Important to respect cycle chemistry and to not underestimate the complexities of ZLD

The two-and-a-half hour session on water management/chemistry/corrosion control, by itself, was worth the registration fee for the 2014 Combined Cycle Users Group meeting last August in San Antonio. Moderator Steve Royall, director of fossil and solar O&M for Pacific Gas & Electric Co, and vice chair of the CCUG steering committee, had the easiest assignment: Just introduce the speakers.

There aren’t many people capable of making presentations/discussions on water treatment entertaining, but Jim Moen, PE, senior plant engineer of PG&E’s Colusa Generating Station, and Dan Sampson, principal technical consultant on power, water, and wastewater for WorleyParsons, certainly proved they could. The long-term colleagues in power generation and water management were well prepared to share with users their extensive knowledge on cycle chemistry, cooling-water treatment, makeup treatment, corrosion control, clarifier problems, ZLD systems, and RO/EDI maintenance.

They were armed with more than 50 slides, virtually all crammed with valuable information for anyone in generation-plant O&M wanting to come up to speed on water. Moen and Sampson spend the better part of their professional lives on the deck plates and knew a formal presentation on all the topics they were prepared to address would have been a snoozer. So Moderator Royall watched as they established a rapport with the attendees, identified the sources of their water pains, and plucked out slides here and there to provide guidance that offered a measure of relief in several cases.

You can benefit from the water management know-how Moen and Sampson shared with CCUG users in San Antonio by reviewing their slides, only a click away. More: Trends in powerplant chemistry, water, and wastewater, covered by Sampson at the CCUG 2013 meeting, and his slides on corrosion product transport monitoring from CCUG 2012 are valuable in-plant teaching aids.

After presenting an outline for the session, Sampson began “stirring the pot” to get the audience involved. He said, “Chemistry is a necessary evil. What you don’t see will hurt you—eventually.” With so few chemistry professionals employed at the plant level these days, he recommended a third-party audit and data review to provide an “honest opinion” on how well a given generating facility is doing regarding water chemistry, corrosion control, etc.

Sampson urged that such an audit be undertaken with an open mind and a willingness to make changes where necessary. A 2014 Best Practices Award recognized the efforts of PG&E’s Colusa and Gateway Generating Stations in this regard. It provides a first-hand look at the extent of the assessment process and positive actions taken to assure that the plants are capable of maintaining availability and reliability at the highest levels possible.

In dollars-and-sense terms, a benefit of a cycle-chemistry evaluation and overhaul program is it almost always reduces life-cycle O&M costs. Said Sampson: It’s certainly less expensive than a forced outage. Some illustrations offered:

      • Bad cooling-tower chemistry is conducive to poor heat transfer and/or failed condenser tubes.

      • Bad chemistry in an air-cooled condenser, depending on design, can mean air leaks and significant transport of iron into the steam cycle.

      • Poor HRSG chemistry reduces heat transfer and causes tube failures.

      • Poor chemistry in the closed cooling-water system is a precursor of bearing failures.

As for the “right time” to do an audit, Sampson said “now,” pointing to the increase in water-use restrictions and higher prices, plus the bans in many areas on plant liquid discharges—all of which have added to the already significant burdens on owner/operators. To prepare for a third-party review, the consultant suggested plant personnel review their best practices for such things as the following:

      • Water-use survey and ongoing monitoring.

      • Demand reduction. Examples include reducing boiler blowdown, increasing cycles of concentration, etc.

      • Onsite treatment of drains for reuse.

      • Treatment and use of municipal and industrial wastewaters for plant makeup.

      • Alternative uses for plant wastewater streams.

      • Sampling and pretreatment of ground and surface waters.

Cooling water. Top plant efficiency depends a great deal on the performance of the heat rejection system. Moen spent quality time reviewing the key performance indicators (KPIs) for cooling towers—including approach to wet bulb; difference between the actual cold-water temperature and the design value; tower capability, actual performance versus design; chemistry limits and control capability, etc.

The speaker suggested attendees use the Cooling Tower Institute “Blue Book” software product for calculating performance parameters, such as the Merkel equation. CTI’s ToolKit represents industry consensus for cooling-tower design and performance evaluations, he said. The cooling-tower performance section of the Moen/Sampson presentation is robust and valuable for plant use.

Moen said that any plant waste stream with a lower TDS value than the cooling-tower water should be returned to the tower, which makes it something like a waste sump. If you do this, be sure your tower is equipped with state-of-the-art controls because water quality is variable.

Open discussion. Here are a few takeaways from the robust exchange between the speakers and attendees before the coffee break:

      • Regarding wet chemistry, Sampson said the only value of it in an operating plant today is to confirm the accuracy of online analyzers. Best practice: Run demin water through all analyzers when the plant is shut down.

      • Proper staffing and training is important, Sampson stressed. Employees have to care about chemistry, he said, and to know why they are taking measurements. It’s important for the staff to know the instrumentation and processes are accurate and reliable.

      • The Chemistry Performance Index was introduced as a valuable tool for keeping plant chemistry within specifications. CPI data should be analyzed weekly, Sampson said, and daily figures should be compared to that baseline. If the daily and weekly numbers deviate by more than 10%, the equivalent of a root-cause analysis (RCA) should be implemented to determine why.

      • One user mentioned a sudden drop in feedwater pH at his plant and wanted to know the likely cause. Sampson said a glycol leak into the steam cycle would do what he described. He recalled an incident where one gallon of glycol leaked into a drum and dropped the pH from 9.5 to 4.5. It took days to figure out what had happened. Once you get glycol into a steam system, the water consultant continued, it’s almost impossible to get it out. Glycol breaks down into a weak mineral acid in the drum and can drop pH to 4.5 or below. The only practical way to correct the problem is to drain the boiler and refill it.

      • The impact of seasonality on clarifier treatment was another discussion topic. It was said that different coagulants and polymers might be required at different times of the year. Regarding uses for sludge, a suggestion was to use it for capping a landfill or as a fertilizer or fill in farming.

ZLD. Following the coffee break, session focus shifted to zero liquid discharge. ZLD was a hot discussion topic given its interest by regulatory agencies. If you are unfamiliar with it, picture a system that allows water to leave the site only as a gas (steam or water vapor); dissolved solids are concentrated until only solid waste remains. The 25 slides on ZLD in the presentation are an excellent primer on the subject.

Sampson strongly recommended that ZLD be considered only as a last resort. Discharge to others, if possible, he said. According to his calculations, a basic 500-gpm ZLD system will cost about $50 million (net present value) over 20 years.

Both Sampson and Moen have deep experience with ZLD, and the scars to prove it. They reviewed some of the pitfalls to avoid in the design and operation of these systems, including the following:

      • ZLD system performance degrades over time and this should be factored into the design. Two 60% modules were suggested, and that assumes confidence in the plant water balance.

      • ZLD systems must operate with constant chemistry and flow, unlike wastewater treatment systems which are designed to receive different plant streams with variable flow and chemistry with the goal of providing an acceptable ZLD systems do not turn up or down, or on or off, well. They cannot tolerate deviations from design chemistry and must be monitored continuously.

      • It would be a mistake to assume a ZLD system can be operated by the same number of personnel as a conventional water treatment plant. Here’s the staffing Sampson and Moen recommended for a simple ZLD system: A minimum of seven to nine experienced people— including a manager (plant chemist or ZLD O&M manager), a dedicated ZLD operator 24/7, and half time for a mechanic and I&C technician. Complex ZLD designs require 12 to 14 extra people—including a manager, two operators 24/7, and a full-time mechanic and I&C tech.

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