Fighting Scale and Corrosion on Balance of Geothermal Plant Equipment

There are three main types of geothermal power plants: dry steam plants that send geothermal steam directly from the ground into the turbine, flash steam power plants that convert hot groundwater to steam through high pressure, and binary plants that run hot groundwater through a heat exchanger to heat another liquid to steam. All three plants face more corrosion issues than most other forms of power generation due to the high impurities in the water and steam coming out of the ground (Figure 1). Corrosion and scale control are therefore critical for geothermal plants to maximize operational efficiency and help equipment last longer.

1. Geothermal power plants are extremely corrosive due to hot water and steam carrying minerals and impurities from the ground. Courtesy: Cortec

While corrosion and scale management strategies exist throughout the entire plant, starting at the wellhead, this article focuses on maintenance and layup strategies for “balance of plant” equipment—those assets located downstream from the main steam stop valve (where steam enters the plant) and that account for approximately 90% of geothermal plant maintenance activities.

A Look at the Harsh Geothermal Environment

Before looking at how to fight corrosion and scale, it is helpful to understand why these two byproducts are such a problem for geothermal equipment. Thinking of geothermal fluid as lava in water form will give a good idea about why geothermal water and steam have so many impurities and corrosive elements.

Originating in the ground, these water and steam sources naturally carry with them abundant scale-forming minerals and corrosive chemicals such as hydrogen sulfide (H ₂ S). Dry steam plant turbines will naturally experience the brunt of the attack from direct exposure to steam impurities; however, all geothermal plants will have some carryover of impurities into the plant, heightening the overall risk for corrosion and scale formation.

Corrosion is a problem because it eats away at metal surfaces, thinning piping and equipment walls to the eventual point of leakage, often clogging systems with corrosion products. Repair and replacement ensue, not only incurring labor and equipment costs but quickly racking up the cost of lost production from unplanned downtime.

The problem with scale buildup is that it reduces efficiency and leads to serious issues if allowed to continue long enough. As scale builds up on heat exchange surfaces, it insulates the metal and reduces heat transfer efficiency. This makes the system work harder. In worst-case scenarios, it can result in ruptured boiler tubes with catastrophic ramifications. Scale also reduces flowrates, turbine performance, and the amount of power being generated. Scale mitigation can therefore contribute to better production and plant safety.

The best solution to corrosion and scale problems is prevention because it allows maintenance personnel to avoid the bigger issues that come when these two processes run their course. The next best option is rust and scale removal for damage control. Since it is impossible to completely eliminate rust and scale in a geothermal plant, a dual approach of preventative maintenance and corrective maintenance is best.

Preventative Maintenance with Corrosion and Scale Inhibitors

The first preventative maintenance step is to reduce the corrosiveness of steam and water coming into the plant. This can be done by injecting a concentrated solution of nitrogen-based heterocyclic compound into the water or steam at the wellhead and then later in the condenser. This compound scavenges H ₂ S and helps capture mercaptans, sulfides, and sulfur compounds.

The next stage of attack is to actively inhibit the effect of impurities and corrosives that remain in the steam and filter throughout the plant. On the corrosion inhibitor side, one particularly useful chemistry is amine carboxylates, also known as vapor-phase corrosion inhibitors (VpCIs). These offer protection in both the contact phase (through direct application to the metal surface) and vapor phase (through vapor diffusion and deposition on the surface).

Amine carboxylates are attracted to metal and adsorb on it by means of an ionic bond. The resulting molecular layer has hydrophobic properties that protect metals from the normal corrosion reaction that occurs in the presence of moisture, oxygen, and other corrosive species. While amine carboxylates are soluble in liquid, their vaporizing abilities allow them to also diffuse throughout voids (such as the headspace above the water level in a heat exchanger), allowing great flexibility of application and more comprehensive coverage.

Scale inhibitors operate by discouraging scaling minerals from accumulating into a larger compound. Scale forms when the level of minerals such as calcium carbonate, calcium phosphate, or magnesium silicate in the solution exceeds the solubility limit. At this point, individual molecules start aggregating into a crystalline formation that coats tubes and turbine blades. When it is not possible to remove these minerals from the water, it is possible to slow down their ability to combine. Scale inhibitors do this by basically “coating” the connecting structure of a scale molecule so that it cannot line up and bond with another molecule to create a scale deposit.

Corrective Maintenance with Rust and Scale Removers

Existing rust and scale can be cleaned away with rust and scale removers. Bio-based products are a nice option—especially for organizations already promoting the use of renewable resources through geothermal power generation. VpCI-422 is a liquid rust remover that contains 92% U.S. Department of Agriculture (USDA)-certified bio-based content and can be reused many times in a dip bath application where workers immerse entire rusted parts for cleaning and restoration. It can also be circulated through heat exchangers, pumps, and piping to remove rust from internal surfaces. This should be followed by an alkaline rinse that neutralizes the bio-based acid and preferably contains flash corrosion inhibitors. VpCI-423, a rust remover in gel form, contains 91% USDA-certified biobased content and may be used to remove rust on surfaces that cannot be immersed.

As its name suggests, EcoClean Biodegradable Scale and Rust Remover Powered by Nano VpCI may be used for scale removal. This is a more worker-friendly option than harsher chemicals on the market. It contains 100% USDA-certified bio-based content and works well for routine equipment cleaning every few years.

Specific Equipment Recommendations

After understanding the overall scale and corrosion concerns, and general mitigation technologies, it is time to look at specific components in a geothermal power plant, and create a routine maintenance and layup plan for each. The goal is to increase efficiency, uptime, and equipment service life by limiting corrosion and scale damage, and keeping components in good working order.

Turbines. Most geothermal plants have at least one or two steam turbines and could have more depending on the size of the plant. Turbines are part of the plant’s “centerline equipment,” the primary equipment that drives power generation.

As mentioned above, preventative maintenance for turbines starts by adding an H ₂ S scavenger to the steam before it enters the turbine. Despite this precaution, the turbine should still be periodically inspected for rust and scale. Scale buildup on turbine blades is very common and should be cleaned off to avoid vibration (which damages the turbine), and maintain peak performance and power generation. Rust should also be removed and a clear water-based corrosion inhibitor coating with high temperature resistance applied to the shaft outside the turbine casing. Adding a corrosion inhibitor additive to the oil system can be another good preventative maintenance step provided the additive passes compatibility testing with the operating oil.

When the turbine is disassembled for repair work, the casing, blades, and rotor may be sprayed with a very thin corrosion inhibiting coating called ElectriCorr VpCI-239. This will dissolve soon after the turbine is back in operation but provides an excellent protective layer for interim protection. Turbines that are not disassembled but enter temporary shutdown or are kept as critical spares should be preserved in readiness for immediate use by fogging the flow path with VpCIs and shrink wrapping the turbine in a vapor-phase corrosion-inhibiting film material. Adding a corrosion inhibitor to the oil system and circulating it before shutdown is also a best practice, although compatibility must be confirmed before leaving the additive in for operational use.

Pumps and Valves. For the hundreds of pumps and valves around a power plant, workers should keep a corrosion-inhibiting super penetrant on hand to lubricate stems, bushings, nuts, or any linkages during operation and maintenance. This is important when adjusting a valve packaging nut or removing parts for maintenance, as too much force placed on rust-locked connectors may cause stems to break off and create a whole new maintenance crisis. A super penetrant/lubricant will help loosen parts so this is less likely to happen.

On the layup side, spare pumps/valves should always be ready for backup installation in case of emergency or routine maintenance replacement. The basic process for pump and valve layup is to remove rust, fog the flow path with vapor-phase corrosion inhibitors, add a corrosion inhibitor to the oil system, and wrap the entire package in a shrink film containing VpCIs.

2. Heat exchangers and cooling towers are a common sight at geothermal power plants and should be regularly guarded against scale buildup. Courtesy: Cortec

Heat Exchangers and Cooling Towers. Heat exchangers and cooling towers (Figure 2) are another key focus area for scale prevention and removal. Because scale easily goes undetected, it is important for maintenance personnel to pay attention to pressure and temperature readings since these can indicate a problem with heat transfer efficiency and provide a warning sign that scale removal is needed. Scale inhibitors added to the standard water treatment package will reduce buildup; periodic scale removal will clean surfaces where scale does accumulate. The addition of a corrosion inhibitor will keep the overall system in better condition by discouraging metal oxidation.

Spare heat exchangers can be protected by fogging with VpCIs and wrapping the equipment in corrosion-inhibiting shrink film. Cooling tower systems may be preserved by circulating specific corrosion inhibitor water treatments through the system to form a protective film and then either draining the basin (dry layup) or leaving it full (wet layup).

Generators and Electric Motors. There are multiple ways to protect generators during operation. One is to place a breathable pouch of VpCIs inside the generator housing. This will allow the formation of a protective molecular layer on metal surfaces inside. The generator core can be protected by powering it off and fogging it with VpCI-239, a corrosion inhibiting fluid that forms a thin permanent coating and can be applied without requiring a direct line of sight through the generator core. This can also be done to electric motor cores (power must be off for application). Water-cooled generators can be protected inside hollow copper conductors by adding a low-conductivity copper corrosion inhibitor to the cooling water during operation.

3. Geothermal plants contain many electrical cabinets that can be protected against corrosion on metal contact points by placing a vapor-phase corrosion-inhibitor-emitting device inside. Courtesy: Cortec

Control Panels. A basic geothermal power plant will have 50 to 100 control panels of various sizes (Figure 3), and larger plants will have proportionally more. Wires and metal contacts inside these panels are vulnerable to early replacement in corrosive environments. This effect can be minimized by placing an H ₂ S absorbing cup (Figure 4) inside the panel for passive protection along with a small cup or emitting device that releases VpCIs for active protection.

4. This small self-adhesive cup contains material that absorbs hydrogen sulfide and mercaptans, and can be easily placed inside a control panel to reduce the corrosiveness of the environment. Courtesy: Cortec

Corrosion Under Insulation (CUI)

Hot insulated piping is another common sight at geothermal plants and creates a natural breeding ground for corrosion when pipes leak, corrosives filter in, or simple condensation builds up on the insulated surface from temperature contrasts. Out of the many corrosion under insulation protection strategies available, almost all require insulation removal for application. Vapor-phase corrosion inhibitors have emerged as a less-intrusive method that can offer protection at temperatures up to 662F (350C). The inhibitors can be injected into existing insulation with the object of vapor migration through the insulation and along the insulated metal surface to form a hydrophobic corrosion-inhibiting layer (Figure 5).

5. The two small circles to the left of the wires are vapor-phase corrosion-inhibitor-emitting devices that have been placed inside the electrical unit at a geothermal power plant for corrosion protection of metal surfaces. Courtesy: Cortec

Corrosion and scale are sure to be issues at any type of geothermal power plant. Although they cannot be entirely eliminated, they should be mitigated. Implementing the suggestions above as part of routine best practices will allow geothermal plants to take a major step toward better plant maintenance and operations by minimizing corrosion and scale problems that would otherwise be much more serious.

—James (Jim) E. Holden, PE is technical director of Energy and Engineered Services at Cortec Corp. Scott Bryan is Cortec’s technical sales manager for water treatment products. Julie Holmquist has been a content writer at Cortec Corp. since 2015, writing about corrosion-inhibiting technology for a variety of industries.