CA1202218A - Start-up system for once-through boilers - Google Patents

Abstract

ABSTRACT

A start-up system for once-through boilers including the reduction of minimum flow during start-up to 15% full load by the use of multi-lead internal ribbed tubing to the enclosure tubes in the high heat input zones, the use of a variable pressure by-pass stop valve in the main flow path, a pressure reducing means in the by-pass system and two steam spray attemperators from the flash tank or separators. The first steam attemperation is to the outlet of the secondary superheater and the second to the outlet of the reheater.

Description

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START-UP SYSTEM FOR
ONCE-T~ROUGH BOILERS

~AC~GROUND OF THE rNVENTION

Field of the Invention:

The present inven~ion relates ~o a once-through vapor generator, and in par~icular to a ~o~el start-up bypass system for a once-through vapor generator.
A typical once-through vapor generator, o the type to which the pTesent invention pertains, will include an inlet end and an outlet end, wi~h a plurality of heat transfer surfaces between ~he ends. As a general rule~ these will include an economizer pass, urnace passes defining ~he high temperature ~adiant heat transfer portion of the generator, a reheater, and primary and finishing superheating passes~ ~he ou~let end of the generator being connected to a suitable point of use such as a high pressure steam turbine. The exhaust flow from the turbine or turbines is transmitted to condens-ing meansj a deaerator, and from ~here through heat recovery surfaces to the inlet end of the generator.
During start-up of the vapor generator, the low enthalpy fluid cannot be handled by the high pressure turbine, and or this reason, the generator usually is provided with a bypass system to recirculate the flow until the flow is at the enthalpy level required by the turbine. It is known to trans-mit this flow to heat recovery surfaces where it is passed in heat exchange with the feed ~low to the Yapor generator inlet end. It is also known to position a flash tank or separator in the bypass system designed to separate the flow entering the bypass system in~o vapor and liquid streams and to transmit the vapor stream back to the main flow path for warming and roll;ng the high pressure turbine. Other uses are known for the bypass flow, including turbine gland sealing, and pegging the deaera~or.

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2~ 3 ~ 4399 Depending upon the design of the vapor genera~or, there may be no flow during at least ~he initial s~ages of star~-up through certain surfaces, for ins*ance the reheater surfaces of the generator, and perhaps e~en through primary and finish ing superhea~ing surfaces. These surfaces usuall~ are posi-tioned in lower temper~ture or convectîon heating zones, so that during the initial stages o start-up~ cooling of the surfaces is not necessary. Accordingly, the bypass system usually is connected to the main flow path upstream of at least the finishing superheater surface. This has the advantage that ~he vapor flow returned to t~e main flow path from the ~ypass ~ystem flash tank or separator can be subjected to further heating and superheating for earlier warming and rolling of the high pressure turbine~ reducing the star*-up lS period.-In a once-through boiler~ prior to firin~, sufficien~ -flow is required through the boiler pressure par~s which are exposed to high gas temperatures during start-up. With smooth furnace tubes, a minimum flow at 25 percent of full load flow generally represents a good balance be~ween urnace tube cooling requirements over the load range and furnace enclosure pressure drop. In the low load range, tube fluid mass flow cannot be reduced apprec;ably without ~he possibility of "pseudo film boiling~'l a condition much iike that known as "departure from nucleate boiling" (DNB) at subcritical pressures. Like DNB, ps~udo film boiling represents a sudden deteriora~ion of heat transfer a~ the in~ernal tube surface, which results in unacceptably high levels of tube metal temperatures and must be avoided under all operating conditions.
The multi-lead ribbed tube has found broad acceptance as a tool to prevent ~NB in 2400 psî boilers and has pro~ed very effective in preventing pseudo film boiling at super-critical pressures. Rib~ed tubing use enables reduction of the minimum flow to 15 percent of full load flow of 3500 psi boilers. Lowering the minimum flow means reduced start-up time, reduced heat loss to the condenser~ and reduced auxiliary steam and auxiliary power requirements during start-up. The ; ~3g9 lower m;nimum flow also offers the abili*y to operate the boiler oveT a wider load range, 20 ~o 100 ~, wi~hout going on the bypass sys.tem. This- wider load range can be ~andled for most domestic co~ls wit:hout oil support and wi h S reasona~le net plant heat rates.
On manr units, the high pressure turbine has been sub-jected to ~emperature dips during start-ups when the steam temperature dropped while ramping throttle pressure. The temperature con~rol problems during.star~-up s~em from the practice of ramping superheater pressure to full operating pressure at relatively low loads. W~ile ramping pressure, superheater flow is raised.from approximately 7. to 25~, and s-multaneously7 flow ~s shifted from:-*he flash .tank-to the normal path through khe boiler. All of.this happens a* very l~w loads where iong ~ime lags and-large changes in fluid .
storage and heat storage in ~he koiler preclude effective steam temperature con~rol.
Developments hav0 ~aken place ~o orercome these short-comings; specifically, to reduce minimum start-up flow, simplify and speed up start-up; permit controlled shutdowns~
provide positive control of steam conditions 9 and enable quick load changes over a wide load range. Some of these .developments are; the use of internally ribbed tubes, the . concept of dual pressure operation-(i.e., capabili~y to operate at variable thrott7e pressure while maintaining fluid.pressure in the economizer, boiler enclosure, and primary superheater), capability o ~ariable throttle pressure operation over a wide load range, and use of saturated steam for attemperation of main and reheat steam during star~-up and at ~ow loads.
One problem experienced with conventional bypass systems is that as the vapor generators become larger in size, a~d of much larger capacity, the bypass systems of necessity must ~e designed to handle ever greater quantities of 10w; that 35 , is, the 30~ minimum flow ~ecomes increasingly greater in terms of total mass flow. The flash tanks or separators positivned in the bypass systems also must be capable of

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43~9 handling the increased flows as capacities of the generators increase, and since these flash tanks or separators are heavy walled vessels designed to withstand high pressures, and temperatures, it is apparent that the separators or flash tanks become major items in the capital costs of the generator, particularly in the cost of the bypass system.
It is known, to use a plurality of smaller sized flash tanks OT separator vessels in place of one large very hea~y walled vessel. Howeve~, whether one or seve~al vessels are used, the expense of this part of the system is high and can be out of proportion when compa~ed with the ~emainder of the system.
A further disad~antage experienced with conventional bypass systems concerns switch-over of flow from the bypass system to th~ main flow path at the point in the start-up period when the bypass system is isolated ~rom the flow.
Although the bypass systems and flash tanks sr separato~s can be designed to handle flows up to full operating pressures and temperatures in a once-through vapor generator, which may be in the order of 3,600 psi and about 1,100F., respec-tively, economics (as discussed above) dictate that the bypass system be designed for and utilized up to only about 1,000 psi, at which time or point in the staTt-up period the flow is switched over to the main ~low path. Since the bypass system is positioned upstream of one OT more of the superheating sections, for shorter staTt-up time~ there usually is insuffi-cient surface upstream of the bypass system to produce a ~ully vaporized flow at the normal swi~ch-over pressure of about 1,000~psi, at this point in the start-up period. The result is that the surfaces downstream of the bypass system~ which prior to switch-over, will have received a vapor flow from the flash tank or separator, will now receive a vapor-liquid mixture flow from the upstream surfaces 9 resulting in a tempera-ture drop in the surfaces downstream of the bypass system and an undesirable tlemperatu~e shock to these surfaces.
DESCRIPT_ON OP PRIOR ART
U. S. Patent No. 3,529,580 ~Stevens) describes a once-through vapor generator comprising a main flow path and a

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bypass system which includes a first and second conduit means connectecl to the main flow path, a flash tank means, vapor and liquid return lines from the flow tank, and the second conduit means leading to the heat recovery surfaces and including a valve means therein so as to apportion the flow from said main flow path between the first and second conduit means.
U.S. Patent No. 3,954,087 (Stevens et al) discloses an apparatus and method of start-up of a subcritical and super critical once-through vapor generator This system comprises a plurality of separators which are capable of handling full load flow and auxiliary flow paths, one to the condenser and the other to the main flow path between the condenser and vapor generator.
Other relevant prior art consists of UOS. Pakent Nos.
3,338,053 and 3,338,055 (Gorzegno, et al). Disclosed is a start-up apparatus and methods for a subcritical and super-critical vapor generator. This system is comprised of a pressure reducing means in the main flow path between the vapor generating surface and superheaters, a flash tank means connected between ~he two superh~ater surfaces, and liquid and vapor bypass conduit means from said flash tank means.
In accordance with a preferred embodiment of the present invention, there is provided a once-through vapor generator comprising a main flow path, heating surfaces~ heat recovery surfaces and a bypass system. The bypass system includes a flash tank means sized to handle flow up to at least 25% of full load, a variable superheater bypass stop valve located between the superheating surfaces and sized to handle variable superheating pressure from approximately 15% to full load, and two steam spray attemperators from the flash tank;
one to the outlet of the second superheater and the second to the outlet of a reheater located between a first high pressure turbine and a second low pressure turbine.
It is an object of the present invention to overcome the above problems, and in particular to provide a simplified bypass system capable of avoiding the temperature shock experienced in conventional svstems during switch-over from the byvass system to main path flow.
It is an object of this invention to maintain minimum flow through the boiler furnace parts exposed to high temperature combustion gases during start-up.
It is an object of this invention to provide means of positive control of steam conditions during ~tart-up and shutdown to suit turbine metal requirements.
It is an object of this invention to recover heat during start-up and low load operation. It is an object of this invention to provide for water cleanup during start-up without delays in boiler and turbine warming operations.
It is the final object of this invention to provide means of operating at variable throttle pressure over the load range while maintaining the necessary supercritical pressure in the furnace circuits.
There is provided in accordance with the present invention a once-through vapor generator start-up system comprising a main flow path including, in series ~low relationship, a vapor genera*ing section, a superheating section having a primary and a secondary heating surface, a turbine section, a reheater section, and a condensing section, a collecting and separating section in flow relationship wi.th the main flow path, and a first and a second spray attemperator flow path in flow relationship with the main flow path whereby the first steam spray attemperator flow path is connected from the collecting and separating section to a point downstream of the secondary heating surface and the second steam spray attemperator flow path is connected from the collecting

6 -and separating section to a point downstream of the r~heater section.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic diagram of a once~through boiler start-up system.
FIG. 2 is a schematic diagram of a once-through boiler start-up system with superheater and reheater steam attemperation.
FIG. 3 is a schematic diagram of a once-through boiler start-up system for variable pressure operation.

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DESCRIPTION OF THE PREFERRED EMBODIMENT
Shown in Figure 1 is a schematic diagram of a once-through boiler start-up system for constant pressure unit 10.
Included in the unit 10 is a vapor generating section 12 in parallel flow relationship with a col:Lecting and separating section 14, and in series flow with a superheating section 16, a turbine section 18 and a condensing section 20. It is understood that the connections between these sections, which are shown schematically in the drawings, are achieved by fluid circuitry in the form of tubesr condu:its, risers, headers, etc.
to transfer heat exchange fluid either in a liquid form or a vapor form throughout the various sections.
The vapor generating section 12 consists of an economizer 22 adapted to receive the heat exchange 1uid/ which is preferably water, and pass it to a furnace 24, after which it is passed to the superheating section 16.
The superheating section 16 includes a primary superheater 26 and a secondary superheater 28 which are connected in the vapor circuit in series flow relationship in the vestibule section and the convection section of the vapor generating unit. A full capacity stop valve 30 and a stop by-pass pressure reducing valve 31, in parallel flow with each other, are located in the vapor circuit between the primary and secondary superheater, 26 and 28 respectively. Note that although only one stop or steam flow valve 30 and one stop bypass valve 31 are shown there could be a plurality of each valve. The function of valves 30 and 31 could be combined into one inline valve.
The vapor output from the secondary superheater 28 is adapted to be connected to the turbine section 18 which includes a high pressure turbine 32 and a low~pressure turbine 34. A reheater 36 which is also located in the convection section, is flow connected between the high pressure turbine 32 and the low pressure turbine 34. The turbines 32 and 34 are driven by the vapor from the secondary superheater 28 and ~0~

the reheater 36 respectively and are adapted to drive an electric generator or the like (not shown) in a conventional manner. A drain line 38 is connec:~ed to the vapor circuit between the secondary superheater 28 and the high pressure turbine 32 to enable the ~apor circui~ to be warmed prior to rolling of the high pressure turbine 32 as will be e~plained later.
The outlet fTom the low pressure turbine 34 is connected to the condenser section 20 which includes a condenser 40, a pump 41 and a condensate polishing system 42. The exhaust vapor from the *urbine section 18 is passed to the condenser 40 where it is condensed, then pumped by pumps 41 through the condensate polishing system 42 and then on to a plurality of external low pressure heaters designated by reference numeral 44. A deaerator 46 is connected to the output of the low pressure heaters 44 for ~eceiving the condensate before it is circula~ed, via feed water pump 4g, to hig~ pressure heater S0 to further heat the csndensate before it enters ~he econo-mizer of the vapor generating section 12.
The boiler feed pump 48 supplies the minimum required flow of feedwater during start-up and low load operation as required to the furnace circuitry. The addition of multi-lead internal ribs of the kind well known in the art to the enclosure tubes in the high heat input zones within the con-vection pass permits lower velocity limits, so the same size enclosure tubes can be used, and the minimum flow reduced from 25% to 15%. Since the velocities of the liquid in the tubes at full load are the same as before the addition of the ribs, there is only a slight increase in the pressure drop due to the slightly higher friction factor created by the ribbed tubing. During early start-up, all of the flow goes through thle primary superheater 26 and bypasses the secondary superheater 28 through the bypass system's pressure reducing valve 52 to the 1ash tank 54 where the water-steam mixture is separated. The water flows to a deaerator 46 and a condenser 40. The steam flows from flash tank 54 through ZQæ~

two attemperator lines 56 and 58. The overall water level in the flash tank 54 is controlled by drain valves 60 and 62. The drain valve 60 controls the flow to the deaerator 46 for maximum heat recovery. Excess water above the capacity of the deaerator 46 is discharged to the condenser 40 through drain valve 62. If the flow through drains 60 and 62 is not within water quality limits, all of the flow is through valve 62 to the condenser 40 and a polishing system 42.
The return block valve 64 from the flash tank 54 remains closed until a level is established in the flash tank 54 to assure that water will not enter the steam attemperator lines 56 and 58. Once a level is es-tablished the block valve 64 is opened and then the deaerator steam pegging line 66 from the flash tank 54 can be used to hold pressure in the deaerator 46 (controlled by valve 66). This permits returning all of the excess flow to the condenser 40 through the drain valve 62 during a hot cleanup without using an auxiliary steam source for maintaining deaerator pressure and also serves to recover the heat in the steam during cleanup.

After a predetermined water~steam level is established in the flash tank 54 the bypass return steam valve 68 is opened and the dry steam flows to the secondary superheàter 28. Steam separated in the flash tank 54 in excess of that required is relieved through the steam relief valve 70 to the condenser 40.
The steam relief valve 70 also acts as an over pressure relief valve to avoid popping spring loaded safety valves (not shown) on the flash tank 54. Steam relief valve 70 has an adjustable set point which can be set to hold flash tank 54 pressure at desired levels at particular load points during start-up.

The entire bypass system is sized to handle 25% flow during start-up and to permit up to 25~ load on the flash tank 54. The transition from operation on the flash tank 54 to straigh through flow is made at approximately 25~ load. As the steam entering and leaving the flash tank 54 at this time is dry steam, the .....

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transition from flow through the pressure reducing valve 52 to flow through the steam flo~ valve 30 is accomplished with-out fluctuations in steam temperature. The transition is accomplished by opening the superheater stop valve by-pass valve 31 and the closing of the by-pass valves 52 and 68. As no other transients are occurring at this time, there are no changes in steam temperature and pressure.
Above 25% load the steam flows through the steam flow valve 31 bypassing stop valve 30. The steam flow valve 31 permits variable pressure operation of the superhe~tex section 16 up to full load.
The variable throttle pressure feature is well known in the art and permits operating the unit with the throttle valves almost wide open. This feature eliminates the turbine metal temperature changes resulting from valve throttling and permits rapid load changes without being limited by turbine heating-cooling rates. Shutdown with variable throttle pressure maintains high temperature in the turbine metals and permits rapid ~estarting.
The start-up system shown in Figure 2 includes provision for steam attemperation to the secondary superheater and reheat steam outlet headers, points 57 and 59 respectively, for positive control of the st~am conditions during start-up to meet the turbine metal requirements. The superheater out-let steam attemperator line 56, is used at low loads to introduce saturated steam from the flash tank 54 to the superheater outlet header 57. Initial rolling of the turbine 32 may be started with saturated steam through steam attempera-tor line 56, mixed with a limited quantity of steam passing through the return bypass steam valve 6~ and the secondary superheater 28 to control the high pressure turbine 32 inlet temperature down to about 550F.
The steam return control valve 68 is used to acquire the necessary pressure drop between the flash tank 54 and the secondary superheater outlet 57 for attemperation. The ~z~

reheat outlet steam attemperator :Line 58 is used at low loads to introduce flash tank ste~m to the reheat outlet header 59. The ratio of flow through the attemperator line 58 and the high pressure turbine 32 is limited to 1:1, for turbine consideration mainly to limit windage heating in the high pressure turbine 32.
Figure 3 is a schematic diagram of a power plant with a variable pressure once-through vapor generator unit ll.
Included in the unit 11 is a vapor generating section 12 connected in a series flow relationship with a collecting and separating section 14, a superheating section 16, a turbine section 18, and a condensing section 20~
The collecting and separating section 14 is composed of vertical separators 55 and is connected in series flow with the vapor generation section 12 and the superheating section 16.
The superheating section 16 includes a primary superheater 26 and a secondary superheater 28. A stop valve 30 and a stop bypass valve 31, in parallel flow with each other, are flow connected between the two superheaters.
In the vapor generating unit 11 a pressure reducing valve 52 is connected to the main steam line before the primary superheater 26. Flow through valve 52 is used as the source of steam attemperation, as will be discussed later.
The vapor output from the secondary superheater 28 is adapted to be connected to the turbine section 18 which is shown to include high pressure turbine 32 and low pressure turbine 34. An intermediate pressure turbine 33 can be used if desired and located between the high and lo~ pressure turbines, 32 and 34 respectively. The reheater 36 is flow connected between the high pressure turbine 32 and the inter-mediate pressure turbine 33. The outlet from the low pressure turbine 34 is f:Low connected to the condenser section 20.
The boiler fesdwater pump 48 supplies the minimum required flow of feedwater during start-up and low load operation as required to protect the furnace circuitry. Included in the i ~39g .

circuitry for start~up are an economi~er 22, a urnaee enclosure 24, and a separating vessel 55. The water-steam mixture leaving the fu~nace is separated in the separating vessel 55 during start-up and low load operation.
Drain valves 60 and 62 control the liquid drainage rom the separator 55. Valve 60 contTols the flow to the deaerator 46 for maximum heat recovery. Excess water no~ sent ~o the deaerator 46 is discharged to the condenser 40 thTough valve 62, which controls the water level in the separator 55. If the flows through the drains are not within water quality limits, all of the flow is through Yalve 62 to the condenser 40 and then to polishing system 42.
The dry steam 7eaving the separator 55 flows through the convection pass to the primary and secondary supeTheaters J
26 and 28 respectively. During the initial phase of start-up, the flow through the main steam line drain 38 is used for warm up of the steam lines.
Generally during hot starts, including starts ollowing overnight shutdowns, the gas temperature leaving the furnace 24 has to be kept high to maintain high main steam and reheat temperature. This results in too rapid a rise in thro~tle pressure which is undesirable because of the resulting large throttling temperature drop when admitting steam to the turbine 32. By means of the superheater stop valve 30 and bypass means, the saturation pressure surface can be isolated from the secondary superheater. The overfiring required to raise and maintain steam temperatures can be allowed to raise satura;tion temperature or boiler pressure while maintaining the desirable low pressure entering ~he high pressure turbine 32 and in the secondary superheater 2~.
When reaching the maximum desired boiler pressure or when starting up with the superheater stop valve 30 open, the superheater by-pass system to the condenser 40 (or low pressure auxiliary steam system) provides the means to con~rol or limit z~

boiler pressure durin~ hot start conditions. During the transient loading period for the unit following a hot restart~
the superheater by-pass valve 70 may be opened to permit higher firing rates and thus sustain rated steam temperature until -the boiler control load is reached. Thus the by-pass and control system has the capability to continuously maintain desired steam temperatures during the transient time of loading the unit 11 from start-up to full load following a hot restartO

Before steam is taken to the turbine 32, the reheater 36 is without flow. The reheater metal absorbs heat from the flue gas and eventually reaches the flue gas temperature whlch can be as high as 1000F (538C). When steam is first admitted to the turbine 32 and thereafter passing through the reheater 36, reheat outlet steam temperature rises very rapidly to the gas temperature level, resulting in a poor match with turbine metal temperatures for cold starts.

Reheat steam attemperation with saturated steam from the separator 55 permits reducing reheat outlet steam temperature when steam is first admitted to the turbine 32, and offers positive reheat steam temperature control at low loads. Control of the saturated steam flow to the reheater outlet attemperator is provided by line 5~. The ratio of flow through the attemperator line 58 and the high pressure turbine 32 is limited to 1:1 for turbine considerations.

For cold starts andstarts following weekend shutdowns, the requirements of low steam temperature for a temperature match with the metal component and high heat input for a quick start-up, are not compatible. With low steam flows, the main steam temperature approaches the gas temperature in the vicinity of the secondary superheater outlet 57. Therefore -to keep the steam temperature low, the gas temperature must be kept low, however the heat input must be high enough to generate sufficient steam for rolling and initial loading of the turbine section 18.

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-- 1'1 --The superheater outlet steam attemperator 56 utilizing steam from the separator 55, overcomes this problem by permitting steam temperature control independent of heat input to the vapor generating section 12. In order to by-pass saturated steam from the vapor generating section 12 to the secondary superheater outlet 57, it is necessary to use stop valve 30 and stop by-pass valve 31 between the primary and secondary superheater, 26 and 28 respectively, -to provide flow resistance and to control flow through the secondary superheater 28.

The superheater outlet steam attemperator line 56 is used at low loads to introduce saturated steam from the separator 55 to the superheatex outlet 57 for rolling the turbine 32. Initial rolling of the turbine 32 may be started with saturated steam through the superheater outlet steam attemperator line 56, mixed with a limited quantity of steam passing through the stop by-pass valve 31 and through the secondary superheater 28 to control the high pressure turbine 32 inlet temperature to the desired value.

While in accordance with the provisions of the statutes there is illustrated and described herein a specific embodiment of the invention and those skilled in the art will understand that changes may be made in the form of the invention covered by the claims, and that certain features of the invention may sometimes be used to advantage without a corresponding use of the other features.

Claims (6)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY
OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A once-through vapor generator start-up system comprising;
a main flow path including, in series flow relationship, a vapor generating section, a superheating section having a primary and a secondary heating surface, a turbine section, a reheater section, and a condensing section, a collecting and separating section in flow relationship with the main flow path, and a first and a second steam spray attemperator flow path in flow relationship with the main flow path whereby the first steam spray attemperator flow path is connected from the collecting and separating section to a point downstream of the secondary heating surface and the second steam spray attemperator flow path is connected from the collecting and separating section to a point downstream of the reheater section.

2. The start-up system according to claim 1 wherein the vapor generator is of the constant pressure type and the collecting and separating section is flow connected downstream of the first heating section in a parallel flow relationship with the main flow path.

3. The start-up system according to claim 2 further comprising a pressure reducing means flow connected between the main flow path and the collecting and separating section.

4. The start-up system according to claim 1 wherein the vapor generator is of the variable pressure type and the collecting and separating section is flow connected upstream of the superheating section in a series flow relationship with the main flow path.

5. The start-up system according to claim 4 further comprising a pressure reducing means flow connected between a point downstream of the collecting and separating section and the first and second steam spray attemperator flow paths.

6. The start-up system according to claim 1 further including a variable pressure by-pass stop valve in parallel flow relationship with a full capacity stop valve, the full capacity stop valve in the main flow path between the heating surfaces, the variable pressure by-pass stop valve sized to accommodate variable superheating pressure from at least 25% to 100% full load.

capacity stop valve in the main flow path between the heating surfaces, the variable pressure by-pass stop valve sized to accommodate variable superheating pressure from at least 25% to 100% full load.