By Brad Buecker
Even minor traces of chloride, sulfate, and sodium hydroxide can cause severe problems in steam systems, and especially turbines. In the case of sodium hydroxide, stress corrosion cracking of turbine blades and rotors may occur very rapidly. Chloride and sulfate salts deposit in the last rows of the LP turbine (the phase transition zone, PTZ) and can induce pitting that in turn leads to stress corrosion cracking and corrosion fatigue.
The Electric Power Research Institute’s (EPRI) recommended steam sodium limit is 2 parts-per-billion (ppb), with a concentration of 1 ppb “normally achievable in both drum and once-through units with good control of mechanical carryover or operating on AVT or OT.” The 2 ppb limit also applies to chloride and sulfate, again with the understanding that concentrations should normally be much lower.
Reliable on-line sodium analyzers have been available for years, but trace chloride and sulfate monitoring has been more difficult. Ion chromatography is a valid technique, but the instrumentation is expensive and requires much operator attention. The surrogate measurement for these two impurities has been cation conductivity, (or more properly termed, conductivity after cation exchange [CACE] to emphasize the fact that the sample is routed through a cation resin column to exchange all cations, e.g. ammonium, sodium, calcium, etc., for hydrogen ions).
This ion exchange process creates a sample of a dilute acid solution of chloride and sulfate, whose conductivity is then measured. An advancement that has become popular is degasified CACE, which utilizes a sample reboiler or nitrogen sparging compartment to scrub CO2 from the sample and minimize interference from this compound.
The common normal CACE or degassed CACE limit for steam is 0.2 µS/cm, but laboratory data has shown that the concentration of chloride and/or sulfate can be considerably greater than 2 ppb with CACE at the 0.2 µS/cm threshold. Thus, direct measurement of these troublesome impurities offers significant advantages, and a new instrument is now available to monitor trace concentrations (down to 0.5 ppb). The analyzer combines two well-established technologies to provide on-line measurements of these ions – capillary electrophoresis to separate the ions and conductivity to measure and calculate their concentration.
A sample and a known reagent are collected in a cartridge with a capillary, and voltage is applied across the cartridge. The ions in the mixture start separating and moving through the capillary towards respective electrodes at the ends of the capillary, thus causing separation of ions. Based on the size-to-charge ratio, different ions move at different speeds, forming clusters of each ion as they flow towards the electrode. Each cluster of ions passes a conductivity sensor on the cartridge just before they reach the electrode, and the sensor records a measurement based on the concentration of the ion. By comparing the measurement for the ions with the measurement for the known reagent, the analyzer calculates the concentration of the ions in ppb. This measurement can be repeated as frequently as every 13 minutes, drawing a fresh sample each time.
This ability to accurately monitor trace concentrations of chloride and sulfate offers an excellent enhancement to steam monitoring capabilities. Looking towards the future, company personnel have been asked if the instrument capabilities could be expanded to analyze for other trace compounds, most notably two of the primary decomposition products, formate and acetate, that are generated in steam boilers if any organic compounds (neutralizing or filming amines) are employed for pH control and corrosion protection. Research is underway, but a definitive answer is not yet available.
What About the Rest of the Steam Generating Network?
While protection of the steam system and turbine is extremely important, maintaining proper chemistry in the other sections of the steam generator is also vital to minimize corrosion and fouling. Impurity ingress from condenser tube leaks or malfunctioning makeup water systems can introduce impurities, including our old friends, chloride and sulfate, that can cause rapid damage in conventional boilers and HRSG evaporators. Poor condensate/feedwater chemistry control will induce corrosion that not only can cause catastrophic failures (flow-accelerated corrosion, FAC) within these systems, but introduce corrosion products to the boilers that then precipitate on waterwall tubes and influence corrosion chemistry. So, with those themes in mind, the following sections outline guidelines for continuous sampling of the other systems within the steam generator
Makeup Treatment System
The core process of most power plant makeup water systems is reverse osmosis (RO) followed by either mixed-bed ion exchange (MBIX) or electrodeionization (EDI) to “polish” the RO effluent to meet utility steam generator requirements. RO units typically are equipped with a number of instruments to monitor system performance, including pressure, temperature, flow, and specific conductivity. We will focus upon the recommended analyses of the final effluent from either a MBIX or EDI polisher.
Note: In this and several of the following sections, the normal limit for each parameter is included.
- Specific conductivity: ≤0.1 µS/cm
- Silica: ≤10 parts per billion (ppb)
- Sodium: ≤2 ppb
Control within these guidelines ensures that high-purity water is being distributed to the steam generator. A rise in any of the values indicates that either the MBIX resin has reached exhaustion or that a failure has occurred in the EDI unit. Prompt corrective action is necessary.
Often the design specifications for new plants call for continuous pH monitoring of makeup system effluent, but pH measurement of high purity water is very difficult. The analyses listed above are sufficient for evaluating process conditions.
Condensate Pump Discharge
Condensate pump discharge (CPD) is an absolutely critical monitoring point, particularly for systems with water-cooled condensers, as this is the most likely source for major condensate contamination. A condensate polisher will provide a buffer against contaminant ingress, but unfortunately polishers are often not considered necessary for drum units, when in fact they can be of great benefit.
Recommended CPD analyses are:
- CACE or degassed CACE: ≤0.2 µS/cm
- Specific Conductivity: Consistent with pH
- Sodium: ≤2 ppb
- Dissolved Oxygen: ≤20 ppb
- pH: 9.6 to 10.0 (This is the pH range for triple-pressure feed-forward low-pressure [FFLP] HRSGs, where the LP circuit basically serves as a feedwater heater for the intermediate-pressure (IP) and high-pressure (HP) evaporators. The range may be a bit lower for other HRSG designs.)
Sodium monitoring is very effective for detecting condenser tube leaks. With a tight condenser, sodium levels in the condensate should be below 2 ppb, and in many cases less than 1 ppb. Excursions of course suggest a leaking tube(s).
As with sodium, a rise in CACE indicates impurity in-leakage, although this measurement is also influenced by carbon dioxide ingression, typically via air in-leakage at the condenser. The CACE limit of 0.2 µS/cm is a standard requirement for implementation of all-volatile treatment oxidizing [AVT(O)] chemistry, which is the best choice for condensate/feedwater systems that do not have copper alloys (virtually all HRSG systems have no copper alloy components). No longer is oxygen scavenger feed recommended in all-ferrous systems, as this chemistry can induce flow-accelerated corrosion in HRSG low-pressure and intermediate-pressure economizers and evaporators, and attemperator lines.
Dissolved oxygen (D.O.) monitoring is important for evaluating condenser air in-leakage. A sudden increase in D.O. may indicate a structural or equipment failure in the condenser shell, at penetrations to the condenser, or even at remote locations such as the gland steam condenser. However, some air in-leakage is desired, as it provides the oxygen necessary for AVT(O) chemistry. In this regard, the D.O. normal limit was increased from 10 ppb to 20 ppb several years ago.
Previously we noted that pH measurement of high-purity water is difficult, and is not practical for demineralizer effluent. While direct pH monitoring is recommended for condensate and feedwater, the measurement is still difficult. However, in the absence of significant condenser air in-leakage, ammonia concentration, pH, and specific conductivity (S.C.) are directly related. S.C. is a very reliable measurement, and thus is normally utilized to control ammonia feed and pH in the condensate and feedwater.
LP Economizer Inlet/Boiler Feed Pump Discharge
These samples are necessary to monitor the feedwater before it enters the evaporators to ensure that the chemistry is being properly controlled to minimize FAC. Additionally, the measurements provide backup to those of the condensate pump discharge.
Recommended feedwater/economizer analyses are:
- CACE: ≤0.2 µS/cm
- S.C: Consistent with pH
- Sodium: ≤2 ppb
- Dissolved Oxygen (range): 5 to 10 ppb
- pH: 9.6 to 10.0 (This is the pH range for FFLP HRSGs. The range may be a bit lower for other HRSG designs.)
- Iron: ≤2 ppb
The discussion for CACE, S.C., pH, and sodium mirrors that for the condensate pump discharge, and also the first three, along with dissolved oxygen, are critical measurements for AVT(O) chemistry.
The reader will note the inclusion of iron in this set of parameters. Iron monitoring provides a direct measure of flow-accelerated corrosion and the effectiveness of the feedwater chemistry program. Typically, 90 percent or greater of iron corrosion products generated by FAC are particulate in nature. Several methods exist to monitor steel corrosion, and include:
- Continuous particulate monitoring
- Corrosion product sampling
- Grab sample analysis
A particulate monitor simply measures particle count in real time, and does not chemically differentiate what those particles might be. But, in high-purity feedwater systems, and in the absence of any copper alloy components, virtually all of the particles will be iron oxide.
A corrosion product sampler utilizes both a filter and ion exchange resin to capture suspended and dissolved iron. (It can also capture other metals including copper, if necessary.) After a designated run time, where the instrument also has a flow totalizer, analyses of the filter and ion exchange resin reveal the amount of metal captured. Straightforward calculations determine the corrosion rate over the period of time the sample was collected.
Finally, improved grab sampling techniques are available, in which, with proper sample treatment, iron measurements down to 1 ppb are possible. This method can provide near real-time data of corrosion rates, although on a snapshot basis.
Iron monitoring is often also recommended for CPD and boiler water samples to determine corrosion rates in other locations. With the on-line instruments, some form of sample sequencing can be arranged. Grab samples for each location can obviously be collected at any time.
Evaporator (Boiler) Water
Evaporator water sampling is critical for two primary reasons. First, whether the steam generator is an HRSG or conventional unit, the highest heat fluxes occur within the boiler. Thus, the effects of impurity ingress or poor chemistry are magnified by the high temperatures and pressures in these circuits. This issue is magnified by the fact that corrosion products from the feedwater system, most notably iron oxides, tend to precipitate on boiler internals. Iron oxide deposits are typically porous in nature, which allows impurities to concentrate underneath the deposits and cause corrosion, sometimes very severe, that would not occur otherwise.
Secondly, boiler water chemistry must be established and monitored to ensure that steam purity matches the guidelines previously shown. Excess impurities in the boiler water can lead to problematic carryover to the steam circuits and turbine.
Recommended boiler water analyses include:
- pH (<8.0, immediate unit shutdown; typical range 9.0 to 9.8 with the precise range subject to steam generator design, pressure, and chemical treatment program)
- Specific Conductivity
- Phosphate (for those units on phosphate treatment)
The reader will notice no absolute limits listed for many of these parameters. This is due to the fact that allowable impurity concentrations vary as a function of boiler pressure. As pressure increases, and the density of water and steam converge, drum moisture separators become less effective, which in turn will allow greater carryover of water droplets into the saturated steam. Within the steam generator(s) proper, temperature and pressure influence corrosion, and in particular affect reactions underneath iron oxide deposits on waterwall tubes and other boiler internals. This influence is magnified at higher temperatures.
Also, the recommended chemistry control ranges will vary depending upon the selected boiler water treatment program, i.e., tri-sodium phosphate, caustic, or AVT-only chemistry. (Note: A concept known as sodium balancing is important for boiler water chemistry control, particularly for units with AVT-only programs. A discussion of sodium balancing is beyond the scope of this article.) Charts and graphs for acceptable control ranges may be found in EPRI guidelines or those from the International Association of the Properties of Water and Steam (IAPWS). In addition, software programs (including an excellent program developed by Mr. Randy Turner of Swan Analytical) are now available to precisely calculate boiler water conditions, and alert chemists and technical personnel to chemistry upsets.
The author wishes to thank Mr. Akash Trivedi of Metter Toledo Thornton for providing the detailed information regarding the chloride/sulfate analyzer.
Brad Buecker is senior technical publicist at ChemTreat.