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Cooling Towers for Nuclear
How To Cool A P
Thermoelectric power plants, whether fossil-fueled or nuclear, require cooling water systems. The fuel source—uranium, coal or natural gas—heats water into steam, which drives a turbine generator that produces the electricity.
The exhausted steam from the turbine must be condensed back to water and recycled to the steam generator or boiler to begin the process anew. This condensation occurs by passing it through a heat exchanger—or condenser—where low-temperature cooling water absorbs the heat of the steam and cools it down to water again.
The majority of power plants use one of two types of cooling water systems. In a once-through or open-cycle cooling system, water is withdrawn from a water source, such as a lake, river or reservoir. The water passes through the condenser and is then returned to its original source, with a negligible amount of heat transferred to the aquatic environment.
In a recirculating or closed-cycle system, cooling water is pumped from the condenser to a “wet” cooling tower, where the heat of the water transfers to the ambient air by evaporation. The resulting lower temperature cooling water is then returned to the condenser, and the amount of water that evaporates in the cooling tower is replenished.
Once-through systems withdraw more water than recirculating systems, but consume little of it—on average, only about 1 percent of the water withdrawn is ultimately consumed. Recirculating systems withdraw much less water than once-through systems, but consume about 70 percent to 90 percent of what they withdraw by evaporation in the cooling towers. Cooling towers consume about twice as much water as once-through systems.
Both systems typically withdraw only a very small quantity of water relative to the overall size of the water bodies on which they are located—typically 1 percent to 2 percent of the average river flow. The cooling water at nuclear plants that is returned to lakes and rivers is never made radioactive and is entirely safe.
Of the 104 U.S. nuclear power plants, 60 use a once-through cooling system, 35 reactors use wet cooling towers, and nine use hybrid systems—a combination of once-through and cooling tower systems. Environmental conditions determine which is used at any given time.
The electric power industry is pursuing strategies to use less water, less freshwater, or no freshwater at all for plant cooling.
For instance, the Palo Verde nuclear plant in Arizona, the largest power plant in the United States, is the only nuclear plant in the world to use recycled, partially-treated municipal wastewater for the plant’s cooling towers. Palo Verde is the only U.S. nuclear plant not situated on a large body of water.
Another innovative cooling strategy is used by the Limerick nuclear plant near Philadelphia, which makes use of mine pool water to augment river flow during shortages.
Some companies planning to build new nuclear plants intend to use hybrid cooling systems, or use water treatment technology, such as seawater desalination to conserve freshwater resources, or even dry cooling systems that would use forced air rather than water.
U.S. oil firms, workers must look far back for strike precedents
BY JESSICA RESNICK-AULT
(Reuters) - The last time U.S. oil workers went on strike in support of a nationwide pact, Jimmy Carter was president and KC and the Sunshine Band flew high in the Billboard chart with "Please Don't Go."
As oil companies prepare to cope with strikes at nine U.S. refineries and chemical plants, accounting for about 10 percent of U.S. refining capacity, they can look to distant lessons from the three-month-long strike that started on Jan. 8, 1980.Then, plants continued to operate despite widespread picket lines by union members fighting for improved pay and benefits such as dental coverage.Much has changed in the industry over 35 years, but the companies are expected to use some of the same tactics as their counterparts in 1980, including calling on managers to don overalls and fill in for workers.
In 1980, the companies also relied upon retired workers and people from other plants to run the refineries where strikes occurred, recalled Bob Landry, a retired refinery worker in Baton Rouge, Louisiana.“Looking back now, we all have all our teeth when we retire. It was a good thing, you have to look at the long run,” he said.On Sunday, union workers took to picket lines after the United Steelworkers union (USW) said Royal Dutch Shell Plc (RDSa.L), the lead industry negotiator, halted talks over pay and conditions.Shell activated a strike contingency plan at its sprawling joint venture refinery and chemical plant in Deer Park, Texas, to keep operating normally, and other companies, like Tesoro Corp (TSO.N) sprung into action at their own plants.Refinery workers in 1980 were represented by the Oil, Chemical and Atomic Workers Union
(OCAW).Since then, the landscape has changed and unions have become less influential: many refineries have shutdown as oil production and processing trends in the U.S. have shifted. Others have expanded to become more efficient processors of crude. Free trade deals have loosened unions' grip.After a refinery’s union membership voted in favor of the strike, 24-hour pickets were set up immediately, according to Donald Erlandson, who researched past strikes for a union local on the West Coast."Picket pay was $25 a week," he wrote. “A comprehensive picket shift schedule was put together. The brothers at Allied Chemical assessed their dues an extra $20 a week to help support the effort."At the end of the strike, the union won a 5 percent pay raise nationally, plus an additional 52 cents for the first year with a 10.5 percent pay raise the second year. And they got their first dental plan.There have been other local work stoppages since then, and companies also came to the brink of strike as recently as 2012.Union workers at Tesoro Corp voted to allow a strike in the wake of difficult negotiations at four of its plants in May 2012.At the time, Tesoro CEO Greg Goff said its 166,000 barrel per day San Francisco Bay refinery in Martinez, California, would temporarily stop production but continue as a refined products terminal while replacement workers were trained.The refinery union locals ultimately reached agreements with the company, so the plans weren't tested Canadian firm Husky Energy (HSE.TO), where employees went on strike, did replace hourly workers with managers and other replacements. That plant also eventually reached an agreement with workers, and they returned.This time, Tesoro said it would continue operating its refineries through the strike, with the exception of its Martinez plant, which it said it would shut because maintenance work is currently underway.
"Heavy" applications are those where it is necessary to foresee and accommodate thermal and fluid dynamic phenomena combined with the interaction between solid bodies and high-speed fluids.
The thickness, material type, and shape of the thermowell can have significant impact on the responsiveness of the sensor to a change in process temperature. The greater the wall thickness of the thermowell, the slower the response time, as the heat from the process fluid must be conducted through the thermowell to the measuring tip of the sensor. A thinner wall thickness provides quicker response times for temperature measurement, but it also lowers the mechanical strength of the thermowell. The specific thermowell material can also affect responsiveness, and, therefore, must be considered as an important design element.
Thermowell geometry can have a profound effect on the mechanical integrity of the thermowell, as it affects the vibration effects induced by the flow of process fluid past the thermowell. These effects can be quantified and evaluated using industry-standard calculations, such as those found in the ASME PTC 19.3 TW-2010 standard. Typical thermowell geometries are tapered, stepped, or straight.
The thermowell immersion depth (U-length) should be sufficient to eliminate any conduction error from the atmosphere or another process temperature. A general rule is to use an insertion length equal to a minimum of 10 times the diameter of the thermowell. In pipes with a small cross section, the sensor tip should reach - or extend slightly past - the center of the pipe.
The ideal installation for a thermowell is one that minimizes conditions that can compromise the mechanical integrity of the thermowell while making sure that the measuring tip is located in a stable, representative portion of the process temperature profile.