By Richard A. Johnson, P.E., Manager of Engineering, Safe Harbor Water Power Corp.
Personnel at the 380 MW Safe Harbor hydroelectric plant discovered a gap between a wicket gate stem and bearing surface in one unit. To temporarily repair the unit, personnel inserted pieces of steel pipe in the gap and welded them in place. The repair functioned successfully during high spring flows. Safe Harbor Water Power Corp. is now working to determine whether to rehabilitate the unit or install a new runner.
Unit 5 at Safe Harbor—on the Susquehanna River in southern Pennsylvania—has been operating since 1934. In 2006, personnel at Safe Harbor conducted a routine annual inspection of this unit. During the inspection, they discovered a sizeable gap between one of the wicket gate stems and the brass bushing. On a unit where the normal clearance is 0.005 inch, the gap was more than 0.5 inch.
The arrangement on this unit consists of a wicket gate stem made of mild steel with a stainless steel sleeve shrunk fit to the stem. This sleeve was designed to rotate with the stem inside the bushing. A more detailed inspection of this wicket gate stem revealed that the stainless steel sleeve had come loose from the stem and was rotating freely. Because of this finding, plant personnel determined that the wicket gate stem had worn away significantly, causing the gap. This problem was not seen during the annual inspection in 2005, so it is unknown how rapidly the wear was occurring.
The gap was a serious concern for several reasons. First, if the wicket gate stem continued to wear at a high rate, it would eventually break and could go through the unit, causing significant damage. Second, taking this unit apart would cost about $1 million in labor and lost generation. Third, there was neither a plan in place nor money budgeted to totally rehabilitate the unit. And fourth, another unit at the plant was being rehabilitated, so there was not sufficient laydown area to take Unit 5 apart.
Based on these considerations, plant personnel decided first to reduce operating time for the unit by making it the last unit on and the first unit off. This was a temporary solution to give personnel time to determine the best way to repair the unit. A repair was needed to allow the unit to continue operating and to give Safe Harbor Water Power time to plan a more extensive outage that would involve either rehabilitating Unit 5 or installing a new runner. The goal was to develop a repair that would allow the unit to operate for at least another two years.
In 2007, engineering and maintenance personnel at Safe Harbor met to discuss possible solutions to the problem with Unit 5. Personnel knew the repair would involve placing some sort of material in the gap. The best solution would be to choose a material that could be welded and that would hold up to significant wear and tear. Personnel considered and dismissed car body filler because it cannot be welded and would not hold up under operating conditions.
The solution chosen was to obtain a pipe with a diameter similar to the size of the wicket gate stem and with sufficient wall thickness to become the interface between the worn stem and the stainless steel sleeve. Personnel chose steel pipe because it can be welded and is strong enough to last for the required two years.
Plant personnel used a jack to move the existing gate stem until it was roughly centered in the gap. They then measured the gap again and obtained a piece of steel pipe 8 inches long (the length of the stem). Personnel machined the inside diameter of the pipe to be slightly smaller than the gap. The next step was to cut the pipe longitudinally into 12 pieces, each about 2 inches wide.
Personnel then inserted these pieces of pipe into the gap and drove them in place with a hammer. There was not room to get all the pieces into the gap, so the gap was not completely filled. Personnel then welded the pieces of pipe to both the wicket gate stem and the stainless steel sleeve. Because personnel only had access to the top of the stem, all welding had to take place in this location. Thus, this was not a perfect repair. However, it only needed to last about two years.
After the repair was complete in the fall of 2007 personnel rotated the unit several times while visually checking the repair. The pipe kept the wicket gate stem centered so that it was able to rotate without binding.
Once the unit was repaired, plant personnel agreed to put the unit back into normal operation. They planned to take the unit out of service in early spring after the high runoff flows had decreased and determine if the repair had held up.
Unit 5 was operated normally during the 2008 high spring flows. In early May, personnel took the unit off line to inspect the repair. Everything held together well and the unit continues to operate.
Safe Harbor Water Power is now working to determine whether to rehabilitate Unit 5 or to install a new runner. This plant was built in the 1930s and contains 14 units. All of the units have some sort of problem. Rehabilitation work has been completed on one unit and another is currently undergoing rehabilitation. The success of this repair to Unit 5 has allowed the company to prioritize repairs required for several units.
Editor’s note. This article originally appeared in Hydro Review magazine.
Non-solvented Two-component Epoxy Systems
By Osmay Oharriz, Chemical Engineer, Belzona Inc.
Epoxy systems are widely used in many industries. Their use is based upon their excellent adhesion to steel and concrete, good mechanical properties, erosion-corrosion resistance and chemical resistance. They can be used as coatings applied to the exterior of structural steel, concrete, wood or any other surfaces subject to chemical condensation, fumes or splashes. They can also be used for lining the interior of pipework, vessels, tanks or any other process equipment in permanent contact with chemicals.
Non Solvented Two-component Epoxy Systems
Non solvented two-component epoxy systems consist of a resin base and a solidifier (also known as hardener or activator). These products contain little or no VOC (volatile organic compounds) and cure through a polymerization process. The base component typically contains an epoxy-functional resin modified with a suitable diluent and the solidifier typically contains a polyamine or polyamide hardener. Both components are mixed and allowed to cure for a certain amount of time dictated by environmental variables such as temperature and relative humidity.
Resin bases are usually more viscous than their respective solidifier. These systems are generally applied by brush, roller, applicator, or airless spray. This article will focus on the application of non solvented two-component epoxy systems using airless spray.
Airless spray is a technique which forces a material through a tip or aperture without the need for compressed air to achieve break-up or atomization. In airless spray, pressure is the driving force and is generally supplied to the spray gun by an air-driven reciprocating pump. It is the velocity at which the product flows and consequent friction forces from the atmosphere which causes the product to atomize in droplets of different sizes.
Airless spraying has several advantages over conventional spray methods.
- Spray is softer and less turbulent which reduces the amount of material lost due to bouncing.
- Droplets formed are generally larger and produce a heavier pattern per single pass.
- Production rate is increased as application is conducted faster than any other manually operated method.
- The coating or lining may be driven into crevices, cracks, corners and hard-to-reach areas easier than with conventional methods.
- The equipment for airless spray can be powered by air, electricity and hydraulics.
Some drawbacks are due to the high pressure levels involved in the operation itself and include:
- Spray gun tips tend to clog and can be dangerous to clean.
- Accidental skin injection is extremely dangerous as chemicals are driven through the skin pores and into the blood flow.
- Spray operators must be fully trained to service and operate the equipment.
As noted, spraying involves driving a stream of fluid from one point to another until it finds atmospheric resistance and breaks up. This resistance tends to overcome three properties of fluids which have a significant effect on airless spraying. These are surface tension, density and viscosity; the latter being the most critical for proper atomization.
Viscosity has a great influence on the final atomizing pressure as it tends to prevent fluids from breaking up. As mentioned, bases of non solvented two-component epoxy coatings or lining systems are usually more viscous than their respective solidifiers. Once mixed, the final product undergoes several rheological stages from semi liquid stage through gelation stage until it is converted into a solid film. During this curing process the viscosity of the material increases, making it more difficult to spray.
Adding solvents to “thin” non solvented two-component epoxy systems as a means to reducing viscosity (making it more sprayable) has a detrimental effect on the adhesion of the system as well as its erosion-corrosion and chemical resistance. This is why this is not desirable. Hence, the only practical possibility lies in increasing the system temperature up to an acceptable level. On the one hand, this allows the coating to be applied at faster flow rates. As the coating breaks up in the air and reaches the surface to be coated, its viscosity diminishes rapidly, helping the coating to remain in place thus reducing sagging or running. On the other hand though, by increasing the temperature the working life (also known as pot-life) of such systems is conversely decreased, which imposes a major risk of failure should the material solidify inside the spray equipment.
Single-component vs. Plural-component Spray Rigs
Two main airless spray systems are in general use for spraying non solvented two-component epoxy systems: single and plural component. These systems are not standardized; most commonly they are custom made to suit specific applications.
Single component airless spray rigs are generally equipped with a hopper containing the final product, that is, base and solidifier mixed mechanically or by hand. From the hopper the product is pumped by a double acting reciprocating pump through the hoses and into a spray gun, which finally delivers the atomized product. A plural-component airless spray rig, on the other hand, is equipped with two hoppers to contain the base and solidifier separately. The spray rig can be set up so pre-fixed proportions of the base to the solidifier are mixed at a fluid mixing manifold, where polymerization is initiated. From this point on, the coating will travel through the hoses until finally reaching the tip of the spray gun.
Automatically proportioning amounts of base and solidifier is a superior alternative to hand-mixing both components. Hand mixing usually leads to leftover material which translates into higher expenses. Besides, as the material is not pre-mixed in the hoppers, should the spray operation be temporarily halted and the lines need to be purged, both base and solidifier can be reused.
Both single and plural systems should be equipped with trace heating bundles. For single spray units these sets consist of three hoses: the main hose through which the mixed product travels, and two hoses through which the heating medium circulates. For plural sprayers, the bundle consists of two individual hoses, one for each component (base and solidifier) and two other hoses through which the heating medium circulates. Such bundles are paramount in ensuring that the temperature along the line does not decrease at any time, and consequently, the viscosity of the coating does not increase.
The heating medium is usually water because of its abundance and cheap cost. This is the reason why the heating hoses are also referred to as the water lines. Water is contained in a reservoir, from which it is pumped by an air operated diaphragm pump into a fluid heater, and back into the line to close the loop. The hoses are finally encapsulated with an insulation material such as nitrile foam and wrapped with adhesive tape.
When using plural component spray rigs for applying these systems, it is common practice to increase the temperature of the base component and keep that of the solidifier unaltered. This is based upon the fact that the viscosity of bases is generally greater than that of their respective solidifiers. Hence, increasing the temperature of the base alone is generally sufficient to achieve good spray application. Nevertheless, both components can be heated should it be deemed necessary.
Whip hoses are used to connect the trace heated bundle to the spray gun. This gives more flexibility and improves operator’s maneuverability at the work face. Several static in-line mixers are available for plural-component spray rigs. These are usually interspersed along the length of the whip hoses beyond the mixing fluid manifold. In order to enhance mixing operations, these in-line mixers cut the material to be sprayed in half and turn it 90 degrees several times. Filters are also used for both sprayers to remove any contamination in the supply system while reducing spray gun tip clogging.
A schematic comparison between these two spray rigs is provided as follows:
Evaluation of a non solvented two component epoxy system applied by airless spray
To compare via pull off adhesion testing the application of the same non solvented two-component epoxy coating by hand to that carried out by plural component airless spray.
Carbon steel panels of dimensions 200 x 200 x 10 mm (7.9 x 7.9 x 0.4 in)
Two panels, hereinafter referred to as Panel 1 and Panel 2, were used for this trial. Panel 1 corresponds to that onto which the coating material was sprayed and Panel 2 that onto which the coating material was hand-applied.
The material used was a high performance 100 percent solid two-component epoxy barrier coating provided by Belzona. This coating is used for protecting metallic and cementitious substrates against the effects of chemical attack, and as a lining, it is thermally stable up to 140 F (60 C).
Set Up and Application Details
- Panels 1 and 2 were abrasive blasted to Near White Metal as per NACE No. 2/SSPC-SP 10 “Near-White Metal Blast Cleaning.”
- Substrate profile on both panels was determined to be at least 3 mils (75 mm) by using Testex Replica Tape as per NACE RP028702 “Field Measurement of Surface Profile of Abrasive Blast-Cleaned Steel Surfaces Using a Replica Tape.”
- Substrate on both panels was degreased and cleaned using a cleaner degreaser product recommended by the coating manufacturer.
- Air temperature, relative humidity and dew point before, during, and after the application, were recorded to be 58.3 F (14.6 C), 48.6 percent and 39.0 F (3.9 C), respectively.
- Surface temperature for both panels was recorded prior to commencement of the application and determined to be 23.6 F (13.1 C) above the dew point.
- Graco plural XM sprayer equipped with an XTR-7 Spray Gun was used.
- Air operated diaphragm pump and fluid heater used were a Husky pump and a Viscon heater respectively.
- Pump mix proportioning was set up at a mixing ratio by volume of 3.8:1 (base: solidifier) as per the coating’s manufacturer.
- Base for spray application was placed in hopper labeled “A” and heated up to 104 F (40 C).
- Solidifier for spray application was placed in hopper labeled “B” and kept at ambient temperature, 68 F (20 C).
- Temperature of base and solidifier was not altered for hand application and was recorded to be 68 F (20 C)
- Viscon HP Fluid Heater was set up at 149 F (65 C) to maintain the temperature throughout the line at 104 F (40 C).
- Husky double diaphragm pump was used to pump the water throughout the water line.
- Air inlet pressure to the pump was set at 120 psi.
- 30 mesh and 60 mesh filters were used for the base and solidifier lines respectively.
- Heated trace bundle used was 50 feet long.
- Three static in-line mixers were scattered along the whip hose line.
- Spray gun tip used for spraying was Graco XHD-417 and a shortened bristle brush was used for hand application.
- Target thickness was determined to be 10 mils (250 μm) per coat as per the material’s manufacturer.
- Three passes were made per panel.
- Coating was allowed to cure for 14 days based upon curing times specified by coating’s manufacturer.
- Dry film thickness measurement was conducted using Type 2 Electromagnetic DFT Gauges as per SSPC-PA 2 “Measurement of Dry Coating Thickness with Magnetic Gauges.”
- Three 14 mm test dollies were glued onto the cured coating using an adhesive material recommended for adhesion testing by the coating’s manufacturer.
- After the adhesive material was cured, pull off adhesion testing was carried out as per ASTM D 4541 “Standard Test Method for Pull-Off Strength of Coatings Using Portable Adhesion Testers.”
- Pull-off adhesion test results are shown in Table 1.
Discussion of Results
Pull-off adhesion testing was carried out to determine the adhesion of a non solvented two-component epoxy coating to a substrate when applied by plural airless spray and by hand. The results showed that for the panel onto which the coating was hand-applied, the pull off strength (adhesion) of the coating was over 5,000 psi. Failure mode was cohesive, which means that the plane of limiting strength was within the material and not on the interface between the material and the substrate. For the panel onto which the coating was airlessly spray applied, the pull-off strength value was over 6,000 psi and no failure was recorded.
Two conclusions can be drawn from this adhesion test. First, the coating provided proved to have excellent adhesion to metallic substrates. Second, airless spray gives good adhesion values; and therefore, is an acceptable application method for non solvented two-component epoxy systems.
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