US3518830A - Vapor heated tube and shell heat exchanger system and method of purging - Google Patents

Description

y 1970 P. w. VISCOVICH ETAL 3,518,830

VAPOR HEATED TUBE AND SHELL HEAT EXCHANGER SYSTEM AND METHOD OF PURGING Filed Oct. 17. 1968 INVENTORS Poul W.Viscov|ch 8 Allan A. Dunnovoni United States Patent 3,518,830 VAPOR HEATED TUBE AND SHELL HEAT EXCHANGER SYSTEM AND METHOD OF PURGING Paul W. Viscovich, Swedesboro, N.J., and Allan A.

Dunnavant, Wallingford, Pa., assignors to Westinghouse Electric Corporation, Pittsburgh, Pa., a corporation of Pennsylvania Filed Oct. 17, 1968, Ser. No. 768,454 Int. Cl. F01k 7/16 US. Cl. 6073 Claims ABSTRACT OF THE DISCLOSURE A system comprising a vapor heated tube and shell heat exchanger for heating steam to a higher temperature is provided with a novel system for purging air from the heat exchanger tubes upon initiating operation. The system is operable by a novel method which includes venting the tubes to a region below atmospheric pressure to remove most of the air from the tubes, preheating the tubes externally in a uniform manner by the steam to be reheated, and then completing the purging by passing highly heated steam through the tubes at a low rate so as to reduce the tendency of the tubes to warp or buckle and to prevent damage to the tubes and/or the tube supports.

BACKGROUND OF THE INVENTION type to reheat pressurized wet steam to a higher temperature and superheated state. Reheaters of this type are well known and comprise a large vessel defining a chamber to which the steam to be reheated is directed and having a bundle of U-tubes disposed therein for the heating steam. The thus heated tubes reheat the saturated steam to a higher temperature to provide superheated steam.

Usually a moisture separator is incorporated in the reheater to remove the moisture from the wet steam before reheating.

Steam turbine power plants, and especially large electric generating power plans, employ a reheater between the high pressure turbine and a lower pressure turbine to reheat the motive steam after partial expansion to remove moisture from the steam and superheat the steam before admission to the lower pressure turbine, thereby to minimize erosion damage to the blades of the lower pressure turbine that would otherwise be incurred by impact of high velocity moisture droplets on the blades and to increase the thermal efficiency of the turbine cycle.

The tube bundle is preferably provided with U-tubes connected at their open ends to a tube sheet so that, as they are heated by the heating steam, they may expand freely in longitudinal direction within the steam chamber. The tubes are supported against sagging by perforated support sheets and slide to and fro through the tube sheets during elongation incident to heating and contraction due to cooling.

It has been noted that a U-tube of the above type will initially expand progressively as flow of heating steam within the tube condenses and creates a travelling heat front. This progressive heating will permit the first or incoming leg of the tube to elongate freely and cause warping of the second or outgoing leg at the turnaround or U-bend with attendant seizing in the tube support sheets. This constrained condition of the cooler (and therefore shorter) outgoing leg results in permanent dam- Patented July 7, 1970 "ice age to the tubes and support sheets in the form of serious distortion.

BRIEF SUMMARY OF THE INVENTION In accordance with the invention, there is provided a system for reheating steam comprising a moisture separator reheater structure having a shell, a bundle of U-tubes disposed in a chamber formed by the shell, an inlet for directing steam to be reheated into the chamber in heat exchange relation with the tubes, an outlet for the reheated steam, an inlet for directing heating steam to the U-tubes and an outlet for directing the vitiated heating steam and condensate from the tubes.

The tubes are supported at their open ends by a tube sheet that jointly with an enclosing channel head forms an inlet chamber and an outlet chamber for the heating steam. The tubes are also supported along their length by spaced support sheets.

A heating steam supply conduit is connected to the inlet and is provided with a reheat temperature control valve and a bypass valve means in parallel with each other.

A drain conduit is connected to the outlet and a purge conduit is connected to the channel head in fluid communication with the outlet chamber. The purge conduit is provided with a normally closed purge valve and is directed to a suitable region of reduced pressure such as a steam condenser.

When the above system is shut down, for any reason, by closing the control valve and the bypass valve in the heating steam supply conduit and interrupting the supply of steam to be reheated, and the system is allowed to cool down, air and/or other incondensible gases will collect in the U-tubes. These incondensible gases impede the normal flow of heating steam through the tubes upon initiation of operation of the reheater system, with deleterious and undesirable results, as previously noted.

To purge the tubes of air, above, and to allow the tubes to heat uniformly so as to effect matching linear expansion of the incoming and outgoing legs of the tubes, the following method is employed to initiate operation of the system.

l) The normally closed purge valve is opened, thereby venting the tubes and lowering their internal pressure, to effect evacuation of the air in the tubes.

(2) Flow of steam to be reheated is initiated in the reheater, causing this steam to preheat the tubes externally in a uniform manner through out their entire length.

(3) Heating steam flow is then initiated through the tubes at a low rate by opening the bypass valve, displacing the air, to completely evacuate the tubes of any residual air.

(4) After the above steps have been completed, the system is placed in normal operation by opening the control valve to initiate full flow of heating steam to the tubes, and closing the bypass valve and purge valve.

The introduction of heating steam at full pressure will then result in full heat exchange of both legs of the tubes with continuous condensation of the heating steam in both legs at the same temperature, so that both legs of the tubes will expand in unison.

After the system is brought up to the normal operating range, as outlined above, if the flow of steam to be heated is interrupted for any reason, the tubes are available for immediate use as long as the interior of the tubes is maintained in a fully pressurized state by the heating steam.

THE DRAWING The invention, and the objects and advantages thereof, will be better understood upon consideration of the fol- 3 lowing detailed description read in connection with the accompanying drawing in which:

The sole figure is a schematic diagram showing the invention in a steam turbine power plant with steam reheating between turbine units and regenerative feedwater heating.

PREFERRED EMBODIMENT Referring to the drawing in detail, there is shown a turbine power plant of the vapor type, for example steam, employing a high pressure (H.P.) turbine and a lower pressure (L.P.) turbine 12 drivingly connected to a suitable load such as an electric generator 13. Hot pressurized motive steam from a suitable steam generator 15 is directed to the H.P. turbine 10 by suitable conduit structure 16, 17, wherein it is partially expanded to a lower pressure and temperature and thence directed to the L.P. turbine 12 for further expansion by suitable H.P. exhaust conduit structure 19 and L.P. inlet conduit structure 20.

Interposed between the H.P. exhaust and the L.P. inlet conduits, is a moisture separator and reheater structure 22 through which the exhaust steam from the H.P. turbine is directed in a manner to remove moisture and to reheat the steam before admission to the L.P. turbine 12, as generally known in the art and as will later be explained in detail.

The turbine power plant is of the closed loop type. Accordingly, the vitiated steam exhausting from the L.P. turbine 12 is condensed at sub-atmospheric pressure in a suitable steam condenser 24 and then returned as feedwater to the steam generator 15 via a regenerative feedwater heating circuit including conduit 25, a tube and shell type low pressure (L.P.) feedwater heater 26, conduit 27, a tube and shell type high pressure (H.P.) feedwater heater 28 and conduit 29. The shell side of the L.P. feedwater heater 26 is heated by steam conducted thereto by a conduit 31 and extracted from the L.P. turbine 12, after partial expansion therein; and, in a similar manner, the shell side of the H.P. feedwater heater 28 is heated by steam conducted thereto by a conduit 32 and extracted from the H.P. turbine 10, after partial expan sion therein. Thus, the cool condensate from the condenser 24 is initially heated in the tubes of L.P. feedwater heater 26 and then heated to a higher temperature in the tubes of the H.P. feedwater heater 28 before admission to the steam generator 15 and completing the loop.

The moisture separator/reheater structure 22 comprises an outer tubular shell structure 33 having an inlet fitting 34 connected to the H.P. exhaust conduit 19, an outlet 0 fitting 35 connected to the L.P. inlet conduit and means including a moisture separator 36 (for example a blanket of fine filamentary metal material in matted random form) separating the shell into a lower chamber 38 and an upper chamber 39. The inlet fitting 34 is in direct communication with the lower chamber 38 and thus receives the moisture-laden steam, while the outlet fitting is in direct communication with the upper chamber 39. The lower chamber 38 is provided with a drain fitting 40 connected to the shell side of the L.P. feedwater heater by a conduit 40a.

Associated with the upper chamber 39 is a reheater structure, generally designated 41, including a bundle of U-tubes 42 (only one shown) extending longitudinally across the upper chamber 39 and having their adjacent open ends received in a suitable tube sheet 43. The tube sheet 43 is encompassed by a channel head 44 having an internal partition 45 and jointly therewith forms an inlet chamber 46 and an outlet chamber 47. Since the tubes 42 are relatively long and are supported at one end (in cantilever beam fashion) a plurality of tube support sheets 48 are disposed at spaced intervals along the length of the tubes to provide additional support against sagging or buckling. As well known in the art, the support sheets are provided with suitable openings through which the incoming tube leg portions 42a and the outgoing tube leg portions 42b extend in slidable relation, to permit expansion and contraction in operation.

High pressure, high temperature steam from the steam generator 15 is directed through an inlet fitting 50 into the inlet chamber 46 of the reheater by conduit structure 49. The conduit structure 49 includes two parallel conduits 51 and 52 having valves 53 and 54, respectively disposed therein. Valve 53 is a control valve of large mass flow capacity movable from a fully on position to a fully ofi position and is further identified as a reheat temperature control valve. Valve 54, on the other hand, is a bypass valve of smaller mass fiow capacity and is employed in conjunction with a suitable metering orifice plate 55. If desired, however, the orifice plate 55 may be omitted and the valve 55 may be of the adjustable or metering type.

The outlet chamber 47 of the reheater is provided with three outlet fittings 56, 57 and 58. The outlet fittings 56 is connected to the shell side of the H.P. feedwater heater 28 by a conduit 60 having a suitable orifice plate 60a disposed therein.

The outlet fitting 58 is connected to a suitable drain receiver vessel 61 by a conduit 62. The drain receiver 61 is provided with a drain outlet 63 connected to the shell side of the H.P. feedwater heater 28 by a drain conduit 64 and is arranged to maintain a liquid level 65 of condensate therein in operation. The liquid level is controlled by a drain valve 66 disposed in the drain conduit 64 and regulated by a suitable liquid level controller 67. The drain receiver vessel 61 is connected to the outlet fitting 57 of the reheater 40 by a conduit 71 and is also connected 'to a suitable region of below atmospheric pressure, such as the steam condenser 24, by a parallel conduit 69 having a normally closed purge valve 70 to be later described.

Under steady state operating conditions, the reheat control valve 53 is in its open position, the bypass valve 54 is in the closed position and the purge valve 70 is in the closed position. Steam generated in the steam generator is directed by conduits 16 and 17 to the H.P. turbine 10 to motivate the same, and after expansion therein is directed to the lower chamber 38 of the moisture separator/ reheater 22, as moisture-laden steam, by conduit 19. The moisture-laden steam flows from the lower chamber 38 to the upper chamber 39 as moisture-free saturated steam, after passing through and being subjected to the moisture separating function of the moisture separator 36. The moisture removed from the moisture-laden steam by the separator 36 is collected at the bottom of the lower chamber 38 and is withdrawn therefrom and directed to the L.P. feedwater heater 26 by the conduit 40a.

In addition, steam from the generator 15, reheat control valve 53 and conduit 51 is directed by conduits 16, 49 into the inlet chamber 46 of the reheater 41. From the inlet chamber the steam is directed through the U-tubes 42 to heat the same and the condensate formed during the resulating heat exchange with the moisture free steam in the upper chamber 39 is directed through the outlet chamber 47 to the H.P. feedwater heater 28 by way of conduit 60 and/ or by way of conduit 62, drain receiver 61 and conduit 64.

The moisture free saturated steam, thus reheated to a superheated state by the reheater 41, is directed through the outlet 35 and conduit 20 for further expansion in the L.P. turbine, and is then directed to and condensed in the steam condenser 24. The resulting condensate is then returned to the steam generator 15 after partial reheating in the feedwater heaters 26 and 28.

In the event of a shutdown of the system, the steam supply to the H.P. turbine 10 is terminated in any suitable manner (not shown) and to the reheater 41 by closing the control valve 53. As the system cools down, it is inevitable that air and other incondensible gases will collect in various parts of the system including the reheater U-tubes 42, so that, on start up the problems previously described are present.

In accordance with the invention, on start up, before the motive steam supply to the H.P. turbine and the reheater 41 is initiated, the system is operated in the following manner:

(1) The normally closed purge valve 70 is opened, thereby venting the U-tubes to the condenser 24 by way of the outlet fitting 57, conduit 71, purge valve 70 and conduit 69. Since the condenser 24 is maintained at subatmospheric pressure, the internal pressure within the tubes 42 is lowered, thereby venting the tubes to eifect evacuation of the air in the tubes to the condenser.

(2) Flow of motive steam to the H.P. turbine from the steam generator is initiated, thereby initiating flow of moisture laden steam from the H.P. turbine to the moisture separator/ heater 2-2. As this steam flows through the separator 36 with attendant moisture removal, the moisture free steam flows past and around the U-tubes 42 in the upper chamber 39 and is effective to preheat the tubes externally in a uniform manner through their entire length, and the incoming and outgoing legs 42a and 42b, respectively, are free to elongate without buckling or jamming at the tube support sheets 48. This heating of the tubes is further effective, because of the natural temperature of the steam, to heat the walls of the tubes to a temperature greater than the dew point temperature corresponding to the reduced pressure within the tubes. Accordingly any condensate within the tubes is vaporized and evacuated to the condenser leaving the tube interior in a dry warm state.

(3) Heating steam flow through the reheater 41 is then initiated, by opening the bypass valve 54. This flow is a low metered rate, as determined by the orifice 55, suflicient to purge the tubes, i.e. displace and drive any residual air from the tubes to the condenser. The heating steam upon entering the reduced pressure ambient of the tubes will be at a saturation temperature corresponding to reduced pressure within the tubes and is effective to heat the tubes to a temperature greater than that of the steam surrounding the tubes, thereby initiating heat exchange to the surrounding steam. When the above is attained, the atmosphere Within the tubes will stabilize at a partial pressure that is wholly that of the purging steam.

(4) After the U-tubes are vented, heated and purged, as outlined above, the reheat control valve 53 is opened to direct heating steam at the full flow rate to the U-tubes 42, thereby to initiate the heating function of the reheater '41. The bypass valve 54- and the venting valve 70 may then be closed. As full heat exchange occurs, condensation of the heating steam will occur substantially uniformly throughout the lengths of the incoming and outgoing U-tube legs 42a and 4212, thereby obviating unequal expansion of the legs with their former buckling and jamming at the tube support sheets 48. The condensate thus formed is directed from the outlet chamber 47 to the drain receiver 61. The internal pressure of the drain receiver is maintained at the same value as the internal pressure of the outlet chamber 47 of the reheater by the pressure equalizing elfect of the conduits 69 and 71. Also, the liquid level 65 within the drain receiver is maintained within operational limits by the controlling effect of the liquid level control 67, which control is elfective to regulate the rate of flow from the drain receiver 61 to the LP. feedwater heater 28 by regulating the valve 66.

If the turbines 10 and 12 are shut down for any reason such as loss of load on the generator 13, so that flow of expanded steam from the H.P. turbine 10 is interrupted and heating steam flow to the reheater 41 through the reheat control valve 53 is interrupted due to closure of the valve incident to shut-down, the bypass valve 52 may be opened to maintain the tubes 42 of the reheater 41 in a hot and fully pressurized state. Accordingly, the moisture separator/reheater device 22 is maintained in condition for immediate use when the power plant is subsequently restored to operation, without requiring venting and purging of the tubes, as previously described.

We claim:

1. In a steam turbine power plant comprising:

a first turbine motivated by steam at a first pressure,

a second turbine motivated by steam at a second pressure exhausted by said first turbine,

a reheater interposed between said first and second turbines for reheating the steam from the first turbine before admission to the second turbine,

said reheater comprising a vessel defining a chamber for the steam to be reheated and a bundle of heat exchange tubes disposed in said chamber, first conduit means for admitting heating steam to said tube bundle thereby to heat the steam in said chamber, and means for withdrawing condensate from said tube bundle attained during the resulting heat exchange; the improvement comprising means for purging air from said tubes, said means including a second conduit providing a fluid communication between said tubes and a region of lower than atmospheric pressure, and a purge valve disposed in said conduit and movable to an open position to permit said purging and to a closed position to terminate such purging. 2. The apparatus recited in claim 1, and further including:

valve means in said first conduit for interrupting the supply of heating steam when the purge valve is in an oepn position, and means for admitting steam to be heated to the chamber when the purge valve is in the open position, thereby to heat the tubes externally to a first temperature in a uniform manner and enhance the purging of air from the tubes. 3. The apparatus recited in claim 2 wherein: the valve means in the first conduit include a control valve and a bypass valve disposed in parallel flow relative with each other, and said control valve is movable to a closed position and said bypass valve is movable to an open position to admit heating steam to the tubes at a low rate to preheat the tubes uniformly to a temperature higher than said first temperature. 4. The apparatus recited in claim 1, wherein: the power plane further includes a condenser for condensing the steam exhausted in the second turbine, a steam generator for employing the condensate from said condenser as feedwater to generate steam for the first turbine and a feed-Water heater for heating the condensate before delivery to said steam generator, and the improvement further comprises:

valve means in said first conduit for interrupting the supply of heating steam when the purge valve is in an open position, means for admitting steam to be heated to the chamber when the purge valve is in the open position, thereby to heat the tubes externally to a first temperature in a uniform manner and enhance the purging of the air from the tubes, the valve means in the first conduit include:

a reheat control valve and a flow metering strutcure disposed in parallel flow relative with each other,

said control valve is movable to a closed position and said metering structure is movable to an open position to admit heating steam to the tubes at a low metered rate to preheat the tubes uniformly to a tem perature higher than said first temperature, and

means for directing the condensed steam from the tubes to the feedwater heater to impart additional heat to the feedwater before delivery to the steam generator.

3,518,830 7 8 5. The apparatus recited in claim 4, wherein: References Cited the control valve is movable to an open position to ad- UNITED STATES PATENTS mit heating steam to the tubes after preheating, to

reheat the steam to be reheated before admission to 1,769,472 7/1930 Spear 60 73 the second turbine, with attendant condensation of 5 2191879 8 12/1959 Schrqder 60 '73 the heating steam in the resulting heat exchange, 3,030,779 4/1962 Hrymszak et 60 73 X a gi g fg jg ig gig the condensate imme- CARROLL B. DORITY, JR., Prlmary Examiner means for directing the condensate to the feedwater US. Cl. X.R.

heater to impart additional heat to the feedwater. 60107, 67

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