in the second connection is controlled in response to the flow in the first connection to the feedwater heaters.

DESCRIPTION This invention relates to apparatus and a method for starting up a once-through vapor generator, particularly of the type which includes a bypass line between heating surfaces of the generator and heat recovery surfaces thereof.

It is known to provide a bypass line in a once-through vapor generator to bypass the flow in the generator around the unit turbine. The reason for this is that during the start-up period for the generator, the quality of the fluid is not such that it can be handled by the tur bine. Generally, the bypass line includes a flask tank, with a connection to return vapor flashed in the flash tank to the generator main flow path for early starting of the generator turbine. The bypass line and flash tank may also be provided with connections between the vapor space and liquid space of the flash tank and heat recovery components, such as a generator feedwater heater and deaerator. Surplus flash tank heat may be discharged to a heat sump such as a condenser through vapor or liquid line connections between the flash tank and the sump.

Normally the exhaust flow from the feedwater heater, which is of the shell and tube type, is valved in a way to produce a desired pressure in the bypass line and at the turbine throttle during the start-up period. This can be done by controlling the liquid level in the feedwater heater and in that way the amount of condensing surface in the heater which is uncovered. A predetermined amount of surface condenses the flow to the heater, either liquid flow flashed in the heater or vapor flow, and as the level drops because of insufficient condensing surface, the exhaust flow valve from the heater is closed to raise the level. This increases the presssures in the heater and correspondingly the pressure upstream of the heater. By controlling the level in the heater, the desired pressure in the bypass system and at the turbine throttle can be obtained.

In the present invention, it is contemplated that the bypass line and feedwater heater will be operated during start-up at a constant pressure, for instance 1100 p.s.i., holding turbine throttle pressure constant at, for instance, about 1000 p.s.i.

The primary problem in such a pressure control for start-up is that it does not protect the feedwater heater from excessive flow from the flash tank. Normally, a generator of today will be provided with at least two high pressure heaters in parallel, each being designed or sized for half of normal load extraction steam rates with some margin of safety. A bypass flow during start-up can be much higher, as high as 25% to 30% of full load flow, and if one of the heaters is out of service, and the other is required to accommodate the full 25 to 30% flow, the resultant high velocity flow in the heater can damage the subcooling section of the heater. The heater exhaust flow valve is not operative to protect the heater since it will merely pass the additional flow to maintain a predetermined level. This condition, plus the situation where both heaters may be out of service, requires a bypass around the heaters. Such a bypass also is necessary early in the start-up period, prior to burner light off, when the flow to the generator is heated by pegging the deaerator. A bypass flow from the flash tank to the heaters would only cool the feedwater.

The sump connection between the flash tank and a sump such as the condenser provides such a bypass, this usually is valved to control water level in the flash tank, between minimum and maximum levels, and obviously would not suffice to control flow to the heaters whether one, two, or no heaters were in service.

It is an object of the invention to provide and improved control of drain flow from the flash tank to the feedwater heaters.

It is also an object of the invention to obtain automatically the maximum possible recovery of heat during the start-up period.

In accordance with the invention there is provided, for a once-through vapor generator which includes a main flow path including heating surfaces therein, condensing means and feedwater heating means also in said main flow path, a bypass line between said heating surfaces and said condensing means and feedwater heating means, the bypass line including flash tank means and drain connections between said flash tank means and condensing feedwater heating means, the improvement comprising controlling the flow in the connection leading to said condensing means in response to maximum and minimum levels in said flash tank means. At levels intermediate said maximum and minimum levels, the flow in said connection leading to said condensing means is controlled in response to flow in the connection leading to said feed water heating means.

The invention and advantages thereof will become apparent upon further consideration of the specification, with reference to the accompanying drawing, in which the figure represents schematically a flow arrangement and start-up system for a once-through vapor generator in accordance with the invention.

Referring to the figure, the once-through vapor generator comprises a main flow path which includes an economizer 12, furnace passes 14 receiving a fluid from the economizer, a primary superheater 16 downstream of the furnace passes, and a finishing superheater 18 downstream of the primary superheater. A high pressure turbine 20 receives fluid from the finishing superheater 18, and the exhaust flow from the high pressure turbine 20 is reheated in reheater 22 and then transmitted to a low pressure turbine 24. From the latter, the exhaust flow passes to condenser 26. Between condenser 26 and the economizer 12, the main flow path includes demineralizer 28 receiving flow from the condenser, a low pressure heater 30, and a deaerator 32, the latter transmitting the flow to feedwater pump 34. In the feedwater pump 34, the fluid, which can now be considered the feed flow for the generator, is pressurized, and transmitted to the tube sides of high pressure heaters 36-40 inclusive, and from there to the ecohomizer 12. The high pressure heaters 36 and 38 are in parallel with the heaters 40 and 42, and either set or both sets of heaters can be used. On the shell side of the heaters, the drain flows from the nos. 1A and 1B heaters (40 and 38) via lines 38a and 40a to the nos. 2A and 2B heaters (42 and 36), and from there to the deaerator 32.

Between the outlet of the primary superheater 16 and the heat recovery and heat sump portions of the main flow path, which for purposes of this invention, will be considered those portions of the path including the condenser 26 and high pressure heaters 36-42, is a bypass line means generally designated by the number 44. The bypass line means primarily comprises a conduit 46 connected to the outlet of the primary superheater leading to a flash tank 48. The flash tank in turn is provided with a number of conduits leading therefrom, including a steam conduit 50 adapted to return a vapor flow from the vapor space of the flash tank to the inlet of the finishing superheater, and a drain line 52 leading from the liquid space of the flash tank. The vapor line '50 is provided with branch conduits 50a leading to the condenser, 50b leading to the turbine gland seal, and 50c leading to the deaerator, the latter being for the purpose of pegging the deaerator. The drain line 52 is provided with a branch conduit 52a leading to the condenser, and a second branch conduit 52b leading to the high pressure heaters. Also leading from the flash tank is conduit 54, from the vapor space thereof to the high pressure heaters.

In the conduit 46 leading from the outlet of the primary superheater to the flash tank is a shut-off valve 56, and in the conduit 50 leading from the flash tank to the inlet of the finishing superheater, there is positioned valve 58, both valves functioning to isolate the bypass system from the main flow path of the generator during normal load operation of the generator. During use of the bypass system, remote manual control valve 60* in the main flow path between the primary superheater outlet and finishing superheater inlet can be closed to direct the flow through the bypass.

Also of interest with respect to the bypass system is pressure reducing valve means 62 disposed in the main flow path between the furnace passes and primary superheater designed to reduce the pressure in the generator during start-up. As an example in accordance with the invention, the feedwater pump may pressurize the generator furnace passes during the start-up period to normal operating pressure, Which may be subcritical or supercritical. The high pressure in the furnace passes is necessary for protection of these passes. To obtain a steam flow to the turbine earlier in the start-up period, the pressure reducing valve 62 reduces the pressure at the outlet of the furnace passes to about 1100 p.s.i., the steam at the lower pressure flashing in flash tank 48 earlier in the start-up period. Reducing the pressure upstream of the primary superheater has the advantage of improved heat transfer at lower pressure in the primary superheater, and thus additional heat pick-up and a shorter starting-up period for the working fluid in the generator.

The present invention is concerned primarily with the portion of the start-up period in which there is a liquid or liquid and vapor flow into the flash tank, and a liquid level established in the flash tank. For this period, the branch line 52b is provided with connections 52b and 52b" to Nos. 1A and 1B high pressure heaters 40 and 38, respectively, each having a shut-off or isolation valve, items 64 and 64' respectively, so that sets of heaters, 40 and 42, or 36 and 38, can be used individually or in paral lel, or both sets of heaters can be out of service.

In the situation where none of the high pressure heaters is in service, all of the drain flow from the flash tank would have to be dumped through line 52a to the condenser. Alternatively, where only one of the sets of high pressure heaters is in service, the other being out of serv- 4 ice, the one in service is still incapable of handling all of the start-up flow, in which case some of the drain flow from the flash tank would have to be dumped to the condenser.

A particular example of the situation where no feedwater heater is in service, is during the very early stages of the start-up period. The deaerator 32 may be pegged with steam from a separate or auxiliary source, even prior to firing the burners, resulting in a heated flow of feedwater from the deaerator through the high pressure heaters, on the tube side thereof, to the economizer. The flow in the flash tank is still cold and unless the heaters are isolated from the flash tank (by valves 64 and 64) the undesirable condition of extraction of heat from the feedwater will exist.

A further reason for keeping the high pressure heaters out of service during the very early stages of start-up is to isolate the heaters (using valves 64 and 64') until flash tank drain temperature reaches a value that will subsequently flash steam in the heaters to produce the necessary pressure to force a flow to the condenser.

To explain this in more detail, the heaters, as mentioned above, are of the shell and tube type with means for passing the feedwater through the heater tubes, and for introducing the bypass flow to the heater shell side. Usually the exhaust port for the heater shell-side flow is in the subcooling section of the heater. Referring to the figure, the exhaust lines from the No. 1 heaters are provided with flow control valves 66 and 66 actuated in response to liquid level in the heaters. During the early stages of startup, there is insufficient pressure in the flash tank and/or in the high pressure heaters to force the fluid from the heaters to the condenser. The only way the flow can be forced to the condenser may be to flood the heater and possibly the flash tank, and this may be considered undesirable in certain installations.

Various reasons well known to those skilled in the art may exist why a single heater may or may not be in service. For instance, it may be out of service for repair work. In any event variable amounts of fluid will be diverted through conduits or connections 52a and 52b depending upon the number of heaters in service and the point in the start-up period. Automatically controlling this is achieved in the following way.

The sump conduit or connection 52a, which in essence is a bypass connection around the heaters, lead from the flash tank to the condenser or sump 26, and is provided with a flow control valve 68 actuated by flow controller 70. As shown, the flow controller receives maximum and minimum level signals from the flash tank and actuates the valve 68 so that the liquid level in the flash tank is main tained between maximum and minium points. As the level in the flash tank drops below a minimum level, the flow controller 70 closes the valve 68 raising the level in the flash tank, and conversely, as the water level in the flash tank exceeds the maximum level point, the flow controller 68 opens the valve lowering the level. The purpose of this is to provide a pressure seal against steam flow or leakage of steam through the conduit 52a to the condenser. Also, depending upon flash tank design, a minimum level may be necessary to obtain separation of vapor and liquid in the flash tank. By the same token, there is usually a maximum liquid level in the flash tank which must not be exceeded if flashing and separation is to occur.

In the conduit 52b, the start-up system is provided with a flow meter 72, adapted also to transmit a signal to the flow controller 70. As long as the level in the flash tank is between maximum and minimum points, the flow controller is responsive to the signal from the flow meter 72. The controller 70 has three set points for this signal, one when all heaters are in operation, another when only one set of heaters is in operation, and the third when no heaters are in operation. These set points are determined for maximum permissible heater flow capacity to obtain maximum heat recovery and still avoid damage to the heaters. Initially during start-up, in the very early stages of the start-up period, there will be no flow to the high pressure heaters, and the set point will cause the controller 70 to divert all of the drain flow to condenser 26. If only one of the high pressure heaters is subsequently placed in service, the second said point will adjust the flow in conduit 52b to a predetermined amount within design limits of this heater, the flow in conduit 52a increasing or decreasing only if maximum or minimum level signals are received. When both high pressure heaters are in service, all of the drain flow will normally pass to the high pressure heaters by virtue of the first set point for the flow controller. However, the minimum level set point in the flash tank is such that the signal from the flow meter will be locked-out if the level drops below minimum with the level signal then taking over and maintaining minimum level in the flash tank, or at least completely closing the valve 68 in line 52a. For instance in the later stages of start-up when the unit is at equilibrium and flash tank drain flow approaches zero, the steam flow through the flow meter could reach a magnitude calling for opening of valve 68 and dumping of valuable heat to the condenser. To prevent this possibility, it is necessary to lockout the flow controllers effect on the positioning of the valve 68.

EXAMPLE As an example in accordance with the invention, the generator is started with a pumping rate of about 25% of full load flow. The burners are placed in service and fired at a rate of approximately of full load firing rate, subject to a minimum fuel limit for combustion stability and to a furnace exit maximum gas temperature of about 1l00. The control valves 62 hold fluid pressure within the furnace passes at about 3550 p.s.i.g., in a supercritical unit, and valves in the bypass system and at the turbine inlet hold pressure downstream of the furnace passes at about 1100 p.s.ig. The control loops in the bypass system automatically distribute the flow of water and subsequently the flow of steam from the flash tank for maximum heat recovery. This is accomplished as fol lows:

Initially the drain line valves 64, 64' and 68 are closed. All other outflow valves connected to the flash tank are closed allowing the water level to build up in the flash tank. When the fluid temperature leaving the furnace circuit reaches about 200 F., the minimum fluid temperature to obtain flashing in the high pressure heaters, the valves 64, 64', 66, and 66' are opened and the high pressure heaters are placed in service. Depending upon whether one, two or no heaters are in service, the flow controller regulates the opening of valve 68 to maintain both a prescribed maximum permissible heater drain flow and flash tank level between maximum and minimum points. With both high pressure heaters in service, the flow controller 70 will close control valve 68 and all the flash tank drain flow will usually be to the heaters. With one high pressure heater in service, the flow controller will regulate the closure of valve 68 until the maximum permissible drain flow to the heater for maximum heat recovery is obtained.

The admission of hot water to the high pressure heater or heaters, if both are in service, will result in flashing of steam and the establishment of a pressure suitable for forcing the heater drain flow to the condenser. In the event insufficient pressure is available to pass all of the drain flow, the flash tank level will rise above the maximum level set point, and valve 68 will open to dump or bypass a portion of the flow to the condenser. However, the level controllers in the high pressure heater drain lines are set to control heater level or heater condensing surface at a point that will subsequently or eventually permit heater pressure and flash tank pressure to reach approximately 1100 p.s.i.g., pressure upstream of the heaters being a function of heater level or condensing surface with a predetermined heater drain flow. During this stage of start-up, the turbine can be appropriately loaded.

With one heater in service, flash tank drain flow will be distributed to both the heater and the condenser until the flash tank drain flow decreases (because of increased flashed steam supplied to the turbine and other sub-loops) to a flow equal to the maximum required by the heater. When this occurs, control valve 68 will be closed and all of the drain flow will be to the heater. This continues to maintain a pressure seal against vapor flow to the condenser assured by lock-out of any signal from flow meter 72 which might tend to cause an opening at valve 68.

Among advantages of the invention is the advantage that the heater of heaters receive the maximum permissible flow for optimum heat recovery, whether one or all heaters are in service, bleeding to a heat sump only the amount of flow necessary to protect the heaters. At the same time, the bleed flow to the heat sump is controlled to maintain a desired liquid level in the flash tank to pressure seal the tank against loss of steam to the sump, and for effective flash tank performance.

Although the invention has been described with respect to specific embodiments, variations within the scope of the following claims will be apparent to those skilled in the art.

What is claimed is:

1. A method for starting up a once-through vapor generator of the type comprising a main flow path including heating surfaces, condenser means and feedwater heater means in series, a bypass line between said heating surfaces and said condenser and feedwater heater means, including flash tank means and drain conduit means between said flash tank means and condenser and feedwater heater means, comprising the steps of:

controlling the flow in said drain conduit means to said condenser means in response to maximum and minimum levels in said flash tank means; and

at levels intermediate said maximum and minimum levels controlling the flow in said drain conduit means to said condenser means in response to flow to said feedwater heater means.

2. A method according to claim 1 wherein said feedwater heater means includes at least two high pressure heaters in parallel, said method including the step of controlling the flow to said condenser means in response to three set-point levels of flow to said feedwater heater means, one being for no flow to said feedwater heater means, the second for flow only to one of said heaters, and the third for flow to both of said heaters.

3. A method according to claim 1 wherein the flow to said feedwater heater means is substantially the maximum permissible for optimum heat recovery.

4. A method according to claim 1 wherein the control in response to flow to said feedwater heater means is locked-out when level in the flash tank means drops below minimum to avoid control of the flow to said condenser means in response to a high vapor flow to said feedwater heatermeans.

5, A method according to claim 1 wherein said gen erator includes a deaerator in said main flow path, the method further including the steps of pegging the deaeratorliwith auxiliary steam very early in the start-up period an flowing all the flash tank drain flow to the condenser means until the temperature of the fluid in the flash tank means reaches a predetermined level suflicient for flashing of the fluid in said feedwater heater means.

6. A method according to claim 5 wherein said temperature is about 200 F. and the pressure in the by-pass line is about 1100 p.s.i.

7. A method for starting up a once-through vapor generator of the type comprising a main flow path including heating surfaces, condenser means, deaerator means and feedwater heater means in series, a by-pass line between said heating surfaces and said condenser and feedwater heater means including flash tank means and drain conduit means between said flash tank means and condenser and feedwater heater means, comprising the steps of:

initially in the start-up period flowing all the flash tank drain flow to the condenser means until the temperature of the fluid in the flash tank means reaches a predetermined level sufficient for flashing of the fluid in said feedwater heater means;

thereafter flowing the maximum permissible drain flow to said feedwater heater means for optimum heat recovery depending upon the feedwater heater means design limits;

controlling the flow to said condenser means in response to a flow signal proportional to the flow to said feedwater heater means as long as the liquid level in the flask tank means remains between maximum and minimum points;

at levels below said minimum point locking-out said flow signal from control of the drain flow to said condenser means; and

at levels above said maximum point controlling the drain flow to said condenser means to maintain the flash tank level at or below said maximum point.

8. A method according to claim 7 wherein the feedwater heater means drain flow is controlled to maintain a predetermined level in the heater means and therefore pressure in the bypass line.

9. A once-through vapor generator start-up system comprising:

a generator main flow path including heating surfaces therein, condenser and feedwater heater means in series flow relationship with said heating surfaces;

a start-up bypass line between said heating surfaces and said condenser and feedwater heater means;

flash tank means in said bypass line including a liquid space and a vapor space;

first and second drain conduit means connecting said flash tank means liquid space to said condensing and feedwater heater means respectively;

flow control valve means in said first conduit means;

the improvement comprising control means for opening and closing said flow control valve means;

first signal means responsive to maximum and minimum liquid levels in said flash tank means;

second signal means responsive to the flow rate in said second conduit means;

both said signal means being operative to actuate said control means, the second signal means being operative when the liquid level in the flash tank is between said maximum and minimum liquid levels.

10. A start-up system according to claim 9 wherein said feedwater heater means includes two feedwater heaters in parallel of the shell and tube type, said second signal means having three set points for alternatively one, two or no heaters in service.

11. A start-up system according to claim 9 wherein said feedwater heater means is of the shell and tube type, further including means to control the liquid level in said feedwater heater means and thereby the pressure in said bypass system.

12. A once-through vapor generator comprising:

a generator main flow path including in series heating surfaces, condenser means following said heating surfaces, and feedwater heating means preceding said heating surfaces, said feedwater heating means being of the shell and tube type;

a start-up bypass line;

means connecting said bypass line to the main flow path at a point intermediate portions of said heating surfaces;

flash tank means in said bypass line including a liquid space and a vapor space;

said bypass line including a first drain connection between said flash tank means and condensing means and a second drain connection between said flash tank means and feedwater heating means;

flow control valve means in said first drain connection;

stop valve means in said second drain connection;

flow controlling means opening and closing said flow control valve means;

means responsive to maximum and minimum liquid levels in said flash tank means transmitting a first signal to said flow controlling means when the liquid level in said flash tank means is above or below maximum and minimum points;

flow rate measuring means responsive to the flow rate in said second drain connection transmitting a signal to said flow controlling means operative when the liquid level in said flash tank means is intermediate said maximum and minimum points;

means to maintain portions of the heating surfaces upstream of said bypass line at a high pressure; and

level control means for said feedwater heating means maintaining the bypass line at a pressure lower than said high pressure.

13. A once-through vapor generator comprising:

a generator main flow path including in series heating surfaces, heat sump means, and feedwater heating means of the shell and tube type;

a start-up bypass line between said heating surfaces and heat sump feedwater heating means;

flash tank means in said bypass line;

flow means to transmit during start-up a drain liquid flow from said flash tankmeans to said feedwater heating means;

said flow means including flow control means controlling a rate of bleed flow from said drain liquid flow to said heat sump means to maintain said drain liquid flow to said feedwater heating means at a maxi mum permissible level dependent upon design limits of the feedwater heating means for optimum heat recovery;

said flow control means also being responsive to maximum and minimum liquid levels in said flash tank to maintain the flash tank liquid between said levels. said levels.

References Cited UNITED STATES PATENTS 3,021,824- 2/1962 Profos 122'406 XR 3,183,896 5/1965 Lytle et al l22406 3,159,145 12/1964 Strohmeyer 122-406 XR 3,286,466 11/1966 Stevens 122-406 XR KENNETH w. SPRAGUE, Primary Examiner

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