Turbine interstage sealing arrangement

A gas turbine sealing arrangement which includes a seal housing having at least one seal in close proximity to the turbine rotor. The seal housing is preheated prior to turbine start-up to thermally move the seal housing and its seal radially away from the rotor.

BACKGROUND OF THE INVENTION
 1. Field of the invention
 The invention in general relates to multistage rotating machinery and more
 particularly to an arrangement for protecting the seals normally provided
 between stages and for establishing improved interstage leakage control.
 2. Description of related art
 In various multistage rotating machines used for energy conversion, such as
 turbines, a fluid is used to produce rotational motion. In a turbine
 stage, high pressure-low velocity fluid is expanded through stationary
 nozzles, or vanes, producing a lower pressure-higher velocity jet which is
 directed onto the blades of a rotor assembly causing rotation thereof. The
 turbine is constituted by a plurality of such stages and in each stage the
 kinetic energy of the fluid is converted into rotational kinetic energy of
 the rotor assembly.
 Any fluid leakage between stages reduces turbine performance and efficiency
 and, therefore, annular interstage seals in seal housings, are provided to
 reduce such leakage. In general, flow leakage is reduced when the gap
 between the seal and rotating rotor is minimized. During turbine start-up
 the rotor assembly expands radially and may actually contact the seal,
 causing deformation thereof. In order to prevent this unwanted contact
 from occurring, the seal housing is built a sufficient distance from the
 rotor assembly so as to allow for this initial expansion of the rotor
 assembly. However, at steady state operation, due to the initial
 positioning of the seal housing, the distance between the seal and rotor
 assembly is not optimal, thus reducing turbine performance.
 The present invention provides an arrangement whereby the seals may be
 located closer to the rotor assembly during steady state operation, to
 minimize fluid bypass and to therefore increase overall efficiency.
 SUMMARY OF THE INVENTION
 An interstage sealing arrangement for a multistage turbine is provided and
 includes a seal housing connected to a stationary portion of the turbine.
 The seal housing contains at least one seal, normally in close proximity
 to a rotor assembly of the turbine. A heating means is provided in thermal
 contact with the seal housing and is operable, when supplied with
 electrical energy, to heat the seal housing, causing it to move radially,
 along with its seal, at least prior to the start-up period of the turbine.

DESCRIPTION OF THE PREFERRED EMBODIMENT
 In the drawings, which are not necessarily to scale, like or corresponding
 parts are denoted by like or corresponding reference numerals.
 Although the present invention is applicable to a variety of rotating
 machinery, it will be described with respect to a turbine, more
 particularly, to a gas turbine, a portion of which is illustrated in FIG.
 1.
 FIG. 1 shows two turbine blades 10 and 11 connected to respective rotor
 disks 12 and 13 of a rotor assembly, and each having a respective disk arm
 14 and 15. An annular band 16 mates with disk arms 14 and 15 and serves to
 minimize rotor cooling gas in cavity 17 from passing into disk cavity 18.
 A vane 20 is connected to the stationary portion of the turbine and
 includes a front annular flange 22 and a back annular flange 23, between
 which is positioned a seal housing 24. The seal housing 24 is maintained
 in position between flanges 22 and 23 by means of an adjustable
 spring-loaded locating mechanism 26 which allows the seal housing 24 to
 grow thermally, independently of the vane assembly.
 Seal housing 24 includes at least one seal such as labyrinth seal 30 having
 a plurality of fingers 31 extending toward the disk arms 14 and 15. In
 addition, FIG. 1 also shows another type of seal, a brush seal 34 having
 bristles 35 which can contact the disk arm 15 to aid in minimizing
 upstream to downstream gas flow (from left to right in FIG. 1) through the
 seal.
 Cooling gas is also provided, via internal passages in vanes 20, to annular
 chamber 40 above the seal housing 24 and this gas passes between flanges
 22 and 23 and the seal housing 24 into back disk cavity 18, as well as
 front disk cavity 42. In addition, the cooling gas from chamber 40 is
 directed into the volume to the left of seal 30 by means of gas passageway
 44 in seal housing 24. A portion of this gas also finds its way into disk
 cavity 42 via a knife seal 46.
 The gas in disk cavities 18 and 42 not only helps in cooling the base of
 the turbine blades 10 and 11, but also functions to balance against the
 pressure of hot gas driving the turbine blades, and represented by arrows
 50 and 51. That is, the hot gas is prevented from entering the disk
 cavities 18 and 42 by means of the pressure conditions established. If the
 seals are worn or are otherwise too far away from the rotor assembly, as
 represented by disk arms 14 and 15, then the cooling gas requirements
 would become excessively large, thus reducing the overall efficiency of
 the turbine.
 By way of example, and with additional reference to FIG. 2, there is
 illustrated a start-up cycle for a typical turbine interstage location
 such as illustrated in FIG. 1. Time is plotted on the horizontal axis,
 normalized seal-to-rotor distance (i.e. seal tip-to-disk arm), represented
 by curve 60, is plotted on the left vertical axis and turbine speed,
 represented by curve 61, is plotted on the right vertical axis. The curve
 is plotted for a labyrinth seal, although a brush seal would be similar,
 except that the clearances would be smaller.
 Various speed profiles may be followed and by way of one example, the
 turbine is started at time t0 and increases in speed up to time ti where
 it is held constant from t1 to t2. During the time from t0 to t1 the rotor
 assembly increases in diameter by centrifugal force and so the
 seal-to-rotor distance correspondingly decreases and then levels out for
 time t1 to t2. During a second speed ramp from t2 to t3, the rotor
 continues to grow and a maximum closure (minimum clearance) is experienced
 at time t3. From time t3 and as the turbine reaches steady state speed, at
 time t4, the seal housing is starting to expand due to heating effects,
 thereby increasing the seal-to-rotor spacing.
 At time t5, the seal housing has moved to its maximum radial position
 (normalized to a value of 1) and as the rotor continues to grow by thermal
 action, the spacing between them continues to decrease until a steady
 state condition is reached at time t6 wherein the seal-to-rotor distance
 is approximately 0.75 (compared to a maximum of 1 at time t5 and a minimum
 of about 0.33 at time t3). A much improved sealing function could be
 achieved if this steady state distance between the seal housing and rotor
 were smaller. However, if the initial cold build distance between the seal
 housing and rotor assembly is made smaller, then there is a danger that
 the seal will actually contact the rotor assembly at t3, as the rotor
 assembly initially grows, and be damaged thereby. The present invention
 obviates this potential damage situation and allows for a smaller
 clearance during steady state operation.
 With reference once again to FIG. 1, the present invention, in effect,
 radially moves the seal housing 24, along with its seals 30 and 34, away
 from the rotor assembly prior to turbine start-up. This is accomplished
 with the provision of a heating means which causes thermal expansion, and
 a corresponding radial movement of the seal housing 24. More particularly,
 a heater cable 70 is positioned in thermal contact with the seal housing
 24, and when supplied with electric energy causes the seal housing to grow
 radially.
 In the embodiment of FIG. 1 the heater cable 70 is positioned within a
 circumferential groove 72 machined into the seal housing 24 and held in
 place by means of a thermally conductive adhesive, for example. In order
 to obtain an indication of the temperature condition of the seal housing
 the arrangement includes one or more temperature sensors 74 which may be
 connected to the heater cable 70.
 FIG. 3 illustrates a commercially available heater cable which includes
 heating elements 76 and 77 contained within a magnesium oxide insulation
 78 and surrounded by an alloy sheath 80. Temperature sensor 74 is affixed
 to the outside of sheath 80. The cable is fabricated to a desired
 curvature to match that of groove 72, or if flexible enough, the cable may
 be bent as inserted.
 With a seal housing 24 in two arcuate 180.degree. sections, the heater
 cable 70 may also be in two arcuate 180.degree. sections, 70a and 70b, as
 illustrated in FIG. 4. The heater cable sections 70a and 70b are
 electrically connected to a heater control system 82, which may also
 supply electrical energy to the cable, as further illustrated in FIG. 5,
 to which reference is now made.
 In FIG. 5, a power supply supplies electrical energy to the heater cable
 70, i.e., to the two halves 70a and 70b, via respective thermostats 86a
 and 86b which control respective line switches 88a and 88b.
 The opening and closing of switch 88a is governed by a controller 90a
 having a positive input 91a for receiving, from setpoint control 92a, a
 voltage indicative of a desired heater temperature. A negative input 93a
 receives an indication of present heater temperature via decision circuit
 94a. In the embodiment of FIG. 5 two spaced-apart temperature sensors are
 utilized to generate signals indicative of heater temperature values.
 These signals are provided, via electric leads 95a and 96a, to decision
 circuit 94a, which may then pick the higher value or lower value, if there
 is one, or may output some average of the two signals.
 The difference between the signals applied to inputs 91a and 93a is
 provided to proportional circuit 97a of controller 90a, and when the
 signals are equal or within some predetermined threshold of one another,
 indicating the desired temperature has been reached, the proportional
 circuit 97a will cause switch 88a to open.
 The operation described above with respect to thermostat 86a is equally
 applicable to the operation of thermostat 86b for governing electrical
 energy supplied to heater 70b.
 FIG. 6 illustrates the start-up cycle for the turbine interstage location,
 incorporating the present invention. The axes, as well as the turbine
 speed curve 61 are the same as that shown in FIG. 2. The seal clearance
 curve 99, however is quite different from its counterpart curve 60 in FIG.
 1.
 More particularly, prior to normal turbine start-up which is at time t0,
 the seal heater 70 is energized at some time -t which causes thermal
 expansion of the seal housing 24 and movement, together with its seals,
 away from the rotor assembly so that at actual turbine start-up at time
 t0, the seal clearance is already at a value of about 0.89. It will not
 reach the maximum value of 1, as was the case in FIG. 2, since it is
 initially cold built closer to the rotor. During the first speed ramp from
 t0 to t1 the rotor assembly grows by centrifugal force reducing the
 clearance. The clearance is further reduced during the second speed ramp
 from t2 to t3 and after the turbine has reached steady state speed at time
 t4 thermal growth of the rotor assembly causes steady closure until time
 t6 where steady state clearance is established at around 0.46,
 significantly less than the prior art value of 0.75, shown in FIG. 2. For
 a brush seal, the steady state closure is even greater.
 Thus with the present invention the seal may initially be placed closer to
 the rotor assembly since it is thermally moved away prior to initial
 turbine start-up. If a conventional seal housing was built with an initial
 large seal-to-rotor clearance at time t0, it would thermally grow during
 the start-up process and end up at steady state with an objectionably
 large clearance. Conversely, if the seal housing were initially placed
 closer to the rotor assembly, the seal(s) may experience damage due to
 rotor assembly growth during the start-up process.
 Although the present invention has been described with a certain degree of
 particularity, it is to be understood that various substitutions and
 modifications may be made without departing from the spirit and scope of
 the invention as defined in the appended claims. By way of example, if
 desired, the heater may also be put into operation during a turbine
 shut-down process to move or keep the seals away from the rotor during the
 latter part of such shut-down process.