Retractable brush seals allow sweeping improvements in steam turbine efficiency21 July 2000
Simple modifications can sometimes produce remarkable results. The newest and possibly most significant improvement in shaft sealing technology is the combination of a brush seal, adapted from gas turbine technology, with the proven concept of retractable packing. Mary E Foley, TurboCare, Chicopee, Massachusetts, USA
Significant improvements have been made to the sealing systems of steam turbines in the past decade. Turbocare's Retractable Packing® was successfully introduced in 1986, and is now considered to be a standard in steam turbine upgrades. These improvements have a proven track record in hundreds of steam turbines.
In recent years brush seal technology has been used in the rotating equipment industry. Application of the brush seal in a steam turbine environment presents many new challenges such as holder ellipticity, a segmented application, low flows, large excursions at startup, and long operating cycles where access is prohibited. Retractable packing mitigates these detrimental conditions, providing the longest lived tight effective seal currently achievable. The technology is now field proven, demonstrating the benefits of improved thermodynamic performance through the use of these flexible seals.
Conventional labyrinth packing
Many steam turbines use labyrinth packing to seal between the stationary component and the rotating shaft. These labyrinth shaft seals (packing rings), depending on OEM, are designed to maintain a 0.010in to 0.025in (0.26 to 0.63 mm) radial clearance. The packing rings are held in the closed position by using either coil springs or flat springs during assembly (Figure 1). During operation a combination of spring force and internal steam pressure keeps the seals closed. The packing rings are designed so that as steam flows past each tooth, there is a pressure drop, which is relatively linear in nature. This pressure acts to push the seals away from the rotor, which would result in increased clearance. This opening force is overcome by the closing forces which include the pressure of the steam that naturally leaks to the downflow side of the segment and the force of the springs.
During operation the most likely factor to influence sealing efficiency is for a rub to occur. Such a rub wears the seal and has two detrimental effects. First, it will open the clearance causing an increase in leakage area. Second, the rub will modify the sharp form or shape of the seal, increasing the coefficient of discharge. These effects increase leakage flow and cause a deterioration of steam path efficiency. These rubs will most likely occur during the first run-up after return to service. In fact, the results of a survey presented at the 26th Turbomachinery Symposium indicated that the highest failure mode risk component is rubs of the labyrinth seals (packing rings) that occur during test and start–up.
It is common to initiate rubs at this time since temporary thermal distortion of the stationary component results in deflection of the turbine components, caused by top–bottom temperature differences. Also, as a rotor goes through its natural critical frequency, or speed, vibration amplitudes induced along the rotor will vary. The centre of a long rotor span will deflect the greatest, while deflection at the bearings will be the least. It is common to find that the deflection at the rotor mid span, owing to the vibration at critical speed, will exceed the nominal design shaft seal clearance, initiating a rub. Although great care is taken to properly align a turbine, it is difficult to achieve design alignment for the entire assembly. This is exacerbated when you consider the distortion and age of the stationary parts. Worn out or damaged bearing babbitt over time may also effect alignment that could result in rubs during operation between outages or turnarounds.
The initiation of a packing ring rub at start-up not only increases the design clearance and deforms the sharp efficient seal but also causes uneven heating of the rotor resulting in a bowed rotor, or thermal sag. The shaft seals as well as the blade cover seals are damaged at the rub initiation point and at the adjacent stages. The damage to the blade cover seals, or tip seals is substantial, because of the increased amplitude of deflection at the large diameter.
After rubbing, there is a tendency to seal embrittlement. Under these conditions, relatively light impact from mechanical debris, or even high steam forces can cause teeth to rupture and deteriorate. The leakage area is significantly increased where the material has been lost resulting in further reduction of efficiency. The labyrinth seal’s mating surface, such as the rotor body or the blade covers or shrouds can also be damaged by various mechanisms. Often seal rubs will cause localised heating and consequently hardening making the cover and rotor material more susceptible to corrosive attack from aggressive ions that have found access to the steam path.
The result of a rub that has been initiated by either thermal distortion or vibration as the rotor passes through its critical will of course be a compromise between the tight design shaft seal clearances, and a blunt seal. For example the impact on efficiency in a 500 MW impulse turbine of a 0.020in (0.51mm) rub would result in a loss of approximately 160kW in a single HP stage.
To address the problem of initiating a rub, an improved design would be one that provides a large clearance during the time the rotor has the greatest opportunity to rub the seals, but also provide a tight clearance during normal operation. TurboCare retractable packing (Figures 2, 3) eliminates the springs holding the segments in the closed position during turbine startup. Coil springs are installed in the butts of the segments to force them radially away from the shaft during the startup mode. A large clearance, 0.090in to 0.150in (2.3 to 3.81mm) is available to prevent rub initiation.
The steam slot on the upstream face of each segment of the seal allows for upstream steam pressure to flow behind the packing ring (Figures 4, 5). This steam pressure pushes the segment downstream, sealing it against the steam joint. As the flow through the turbine increases the pressure behind the ring also increases until the closing forces are large enough to overcome all the forces pushing the ring into the open position thus allowing the ring to move to the closed position. The retractable seals are designed to close after the turbine has been evenly heated and has passed through its critical speed, the most common time for the initiation of a rub.
TurboCare retractable packing has been successfully installed into almost 500 units worldwide. Virtually every type of OEM steam turbine that incorporates a labyrinth seal has been retrofitted with retractable packing. Operational experience has been available now for over 10 years; many units have been opened and inspected, two or even three times, and returned to service.
End users have reported that retractable packing has performed well. One user is quoted as saying "(the seal)...has performed remarkably well in the HP-IP turbine over the seven years of service between turbine overhauls. Inspection of the packing ring sealing surfaces conclusively indicates that the packing was in the closed position during the normal operation of the unit. The tight clearances found during the inspection and turbine audit indicates substantial heat rate savings due to the packing design.”
The benefits of retractable packing include increased efficiency, improved output, unit flexibility, or ease of startup, and reduced emissions. Generally no turbine modifications are required to retrofit a unit with labyrinth packing of the TurboCare retractable packing type.
Inspections on numerous units containing TurboCare retractable packing have demonstrated that by reducing the tendency to rub the shaft packing at unit startup, the possibility of bowing the rotor is reduced, and therefore the likelihood of damaging the spill strips. Options are therefore available to improve the original shroud seal design. Depending on the original design of the unit, upgrades include reduction in clearance, tooth redesign, material upgrades and coatings.
Arriving at retractable brush seals
Retractable packing was designed to maintain OEM tight, efficient seal clearances but it would be improved if it could maintain a tighter design clearance, and this has been achieved by the application of a simple idea. The addition of a brush seal to the proven retractable packing has resulted in the first “zero clearance” steam turbine seal that can also tolerate contact during transient rotor excursions (Figure 6). Leakage is limited to the flow that can find its way through the tight maze created by the bristle pack. This new design can dramatically increase unit performance and maintain consistent sealing while optimising component life.
Brush seals have been used in aircraft engines for over ten years. In a Pratt & Whitney 4000 engine, brush seals were compared to the standard labyrinth seals after completing 3000 start–stop endurance cycles. The brush seal radial clearance increased by no more than 0.002in (0.051mm) after the endurance run, and provided an estimated 0.4 per cent improvement in fuel consumption when compared to the original labyrinth seals. These data are quite impressive in the light of the radial magnitude and frequency of the rub cycles to which these seals were subjected.
Over the past few years industrial gas turbines have been adapted to accept brush seals in areas where control of secondary leakage flow can improve power output. Brush seals have been installed in Frame 7, 6, and 9 engines in the high pressure packing area and/or bearing seal locations. Performance improvements of 1.9 per cent in power and 0.7 per cent in heat rate have been reported for brush seals installed in a Frame 7EA engine; the projected seal life is 48 000 hours.
Brush seals are now being successfully installed in both utility and mechanical drive steam turbines. Brush seals are considered a design advantage over the rigid labyrinth packing ring, as the compliant brush can deflect to absorb rotor radial excursions such as those occurring during unit start up. Brush seals provide a tight, rub–tolerant seal that when applied properly, degrades gradually, but minimally, over time. No appreciable permanent loss of performance is experienced after a short term rotor contact. However, long term transient contact between the brush and rotor is not desirable.
Composition of the brush
The brush material is Haynes 25 cobalt based super-alloy. Several thousand extremely fine diameter bristles are packed together, forming a tortuous path for steam flow along the shaft. The bristles are angled in the direction of shaft rotation, preventing them from picking up on the shaft as it rotates. The brush pack is sandwiched between two sideplates or washers, and then welded at the periphery to complete the seal assembly. Sideplate and pack geometries are selected to provide the proper level of seal compliance during excursions. The radial clearance, or “fence height”, between the downstream sideplate and rotor is sized large enough to prevent hard contact, but small enough to support the bristles against the pressure gradient (Figure 7).
TurboCare’s brush seal design specifies sharp “teeth” on the front and back plates, which simulate a typical labyrinth tooth configuration. The brush seal is then inserted into what is essentially a standard retractable seal labyrinth ring (Figure 8). This conservative approach is taken so that in the unlikely event of a brush failure, the remaining labyrinth portion of the seal will maintain the stage sealing.
Adapting to steam
TurboCare has conducted testing to revalidate published brush seal performance parameters, and to adapt the technology to steam turbine applications. Tests were performed with brush seals in conjunction with, and without, labyrinth seals; and in combination with high and low rotor lands. In general, it was found that brush seals are far more effective when installed on a low land, rather than a high land. The addition of labyrinth teeth around a brush seal on a high land does not significantly improve the overall seal leakage. However, adding labyrinth teeth around the brush installed on a low land provided the lowest leakage performance overall.
Testing has also shown that the relative performance of the brush seal is dependent on the operating conditions at its location within the turbine. Brush seals installed with a close clearance to the shaft, (eg conventional labyrinth rings), in low flow conditions, are susceptible to overheating. Also, brush seals in close proximity to the shaft at times when the rotor is travelling through its critical speed, may magnify the amplitude of vibrations at critical. Both conditions, which are common during steam turbine startup, can damage the intended tight clearance of the brush seal and render it ineffective.
Testing has also been performed to identify the optimum wire diameter, bristle pack density, and lay angle of the brush seal. In general, seal wear rate increases with increasing bristle stiffness, rotor speed and excursion depth. Therefore, to maintain the best possible effective seal, excursions of a large magnitude for a prolonged period of time should be avoided.
TurboCare has addressed these design considerations by implementing the brush seal into its patented retractable packing. The retractable feature allows the brush seal, inserted into the packing, to operate at a large clearance at steam turbine startup. This is the period when large radial excursions are experienced, due to thermal distortion and critical speed related vibration.
Brush seals in the field
Retractable Brush Seals™ are currently installed in approximately 20 steam turbines ranging from a 15 MW unit to an 860 MW nuclear turbine.
Tampa Electric Big Bend coal-fired station (Fig 8) retrofitted their 444 MW General Electric G2 type machine with retractable brush seals in the autumn of 1999. This unit is an opposed flow machine with five high-pressure stages and six intermediate pressure stages. The OEM design packing clearance was 0.025in (0.64 mm), but typically has been found worn about 0.025in over design at previous inspections. Retractable brush seals were installed in all stages with 150 psi (1035 kPa) pressure drop or less. The brush seals were sized utilising the actual rotor diameters to achieve a “zero” clearance in the closed position for at-load operation. Tampa Electric reported a 3.6 per cent increase in the turbine output vs steam mass flow figure, most of which they attributed to the effectiveness of the new seals as the unit was in good condition before the retrofit.
Retractable brush seals are designed to fit the existing geometry of the turbine without costly modifications. Other design features that are critical to the success of the seal include location of the seal on the packing segment (to allow properly timed ring closure), adequate rotor land to backplate clearance, distortion, and rotor/holder deviations. Each of the outlined considerations must be carefully considered in the design of each brush seal to ensure a successful application.
Application has so far been limited to 150 psi (1035 kPa) pressure drop per stage, but a new pressure balanced brush seal is slated for installation in the winter of 2000, an improvement that is intended to allow application of the seal to all stages in a high pressure steam turbine.