Scaling is one of the most frequently occurring problems in geothermal power plants and can prohibit the control of well flow if it builds in the well or wellhead. At HS Energy on the Reykjanes Peninsula in Iceland, the problem was minimized utilizing a design developed by a veteran plant worker. The solution could not have been implemented without an open-minded leader who was ready to listen and push forward with the remedy. In this particular case, Erlendur Guðmundsson’s idea required the backing of HS Energy Vice President Albert L. Albertsson to get the ball rolling.
The solution that was developed is a new valve, referred to as Elli-valve, which allows for better flow control and easier cleaning. It also has been shown to have economical advantages over other valves on the market. This solution could be useful in other geothermal power plants dealing with similar issues, but it also demonstrates how the ingenuity of the people most familiar with an issue, such as the plant workers, can be used to provide effective solutions to problems in any plant.
The Reykjanes Geothermal Field
On the Icelandic Reykjanes Peninsula, HS Energy operates two geothermal power plants, Svartsengi and Reykjanes. Svartsengi currently produces 46.4 MWe and 150 MWt. The high-temperature area stretches over 2 square kilometers. The Svartsengi geothermal power plant was constructed in small phases beginning in 1974 and stretching out to 2000. Reykjanes geothermal power plant went into production in 2006. It currently operates two 50-MW dual-flow turbines delivering approximately 100 MWe. Both plants are relatively close to the ocean, which results in brine rich in Na, Cl, K, SiO2 and SO4. A detailed description of the brine can be found in Table 1.
|Table 1. An example of the brine chemical composition at Reykjanes and Svartsengi. Source: HS Energy
At Svartsengi, the average flow rate is approximately 226 kg/s (as reported in a 2000 study) where steam is 32% of the flow at 5.5 bar separation pressure, and temperatures have been shown to be around 240C. At Reykjanes, temperatures typically run between 250C and 320C.
Studies have shown that the chemical composition of the brine increases certain problems in the control valves. (Interestingly, the scaling problem experienced in the control valves at HS Energy has not been seen at such magnitude at other geothermal power plants located relatively nearby in Iceland, namely Hellisheidi and Nesjavellir—likely because the chemical composition of the brine is very different at those other locations.) Previous studies conducted on surface pipes in the Reykjanes area, operating at pressures of 20 and 5 bar-g, have recorded scale forming at a rate of 0.1 to 0.5 mm in 30 days or roughly 1.2 mm to 6 mm per year. Scaling eventually leads to difficulties in controlling the flow from the well.
Types of Valves
A geothermal wellhead assembly consists of many different valves with various functions. Most of the valves are not operated regularly. Many valves are used only when necessary to divert the flow from the collection pipe system, close the hole if it is not used in production, or when isolation is necessary for repairs.
The control valve is regularly operated, however, where its purpose is to control the flow of steam and brine from the well. Controlling the flow from the well is essential for efficient operation of a geothermal power plant. Choosing the right control valve is therefore of great importance.
Issues with Geothermal Control Valves
The problem of scaling is hard to avoid. Because it can hinder efficient operation of the power plant, it warrants special attention. During the early operational years at HS Energy, a butterfly valve, which had been modified to compensate for the harsh geothermal environment, was used to control the well flow manually. A butterfly valve is not well suited to throttling and is simply a disk that can be rotated on an axis to control flow (Figure 1).
|1. A fully opened butterfly valve. Courtesy: Reynir S. Atlason
The bearing house of the butterfly valve was normally cooler than the brine that flowed around it, which led to scaling on the bearing house and the bearings. Once the scale formed, control of the valve was compromised, which forced the staff to deal with less-than-optimum operating conditions. The valve also proved difficult to clean, which meant that after a certain period of operational time, staff faced the problem of cleaning the scales, as well as repairing or replacing the bearings.
After operating the butterfly valves for several years, it was decided to abandon them and convert to fixed blend disks. The disks could be removed and cleaned without too much effort, but they did not allow for the flow to be controlled in any way. Therefore, the flow required from the wells was approximated when the disks were installed. Disks with different diameters were installed on different wells, depending on their flow rate. Such disks, because of their fixed nature, did not allow the optimum amount of brine and steam to the power plant, which led to steam being wasted in the production process.
Neither the butterfly valve nor the fixed blend disks were considered acceptable for long-term operation, so the company continued to search for alternative solutions. Because it is almost impossible to alter the chemical composition of the brine to avoid scaling, the valves needed to adapt to the conditions.
Using Old Concepts to Develop New Designs
The Giffard’s injector was invented as a replacement for the mechanical pumps used to supply boilers with feedwater. Patented in 1858 by Henri Gifford, its simple design and efficient operation made previously used forced pumps almost obsolete. In essence, it controlled the flow of the fluid or gas with a cone. A section view of a Giffard injector can be seen in Figure 2. The Elli-valve can be considered a derivative of the Giffard injector. This type of valve has not been used under geothermal conditions before.
|2. A section view of the Giffard injector. Courtesy: Reynir S. Atlason
An employee who operated the steam equipment on a daily basis developed the idea. The Elli-valve allows a cone, which can be controlled remotely, to regulate the brine and steam flow through a blending seat. The valve consists mainly of three parts. First is the cone, shown as number 1 in Figure 3. The cone can be adjusted to allow for the required flow from the well to the separators. The cone is manufactured out of steel with a Stellite hard facing. (Early designs had problems with vibration in the cone head, and it would eventually become loose, but that problem was solved by strengthening the attachments between the cone and the cylinder it is attached to.) Figure 4 shows cones ready for use in the valve.
|3. A 3D model of the Elli-valve. The arrows indicate the flow direction. The model has been cut to show the moving cone. Courtesy: Reynir S. Atlason)
|4. Cones ready for installation. These cones produced by Renniverkstaedi Jens Tomassonar ehf and Framtak ehf are used for flow regulation in the Elli-valve. Courtesy: HS Energy and Reynir S. Atlason
Second is the seat, shown as number 2 in Figure 3. Adjusting the relative opening between the seat and the cone controls the flow through the valve. In this design, the seat has a tendency to collect scale. By allowing for easy removal of the seat, it can be cleaned relatively quickly or replaced. This feature shortens the overhaul time for the valve. The seat is manufactured out of Hardox 400 steel.
The third part is the hydraulic jack, which is also manufactured domestically, shown as number 3 in Figure 3. The jack allows for remote operation of the valve. Standard parts are also used, such as the T-pipe, shown as number 4 in Figure 3. Figure 5 is a section view of the valve in its closed position; Figure 6 shows the final assembly.
|5. A section view of the Elli-valve in a fully closed position. Courtesy: HS Energy
|6. The Elli-valve after an overhaul, ready for installation. Courtesy: HS Energy and Reynir S. Atlason
This design has gone through refinements since the day the idea surfaced at HS Energy. The main advantage over the butterfly valve is the relative ease of cleaning and good access. Scale is removed using high-pressure washing.
Moving from Concept to Production
The invention of the valve can be traced back to 1996, when the initial idea surfaced. According to the staff member who designed the valve, it was the dedication of the vice president of the company that fast-tracked the development process. The worker was allowed to produce a prototype of the valve, which was then operated for a year. By using the valve design, the flow to the separators became more efficient because it could be continuously controlled remotely.
After the prototype had been constructed and tested, a local machine shop was consulted to gain their expertise. That input was valuable for the valve manufacturing process. Involvement at this stage of the development process also gave the machine shop a thorough knowledge of the components, resulting in a fluid production stream for parts and maintenance supplies.
After getting positive operational experience from the valve, the company began producing them for the majority of its wells. Today, the valve has been operated for more than 15 years, with constant refinements being made over that period.
Cooperation between engineering firms, machine shops, and energy companies is becoming more prevalent in Iceland and is expected to grow with the mutual platform cluster “Iceland Geothermal.” The initiative is based on defined projects that the cluster members have agreed to work on. Members meet and exchange ideas in an effort to further development and growth as well as to identify key skills that can add value within the cluster. The fruit of this initiative is yet to be fully enjoyed because the platform is relatively young, but it is hoped that information will flow more fluidly, pushing for innovations such as the valve described in this article.
Part of the Elli-valve success can be attributed to the fact that the staff member who designed the valve was involved throughout the development process, from the initial idea, through prototyping, and final manufacturing. The process did not take off until the vice president became involved and pushed for further development though. This example shows that devotion to innovation by corporate leaders does not only fast track and push innovative solutions at individual plants, but it also fosters an innovative spirit within the company. This experience has led employees at HS Energy to propose several other ideas, which have also been developed to some extent.
The Elli-valve has effectively combatted the scaling problem experienced at plants on the Reykjanes Peninsula. Scale does still accumulate in the valve, but due to the valve design, it is relatively easy to disassemble the valve and remove the scale. The cone design allows remote and more precise control of the flow of steam and brine to the separators, continuously allowing for a better utilization of the resource. This valve has now become a standard valve for geothermal power plants on the Reykjanes Peninsula.
By looking to the Icelandic experience, scaling problems can be addressed at other plants experiencing similar issues. ■
—Contributed by Reynir S. Atlason and Runar Unnthorsson, University of Iceland, Dept. of Industrial Engineering, Mechanical Engineering and Computer Science.