Contaminated steam can seriously impair the performance and reliability of the turbine. Dissolved minerals can cause an accumulation of deposits on surfaces, impeding fluid flow. A deposit thickness of only 3 mils on the convex surface of buckets can cause an increase of 1 to 2 % in the fuel bill and a 1% reduction in peaking capacity. (See the companion article HERE.)
Deposits can also restrict mechanical operation. A non-operating valve can allow an overspeed event. Other contaminants can induce stress corrosion cracking (SCC) which can result in catastrophic failure of components. SCC is the growth of microscopic cracks in normally ductile metals under tensile stress in a corrosive environment. SCC can be very difficult to detect. The metal surface can appear unaffected while the subsurface is filled with microscopic cracks. High-tensile structural steels, stainless steels and even mild steel can be susceptible to SCC in the presence of chloride, alkali or nitrate contamination of only a few ppm (depending on the steel and the contaminate). SCC is extremely dangerous as it can lead to disintegration of the affected part, including discs, rotors and turbine shells.
Strict monitoring of boiler chemistry can provide early detection of these contaminants. An ongoing water conditioning program can remove the contaminants before they become hazardous. A better approach is to remove the source of the contaminants.
Boiler feedwater can be a mixture of makeup from primary treatment, condensates returning from the turbine and condensates returning from process steam. Each of these can contain its own share of contaminants. A boiler can accept this feedwater and still produce steam containing less than .05 ppm of solids. However, this feedwater should never be used for attemperation as the contaminates will be introduced straight to the turbine. Water for attemperation should be close to distillate quality.
Dissolved solids from boiler chemicals should be separated from the steam before it leaves the drum. Operating the boiler with too much water in the drum, or with foaming or priming in the boiler, will introduce too much water into the steam separator and reduce its efficiency. Efficient separators can approach less than .01 ppm of solids in the steam. Dissolved gases such as ammonia or CO2 cannot be separated and must be controlled in the boiler water.
Condensates can be contaminated by leaks in the heat exchangers used for process steam, or even incorrect piping of the chemical feed system. One should suspect process chemical leaks if organic, sulfide, ammonia, amine or copper contaminants are detected. Suspect leaks in the chemical feed system for the drum if excess OH alkalinity or phosphate is detected.
Extreme care should be exercised when using volatile acids (such as hydrochloric acid) when cleaning the the condenser. Acid fumes entering low pressure areas of the turbine can condense in confined areas such as blade roots and cause stress corrosion. The proper procedure is to close the joint between the condenser and the low pressure areas with plastic or fabric to form a vapor barrier. All remaining acid should be neutralized and removed from surfaces.
Contamination can also be introduced during inspection and repair of a turbine. Manufacturing fluids, lubricants and preservatives can contain sulfur or chlorine which can decompose into acids. Machined areas and replacement parts should be cleaned with solvents (such as denatured alcohol) and then dried off. NDE/NDT chemicals, especially the dye penetrant Zyglo, can also decompose into acids. Parts inspected with these methods should also be solvent cleaned and dried. Environmental pollutants can introduce solids (dirt) and acid forming compounds. Exposed turbine steam path components should be protected by plastic or cloth until reassembled.
Potential turbine contamination by either deposits or acids should be addressed as soon as it is suspected. Several methods for confirmation and ameliorization are available, depending on the type and degree of the contaminant.
Please contact Mr. Turbine® for advice if you suspect or encounter turbine contamination.
Steam Turbine Contamination
/in Steam Turbine Tips /by Mike.LakeDeposits can also restrict mechanical operation. A non-operating valve can allow an overspeed event. Other contaminants can induce stress corrosion cracking (SCC) which can result in catastrophic failure of components. SCC is the growth of microscopic cracks in normally ductile metals under tensile stress in a corrosive environment. SCC can be very difficult to detect. The metal surface can appear unaffected while the subsurface is filled with microscopic cracks. High-tensile structural steels, stainless steels and even mild steel can be susceptible to SCC in the presence of chloride, alkali or nitrate contamination of only a few ppm (depending on the steel and the contaminate). SCC is extremely dangerous as it can lead to disintegration of the affected part, including discs, rotors and turbine shells.
Strict monitoring of boiler chemistry can provide early detection of these contaminants. An ongoing water conditioning program can remove the contaminants before they become hazardous. A better approach is to remove the source of the contaminants.
Boiler feedwater can be a mixture of makeup from primary treatment, condensates returning from the turbine and condensates returning from process steam. Each of these can contain its own share of contaminants. A boiler can accept this feedwater and still produce steam containing less than .05 ppm of solids. However, this feedwater should never be used for attemperation as the contaminates will be introduced straight to the turbine. Water for attemperation should be close to distillate quality.
Dissolved solids from boiler chemicals should be separated from the steam before it leaves the drum. Operating the boiler with too much water in the drum, or with foaming or priming in the boiler, will introduce too much water into the steam separator and reduce its efficiency. Efficient separators can approach less than .01 ppm of solids in the steam. Dissolved gases such as ammonia or CO2 cannot be separated and must be controlled in the boiler water.
Condensates can be contaminated by leaks in the heat exchangers used for process steam, or even incorrect piping of the chemical feed system. One should suspect process chemical leaks if organic, sulfide, ammonia, amine or copper contaminants are detected. Suspect leaks in the chemical feed system for the drum if excess OH alkalinity or phosphate is detected.
Extreme care should be exercised when using volatile acids (such as hydrochloric acid) when cleaning the the condenser. Acid fumes entering low pressure areas of the turbine can condense in confined areas such as blade roots and cause stress corrosion. The proper procedure is to close the joint between the condenser and the low pressure areas with plastic or fabric to form a vapor barrier. All remaining acid should be neutralized and removed from surfaces.
Contamination can also be introduced during inspection and repair of a turbine. Manufacturing fluids, lubricants and preservatives can contain sulfur or chlorine which can decompose into acids. Machined areas and replacement parts should be cleaned with solvents (such as denatured alcohol) and then dried off. NDE/NDT chemicals, especially the dye penetrant Zyglo, can also decompose into acids. Parts inspected with these methods should also be solvent cleaned and dried. Environmental pollutants can introduce solids (dirt) and acid forming compounds. Exposed turbine steam path components should be protected by plastic or cloth until reassembled.
Potential turbine contamination by either deposits or acids should be addressed as soon as it is suspected. Several methods for confirmation and ameliorization are available, depending on the type and degree of the contaminant.
Please contact Mr. Turbine® for advice if you suspect or encounter turbine contamination.
Install Flux Probe with Generator Rotor-In
/in Generator Tips /by Mike.LakeShorts are commonly caused by the dielectric breakdown of the turn-to-turn insulation system within the main field windings. This failure can result from movement and fretting of the conductors caused by coil foreshortening, end-strap elongation, or inadequate end-turn blocking. Metal particles from erosion or rubbing (copper dusting) can also form new and undesirable pathways between turns.
The magnitude and location of turn-to-turn shorts play heavily in whether or not they will even be noticed. Indeed a great many shorted rotors have operated without issue for years. It is those rotors with turn shorts nearest the poles that are most likely to become thermally sensitive; this caused by asymmetrical heating, rotor bowing, and associated vibration. Shorts can also produce unbalanced magnetic forces, which will increase mechanical stresses. This combination of thermal and mechanical stresses can create more shorts and an accelerating pattern of degradation. On-line monitoring can measure this degradation and signal the increasing need for remediation before a forced outage occurs.
For more information, or for a proposal to install a flux probe in your generator, please contact your TGM® Regional Account Manager at 800-226-7557
"Off The Job" Safety
/in Safety Tips /by Mike.LakeAccidents away from work account for 70 percent of all deaths and 55 percent of all injuries to workers. Your contribution would be difficult to replace if you were injured either on or off the job. Add this to the fact that as a spouse and/or a parent you are priceless to your family, so it is easy to see why a 24-hour safety effort is necessary.
The highways are prime areas of concern for safety away from work. Watch your speed on the road. Be patient getting out of the parking lot, and always watch the other driver.
Most of us are do-it-yourselfers around the house and this is where a lot of people are injured. Be careful when using ladders. Make sure your ladder is safe before climbing it – do not overreach or climb too high.
When using tools, pick the right tool for the job, do not use a tool if it is in poor condition. Power tools should be grounded with a three-pronged plug or double-insulated. Remember to stay off wet surfaces when using electric power tools. Always use PPE just as if you were on the job.
Watch weather conditions. For Northern employees, do not overexert yourself when shoveling snow and for Southern employees, do not work too long in the hot sun, especially if you have had a hard week on the job.
(Safety tips provided by Insperity Support Services)
RTD vs Thermocouple – Which is Best?
/in Controls Tips /by Mike.LakeResistance Temperature Detectors (RTDs)
The electrical resistance of metals rises as the metals become hotter, and falls as heat decreases. RTDs are temperature sensors that use the changes in the electrical resistance of metals to measure the changes in the local temperature. For the readings to be interpretable, the metals used in RTDs must have electrical resistances known to people and recorded for convenient reference. As a result, copper, nickel, and platinum are all popular metals used in the construction of RTDs. The easiest way to identify an RTD is by its wire leads. RTDs most often have three wires coming out of them, two of the same color and one of a different color, usually two white wires and one red wire. They can be of other colors but these are the type we most often encounter on a turbine. RTDs can have two wires, however they are not often used in industry any longer as they are not as accurate as three wire sensors.
Thermocouples are temperature sensors employing two dissimilar metals to produce a small voltage that can be read to determine the local temperature. Different combinations of metals can be used in building the thermocouples to provide different calibrations with different temperature ranges and sensor characteristics. They are classified as a “type” of thermocouple such as E, J, K, etc. The different types are made from various types of metals and are used for a wide range of temperatures. The type of thermocouple can only be identified by its lead wire color. Thermocouples always have two wires with dissimilar colors.
RTD vs Thermocouple
Because the terms encompass entire ranges of temperature sensors tailored for use under a range of conditions, it is impossible to conclude whether RTDs or thermocouples are the superior option as a whole. Instead, it is more useful to compare the performance of RTDs and thermocouples using specific qualities such as cost and temperature range so that users can choose based on the specific needs of the application.
In general, thermocouples are better than RTDs when it comes to cost, ruggedness, measurement speed, and the range of temperatures that can be measured. Most thermocouples are half the cost of RTDs. Furthermore, thermocouples are designed to be more durable and react faster to changes in temperature. However, the main selling point of thermocouples is their range. Most RTDs are limited to a maximum temperature of 1000 degrees Fahrenheit. In contrast, certain thermocouples can be used to measure up to 2700 degrees Fahrenheit.
RTDs are superior to thermocouples in that their readings are more accurate and more repeatable. Repeatable means that the same temperatures produce the same readings over multiple trials. RTDs produce more repeatable readings which are more stable, while their design ensures that RTDs continue producing stable readings longer than thermocouples. Furthermore, RTDs receive more robust signals and it is easier to calibrate RTD readings due to their design.
Conclusion
In brief, RTDs and thermocouples each have their own advantages and disadvantages. Furthermore, each make of RTDs and thermocouples possesses its own advantages and disadvantages. In general, thermocouples are cheaper, more durable, and can measure a larger range of temperatures, while RTDs produce better and more reliable measurements.
Overhead Loads
/in Safety Tips /by Mike.LakeLet’s face it, our job is dangerous within itself – we don’t need Murphy’s Law in the mix as well. Be aware of the load at all times, no matter how large or small it is. Remind yourself of this slogan the next time a load is lifted – “IF IT’S IN THE AIR, IT’S DANGEROUS”.
Let’s review some of the rules that can help keep us from getting injured by failing loads:
REMEMBER …..”If it’s in the air, it’s dangerous.”
Correct Gap Critical to Rotation Speed
/in Controls Tips /by Mike.LakeMaintaining the correct gap between the sensor and the rotor is critical to correctly measuring turbine rotor speed. Setting the gap can be problematic if the correct gap is unknown or if it cannot be accessed with a feeler gauge.
First a little background on why the sensor gap is critical: The sensor, or “pickup”, is mounted perpendicular to the shaft, facing a toothed gear fixed to the rotor. Pickups can be either “active” or “passive” (see below). In either case, the pickup counts the teeth by sensing the difference in height between the tip of the tooth and the valley between the tooth. If the sensor is too close, it can’t reliably distinguish between the tip and the valley. If it is too far away, it can’t reliably register the tip. The correct gap will register a voltage differential which can be counted. The electronic circuits determine shaft rotation speed by dividing the number of teeth on the gear into how fast the voltage changes over a period of time.
Always use the manufacturer’s specification data to determine the correct gap spacing for the pickup. If the specification is not available, an initial setting of 0.025″ (0.64 mm) will work in most cases. Once the unit is running, the controls engineer can determine if the output voltage or signal level is sufficient for the type of control used.
Sometimes the pickup gap cannot be accessed with a feeler gauge. If so, an accurate setting can be obtained with a little math and a technique called “counting the flats”. The two most often found pickup sizes are the 5/8″ – 18 and the 3/4″ – 20 thread sizes. If you do the math, the 18 threads per inch (TPI) device will move 1 inch in the mounting hole if it is rotated 18 times. Looking at it the other way, it will move in the hole 0.055 inches if it is rotated one time. Breaking it down further, it will move 0.009″ if it is rotated one “flat” of the hexagonal shaped body or hex nut. In the case of the larger 3/4″ inch device, it will move 0.050″ per one rotation and 0.008″ per “flat”.
To “count the flats”, line up the tooth of the gear as close as possible to the center of the mounting hole until it looks like the picture above. Once the gear tooth is aligned with the center of the hole, screw the pickup down BY HAND until the face of the pickup gently contacts the tooth. Set the gap by unscrewing the pickup while counting the flats from a fixed reference point (can even be a line made by a Sharpie pen). For a 0.025″ gap unscrew it by 2 ¾ flats. The math would be 0.025″ (gap) / 0.009″ (movement per flat) = 2.77 flats or approximately 2 ¾ flats. For the larger pickup size that would be 0.025″ / 0.008″ = 3.1 flats or just tad over three full flats. Tighten up the locknut and you’re done!
There are many different types of pickups out in use today but the most common types used on steam and gas turbines are the “passive” type (sometimes called inactive pickups) and the “active” type. They look very similar but operate quite differently. Very simply, the typical magnetic or “passive” pickup is simply a coil of fine wire wrapped around a magnetized iron core that self generates a voltage. When the tooth of a gear passes in front of the iron core, a small voltage is generated and when the valley between the teeth passes the iron core, the voltage falls off. The “active” type pickup receives power from an outside source instead of self generating it. There is a small transmitter and receiver inside of the device that sends out a signal from the end of the pickup. When a gear tooth passes this signal, it changes the characteristic of the signal that is reflected back to the receiver. The internal electronics then interpret this and send out a voltage pulse. Although the passive sensor generates a sine wave and the active sensor generates a square wave, both sensors count cycles over time, which represents teeth rotation speed.
Please contact Mr. Turbine® for answers to any issue with Steam or Combustion Turbine Controls, or Generator and Exciter Controls for any motive power system.
Heat Stress
/in Safety Tips /by Mike.LakeWhy is heat a hazard to workers?
When a person works in a hot environment, the body must get rid of excess heat to maintain a stable internal temperature. It does this mainly through circulating blood to the skin and through sweating.
When the air temperature is close to or warmer than normal body temperature, cooling of the body becomes more difficult. Blood circulated to the skin cannot lose its heat. Sweating then becomes the main way the body cools off. But sweating is effective only if the humidity level is low enough to allow evaporation and if the fluids and salts that are lost are adequately replaced.
If the body cannot get rid of excess heat, it will store it. When this happens, the body’s core temperature rises and the heart rate increases. As the body continues to store heat, the person begins to lose concentration and has difficulty focusing on a task, may become irritable or sick, and often loses the desire to drink. The next stage is most often fainting and even death if the person is not cooled down.
Excessive exposure to heat can cause a range of heat-related illnesses, from heat rash and heat cramps to heat exhaustion and heat stroke. Heat stroke can result in death and requires immediate medical attention.
Exposure to heat can also increase the risk of injuries because of sweaty palms, fogged-up safety glasses, dizziness, and burns from hot surfaces or steam.
How can heat-related illness be prevented?
Heat-related illnesses can be prevented. Important ways to reduce heat exposure and the risk of heat-related illness include engineering controls, such as air conditioning and ventilation, that make the work environment cooler, and work practices such as work/rest cycles, drinking water often, and providing an opportunity for workers to build up a level of tolerance to working in the heat. Employers should include these prevention steps in worksite training and plans. Also, it’s important to know and look out for the symptoms of heat-related illness in yourself and others during hot weather. Plan for an emergency and know what to do – acting quickly can save lives!
Remember, refrain from alcohol intake the night prior and drink plenty of fluids during the shift.
Common Sense Safety
/in Safety Tips /by Mike.LakeThe experts say at least 80% of industrial accidents are caused by unsafe acts on the part of employees–and not by unsafe conditions. Although employers are required by law to provide a safe and healthful workplace, it is up to you to be aware of your work environment and follow safe work practices. Statistically, most accidents are caused by unsafe acts, including:
Being In A Hurry – Sometimes there is more concern for completing a job quickly instead of safely. Take time to do a good job and a safe job.
Taking Chances – Daring behavior or blatant disregard for safe work practices can put the whole work team at risk. Follow all company safety rules and watch out for your fellow employees. Horseplay is never appropriate on the job and can lead to disciplinary action.
Being Preoccupied – Daydreaming, drifting off at work, thinking about the weekend, and not paying attention to your work can get you seriously hurt or even killed. Focus on the work you are paid to do. If your mind is troubled or distracted, you’re at risk for an accident.
Having A Negative Attitude – Being angry or in a bad mood can lead to severe accidents because anger nearly always rules over caution. Flying off the handle on an outage is potentially dangerous. Keep your bad moods in check, or more than one person may be hurt. Remember to stay cool and in charge of your emotions.
Failing To Look For Hidden Hazards – At many jobsites, work conditions are constantly changing. Sometimes new, unexpected hazards develop. Always be alert for changes in the environment. Hidden hazards include spilled liquids that could cause slips and falls; out-of-place objects that can be tripped over; unmarked floor openings one could step into; low overhead pipes that could mean a head injury; and other workers who don’t see you enter their hazardous work area.
Remember to stay alert for hazards, so you won’t become one more accident statistic: You can do a quality job without rushing. Maintain a positive attitude and keep your mind on your work. This is just common sense–something smart workers use!
Copper Resistance Testing
/in Generator Tips /by Mike.LakeTest Setup & Execution. The main and neutral lead connections should be broken and open. The lead ends should be free and clean of surface contamination so that the test probes make good contact. Copper resistance testing is performed with a Digital Low Resistance Ohm Meter (DLRO) test set. The DLRO instrument is extremely sensitive. Poor contact and circuit set-up can either produce erroneous readings or no readings at all.
One probe of the DLRO is connected to one lead of an individual phase, and the other probe connected to the other lead of the same phase. A reading (generally to the fourth decimal place) in ohms resistance is measured and recorded. This same process is repeated on the second and third phase. The ambient air temperature and humidity should be recorded as well.
Interpretation of Results. Temperature significantly influences the resistance of a dielectric as well as a conductor. For this reason, the copper resistance measurement should be corrected to standard (typically 40°C).
The original equipment manufacturer normally records and documents the as-built phase-by-phase copper resistance measurements. These are used as the baseline by which all future readings can be compared.
An increase in in copper resistance indicates the presence of some form of high resistance issue (i.e. broken conductors, cold braze joints, turn-to-turn shorting, incorrect connection, incorrect number of turns or stranding, open circuit). Additional testing will be required to determine the specific cause of the variant reading.
Standard. IEEE Standard 11 8TM-1 978, IEEE Standard Test Code for Resistance Measurement.
Test Equipment. A Megger, Model DLRO-10 or comparable is recommended. Kelvin and Wheatstone bridges are also used to measure resistance.
Slips, Trips, and Falls
/in Safety Tips /by Mike.LakeMost common locations for falls:
Falls can be prevented – Ladder Safety