*Editor’s Note: Unico Mechanical Corp. will be exhibiting during Clean Currents conference + tradeshow; visit them at booth # 911, and be sure to catch the Facebook Live interview on NHA’s Facebook page at 1:30 p.m. PST on 10/19/2022.
The major equipment found in hydropower generating plants, such as the turbines, generators, and large valves, have longer lifecycles than equipment used in other types of power generating plants and in other industries. It is also common for hydroelectric power plants to utilize water hydraulic systems supplied from the penstocks to operate turbine shutoff valves, pressure regulating valves, and turbine hydraulic controllers.
Both the longer life cycles and water hydraulic systems can cause hydropower equipment to experience wear and degradation modes that do not appear in equipment operating over shorter life cycles or without continuous exposure to water.
During the time between major disassembly and overhaul work, many things can change within hydropower plant owner organizations and within the greater industry. Often, there is significant turnover of internal staff and a loss of first-hand knowledge of the equipment when the time comes for overhaul or major repair work.
The original equipment manufacturers (OEMs) can also experience staff turn-over, and in some instances, OEMs may be merged, or intellectual property will be transferred to other companies which makes it difficult to locate original design documents or order replacement components. Changes in technology and materials can provide new options that were not available at the time the equipment was designed and manufactured. New or updated regulations can make some options for replacement in-kind obsolete; for example, banning the use of lead coatings and asbestos insulation.
Replacing old equipment with new equipment can appear to be an attractive option when the time comes for a major overhaul. However, in many cases, the refurbishment of existing equipment as part of restoring it to ‘like-new’ condition may provide significant benefits over the challenges that could arise from integrating new replacement equipment with the existing plant.
KNOWING WHEN TO REPAIR AND REFURBISH
In many hydropower plants, the major equipment, such as the turbines, generators, turbine shut-off valves (TSVs), and pressure regulating valves (PRVs), use customized designs that have been scaled for the head, discharge, and capacity of that specific plant – this is the reason hydroelectric turbines are often referred to as prototypes. Replacement equipment would therefore require customized designs from specialized and experienced equipment manufacturers.
Record drawings can be limited for existing plants, especially older plants, and field conditions often differ from record drawings. This can present significant challenges that are difficult to anticipate when interfacing new equipment with existing equipment and structures. The refurbishment of existing equipment can reduce the quantity and severity of conflicts resulting from changed or undocumented field conditions.
When an existing component is repaired or refurbished, then its interface points with other components and the structure are known, and the repair work can be managed to ensure those interfaces are not affected. An existing design or piece of equipment has been proven by operation over its service life, and the owner’s operational experience with the equipment can inform the plan for repair and refurbishment to address specific issues without changing parts of the design that are operating reliably. A new design has the risk of missing an important aspect or feature, one that has allowed the equipment to operate safely and reliably with the other equipment in the plant.
HANDS ON: A LOOK AT ON-SITE REFURBISHMENT
A good example of a refurbishment and repair occurred at a hydropower plant in northern California, where a 55-inch-diameter fixed cone valve is installed in parallel with a vertical Francis turbine-generator. The valve is mechanically linked to the turbine wicket gate servomotors to function as a pressure reducing valve (PRV) while also having the ability to operate independently to bypass water around the turbine.
The original valve had a history of binding and did not operate reliably over its full range of travel. The valve had also experienced significant corrosion damage over is 46-year operating life, especially in locations where bronze had been brazed to carbon steel. The valve was removed and disassembled during the same outage that the turbine was overhauled and upgraded. As an employee of the utility that owns the plant, I was the engineer responsible for the planning and oversight of both the turbine overhaul and the valve refurbishment.
After the removal and disassembly of the valve, the extent of the corrosion damage was assessed and the repair plan was revised to account for the as-found conditions. The four vanes between the valve body cylinder and fixed cone were completely replaced with carbon steel plates while the original castings for the body cylinder and cone were reused, significantly reducing the lead time for the material procurement. The original bronze sleeve was removed from the body cylinder and replaced with a stainless steel weld overlay. Some of the locations where bronze had been brazed onto the carbon steel were redesigned to utilize bolted connections between stainless steel and carbon steel to reduce the galvanic reaction in the future.
The fixed cone valve is mounted to a bulkhead embedded in the powerhouse structure, and the gate ring on the valve is actuated by two large operating rods passing through the bulkhead. After the valve was removed, the embedded components were measured, and it was determined that the four bushings that guide the two operating rods for the gate ring were not properly aligned with the mounting flange for the valve. The bushing alignment was corrected by machining two of the four new bushings eccentrically using the field measurements. The two operating rods were also found to be bent and were replaced in-kind with new operating rods. The refurbished fixed cone valve was reinstalled and continues to operate smoothly and reliably to this day without any signs of the previous binding issues.
WHY IT MATTERS
Hydropower plants possess unique equipment, which makes it difficult to maintain a library of spare or standardized parts; therefore, the act of replacement is inherently challenging.
Hydropower operators should consider repairing and refurbishing existing components, as the alternative is full replacement with new equipment, which can represent a sizeable financial investment.
While there are instances when full component replacement makes sense for the scenario, asset owners, especially those newer to the industry, should take advantage of industry experts, who can help drive successful decisions.
By learning more about the potential options for repair and refurbishment, machines suffering from age and degradation can be granted a new lease on life.