A measure of success in the fight against corrosion

Dissolved impurities in water and steam can cause damaging corrosion and deposition in modern power plants operating at increasingly high temperatures and pressures. To control these impurities, it is first necessary to measure them accurately.

Nitin Kirloskar, Forbes Marshall Pvt Ltd, India

In countries such as India and China, which are still growing despite the economic slowdown, power generation or, to be more precise, the ‘quality’ of power generation matters a lot.

One way to improve power generation would be to increase generation capacity. Another would be to save power by operating power plants more efficiently, reducing outages and loss of capacity. The latter strategy sounds more economical, as the cost of maintaining the health of a power plant is far lower than that of continually adding capacity.

A steam and water analysis system is vital in the battle to maintain steam and water purity, and keep corrosion at bay.
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Corrosion and deposition play a major role in destabilizing power plants, reducing both power generation efficiency, leading to reduced output, and the effective life of a power plant.

Maintaining water and steam quality at ultra-pure levels is one of the best ways to reduce the effects of corrosion and deposition. A water quality management system is therefore vital for any power plant running on steam.

In such a plant, impurities such as dissolved oxygen, silica, phosphate, sodium, chlorides and many others, slowly eat into the boiler tubes/super-heaters and, more importantly, the turbines. These impurities ultimately result in corrosion and deposition, which can lead to turbine failures and boiler shutdowns, and, in extreme cases, permanent damage to equipment. The cost of unplanned shutdowns can be high. In fact, more than 50 per cent of unplanned shutdowns are due to inadequate analysis and control of impurities, and the corrosion and deposition resulting from it.

Corrosion, if not monitored properly, can result in a loss in turbine efficiency of 10 per cent. This means a recurring loss of 10 per cent of revenue. If not enough attention is paid to the corrosion/deposition problem, this loss can be compounded by damages to the turbine or even turbine failure.

Corrosion and deposition have similar effects to those of cholesterol levels in our bodies. Our cholesterol levels need to be controlled, and to do this a proven, reliable and accurate method of measuring and monitoring cholesterol levels is required. It is the same with deposition and corrosion. More specifically a reliable and accurate method of online analysis is needed. Good online analyzers supported by a well-engineered sample conditioning system is the ideal solution. A sample conditioning system and online analyzer system is called a steam and water analysis system (SWAS). With a well-engineered SWAS, it is possible to monitor various dissolved impurities that damage the turbines and other steam equipment to parts per billion (ppb) levels à‚— continuously and online.

Forbes Marshall is one of the largest manufacturers of SWAS packages. The company’s integrated sample conditioning and analysis system includes conditioning equipment and world class online analyzers.

Any good SWAS package depends on expertise in steam engineering as well as control instrumentation. Forbes Marshall can provide this expertise. With an installed base of over 350 SWAS packages, the company has solutions for the most complex steam and water analysis requirements.

Water quality improvements

In the power industry, control of water purity is a prerequisite for the safe and efficient operation of the boiler plant. Typically, up to 20 different samples per boiler unit are taken from various parts of the steam/water cycle.

Sample conditions in today’s supercritical boilers and plant, with capacities of 800 – 1000 MW, can be as severe as 200 bar at 560 à‚°C, and equipment must be of highest quality and integrity. The equipment used varies in complexity from a single sample probe with cooler, pipe work and valves for manual sampling, to fully automatic multi-stream sample conditioning consoles for centralized monitoring.

Water and steam samples are analyzed using a dry-panel analyzer.
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In today’s power industry, online monitoring of various boiler parameters has become common. In the past, these parameters were monitored in laboratories using grab samples, a method that was error prone due to contamination. However, online analyzers have eliminated most of these errors.

With the advent of online analyzers, it became essential to condition the sample, because the sensors used for online analysis were not able to handle the water/steam sample at high temperatures or pressures. To maintain consistency, it was made mandatory to cool the sample to 25-40 à‚°C.

The next development in analyzers was temperature compensation. With this, it became easier to monitor a parameter at any temperature and interpret it as if it was at a particular temperature, say 25 à‚°C.

Today we have state-of-the-art equipment to take care of sampling, sample conditioning and sample analysis. The most popular equipment for online analysis is the SWAS. The SWAS package consists of two parts: the sample conditioning unit and the analysis system. Most of the sample is handled in the former. The analysis of the samples is done in the latter, which mostly handles the signals and gives various outputs, for controls or alarms, for example.

Sample conditioning system

The online analyzers used for analyzing steam or water only work with the required accuracy and reliability if the input conditions are stable. The sensors are delicate and can only handle the water/steam samples at particular temperatures and pressures. This means we need to control and stabilize the temperature, pressure and flow conditions of the sample using a well-engineered sample conditioning system.

To accurately and reliably analyze a steam sample, it is important that the sample is withdrawn from the process, transported, conditioned, analyzed and disposed of properly à‚— all without changing its composition.

Probably the most common problem in sample system design is the lack of realistic information about the properties of the process at the sampling point. Also, while the sample is being conditioned, utmost care has to be taken to see that the sample does not become contaminated and is truly representative. The condition of the sample reaching the analyzer should match exactly with that at the tapping point. Unless this condition is met, we might end up with highly accurate analysis of the wrong sample.

The design of a sampling system depends on a variety of factors, such as conditions to which the sampling system will be exposed. Materials selection, mechanical design, thermodynamic calculations and so on greatly depend on these conditions. Similarly, the end user’s requirements, space constraints, and the consultant’s design philosophy have design implications. Systems can be completely enclosed or placed in walk-in cabinets, or arranged in open-frame, free-standing configurations.

Sample extraction

An important aspect of accurate steam analysis is sample extraction. It is vital to choose the correct sample extraction probe. The validity of the analysis will undermined if representative samples are not taken. Being directly attached to the process pipe work, the probe may be subject to severe conditions, and for most applications, this item is manufactured to the stringent codes applicable to high-pressure, high-temperature pipework.

The precise type of probe to be used will depend on the process stream parameter to be measured, the required sample flow rate and the position of the sampling point in the system. The guiding principle in sample extraction probe design is that the steam must enter the probe at the same velocity as the steam flowing in the pipe from where steam is extracted.

Generally, when sampling from pipes for suspended solids an isokinetic probe is used. This class of probe is designed to ensure that the sample enters the port(s) at the same velocity as the main process stream, reducing kinetic segregation of suspended particles to a minimum. A more important factor in obtaining representative samples is the maintenance of a sufficiently high transport velocity in the sample line to prevent hideout of the suspended species. The isokinetic probe is the right choice for these applications. Isokinetic probes may be of single port, multiport or capillary types, and should be installed with the port(s) facing upstream into the oncoming flow.

Sample cooling

The sample coolers form the heart of the sampling system. The preferred design these days is a double helix coil-in-shell-type design, as this provides the required compactness, good approach temperature and optimum cooling water consumption. Forbes Marshall provides an optimized cooler that offers the best combination of low-approach temperature and minimal cooling water consumption, and thus the best performance. The design and performance of the cooler is validated by the Indian Institute of Technology.

A chiller provides cooling, bringing analysis samples down to a temperature where they can be analyzed.
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Safety is another important aspect of good sample cooler design. The traditional stainless steel AISI 316 coils are susceptible to failure if the cooling water is not of the required quality and contains chlorides. Forbes Marshall provides a built-in shell pressure relief valve on its coolers, making it extremely safe for operators.

Sample filtering

The sample conditioning system delivers the sample under controlled temperature, pressure and flow conditions to the online analyzers. The majority of online analyzers are for dissolved solids or volatile species, and the presence of unwanted particulate matter can damage the instrumentation and certain components in the sampling system. To avoid blockages in the sample path, a small high-pressure filter with a sintered stainless steel element is required. However, for plants with a high particulate burden, larger stainless or alloy steel high-pressure Y-filters may be necessary. These can be supplied with integral valves to permit regular cleaning without disassembly.

Pressure conditioning

An important aspect of sample conditioning design is ensuring that analyzers are never subjected to a pressure higher than the safe limit. With inlet pressures conditions as high as 250 bar, this is a serious safety consideration. Pressure reduction is not enough, as sample pressures may fluctuate, and regulation is required.

A pressure regulator that can maintain the downstream pressure at a constant set limit, irrespective of upstream fluctuations, is ideal. On closing fully, the regulator should ensure zero flow condition and be able to withstand the total upstream pressure. A safety valve built into this pressure regulator makes the unit safe for operators as well as analyzers downstream. For high-pressure reduction and regulation, piston-type pressure regulators are needed. For low-pressure reduction and regulation, spring-loaded diaphragm-type pressure regulators should be considered.

Back-pressure regulation

Just as the pressure regulator regulates the downstream pressure constant, irrespective of upstream fluctuations, the back-pressure regulator maintains the upstream pressure constant. This is needed to maintain the flow characteristics of sample flowing to the analyzers. One of the important requirements of sampling system design is sample flow regulation. The back-pressure regulator ensures priority flow to all analyzers and maintains the flow characteristics of the sample at a constant level.

High temperature and pressure protection

During the operation of a sample conditioning system, high temperature and/or high pressure alarm conditions may be encountered. For example, there can be a cooling water failure or for some reason sample flow may increase, leading to a high temperature/pressure sample reaching the components and coolers downstream. As explained earlier, online analyzers have very delicate sensors and must be protected from high-temperature samples by a reliable shut-off and alarm system. Temperature sensing can be done with electrical contacts on the outlet temperature gauge or using a dedicated sample thermo switch. Pressure sensing can be achieved using a pressure switch. The electrical output of both can be used for sample shut off, using a solenoid valve. This arrangement should be able to withstand the high temperatures of a sample in alarm condition, which can be as high as 250 à‚°C.

Cation column

The principle of cation exchange is the same as any ion exchange process. This process is used in any water demineralization plant. The demineralization plant uses anion exchanger and cation exchangers to remove anions and cations from the water, leaving it free from salts and dissolved impurities.

The cation exchange principle alone is used to eliminate the masking effects of chemicals such as ammonia or dissolved amines, as explained in detail below.

The conductivity measurement is a blind method of crosschecking whether dissolved impurities are present in a water sample or whether it is ultra-pure. The ultra-pure water is poorly ionized and, hence, has extremely low conductivity. The moment one adds even a small amount of salt (say NaCl), the conductivity shoots up drastically. This can even happen with the addition of desired chemicals. The typical reaction on the cation column will be:

1) Na(+) + Cl(-) => H(+) ions => HCl + Na(+) ions
2) NH4(+) + OH (-) => H(+) ions => H2O + NH4(+) ions

In the cation columns, resins are present, charged with H(+) ions. These ions replace the +ve ions of any salt or dissolved impurity as it dissociates in water.

In case (1), the cations replace Na ions and the outcome is HCl, that is the corresponding acid. In case (2), the cations replace NH4 ions and the outcome is pure water (H2O). The conductivity in case (1) is three times that imparted by the salt (here NaCl), while in case (2), the conductivity imparted by the chemical (here NH4OH) gets eliminated, as the outcome is pure H2O. Thus the cation conductivity measurement eliminates the masking effects of known/desired chemicals.

The cooling question

Systems are designed to condition samples to the temperature required by the analyzers. The question is: what is this temperature? The temperature should be 25+1 à‚°C, as analysis needs to be done at this temperature.

There are two ways of approaching this issue. One way is to cool the sample with the available cooling water temperature (cooling water is generally available at 32-36 à‚°C if it is coming from cooling towers). With this cooling water, one can cool the sample to (say) 40 à‚°C. This sample can be fed to analyzers, which in turn sense it and interpret the results as if the sample was at 25 à‚°C. A temperature compensation algorithm is used for this purpose inside the analyzer.

An alternative option is to cool the sample to 25+1 à‚°C. Some users and consultants believe in this method. But if we are investing in a sampling system for conditioning the sample, why not do the whole job in the sampling system itself? Chilled water becomes necessary in such a case. This is because cooling water available onsite is not capable of cooling the sample to 25+1 à‚°C. Normal practice is to use available cooling water to extract as much heat as possible from the sample and use chilled water to remove the remaining fraction of heat. Thus it becomes two-stage sample cooling. In the first stage, available cooling water is used, and in the second stage, chilled water.

A chiller with an isothermal bath is a compact unit. The chiller provides the chilled water to a container, the isothermal bath, where the sample coils are immersed, thus avoiding the use of individual heat exchangers. A SWAS vendor who can manufacture chillers/isothermal baths units is an ideal choice for buyers looking for a single point of responsibility arrangement.

Reliable and accurate analysis

The ultimate purpose of buying any sampling system is to achieve reliable and accurate analysis. The analyzers are therefore the most important elements of any SWAS package. The analysis parameters that are most commonly monitored in power stations are conductivity, pH, dissolved oxygen, silica, hydrazine and sodium. Others include alkalinity, hardness, calcium, chloride, phosphate, dissolved ozone and so on. Let us look the significance of some of these parameters.

The power of hydrogen

The steam that goes into the power generating turbines has to be ultra-pure. Thus the water used for generating this steam should be in its purest form. Monitoring the pH value of the feed water gives a direct indication of the alkalinity or acidity of this water. The pH scale goes from zero to 14: zero indicating strong acidity and 14 indicating strong alkalinity. The ultra-pure water is pH7; this is supposed to be neutral. Unfortunately, it never remains neutral and tends to become acidic, due to the ingress of various impurities. Acidic water causes corrosion, especially under high temperatures and pressures. In a steam circuit it is normal practice to keep the pH value of the feed water slightly alkaline to prevent corrosion of pipework and other equipment. In the steam circuit, pH should be monitored in the drum water, high-pressure heaters, make-up condensate, plant effluent, condenser and cooling water.

Conductivity test

Conductivity measurements can detect salt contamination from the atmosphere or due to leakages in heat exchangers in water and steam. The conductivity of ultra-pure water is close to zero (say 1-2 microsiemens/cm), but with the addition of even 1ppm of salt, the conductivity can shoot up to more than 100 microsiemens/cm. Thus conductivity can give a quick indication of a plant malfunctioning or leakages. Typical points in the steam circuit where conductivity should be monitored include the drum steam, drum water, high-pressure heaters, low-pressure heaters, condenser, plant effluent, demineralization plant, and the make-up water to the demineralization plant.

The biggest enemy

The presence of silica in the steam and water circuits of power generation plant is associated with a number of problems in the superheater and turbine sections. Silica present in water or steam never exists in a completely dissolved form. Some part of it is always undissolved. The solubility of silica in stream increases with pressure. The presence of silica in the steam can lead to deposition in superheater tubes and on the turbine blades. Small deposits on the turbine blades can result in a loss of efficiency, while larger deposits can cause permanent mechanical damage.

To ensure that the turbines operate at maximum performance, continuous monitoring of silica in steam, boiler water and feed water is highly recommended. The monitoring of anion and mixed-bed ion exchanges safeguards and optimizes the operation of demineralization plant. Silica analysis is required at this stage also.

The typical points in steam circuits where silica analysis is required, are the high-pressure and low-pressure turbines, drum steam, drum water, CEP discharge, make-up water, demineralization plant and the supply water to the demineralization plant.

The silica that may appear at the boiler drum can be eliminated by Blowdown action in the case of subcritical boilers. Unfortunately, with supercritical boilers, which are of once-through type, there is no boiler drum. Hence, controlling silica and removing it at the demineralization plant level becomes of the utmost importance.

Oxygen control

Within a temperature range of 200-250 à‚°C (feed water), dissolved oxygen causes corrosion in components and piping (condensers, low-pressure preheaters, feed-water tanks, high-pressure preheaters and economizers. The resulting pitting may eventually cause puncturing and failures. Dissolved oxygen also promotes electrolytic action between dissimilar metals, causing corrosion and leakage at joints and gaskets.

To minimize corrosion under alkaline operating conditions, mechanical de-aeration and chemical scavenger additives are used to remove the dissolved oxygen. An analytical check of process efficiency is then essential.

Dissolved oxygen monitoring is imperative in power stations using neutral or combined operating conditions (pH7.0-7.5 or 8.0-8.5). The typical points in steam circuit where dissolved oxygen monitoring is required are the condenser outlet, low-pressure heaters and economizer inlet.

Hydrazine measurement

The use of hydrazine as an oxygen scavenger and a source of feed water alkalinity has well-known advantages: it prevents frothing in the boiler and minimizes deposits on metal surfaces.

As well as scavenging oxygen, hydrazine helps maintain a protective magnetite layer over steel surfaces and controls feed water alkalinity to prevent acidic corrosion. The nominal dosage rate for hydrazine in feed water is about three times its oxygen level. Underdosing of hydrazine leads to increased corrosion. The monitoring of dissolved oxygen levels is insufficient to control the optimum concentration, because it can’t measure excess hydrazine. Typical points in steam circuit where hydrazine monitoring is required are the re-heaters, economizer inlet and low-pressure heaters.

Sodium attack

The measurement of sodium is an effective way of assessing the condition of a high-purity water/steam circuit. The presence of sodium signals contamination with potentially corrosive anions, (chlorides, sulphates etc.). Under conditions of high pressure and temperature, neutral sodium salts exhibit considerable steam solubility. NaCl and NaOH, in particular, are associated with stress corrosion cracking of boiler and superheater tubes.

The measurement of sodium, acting as a carrier of potentially corrosive anions, is now recognized as an effective means of monitoring steam purity.

The ubiquitous character of sodium in the environment makes it useful in spotting leak conditions in the circuit, particularly in the condenser section. Typical points where sodium monitoring is required, are the demineralization plant, condensate pump, condenser, drum steam and demineralization plant output. The monitoring of other parameters, such as alkalinity, hardness, calcium, chloride, phosphate and dissolved ozone, is also required, depending on the size of the plant and the quality of water and steam equipment.

Measurement and control

With increasingly high temperatures and pressures it is important to control key dissolved impurities in the feed water online. To get effective, reliable and accurate measurements, select a supplier who has expertise, a proven track record, the required manufacturing and testing set-up, plus domain knowledge in both steam engineering and control instrumentation.

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