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Power plant operators demand improved steam turbine efficiency when retrofitting turbine steam
paths. In response, suppliers have introduced innovative methods for minimizing steam leakage within the units. Other new technologies minimize or help eliminate various forms of damage which, themselves, lead to inefficiencies. These patented devices 1,2,3 provide a means of reducing steam leakage over blade tips and through shaft end packing, and leakage past the stationary blade rows, all of which help reduce losses.
GUARDIAN RING
There are two basic inventions involved in providing these new seals. The first is the "guardian ring" which employs special "guardian strips" inserted into the gland ring carriers. These strips, used in conjunction with the normal knife edge seals, have a minimally smaller radial clearance and contact the rotating shaft preferentially. Thus, they hold the conventional knife edges away from the rotating surface during any operating transients. The guardian strips material is an antigalling, low coefficient of friction steel which, when making initial contact during any start-up, shutdown or transient condition, will cause the spring loaded gland ring to retract from the rotating surfaces, protecting the knife edge teeth from rub damage.
The guardian rings can be installed into existing ring locations. However, it is always recommended that the locator slots be examined for surface condition and concentricity and, if necessary, machined or cleaned to maximize the effectiveness of the sealing system. (This applies regardless of the sealing devices being used-conventional or guardian-to increase, or help maintain their effectiveness.)
At start-up and shutdown, the guardian rings are held in a radial inward position, with "soft" close-coiled springs maintaining the design clearance between the rotating and stationary portions of the unit. At these no-load conditions the steam pressure behind the gland ring is lower than normal. Therefore, significant contact does not occur while passing through critical speeds, and vibration levels do not increase above normal. In the event of a transient, the guardian rings provide protection for the critical knife edged teeth.
VORTEX SHEDDERS
The second invention minimizes steam leakage over the rotating blade tips. The "vortex shedders" reduce leakage by incorporating a means of lowering the pressure at the inlet to each individual seal. Because the quantity of steam which leaks past any labyrinth seal is a function of the pressure ratio across it, lowering the inlet pressure reduces the pressure ratio and, therefore, the leakage. For this reason, steam which would have bypassed the blade rows now flows through them, increasing the power generated and raising stage output. Small deflectors (the vortex shedders) on the first radial seal strip tooth lower the inlet pressure. These deflectors shed vortices upstream from. the inlet side of the seal strip.
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The vortices, high energy (velocity) phenomena, will lower the mean pressure ahead of the strip, thus lowering the pressure ratio across it and reducing leakage flow. Field installations have shown the effectiveness of this device.
At this time it is difficult to quantify exactly the actual leakage reduction attributable to the vortex shedders. The number and form of the strips installed at any particular location is dependent upon a number of variables, including the stage geometry, blade row configuration and the number of elements. One can determine the potential savings by relating the kilowatt savings to the pressure ratio in a typical stage. Figure I shows a typical high pressure stage with steam conditions at the outer diameter and seal dimensions indicated. In this stage the pressure ratio, at original design condition, is 1,725 psia divided by 1,662 psia, or 1.0379.
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Figure 2 shows the reduction in leakage kilowatts for various initial radial clearances if the inlet pressure goes from 1,725 psia to 1,675 psia. These numbers are general, but can be determined for any stage or location with relative ease. Therefore, if this stage had an initial radial clearance of 0.075 in., with a pressure ratio of 1.0379, and the vortex shedders reduced this ratio to 1.02, The leakage kilowatts would reduce from 203.56 to 147.31. This represents a saving of 56.25 kW. (Indications are that a 2.0 percent reduction is particularly conservative.) Figure 3 shows the value of savings per kilowatt as a function of fuel cost, in cents/MMBtu, for various load factors. These calculations assume a station heat rate of 10,000 BTU/kWh.
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EARLY SUCCESS
To date three units in North America have installed the seal systems, considered to be still in the developmental stage. The first is Connecticut Light and Power's Norwalk Harbor station, a 150,000 kW unit that installed the guardian rings throughout all stages, and at shaft end seals. In addition, the total steam path had the vortex shedder tip seals above all rotating rows. At start-up, this unit developed approximately 6,000 kW more than it had on its initial start twenty years earlier. No other steam path efficiency improvements had been made in the unit. After one year of operation this unit continues to operate at the same level with some minor deterioration consistent with, and attributable to, blade deposits. This unit is not base loaded and has, therefore, seen load swings.
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IMPULSE UNIT
The second unit, an 82,500 kW two-section impulse unit at an industrial plant in Louisiana, had always suffered deterioration immediately upon initial start-up after the installation of new gland rings. At its 1998 outage guardian rings were installed throughout the high and low pressure sections. Unfortunately, the layout of the steam path would not permit attachment of the vortex shedder tip seals without significant field modification, and there was insufficient time to complete this modification during the planned outage.
During this outage, the contractor for the rotor work failed to machine the high pressure journal on this unit concentric with the rotor body. Therefore, the unit ran with an out-of-round condition of 0.002 in. at the high pressure bearing. The outage schedule did not allow sufficient time to make corrections to this machining. At restart this unit showed unusual vibration, reaching 0.004 in. at the low pressure bearing.
This unit's single span rotor had a low pressure bearing with a large overhang. There was extensive downward deflection of the rotor due to vacuum pull on the low pressure casing, so the bearing deflected causing contact between the rotor and guardian strips. Also, during the initial start-up, the low pressure fabricated gland carrier, which was weld attached to the low pressure hood, suffered a I-IOC weld failure and collapsed onto the low pressure rotor. The gland carrier was reattached to the casing by spot welding, but the spot welding failed again, with the gland housing again collapsing onto the rotor. The gland carrier was reattached by complete weld reattachment, the seals were realigned, and scraping adjustment of the seal strips completed. Under the high impact loads the guardian strips experienced in these initial runs, four guardian strips had become loose and were removed. The inventor has now developed a new attachment method.
When restarted and taken to maximum load after the second weld failure, this unit developed 85,000 kW with the valves not wide open. At this point boiler fan capacity limited the load. Previous to the modification the unit had reached a maximum load of only 80,000 kW at valves-wide-open conditions. Operators had not expected this increase in capacity. It was almost certainly a consequence of the low pressure glands not rubbing and allowing excessive clearance. The normal glanding material was a bronze, which would have rubbed open easily without causing excessive vibration. The guardian strips are a martensitic steel and could have rubbed, but still would have preserved the integrity of the clearance. This would make the start more sensitive, but preserve the clearance and, therefore, unit efficiency. At the next outage, operators expect to make a small increase in the radial clearance of the upper glanding segments.
THIRD UNIT
The third installation was on a 275,000 kW reaction unit at Tennessee Valley Authority. A redesigned tip seal system above the two rotating blade rows of the Curtis stage includes a larger number of strips, reduced clearance and vortex shedders. This unit also has redesigned vane profiles on the rotating elements of the Curtis stage, improving the profile from an "arc of circle and straight line" shape to one conforming more with aerodynamic principles.
At restart of this unit, operators measured the high pressure section output using calibrated test instrumentation. The high pressure section efficiency improved by 5.6 percent, with heat rate improvement of 20 to 25 BTU/kWh attributed to the seals. The performance tests were conducted over the entire high pressure section, and it was not possible to apportion the total gains to individual improvements. However, the gains were in excess of what was anticipated from the calculable improvements alone. This utility is now preparing to modify a second unit, making the same improvements but extending the use of the new sealing systems.
To date, field experience on both the guardian rings and vortex shedder have been encouraging, and improvements and adjustments on both designs are now being considered. These modifications are intended to extend the gains in both efficiency and output beyond those which have already been experienced. This must be done while minimizing the time it takes to install these components, and without introducing cost increases that cannot be recovered within a reasonable time.
THE LATEST
One of the first units to go into operation with the guardian seal system throughout the high and low pressure sections was opened recently. This unit was experiencing high vibration (.004 in.) and the owner was concerned there was contact at the diaphragm or shaft seals. When opened, it was revealed that there had been significant rubbing contact over the eight months of operation. These rubs were because the L-0 stage diaphragm was misaligned to the extent that both the guardian strips and the seals had worn by up to 0.076 in. (0.076 in. on the guardian strips and 0.071 in. on the conventional seals) on one side (Photo 1). Despite this continuous heavy rubbing, the shaft was not scored, nor was there bend in the rotor. However, there was a light deposit of Nitronic 60 on the shaft surface (Photo 2).
FUTURE DEVELOPMENTS
The designers are considering several design improvements, including:
- the use of vortex shedders as the first tooth in the guardian rings;
- the use of the vortex shedder as the first strip on the stationary blade rows of reaction units; and
- the possible use of vortex shedders attached by welding small deflectors to the normal seal strip as opposed to the current elements where the single, or first strip in the seal group is cut and deformed.
These design improvements could have significant impact. The potential savings at all seal locations could be considerable without a significant increase in manufacturing or installation costs. Likewise, the potential gains at N2 packing is considerable: the losses that occur from leakage past the balance piston of reaction units suggest there could be significant savings in this area.
Authors:
William P. Sanders is a consulting engineer for Turbo-Technic Services Inc. of Aurora, Ontario. He is a licensed professional engineer with more than 40 years experience in the design, manufacture and repair of steam turbines.
Anthony F. Mitola is president of Turbo Parts Inc. of Clifton Park, New York. He has 30 years' experience in the power industry, originally with GE and currently as an independent turbine parts supplier.
References:
1. U.S. Patent 5,599,026. Awarded to W. P. Sanders and A.F. Mitola. "Turbine Seal with Sealing Strips and Rubbing Strips," Feb. 4, 1997.
2 U.S. Patent 5,704,614. Awarded to W P. Sanders and A. E Mitola. "Method of Servicing Turbine Seals," Jan. 6, 1998.
3. U.S. Patent 5,735,667. Awarded to W. P. Sanders and A. F. Mitola. "Vortex Shedding Tip Seal," April 7, 1998.
4. Martin, H. M., "Steam Turbines," Published in The Engineer, London 1913, Pl 610.
5. Shuster, L., telephone conversation with W. P. Sanders, July 3, 1998.
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