Coal, O&M

CNC Machines Go Mobile

Issue 10 and Volume 114.

By Lawrence Rentz, Vice President – Engineering, Climax Portable Machine Tools

The drive by nuclear plant operators to extend the life of aging equipment and to minimize equipment downtime during planned maintenance shutdowns has led to the extensive use of portable machine tools. These portable machines bring the power and precision of stationery equipment to the plant floor and enable machinists to complete demanding machining projects faster and more efficiently.

The newest game-changing refinement to these portable machine tools now includes the integration of computer numerical controlled (CNC) control systems. With this marriage between machine and CNC-software technologies, all the advantages of portable machining now extend to complex geometries—such as refurbishing corroded funnel-shaped, contoured and curved valves on turbines—and significantly expand the use of the machines and types of work that can be done in situ. Until now, intricate machining projects such as these could only have been done at a machine shop. Major power plants that have used these newly designed, intelligent portable machines view them as a boon to productivity.

Only a few tool manufacturers have the experience of collaborating with nuclear plants on challenging repair projects and application solutions. Even fewer have the capability to custom-design these CNC-portable machine tools. So choosing the right supplier must take careful consideration.

CNC in the Machine Shop

Modern machine shops have had the capability for years to use CNC-enabled stationary machines to make or repair multiple parts precisely and consistently using detailed models which provide all necessary machining information. This input to the CNC’s code control language is machine-specific and directs the axis motion of the milling heads. Within the last several years, ongoing research and collaboration has taken place between countries in the European Union and the United States to extend machine tool control language not only to stationary machines but to portables ones. The result is to allow more information about the machining process to be sent to the machine control about the piece being machined such as dimension and tolerance data, further improving the quality of the work.

Pioneers of the portable machine tool concept have had years of experience in developing and improving their products. Integrating features like heavy-duty grade construction and high torque rates to provide more power, precision and metals removal capabilities to the tools have enabled machinists to be more productive and get their projects done faster. Other improvements also included more modular designs for easy setup and greater portability. Miniaturization of machine tools like boring bars, milling machines and flange facers have made it possible to bring the power and precision of stationery tools to the plant floor. As a result, large pieces of equipment can be repaired in situ in less time—and more cost-effectively—than it takes to disassemble equipment and ship them off site to a machine shop to be repaired. In situ machining has become the de facto method for repairing and maintaining turbines and other heavy equipment, cutting outages for equipment repair by hours or sometimes days.

As maintenance teams have gotten more adept at using these machines, there have been more requests by power plant managers for portable machine tools that not only will further reduce machining time but also do more complicated jobs. In response, highly customized machine tools have been developed which combine the precision and power of portable machine tools with the programmability and machining uniformity of CNC machines. These new machines represent a breakthrough in machine tool technology and offer many advantages over standard and even some custom portable machine tools, including the ability to program the tool’s movements to a desired result and to monitor the tool head as it machines equipment.

CNC-enabled Portable Machine Tools

With a CNC-equipped portable machine tool, the machinist can automate the machining process. Detailed controls information can be programmed using 3D models to define all necessary geometric information for boring, aligning and resurfacing a contoured pipe or other fixture. The operator only has to set up the machine once to set the feed rate, cutting speed, location and angle of the machine tooling and the machine does the rest. These coordinates tell the machine how and where to move, when to stop and how much metal to remove. The computer runs the equipment and performs the desired processes, significantly reducing the risk of human error when doing repetitive machining tasks.

CNC-controlled portable machine tools can be mounted onto the piece to be machined where it can reach into areas and machine surfaces that can’t be reached with manually operated equipment, such as piping bores where there are contours or where one section of piping may be smaller or larger than the previous section. The tooling can also reach inside the ends of large pipes or valve fixtures to re-cut the inside diameter to prepare it for new flanges or seals and also replace hand grinding. Moreover, a portable machine tool complete with CNC controls and a closed circuit TV also can check the cutting tool when it is inside a turbine crown or boring shaft fits so that the operator knows where the tool is at any given time and how it is operating.

Custom CNC-enabled portable machine tools—such as CNC-controlled boring bars and CNC-controlled thread milling machines—give machinists the ability to machine geometries never before attempted using portable machine tools. The machines can also be preprogrammed for various application configurations, minimizing the need for programming support or training, and expanding their overall usage capabilities as well as the types of work that can take advantage of portable machining. As a result of this improved productivity, the cost of these tools for power facilities can be easily justified: they get the job done in substantially less time than it would take to machine equipment the “old-fashioned way” by disassembling the piece and transporting it to a machine shop for repair.

Partnering

When a plant operator is faced with extremely challenging repair and maintenance problems that must be completed within a very tight deadline or wants to expand the kinds of machining projects that can be done in situ, then choosing the right supplier is vitally important. Not all repair and maintenance projects call for sophisticated portable machine tools with CNC capabilities, and only recently have stationery tool manufacturers even attempted to design these CNC-portable machine tools.

Plant operators should look for suppliers who have on staff engineering and design experts with years of experience collaborating on major maintenance projects at nuclear plants and who can determine what portable machining solutions and control systems are right for a given application. The supplier should also be able to provide 24/7 customer support since questions about operating the machines can arise at any time of the day or night, and they should also have a track record of fast response to planned and emergency outage situations.

Because training is often required to make the transition from manually operated machines to CNC-enabled machine tools, hands-on training should be available as part of the purchase agreement. This training enables the machinist to be more productive and to produce the kind of tolerances and finishes that may be required to meet and often exceed industry standards.

By partnering with the most experienced machine tool supplier and making an investment in custom CNC-enabled portable machine tools, plant operators can reap the rewards of shorter maintenance periods. They also gain the ability to expand the use of these tools, to do much more intricate machining jobs in situ and to optimize their workforce. It’s an investment that is paying long term dividends.

Author: Lawrence Rentz is vice president of Engineering at Climax Portable Machine Tools. He leads the Engineering and Quality team and has over 16 years of technology experience in nuclear reactor analytical and maintenance solutions, steam and gas turbine design and machine tool design. Mr. Rentz holds a BSME degree and seven patents in design and manufacturing solutions.

 


 

Proper Calibration of Gas Analyzers

By Terrence Kizer, Environmental Instrumentation Technician, American Ecotech

Air monitoring equipment, whether ambient or part of a continuous emissions monitoring system (CEMS) needs to be verified using an outside source. Without proper calibration and maintenance every system will eventually start to give unreliable data. Methods of verification vary depending on the method of measurement and what components are being measured. Therefore, while designing a system it is important to include the necessary verification support equipment. This article contains a brief overview of different verification techniques that are unique to common forms of air monitoring.

After initial system installation and major maintenance a calibration is necessary to verify analyzer precision. Verification of an analyzer is accomplished by comparing the measurement of the analyzer to a standard of a known concentration of gas that can be traced back to National Institute of Standards and Technology (NIST), but other methods include onsite ozone generation and internal permeation tubes.

A proper gas calibration cylinder should have NIST traceability through the NTRM (NIST traceable reference material) program. By using NTRM bottles, proper calibration is assured by maintaining NIST traceability. When setting up a system, an important consideration when using calibration cylinders is the lead time required. While some gases used in calibration can be acquired on short notice, specialty gases can take considerably longer to mix and assay. This also needs to be taken in consideration when bottles near their expiration date, as new bottles need to be ordered before the older bottle expires. Typical lead times for Environmental Protection Agency protocol bottles are around a month. It is also important to note that cylinders will have different certification periods depending on the reactivity of the gas. A system that measures both a reactive gas such as nitrogen oxide, and a non-reactive gas like carbon monoxide, will have a shorter shelf life.

Linearity is checked by testing multiple calibration points along the range of the analyzer. A multipoint calibration of an analyzer requires using either multiple cylinder concentrations or a dilution system. Using a calibration dilution system, a single bottle can be used to achieve multiple points by combining a higher-concentration cylinder with a zero-air, thereby diluting the concentration to the appropriate level. The main advantage of the dilution systems is obvious; using a single bottle cuts down on the long-term operation cost of the calibration system. In addition, using a single cylinder reduces the possibility of error in the system, since the possibility of contamination is confined to a single bottle.

Most dilution calibrator use a series of mass flow controllers to monitor the amount of source and zero flow used to create the different calibration points. Manufactures of mass flow controls recommend the lowest 10 percent of flow controller’s range not be used; the lower range the flow control becomes less stable. It is important to set up a system where the cylinder concentration and mass flow controllers are able to produce concentration in the analyzers range.

Dilution systems require the use of zero-air generators or cylinders of ultra high-purity nitrogen. Since most dilution calibration systems usually mix a high flow of zero air and with a much lower flow of concentrated gas, a zero-air cylinder will run out much faster than a cylinder with the concentrated gas. Because of the high turnover rate of zero-air bottles and the low cost of producing zero air, zero-air generation is almost always the more economical choice. Zero-air generators are comprised of a compressor pump and scrubbing media or converters. Depending on the range of the analyzer being calibrated and the components being tested, the zero-air generator and scrubber configuration should be adjusted. While activated charcoal is common in zero-air systems, as it is able to remove ozone, hydrocarbons and other common contaminants, specialty media is required for some constituents; for example soda lime is required to remove carbon dioxide in a zero-air system.

After an analyzer has been calibrated, daily spans and zero checks are required to ensure there is no instrument drift. A zero check should provide a sample of zero air, while the span is a sample with a concentration at the high end of an analyzer’s range. Similar to the calibration of an analyzer, span checks can be performed by feeding directly off a known concentration cylinder.

The main thing zero and span checks are testing for is drift, which is where the measurements of span and zero points vary day to day. Since the concentration of the cylinder will remain constant, any drift in the measure of the nightly spans can be attributed to drift in the analyzer. While similar in method to a full calibration a span check should never be used to modify the calibration factors of an instrument. In addition to the precision calibration, analyzers have daily span/zero checks to ensure the analyzer and data acquisition equipment can maintain stability between calibrations. In some cases, these zero/span checks follow the same methodology as the precision calibrations. However, these checks should not be used to change the calibration factors of an instrument.

Cylinder calibration is not possible for all forms of air monitoring. Ozone, a highly reactive compound which cannot be stored in known concentration cylinders, needs to be produced on demand for the purpose of calibration. For purposes of verification, an ozone instrument can be brought to a primary standard or a transfer standard can be compared to the analyzer. Transfer standards fall into two categories: analytical instruments and generation devices. An analytical instrument would require a separate device to produce ozone. The ozone produced by this separate device would be read by both the transfer standard and the analyzer being calibrated. Generation devices, most commonly ultraviolet (UV) generation lamps, are able to produce a range of ozone outputs useful in the calibration of ozone analyzers. While generation devices may seem to be the simpler and more cost effective approach, they are less stable than the analytical methods.

Analyzers can be equipped with internal calibration units, either permeations tubes or internal generation. Permeation tube are membrane tubes that contain reference gas in a liquid or solid state, the reference material permeates through the membrane releasing a stable concentration of gas into the analyzer. While permeation tubes are useful for daily span checks, it is not recommended they be used to the calibration of instruments. Permeation tubes require both a constant temperature and a constant flow of zero air to maintain stable output concentrations. Internal ozone generation is accomplished in the same way that ozone generating transfer standards produce ozone, through the use of ozone generating UV lamps.

While most tests require either known concentration cylinders or ozone generating lamps, some specific tests require both. NOX analyzers that are equipped with internal converters allow for the measure of both nitrogen oxide and nitrogen dioxide, the two primary components of NOX. NOX analyzers, in addition to proper calibration of point measurement, require the calibration of their internal converters. This can be accomplished using an external gas dilution system, with the addition of an ozone generating device. The external calibration system supplies a point of NO to set a baseline for the operation of the analyzer. After a baseline is established, the calibration system mixes a concentration of ozone with the NO stream. The reaction of the ozone with nitrogen oxide forms nitrogen dioxide. By varying the amount of ozone generated, the converter efficiency can be tested. Using this method, commonly referred to gas phase titration, an analyzer that measures both NO and NO2 can be calibrated using a single NO bottle.

Proper verification of an air monitoring site can be a daunting task; yet planning in advance for the necessary equipment will make the job much easier. The large amount of option available allow for a correct fit for any monitoring situation, regardless site accessibility, number of analyzers or level of technical experience of the site operators.

References:

1. The NIST Traceable Reference Material (NTRM) Program for Gas Standards, retrieved Aug 30th 2010, From http://www.nist.gov/cstl/analytical/gas/ntrmprogram.cfm

2. GasCal 1100 Dillution calibrator. (Feb 2010). Ecotech

3. Transfer Standards for Calibration of Air Monitoring Analyzers for Ozone, retrieved Aug 30th 2010 From http://www.epa.gov/ttnamti1/files/ambient/qaqc/OzoneTransferStandardGuidance.pdf

4. EC 9841 A&B Series Nitrogen Oxides analyzer. (May 2007). Ecotech


Micron-Size Particles at Lakes Caused Texas-Scale Problem

By Steven London, President, Steven London Associates

For almost 50 years, CPS Energy of San Antonio has drawn water from the San Antonio River to recharge Braunig and Calaveras Lakes, which the utility built in the 1960s to provide cooling water for their power plants. Braunig Lake, built circa 1962, followed by the larger Calaveras Lake in 1969, have a combined capacity of approximately 99,000 acre feet. CPS Energy buys reuse water from San Antonio Water Systems in the amount of 40,000 acre feet per year to maintain lake levels and replenish the draws from the power plants as well as from evaporation caused by the Texas sun.

A major factor over the years that has influenced the water supply was a seven-year drought. This sapped the normally high reserve of the Edwards Aquifer to the point that flows from springs which normally emerge from the honeycombed geology fell to an all-time low. Initiatives were proposed to preserve the historic source for the City of San Antonio’s drinking water. CPS Energy wanted to reduce demand on the aquifer as a source for power plant cooling water, so plans were made to construct the lakes.

A view of one of the mast-mounted mixers. The units can be adjusted for different angles of delivery and raised up the pole for ease of maintenance.

Treated sewage effluent and state waters have been used to recharge the lakes. A new, 90 MGD wastewater treatment plant equipped with advanced primary and secondary treatment technologies entered service in the 1980s and water quality has improved greatly. The new plant produces a high quality effluent that is discharged into the river instead of into a third lake upstream, which was used by the old wastewater treatment facility. This operation of the San Antonio wastewater treatment plants ensures a consistent flow down the San Antonio River and has allowed the utility to reduce its dependence on the Edwards Aquifer for plant water needs. CPS Energy became one of the nation’s first electric utilities to use treated effluent for cooling water. Beyond their primary role for the utility, the CPS lakes have become habitat for wildlife, fishing and other recreation.

As is often the case with flat-water streams, the San Antonio River can experience high water events that churn the meandering channel into a raging torrent. Until recently, these incidents have left a buildup of silty muck on the intake structure’s floor of the CPS pump station, which draws from the river to recharge Calaveras Lake.

“A three- to four-foot-deep buildup of sediment would cover the floors of the 36’ x 20’ x 50’ intake’s two pump chambers and block the suction bells of the 1500-HP vertical submersible pumps in each chamber which are 16 inches above the floor,” said Ron Christian of CPS Energy’s Field Operations.

“The configuration of the trash grate and traveling screens only aggravated the buildup and the originally designed back flushing system lacked enough force to clear the mud from the pump suction bells. This became a chronic and critical problem, especially during summer months,” said Christian.

Without the makeup water, the level of the shallow lake declined daily.

A four-man crew often spent several days clearing the mud from the pits. Most often, their tool of choice was a large compressor unit that would power an air lance to loosen the sediment while flushing it out of the diversion structure. It was a tedious task repeated after nearly every high water event.

The problem was resolved when CPS Energy learned how another Texas utility’s ingenuity had rectified a similar problem. During a meeting with that utility, CPS learned how it had dealt with an almost identical problem at their raw water pump station serving their power station. They explained how they stopped the silt settling problem by installing Flygt submersible shrouded mixers, manufactured by ITT Water & Wastewater. The compact mixing units stopped the sediment from settling and blocking the intake to the pumps. At the other facility, four Flygt brand model 4640 mixers, once installed, prevented silt from settling and building up. It seemed logical that the mixers would solve the same problem at the CPS Energy intake along the San Antonio River.

CPS Energy’s Jason Wauson and Will Warnke worked closely with an ITT Water and Wastewater representative and the manufacturer’s applications engineers to determine the correct mixer selection and integration into the pumping station. This led to purchasing four, mast-mounted ITT Flygt Model 4640 mixers that were installed by a local contractor.

“During the installation, the pump station’s intake was closed off and the pits drained and cleared prior to receiving the shrouded axial impeller units,” Christian said. “Two were mounted on masts with cables in each pit about 1 foot above the floor on opposite sides of the intake pump suction bells. The mixers can have their delivery angle reoriented 180 degrees off the wall line and the units raised up and down using their cables. The utility’s personnel installed new water level sensors and mixer controls for automatic start/stop operation whenever river levels are 401- to 403-ASL and the river pumps not in service.

“The intake chambers were flooded and stop logs removed,” he said. “When the mixers were activated in the first pit early in 2008, even the small amount of sediment remaining on the floor was immediately churned up and flushed down river.”

Following a series of recent high water events, the Texas-scale problem caused by micron-size silt buildup has been mitigated, saving man-hours once needed to clear the pits to help to insure a reliable source of makeup water from the river.


Breathing Added Life into Failing Heat Exchangers

By Ed Sullivan, Freelance Writer

When heat exchanger tubes, sometimes numbering a thousand or more per unit, begin to crack or wear the effects can lead to a cascade of subsequent failures in adjacent tubes. If too many tubes are plugged, heat exchanger effectiveness is compromised and power generation may be curtailed by a substantial percentage. If conventional mechanical plugs are used they can break loose, leak and fail. At that point the replacement of a costly heat exchanger may be imminent.

However, if the latest sleeve installation technologies and techniques are used the results can lower material and installation cost and can improve heat exchanger performance without the need to take generation units offline while the repair work is done. Perhaps most importantly, the sleeved heat exchanger can operate reliably for added years saving operator’s capital until a planned rebuilding or replacement unit is installed.

AEP Tanner’s Creek

In the fall of 2009 at AEP’s Tanner’s Creek power station, a 995 MW facility on the Ohio River near Lawrenceburg, Ind., thermal stress cracking and wall loss indicated impending failures of 60 percent of the feedwater outlet tubes in the heat exchanger on the plant’s 500 MW supercritical Unit 4.

“We had done some testing and were hopeful that the heater could be repaired,” said Jay King, process supervisor. “It was not feasible to replace the heaters at the time. We had to have a certain amount of time to put together a replacement package to present to the board and show that is was justifiable to replace the heaters in the future.”

“We were at the point where we had to discuss abandoning the heaters until we could replace them. Unfortunately, this would possibly cut down our output because it would not be desirable to operate the heater in each string. This would greatly accelerate the end life of these heaters. To operate without any heaters in service would have meant as much as a 50 MW curtailment,” King said.

The two high-pressure feed water heaters were eddy current tested and were found to have severe stress cracks from the back face of the tube sheet in the de-superheating zone extending approximately six feet to the back of the zone.

To verify the severity of the stress cracking, a tube sample was taken for laboratory analysis which proved that the eddy current test was accurate. It would have been difficult, if not impossible, to cut and pull a tube sample from these heaters via conventional methods due to the heavy tube wall thicknesses. However, American Power Services (APS) was able to extract a tube sample using its plasma arc tube cutter (PATC). APS’s plasma arc tube cutter enables cutting heavy wall tubes at any length up to the tangent point of the U-bend in order to facilitate tube sample removal. Since the thickness of the tube walls ranged between 0.083” and 0.115” thick, cutting and removing a tube sample would otherwise be tough to cut or even access.

“They came up with the idea of sleeving the outlet tubes to increase the longevity of the heaters and return the heaters to service,” said King. “They sleeved all the feedwater outlet tubes that had 70 percent or greater wall loss and had signs of stress cracking.”

More Advanced Sleeving

“At Tanners’ Creek the tube defects were roughly up to six feet back behind the tube sheet,” said APS sales and services engineer David Grimes. “Instead of plugging the problem tubes we went in with sleeves that were roughly seven feet long and installed the sleeves in the ID of the tubes and the sleeves extended past the area where the stress cracking or excessive wall loss was located. So, if the tubes continued to weaken and fail, then we have those liners installed that should prevent the tubes with through wall cracks from leaking.”

Grimes said that APS chose to recommend installing the sleeves because plugging such a large percentage of tubes would have led to reduced thermal performance and increased feedwater velocity in the inlet of the unplugged tubes. Additionally, the plant may have had to cut a bypass orifice in the pass partition plate in order to prevent higher tube inlet velocities that would have led to further heat transfer degradation.

As a result of APS’ approach, workers were able to sleeve the Tanner’s Creek feedwater heaters despite the limited access within the feedwater heater and presence of heavy wall tubing, representing a substantial savings to AEP over replacement heaters.

“I would say that, using this technology, our customers can realize savings of over 80 percent of a cost of a new heat exchanger if they elect to install sleeves rather than replace their problem heat exchangers,” said Grimes. After the sleeving project at Tanner’s Creek unit was finished, King said he was surprised to see that heater performance was substantially maintained.

“We would have expected to see a slight performance decay, since you have the sleeves in place in the de-superheating zone, because it’s now a heavier-walled tube,” he said. “But that didn’t happen. We maintained the same performance levels. The terminal temperature difference and the saturation temperature of the heater (based on the pressure) versus the feedwater outlet, have not decayed at all.” Quite possibly that resulted from the selective, limited use of sleeving.

“The thing I liked best about going this route is that we didn’t have to abandon the heaters,” said King. “That would have presented us with a difficult situation on how we would start and shut down the unit without some serious pipe modifications and major changes in operating procedures. These heaters are used during start-up and are equipped with alternate drains that drain back to the condenser that must be utilized during start-up. The piping modifications would have meant a large added O&M expense and time lost for engineering to assess needed operational changes. The installation of the sleeves allowed us to maintain efficiency and return the unit to service maintaining a design basis.”’

In installing the sleeves, APS used an advanced hydraulic expansion method with a flexible hydraulic expansion mandrel. The use of a hydraulic expander provides a more uniform expansion and superior contact of the sleeve OD with the ID of the parent tube. Additionally, the sleeves were strength welded to the parent tubes at the face of the outlet tubesheet. Grimes said that with projects that utilize the advanced tube testing and sleeving technologies, there are benefits that are somewhat immeasurable.

“In most cases, sleeving can be performed with the heater isolated while the unit remains online which allows the utility to continue to generate electricity,” Grimes said. “The power generating capacity that these companies retain, as well as how much more heat transfer they are able to get out of that heat exchanger is an additional cost savings that the utilities realize from this process. So, you would have to calculate that and how much less coal you would have to burn to maintain the same megawatts. Plus, the capital expenditures have to be considered regarding heat exchanger replacement.”


Mixer Selection

Selecting mixers for a water intake station (WIS) creates challenges even in the kind of mixer which can be used. Top entry-type mixers are often difficult or even impossible to mount. Submersible mixers, because of their mounting flexibility, are often the only useable design.

Determining the size of a mixer required for a WIS presents more challenges. A WIS is not a well defined tank or basin; one side is open to the river. Also, depending on the river’s flow, deposition of material on the station’s ramp can range from some fine silt to massive amounts of silt and sand.

The key in making the mixer selection is to recognize that a shear stress must be applied to re-suspend the accumulated solids. The magnitude of the total mixer thrust required to achieve this shear stress is a function of the area to be cleared, the consistency of the settled solids and the desired time to achieve the removal of the solids.—SL

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