Brad Buecker, AEC PowerFlow, USA
For those of us who work or have worked in the power industry, it is sometimes easy to focus exclusively on the big picture of power production, while forgetting that difficulties in auxiliary systems can cause equipment damage and forced outages. Power generating units have many fluid systems that are critically reliant on fluid purity. These include the condensate/feedwater system, turbine lube oil system, electro-hydraulic control (EHC) system and others. Modest, short-term investments in auxiliary system performance can potentially save a power plant from disaster.
The cost of corrosion
Power plant chemists and other plant personnel are aware that contaminants that enter steam generator condensate can cause severe corrosion in the boiler and carry over impurities into the steam. Impurities may come from several sources, although condenser tube leaks are the worst culprits.
Regardless of their source, corrosion mechanisms in a boiler are exacerbated by the presence of porous deposits, which can serve as concentration sites for impurities that directly attack the base metal of waterwall tubes.
Figure 1. External view of a condensate particulate filter (Source: Pall Corporation)
During normal steam generator operation, condensate/feedwater piping and boiler waterwall tubes develop a layer of iron oxide, which, while being a corrosion product itself, protects the underlying base metal against further corrosion. Even in the normal course of operation, this corrosion layer will gradually increase in depth, but during periods of chemistry upsets, thermal transients and forced outages, additional corrosion products are generated. When major work, often carried out during scheduled maintenance outages, is carried out, hundreds, or even thousands, of pounds of loose particulates may lodge in the condenser hotwell, condensate and feedwater systems.
Some plants can remove at least a portion of this debris at start-up, but in many cases, particulate removal is inadequate at best, where perhaps the only method is to withdraw material through the drum blowdown, for example. Particulates that cycle through the waterwall tubes will, as the temperature increases to normal load condition, deposit on the tubes. These porous deposits subsequently influence heat transfer and, much more importantly, serve as sites for possible under-deposit corrosion and premature tube failure.
At some plants, and particularly those with once-through steam generators, start-up holds are used to allow debris to be cleaned from the system. These holds may last for days following a particularly intense maintenance outage. As plant personnel well know, any delay in start-up can cost a utility hundreds of thousands of dollars, or more, in lost power production.
An investment in a condensate particulate filter can pay for itself several times over after just one use. These straightforward mechanical devices can be easily equipped with filter cartridges that remove particulates in the single-digit micron range at very high efficiencies.
A particulate filter is usually located just after the condensate pumps, with the filter placed in a valved bypass loop around the main condensate feed line. The device need not be full flow, as at start-up the condensate circulation is often restricted to half the full-load flow rate or perhaps even less. The device will quickly remove iron oxide particulates and other debris, significantly reducing hold periods.
At a former utility, we once started up a supercritical unit following a boiler chemical cleaning. The only way of removing iron oxide and other particulates from the condensate was filtration through the deep-bed condensate polishers. This process significantly fouled the polisher resin. Moreover, four days of filtration were needed to reduce the solids, the original concentration of which was greater than 1 mg/l, to the relatively low microgram per litre concentration necessary to fire the boiler.
To alleviate this difficulty, we ordered a condensate particulate filter designed to handle half of the full-load flow for installation ahead of the condensate polishers. Plant personnel installed the unit and equipped it with 6-micron (absolute) filter cartridges. The filter was first used at start-up in 2008, following another chemical cleaning. Again, the initial particulate concentration was very high. As it turned out, two filter replacements were required during the particulate cleaning process. While no absolute data is available on particulate concentrations during the two start-ups, the filtration time was reduced from four days to one. An extra three days of operation on a large supercritical unit represented a large revenue increase.
In the simplest terms, steam turbines and combustion turbines are many tons of machinery rotating at 3600 rpm. Very tight tolerances are required at journal or roller bearings, which in turn requires high-purity lubrication oil to prevent bearing wear and premature failure. The most common contaminant in lube oil is water. Water may enter through steam seal leaks, heat exchanger tube failures, condensation in the main lube oil tank or for other reasons. Water can cause corrosion and microbiological fouling in the main lube oil tank and other locations, where the corrosion impurities will then travel to turbine bearings and control valves, piping and so on.
Equipment that has been used in the past to remove water include gravity precipitation systems with filter bags and settling chambers, and centrifuges. Centrifuges use circular motion to separate oil and water, which have different densities.
Typically, these older systems were reasonably efficient at removing free water, but they did not effectively remove emulsified or dissolved water from lubricating oils. A more modern process capable of removing free water and 80-90 per cent of dissolved water is mass transfer vacuum dehydration. The unit is typically installed in a kidney loop on the main lube oil tank. Mild heating of the oil slipstream followed by vacuum dehydration from a small, skid-mounted unit is used to remove virtually all the water in the oil. The tiny amount of dissolved water that remains is at much too low a concentration to convert to free water in the lube oil tank.
Varnish formation in oil is extremely importance at both conventional steam plants and those with combustion turbines. Power Engineering magazine reported on this issue in February 2008 in an article that outlined many of the fundamental varnish removal technologies. However, I understand a number of utilities have achieved widely variable results using these conventional technologies.
A technique that has been recognized for some time, but is now beginning to grab headlines, is varnish removal by adsorption. Varnish occurs when oil and its additives oxidize and polymerize, due to stresses placed on the fluid, which include heat transfer from the equipment, microdieseling and electrostatic energy transfer from particulate filters.
Figure 2. Electrostatic discharge produced by fluid flowing through a particulate filter.
Varnish polymers can reach high molecular weights, and due to their oxidized nature, will settle on internal components, including servo valves. The latter has become a very troublesome issue in many combustion turbines.
While varnish is only slightly soluble in oil, this solubility allows it to be removed from systems without the expense and headaches of periodic off-line cleaning. Adsorption is proving to be an effective technology. It’s is a film-forming mechanism, where the compound to be removed exhibits an electro-chemical affinity for the surface of the collecting media.
The varnish removal compartment contains multi-layer media, the surface of which has been prepared to be especially attractive to oxidized varnish particles. As varnish comes out on the media, deposits within the lube oil system gradually dissolve and are subsequently removed. The progress of this or other technologies can be tracked via the quantitative spetrophotometer analysis test offered by Analysts Inc, Los Angeles, California, USA. The procedure involves filtration of oil samples using a special filter media that collects dissolved varnish to produce a distinct colour. The colour intensity can be directly related to varnish potential. A well-designed and functioning varnish removal system should reduce the VPR to well below the “normal” value.
Electrohydraulic control fluid
Electrohydraulic control (EHC) fluids will also accumulate debris and varnish. A malfunction of turbine control valves, due to contaminated control fluid, can become a serious problem. The most common compounds used as EHC fluids are phosphate esters, that is to say organic compounds where the phosphate addition improves fire resistance.
Figure 3. Varnish deposits on a combustion turbine filter (Source: Analysts Inc)
A common method of filtering EHC fluid is to pass a slipstream through material such as Fuller’s earth. However, this process introduces hardness ions into the fluid, which in turn can react with degraded EHC to produce tenacious deposits such as calcium phosphate. To combat hardness-based deposit formation an ion exchange column can be install on the slipstream, where the exchange media removes the hardness ions. Use of ion exchange for phosphate ester treatment allows the operator to selectively target the acidity and resistivity of the fluid by combining different concentrations of anionic and cationic resins. The flow rate required for these systems is relatively small, so the volume of resin required is minimal. The resin may last for several months before a change-out is needed.
In the September 2005 issue of Power Engineering, I wrote about the very successful application of microfiltration for makeup water pretreatment at a former utility. The machine replaced an aging clarifier and sand filters, where it greatly reduced operating costs and vastly improved the quality of water being fed to a downstream reverse osmosis unit. Since I wrote the article, the system I worked with has continued to operate well. The technology is becoming increasingly popular in the power industry, where equipment is now being installed on high-turbidity waters. Typically, these new units have an integrated automatic cleaning system that cleanses collected solids on a regular basis, minimizing operator labour.
The power of prevention
This article outlines filtration and purification techniques for several important makeup and auxiliary fluids at power generating facilities. Neglect of fluid chemistry or physical integrity can lead to corrosion, fatigue, wear, and other mechanical and chemical failures.
The protection of auxiliary fluid systems is vitally important and the old saying “An ounce of prevention is worth a pound of cure” highly applicable. Relatively inexpensive treatment processes can save a plant from forced unit outages and the subsequent power replacement and equipment repair costs.
Brad Buecker is the technical support specialist with AEC PowerFlow in the USA. Buecker has written many articles on steam generation, water treatment and flue gas desulfurization chemistry.