Wicket gates can stick in their locations during disassembly of a turbine for maintenance or rehabilitation, sometimes causing considerable damage to the unit. Understanding the causes of the situation and methods for dealing with it can help hydro project owners minimize problems.
By Martin A. Bauer and Max E. Spiker
During disassembly of a turbine, wicket gates can “stick” in their locations. In addition to causing considerable damage to the unit, this situation can be dangerous for plant personnel. With many hydro plants undergoing rehabilitation, knowing about the potential for sticky wicket gates and understanding how to deal with the situation can help hydro plant owners minimize problems and avoid costly repairs.
Understanding the wicket gate arrangement
A hydropower unit has two major assemblies — the turbine assembly that uses the flow of water to produce rotational power and the generator assembly that converts this rotational power to electricity. The turbine assembly has several key parts, one of which is the method of controlling the flow of water to the turbine. That control is provided by a set of overlapping vanes, called wicket gates, that are opened to provide water flow to the turbine and closed to shut off this flow of water. The wicket gates usually turn less than a quarter of a circle to control the power output of the generator or power consumption of a pump.
Because of this limited turning radius, wicket gates are prone to problems. However, many of these problems may exist but not be discovered for several years, until either the flow of water cannot be completely controlled by the wicket gates or the turbine is being disassembled. Wicket gates that stick in place are of primary concern when the turbine is being taken apart.
Wicket gates consist of a vane portion and a shaft, which fits into bushing (bearing) surfaces (usually a sleeve made of bronze or brass), on either side of the vane. The position of the gates is controlled by a series of linkages that ensure the same position of all wicket gates in the turbine assembly. The shaft where the linkages connect is longer than the other end. The wicket gate shaft also has a third bushing surface closer to the linkage. If the wicket gates are oriented vertically, the operating linkages generally are located above the vane portion. The linkage is rarely in the water passage.
However, the vane portion of the wicket gate is in the water passage and is subject to water pressure, so a sealing method is employed. The sealing method, called packing, is generally between the two upper bushings and consists of rope-type packing pressed into a recess with a flanged follower. In some cases, a sealing method is used for the bottom bushing.
Finally, the upper two bushings for the wicket gates are installed in a removable plate called the head cover. The head cover supports the turbine bushing, main turbine shaft seal, wicket gates, and linkages. Immediately below the head cover is the turbine. Figure 1 on page 50 shows a cross section of a turbine assembly. Removal of the turbine requires removal of the head cover.
It is in this phase of the unit teardown that sticky wicket gates typically are discovered. The wicket gate stems should remain in their lower bushings when the head cover is lifted. If the gates lift along with the head cover, they can hang up in the scroll case. This can increase the lifting load, damage the lower bushing or wicket gate shaft surface, and cause unequal lifting load when removing the head cover. In addition, the wicket gates could fall out of the head cover and injure plant staff or damage other vital machinery.
Why wicket gates stick
Wicket gates are usually made of steel, and the shafts are subject to corrosion. The shaft above the sealing method also is subject to accumulating dirt. Some of the causes of sticky wicket gates are:
— Corrosion or debris on the wicket gate shafts, caused by leaking wicket gate sealing method or blockage (either partial or full) of the turbine pit drains. The water pressure from the scroll case tends to leak past seals of either or both the main turbine shaft and the wicket gate shaft. Corrosion of the wicket gate shaft due to water is minimized when water that leaks past the seals is allowed to drain away to the plant sump. Another source of debris is old grease. Many wicket gate systems use grease to lubricate the bushing to shaft areas. If this lubricant is allowed to accumulate, it eventually will dry out and harden. Cleaning of the exposed wicket gate shaft before disassembly is helpful.
— Damaged bushing from little or no grease and resulting metal transfer. If the bushing is of the kind that requires grease, the lack of grease can cause the wicket gate shaft and bushing to seize. In this case, considerable force is necessary to push the wicket gate out of the head cover.
— Incorrect bushing alignment. If the wicket bushings were not properly installed, the misalignment can cause the wicket gate shaft to bind in the head cover. Again, considerable force may be necessary to remove the shaft before the head cover is lifted.
Methods to deal with sticking wicket gates
Before removal of the head cover, all wicket gate components need to be removed or loosened. To aid in reassembly, careful match marking and documentation of the sequence of removal are important. The first stage in removing the head cover is to jack the cover using some method other than the plant crane. Many head covers have threaded holes for bolts. The bottom of the hole is the scroll case assembly. By turning the bolt, the head cover is lifted off of the scroll case.
At this time, the wicket gate stems need to be monitored to ensure they are not being lifted with the head cover. If the wicket gate shafts move with the head cover, they need to be pushed back down into lower bushing. If the sealing method is not properly removed, it too can cause the wicket gates to stay with the head cover. One method that has been successful in pressing the wicket gate from the head cover is to use a strong back1 and hydraulic jacks to push the wicket gate down through the head cover. By using existing threaded holes in the head cover, long bolts or threaded rods with a strong back attaching them will allow a hydraulic jack to be placed on the top of the wicket gate stem to push on the strong back and push the gate back down. (See Figure 2 on page 52.) The spacers are used to ensure proper contact between the jack and both the strong back and wicket gate shaft.
Another method is to open the gates so that the edges run past the head cover into the turbine and scroll case. This will block the wicket gates so they cannot move up. Prior to using this method, careful examination of the design of the scroll case and wicket gates is required to ensure that the wicket gates can be opened without doing damage to other parts of the scroll case assembly. A typical limitation is when water passage in the scroll case is smaller than the corresponding dimension of the wicket gate. When lifting the head cover, the gates need to be monitored to ensure the shafts are not bent by the excessive deflection as the head cover is lifted. This method of removal usually is reserved for situations where plant personnel know that the shafts are free.
As the wicket gates are pressed out of the head cover, the upper bushing may pull free from the head cover. In this case, the bushing should be considered an expendable component.
In our experience, the bushings are rarely reusable if the generator has not experienced a turbine assembly teardown in more than ten years. This period of time depends on the disassembly, water quality, maintenance, and operational history. Usually, examination and measurement of the wicket gate assembly tolerances provides a good indication of the need to replace worn wicket gate bushings.
Advice for others
Before performing a project of this significance, there are several considerations that should be taken in account.
First, provide adequate time to perform the job well. We have found that problems encountered during teardown were well-known but were not resolved previously due to a false sense of urgency. In this situation, the time required to correct the original problem is not avoided. It will take longer to repair the subsequent damage.
Figure 2: A strong back and hydraulic jack arrangement can be used to push the wicket gates down into the bottom bushing during removal of the turbine head cover.
Carefully examine the components and look for excessive play of the wicket gate systems. Look for excessive clearances between wicket gate contact surfaces after the hydraulic system is depressurized. Compare measurements with the original manufacturer specifications. These results will help in understanding the potential for problems with wicket gate removal and ordering of materials in case of problems.
Develop plans for critical lifts. These plans ensure a lift is executed properly and that all involved understand the work to be performed. The plans also allow the crane operator and lift personnel to understand the possible weights if wicket gates are hung up in the head cover, allowing the rigging to be oversized to handle this possible occurrence.
Conduct an inventory of and, if needed, purchase spare and replacement components. Bushings may need replaced due to wear, and spares should be available in case of damage during reassembly.
Be aware of potential damage that can occur during removal. If the wicket gates are not pushed back into the lower bushings while the head cover is being removed, damage to equipment and personnel can occur. Wicket gates can fall out of the head cover while being lifted and strike personnel and/or damage the gate itself.
Another potential area of damage is to the bushings. Wicket gate bushings are fastened into place to help alleviate rotation of the bushing inside its fit. When pushing the gates through the bushing, many times the bushing will come out as well. Taking the bushing off the gate stem and refastening it in the journal will allow it to be reused if it is not extremely damaged. Care should be taken to ascertain the position of the bushing to return it to the same orientation and respective shaft.
Assemble equipment and supplies needed to remove sticky wicket gates before lifting the head cover. A through-hole hydraulic jack is commonly used to allow the wicket gate stud to be left in place. The threaded rod holds the strong back assembly to the head cover and can be purchased at many industrial supply houses. A strong back is commonly made out of solid bar stock large enough to give adequate support to the jacking process.
Ensure the gate shafts are as clean as possible from corrosion and old grease/ debris. Attempting to remove debris and corrosion before disassembling the unit can be helpful in minimizing damage due to debris and is a good practice to ensure the bushings are not damaged more by dragging this debris through them.
Mr. Bauer may be reached at Bureau of Reclamation, U.S. Department of the Interior, 11056 West County Road, Loveland, CO 80537; (1) 970-962-4301 (Bauer); E-mail: email@example.com. Mr. Spiker may be reached at Bureau of Reclamation, U.S. Department of the Interior, 125 South State Street, Salt Lake City, UT 84138; (1) 801-524-3735; E-mail: firstname.lastname@example.org.
- A strong back is a steel plate that is strong enough to not deform under high stress, used typically in jacking or lifting applications.
Martin Bauer is chief of operations and maintenance for the Eastern Colorado Area Office of the Great Plains Region of the U.S. Department of the Interior’s Bureau of Reclamation. He has been involved in operations and maintenance for more than 24 years. Max Spiker, deputy power manager for Reclamation’s Upper Colorado Region, has more than 15 years of experience in unit disassembly and repair.
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