Water Steam Chemistry Audit
INTRODUCTION
Well established in the power industry is the knowledge that water/steam chemistry monitoring and control are critical for successful unit availability and reliability. Even seemingly minor impurity ingress to steam generators can initiate individual or multiple and repetitive failures costing millions of dollars in lost production and repairs. Most importantly, some failures have caused injury and death, which is the ultimate cost. Yet, at many plants, including industrial facilities with high-pressure steam generation, only partial measures are in place to monitor and control water treatment and chemical feed processes. A plant audit by an independent third party can often be very beneficial in alerting plant personnel to issues that may have been forgotten or were never emphasized to the proper extent. Also, at the many new combined-cycle and combined heat and power (CHP) plants that are sprouting up around the country, the plants are often minimally staffed with few if any chemistry-trained personnel on board. Water treatment technology and steam generation chemistry issues have evolved dramatically over the last three decades, and insufficient knowledge of the details can be quite problematic. The author participated in audits during his many years at two utilities, and now assists in performing such audits.
Part 1 of this two part series focuses upon high-pressure steam generation issues, and thus its appearance in Ultrapure Water. Equally critical are cooling water and wastewater issues, which the author will discuss in Part 2. That article will appear in UW’s sister publication, Industrial Water Treatment, as the topics apply to many industries.
IMPORTANT AUDIT DETAILS
A plant audit should include one or more site visits and equipment inspections. Items that may be addressed individually, depending upon the needs of plant personnel, or be part of a holistic audit include:
- Plant water balance
- High-purity makeup water treatment configuration and operational details
- Steam generator chemistry programs and chemical feed system arrangement
- On-line instrumentation for water treatment and steam generation chemistry monitoring
- Sample line and sample panel arrangement and conformity to accepted guidelines
- Laboratory equipment and procedures apart from on-line instrumentation
- Plant staff expertise and training programs
- Startup, shutdown and layup procedures
- Open cooling water treatment
- Cooling tower makeup treatment for plants having towers
- Once-through system evaluations for plants with this type of primary cooling
- Evaluation of 316a and 316b issues
- Cooling system process chemistry and history of performance
- Closed cooling water system chemistry and operational issues
- Wastewater discharge and treatment issues
- Effluent sources
- Cooling towers or once-through cooling?
- Ash sluicing systems and ponds?
- Wet flue gas desulfurization (WFGD) ponds and streams?
- Other discharge including drains, holding pond discharges, etc.
The following sections discuss in greater detail items from above.
PLANT WATER BALANCE
Often, an initial step is to examine, and modify if necessary, the plant water balance. A water balance revision may be a key portion of the audit, particularly if changes have been made to water and wastewater treatment processes during the life of the plant. If the water balance has not been updated for years, equipment may have been added, modified, bypassed, or removed that has significantly altered stream volumes and qualities. For example, if reverse osmosis (RO) has been added as a technique for high-purity makeup production, the RO process introduces another wastewater stream, RO reject. On a related note, micro- or ultrafiltration (MF and UF, respectively) is becoming increasingly popular for suspended solids removal ahead of RO units. Suspended solids often cause the most difficulty in RO units, as they clog the spacer material between membranes and may irreversibly foul membranes. MF and especially UF will filter virtually all of these solids and provide a very clean water supply to the RO. However, regular backwashes are required, which introduces another waste stream.
The following diagram outlines a number of the most common discharge streams from a typical combined-cycle plant. Combined-cycle generation has become the primary replacement for the many coal plants that have been or are scheduled to be retired.
Fig. 1. A generic water balance diagram for a common combined-cycle arrangement, with
discharge streams highlighted. Depending upon the raw water supply to the cooling tower, pretreatment might include biological treatment, clarification (perhaps with softening), micro- or ultrafiltration, or a combination of these processes.
HIGH-PURITY MAKEUP WATER TREATMENT SYSTEM
High-purity makeup water treatment always requires several processes. First may be bulk
screening to remove large debris. Next is treatment to remove fine suspended solids that would otherwise foul downstream equipment. These technologies typically include clarification/multimedia filtration and/or micro- or ultrafiltration. If, as Figure 1 illustrates, a potable water supply serves as the high-purity makeup source, this filtration will already have been performed. With regard to the core process, many plants now utilize reverse osmosis for primary dissolved solids removal followed by polishing with ion exchange or electrodeionization (EDI) as the technology of choice. Increasingly, plant personnel are selecting ion exchange and EDI in series to completely minimize the possibility of makeup water impurity ingress to steam generators. An audit can be very beneficial for examining the efficiency and reliability of these systems. Items that are also an important part of an audit include:
A review of process monitoring and control systems for:
- Pretreatment equipment
- RO unit(s), including a normalization program
- Polisher effluent
- Silica, sodium, specific conductivity analyzers
- Are any changes in raw water supply being planned that will have an impact on makeup water treatment?
STEAM GENERATION CHEMISTRY MONITORING AND CONTROL
Power industry steam boilers, and many industrial boilers, operate at high pressures and
temperatures. In these harsh conditions even minor impurity ingress can cause major problems. Also, impurity carryover to steam can be quite problematic. Heat recovery steam generators (HRSGs), an integral part of combined-cycle units, typically are of multi-pressure design, where the chemistry in each circuit is often quite different from the others.
A critical item for an audit is review of water/steam chemistry historical data and comparison to modern guidelines. Surprising is the number of new plant designers and some personnel at existing plants, who are wedded to outdated concepts. For example, in many combined-cycle specifications that this author sees, the documents call for feed of an oxygen scavenger (more properly referred to as reducing agent) to condensate/feedwater systems that contain no copper alloys. Reducing agents generate flow-accelerated corrosion (FAC). Over the last three decades, a number of utility personnel have been killed by FAC-induced failures, and much better chemistry programs have been developed. However, the mindset regarding oxygen scavenger use has been difficult to extinguish.
Another issue that is often not addressed properly is water/steam chemistry monitoring. A major condenser tube leak (or chronic minor leaks) will allow excessive impurities to enter the condensate and subsequently the boiler water, where the high temperatures and presence of deposits can induce corrosion mechanisms that fatally damage boiler tubes. Or, consider steam chemistry. Excessive impurity accumulation in boiler water, failure of water/steam separators in boiler drums, or other factors can introduce impurities to steam that will cause severe corrosion in superheaters, reheaters, and most importantly turbines. A critical aspect in this regard is having in place the proper on-line, continuous instrumentation to detect chemistry upsets quickly. Vital sample points include makeup water system effluent, condensate pump discharge, economizer inlet or outlet, boiler (known as evaporators in HRSGs) water, saturated steam, main steam, reheat steam. Depending upon the sample point, analyses such as cation conductivity, sodium, pH, specific conductivity, silica, and phosphate are absolutely critical for evaluating
water/steam conditions and alerting operators and technical personnel to upsets. Important items for an audit include:
- Sampling locations: Are they comprehensive for analyzing steam generator chemistry throughout the entire system?
- Instrumentation survey: Are the correct instruments in place? Do the analyses conform to the recommendations of EPRI or the International Association for the Properties of Water and Steam (IAPWS)?
- Do sample extraction and conditioning meet approved guidelines? If samples are not collected and conditioned properly, the data may in no way resemble actual process conditions.
- Data acquisition and distribution:
- Are the instrument outputs connected to the DCS for instant display in the control room?
- Are operators and other staff trained to react to upset conditions? Have specific action procedures been developed to deal with upsets?
- If a severe upset occurs, do operators have the authority to shut down the unit? A delay in this decision could be catastrophic.
- Grab sampling and central lab capabilities:
- How are analyses utilized to verify on-line readings and otherwise track chemistry?
- Lab staffing: Are analyses performed by chemists, operators, or other technical personnel? What is the training level of the analysts?
If a steam generator must be shut down due to regular or intermittent load swings, scheduled maintenance outages, or forced-outages, severe corrosion may result if the unit is not protected from air intrusion to a moist environment. Yet, at many plants poor or non-existent layup procedures are in place. Also at many plants if a steam generator has been drained and is then returned to service, the unit is filled with cold demineralized water saturated with oxygen. Corrosion can be quite serious until the unit reaches steady-state operation. Important items in an audit include:
- History, current frequency, and future plans for unit outages
- Short-term (overnight or 1-2 days, common with combined-cycle units)
- Medium-term (several days to perhaps one to two weeks)
- Long-term (two weeks to months or years)
- What layup procedures are currently in place, or will be added per audit recommendations?
- Nitrogen blanketing
- Steam circuits
- Boiler
- Condensate/feedwater system and deaerator
- Steam pegging
- Warm air circulation
- Chemistry adjustments (wet layup)
- pH elevation
- Oxygen scavenger use
- Boiler water circulation to keep chemicals properly distributed
- Boiler fill process
- De-oxygenation of the makeup?
The choice of procedures is very dependent upon common unit operation. For example, with units that cycle frequently, and some actually cycle on a daily basis, nitrogen blanketing and maintaining condenser vacuum may be the only good choice. [2] Note: Nitrogen blanketing for either short- or long-term layups must be given priority status in the plant’s confined space entry procedures. Elemental nitrogen is obviously not poisonous (our atmosphere contains 78 percent nitrogen), but will cause asphyxiation.
If it is known that the unit will be off for an extended period without a fast start requirement, then draining while hot with follow-up warm, dehumidified air circulation can be a very effective approach. This is critical for protection of the low-pressure turbine, among other equipment. Too often, a unit will come off with boiler draining, but the condenser hotwell will be allowed to stand open with water remaining. The resulting humidity moistens salt deposits that have formed on LP blades and the rotor, and the moist salts then in turn can initiate pitting which can evolve into stress corrosion cracking and corrosion fatigue.
Also of importance is the cooling system. Raw cooling water that remains standing in
condensers without treatment allows the formation of microbes that can cause microbiologically induced corrosion (MIC). The author once observed an example where an entire condenser had to be re-tubed due to MIC from water that stood in the condenser for a month-long period. For long outages, the water side of the condenser can be completely drained and dried. For short outages or where the water remains in the condenser, operation of a circulating pump with regular or periodic biocide feed will control microbiological growth.
A PRELUDE TO PART 2
Open and closed cooling water systems play a vital role in plant operation. However, the
chemistry of these systems, and particularly those with cooling towers as a core heat transfer device, may be rather complex. Impurities in raw makeup and circulating water, and other factors, can greatly influence corrosion, fouling, and scaling in cooling systems. Audit items to be addressed in Part 2 of the article will include:
- Makeup and circulating water historical chemistry data
- History of corrosion, fouling, or scaling in:
- Condensers
- Cooling tower fill
- Cooling tower basin
- Other locations
- Current treatment program details and history:
- Reliability
- Upset occurrences
- Chemical costs
- Chemical feed system design and operation
- System materials
- Once-through system evaluations including issues related to 316a and 316b.
- Are any changes in raw water supply being planned that will have an impact on makeup water treatment?
Closed Cooling Systems
- Chemistry program(s)
- Chemical feed system design and operation
- Ability to maintain chemical residuals
- History of leaks
- System materials
- Auxiliary heat exchanger cleaning procedures and chemicals
- Plant wastewater discharges
- Locations
- Past history of violations, or lack thereof
- Impacted by 316a and 316b regulations?
- NPDES or SPDES permit issues, present and future
- Evaluation of current treatment technologies and recommendations for future modifications
- Will discharge be allowed in the future, or is a move to ZLD anticipated? If ZLD is the ultimate scenario, an evaluation of viable technologies is necessary. The choice will be dependent upon physical and regulatory issues, with the following possible technologies on the list.
- Pre-concentration of the waste stream by membrane treatment
- Evaporation ponds
- Deep-well injection
- Thermal treatment (evaporation/crystallization) of the final waste stream
This article outlines many of the major items that should be included in a plant audit. Both
practical experience and research at such organizations as EPRI have demonstrated the extreme importance of proper steam generation chemistry, where even seemingly minor upsets or lack of attention can cause serious problems.
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