EP2141411B1

EP2141411B1 - Header distributor for two-phase flow in a single pass evaporator

Info

Publication number: EP2141411B1
Application number: EP08447033.5A
Authority: EP
Inventors: Christian Fraikin
Original assignee: Cockerill Maintenance and Ingenierie SA
Current assignee: John Cockerill SA
Priority date: 2008-06-30
Filing date: 2008-06-30
Publication date: 2013-08-21
Anticipated expiration: 2028-06-30
Also published as: PL2141411T3; EP2141411A1

Description

Field of the invention

The present invention is applicable to any steam generator, but more particularly to horizontal once-through heat recovery steam generators, where a two-phases flow has to be equally distributed inside all heating tubes through an inlet header.

Background art

In horizontal heat recovery steam generators, the evaporators bundles are usually formed with multiple parallel tubes arranged in successive rows perpendicular to the gas flow direction. Each row of tubes is connected at its lower side to an inlet header and at its upper side to an outlet header.
Horizontal heat recovery steam generators which are designed with a once-through evaporator may have this heat exchanger split in two sections in series.
In this case, the heating tubes of the first section once-through evaporator are fed with hot water coming from an economizer and discharge water-steam mixture with a partial steam fraction.
That fluid is transferred from the first to the second section by means of manifolds, pipes, distributors and feeders.
The heating tubes of the second section once-through evaporator are then fed with two-phases fluid and discharge superheated steam.
It is very important to assure that the feeding of the two-phases flow is equally distributed over all heating tubes of the second evaporator section in order to achieve an uniform temperature profile of the steam over all heating tubes outlets. If a particular tube receives more water than the average, the steam temperature at its outlet shall be lower than in the other tubes and its mean metal temperature shall also be lower inducing unexpected specific tensile thermal stress in said tube. The opposite behavior (so receiving less water than the average) also induces unexpected specific compressive thermal stress. Depending upon the tube configurations, those thermal stresses could also generate at the tubes-to-headers connections high bending stresses with detrimental effects in respect of lifetime.
In steam generators as described in document WO-A-2006/107315 , once-through evaporator sections are usually provided under the form of multiple single-row header-and-tube assemblies. As in such assemblies, each row of tubes is connected to a linear inlet header with a central water inlet tube, there is a risk that portions of the tubes where steam is created run dry. It may also occur in such a configuration that water is segregated in the outlet sections of the central tubes of the row, while steam is segregated in the outlet sections of the external tubes of the row. This produces higher temperatures in some tube walls and less efficient heat transfer. The higher temperatures require then the use of tubes made of expensive high alloy steels
In document WO-A-03/048638 , heated water flows through the tubes of a once-through section at a rate sufficient to maintain the interiors of its tubes fully wetted while enabling steam to develop in that water. This is possible because the once-through section is followed by a circulation section delivering saturated steam to the superheater, while in a steam generator comprising only a once-through evaporator, no liquid water should leave the evaporator. In this case, as the fraction of steam remains always below 100%, the tubes of the once-through section remain fully wetted.
In document US-A-2004/0069244 , a steam generator has a once-through evaporator which converts liquid water into steam in tubes over which hot gases flow. To overcome the drawback of tubes running dry, each tube contains a metal tape which is twisted into a helical configuration to induce turbulence in the mist produced by the boiling, and these turbulences insure that the mist wets the inside surfaces of the tubes, thus producing good heat transfer and moderate temperatures in the tubes.
Document JP-A-2007/298245 is related to an economizer designed to pass almost evenly liquid water, i.e. a monophasic fluid, through a row of heat transfer pipes.
A similar economizer is shown by document JP 57101296 .
Considering the properties of the two-phases flow, it is very difficult to easily achieve an equal distribution inside all the heating tubes because the water and the steam have highly different densities.
In that respect an appropriate distributor device should be installed in the interconnecting pipes in order to supply an identical steam fraction inside each feeder.
A feeder is connected on an inlet header for feeding several heating tubes. A specific distributor device must be installed either inside the inlet header or in each feeder nozzle to facilitate the equalization of the distribution. Such a distributor device is shown by document FR 1549058A .
Document US 5,806,586 A discloses a device for distributing a two-phase refrigerating medium mass flow in a plate evaporator. The evaporator has a distribution channel at the inlet side which may receive a refrigerating medium mass flow coming from an expansion valve and several mutually spaced exchanger channels which branch off from the distribution channel in a substantially perpendicular direction. In order to ensure a uniform distribution of the refrigerant medium mass flow among the exchanger channels, a porous body is arranged in the distribution channel between the refrigerating medium inlet and the branch-off points of the exchanger channels. The porous body is advantageously arranged in an outer throttle insert which extends over at least part of the length of the distribution channel and in whose wall are located additional throttle openings that lead to the exchanger channels.
Document JP 58 173393 A discloses a distributing header for two (vapour-liquid) phase fluid having guide plates in one set of two arranged in the header at the position opposite to the opening part of the inflowing pipe and provided in curved manner symmetrical with respect to the surface, which contains the axis of the inflowing pipe and at the same time is vertical to the axis of the inflowing pipe and to the axis of the header respectively. The guide plates have their upper ends touching to the header. The two (vapor-liquid) phase fluid flowed through the inflowing pipe in the header proceeds along the guide plates 17 while pushing up the liquid lying in the upper part of the inflowing pipe and smoothly changes the direction of its flow in the direction of the axis of the header. At that time, the liquid is broken into fine parts by the vapor phase component and scattered far away in the form of droplets, resulting in supplying the liquid to all the outflowing pipes. Because a part of each outflowing hole is open above the interface between vapor and liquid, the vapor flows out of all the outflowing holes. Consequently, both the vapor and the liquid are supplied to all the outflowing pipes and flowed out thereof.

Aims of the invention

The main object of the present invention is to drastically improve the distribution equalization of the water-steam flow inside all the heating tubes of the second evaporator section.
More particularly for a once-through evaporator, the invention intends to allow achieving a more uniform steam temperature profile at the outlet of the heating tubes and then to improve the lifetime of the components.
Among other objectives, the invention aims at accommodating with different geometries and dimensions of the heating tubes and the inlet headers.
The invention also intends to easily eliminate the stratification effect existing in the feeder nozzles due to water centrifugation induced by the feeder pipe routing.
A further goal of the invention is to permit its use as a retrofit in existing exchangers in order to restore acceptable operating conditions.

Summary of the invention

A first object of the present invention is related to an evaporator section as defined by independent claim 1.
Preferred embodiments of the invention further contain, in combination with the features of Claim 1, one or several of the features disclosed in the dependent claims.
Still another objet of the present invention concerns, as indicated in Claim 11, a heat recovery steam generator (HRSG) comprising an exhaust gas duct connected to a hot gas source, an economizer, a first section and a second section of a once-through evaporator and a superheater, means comprising manifolds, pipes, distributors and feeder pipes for transferring a water-steam mixture, i.e. a biphasic flow, from said first section to said second section of the evaporator, said distributors being configured for equally spreading the biphasic flow into the feeder pipes, characterized in that said second evaporator section is an evaporator section as in Claim 1.

Short description of the drawings

FIG.1 is a schematic cross-sectional view of a horizontal once-through heat recovery steam generator (HRSG) constructed in accordance with and suitable for embodying the present invention.
FIG.2 is a schematic perspective view of the second section of the evaporator with vertical heating tubes and suitable for embodying the present invention.
FIG.3 is a schematic cross-sectional side view of the inlet header embodying the present invention.
FIG.4 is a fragmentary schematic cross-sectional front view of the inlet header embodying a tubular distributor.
FIG.5 is a fragmentary schematic cross-sectional plan view of the inlet header embodying a tubular distributor.
FIG.6 is a fragmentary schematic cross-sectional plan view of the inlet header and the feeder pipe which could have a non-perpendicular orientation in regards of the inlet header axis.
FIG.7 is a fragmentary schematic cross-sectional front view of the inlet header embodying a conical-type distributor.

Description of a preferred embodiment of the invention

Referring to FIG. 1, a steam generator according to a preferred embodiments of the present invention includes an exhaust gas duct 1 connected to a hot gas source such as a gas turbine. The hot gas flows in series respectively through a superheater 2, a second section 3 of a once-through evaporator, a first section 4 of said once-through evaporator and an economizer 5.
The water flows in the opposite direction and is forced by the feedwater pump 6 to the cold side of the economizer 5. The heat extracted from the hot gas elevates the temperature of the water which leaves the first exchanger hotter than when entering.
The liquid water then flows to the first section of the evaporator 4 which converts part of the water into steam at saturated conditions after further extraction of heat from the hot gas.
The water-steam mixture (i.e. a two-phases fluid) is transferred from said first section 4 to said second section 3 by means of manifolds 7, pipes 8, distributors 9 and feeders 10. The two-phases flow is equally spread into the feeders 10 by means of the distributors 9.
In the second section 3 of the evaporator, the extracted heat from the hot gas terminates the evaporation and slightly superheats the steam.
Finally the heat extracted inside the superheater 2 raises the temperature of the superheated steam up to the live steam conditions needed for powering a steam turbine or any other process.
Referring to FIG.2, the second section 3 of the once-through evaporator includes a plurality of heating tubes 12 arranged along a single row - however a multiple tube rows arrangement is also possible - connected on the inlet header 11 at the lower side and on the outlet header 13 at the upper side.
The inlet header 11 has one or several feeder nozzles 14 onto which the feeders 10 are connected and receiving the two-phases flow from the distributors 9.
The outlet header 13 has one or several connector nozzles 15 for conveying the superheated steam to the superheater 2.
FIG.3 to FIG.7 illustrate preferred embodiments of the device of the invention which is called a "two-phases flow header distributor" for a tubular-type 16 or a conical-type distributor 20 respectively, where said two-phases flow is passing through the feeder nozzle 14 and entering partly inside the inlet header 11.
Referring to FIG.3 to FIG.5 , the tubular distributor is fitted with an end cap 17 in order to eliminate any water jet effect impacting the distribution equalization, especially in the nearby heating tubes 12 and more generally in all other heating tubes.
The distributor 16 is fitted in this example with two opposite main orifices or openings 18 located along the inlet header 11 axis. The purpose is to spread equally the water onto the sides of the inlet header 11 regardless of the possible water stratification in the feeder pipe 10 and feeder nozzle 14 sections. The type, number, dimensions and position of those openings 18 may vary according to the heating tubes- inlet header 11, 12 configuration, to the feeder pipe 10 routing and to the real operating conditions.
Moreover two small holes or orifices 19 are fitted on the distributor end cap 17 along the inlet header 11 axis. The purpose is to allow a direct and controlled water feeding of the nearby heating tubes 12 if they are partially hidden by the distributor according to the arrangement. The type, number, dimensions and position of those openings 19 may also vary according to the heating tubes- inlet header 11, 12 configuration, to the feeder pipe 10 routing and to the real operating conditions.
Referring to FIG.6 , the orientation of the feeder pipe 10 has a direct impact on the water stratification inside the pipe due to the centrifugation effect taking place in the last pipe elbow. The header distributor 16 shall be preferably oriented for having the lateral openings 18 perpendicular to the last elbow in order to force the water film present on the extrados side of the upstream elbow to remix in the pipe end cap before being equally spread through the openings. The type, number, dimensions and position of those openings 18 may also vary according to the heating tubes- inlet header 11, 12 configuration and to the real operating conditions.
Referring to FIG.7 , a conical-type distributor 20 is an alternate design for the same purpose. It consists substantially in an inverted cone 21 passing through the feeder nozzle 14.

Claims

Evaporator section (3), suitable for application in a steam generator, comprising an inlet header (11), an outlet header (13), a plurality of heating tubes (12) connected in parallel at a first end to said inlet header (11) and at a second end to said outlet header (13), at least a feeder pipe (10) terminated by a feeder nozzle (14) connected to said inlet header (11) and a header distributor (16, 20) for conveying a biphasic fluid through the inlet header (11) from the feeder pipe (10), said header distributor (16, 20) being suitable for transversally leading, inside said inlet header (11), the biphasic fluid through a protruding section of said header distributor (16, 20) and comprising means (17, 18, 19, 21) to spread equally the biphasic fluid onto the sides of the intet header (11) and further inside said plurality of heating tubes (12), characterized in that said header distributor (16, 20) is passing through the feeder nozzle (14) at the connection of the feeder pipe (10) to the inlet header (11).
Evaporator section (3) according to Claim 1, characterized in that said header distributor (16) is tubular, and in that the protruding section is provided with an end cap (17) and has at least two diametrically-opposed main orifices (18) located on the lateral surface of the distributor (16).
Evaporator section (3) according to Claim 2, characterized in that said main orifices (18) are partly located in the protruding section of the distributor (16) inside the inlet header (11) and partly inside the feeder nozzle (14).
Evaporator section (3) according to Claim 2, characterized in that said end cap is provided with at least two secondary orifices (19) aligned along the inlet header axis.
Evaporator section (3) according to Claim 4, characterized in that the size of the secondary orifices (19) is smaller than the size of the main orifices (18).
Evaporator section (3) according to Claim 2, characterized in that the line joining the centres of the main orifices (18) is perpendicular to the axis of the feeder pipe (10) at the location the latter is entering the inlet header (11).
Evaporator section (3) according to Claim 1, characterized in that said header distributor (20) is a conical or plug-like distributor, substantially comprising an inverted plug (21) and having a terminal part protruding into the inlet header (11).
Evaporator section (3) according to Claim 1, characterized in that said evaporator is a once-through evaporator.
Evaporator section (3) according to Claim 1, characterized in that the plurality of heating tubes (12) is arranged along a single row of tubes connected to the inlet header (11) and to the outlet header (13).
Evaporator section (3) according to Claim 1, characterized in that said biphasic fluid is a mixture of steam and water.
Heat recovery steam generator comprising an exhaust gas duct (1) connected to a hot gas source, an economizer (5), a first section (4) and a second section (3) of a once-through evaporator and a superheater (2), means comprising manifolds (7), pipes (8), distributors (9) and feeder pipes (10) for transferring a water-steam mixture, i.e. a biphasic flow, from said first section (4) to said second section (3) of the evaporator, said distributors (9) being configured for equally spreading the biphasic flow into the feeder pipes (10), characterized in that said second evaporator section (3) is an evaporator section (3) as in Claim 1.