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.
Overlooked Gland Seal Can Be Big Trouble
/in Steam Turbine Tips /by Mike.LakeNo matter the configuration of your system, there is a delicate balance between the high pressure and low pressure ends of the turbine. The labyrinth-type seal rings in the gland housings are designed for a certain amount of pressure drop which coincides with the designed operating conditions of the unit. If too much vacuum is being drawn across the seal rings premature wear and loss of vacuum will be experienced at the labyrinth seal. If too much pressure is present at the labyrinth seal then steam leakage, corrosion and premature wear will be evident.
We recommend checking the gland system for proper operation during all scheduled inspections. Corrections to the system will usually be performed during a major inspection, when all of the components are accessible. Maintenance items can include partially plugged gland leak off lines, improperly adjusted balance butterfly valves, severe wear of the spray chamber nozzle, worn out vacuum pumps, gland condenser tube leaks, eroded air blower impellers and malfunctioning steam seal regulators.
Neglect of the perceived small things can lead to bigger and more costly problems. All of the auxiliary systems that support the steam turbine are of a critical nature especially when overlooked and not properly maintained.
Compressor Failures – Stator Vane Lock-up
/in Combustion Turbine Tips /by Mike.LakeSome compressor failures have been attributed to “lock-up” of the stator vanes. The vane roots are designed to rock slightly at their roots when moved with your hand. Rust and debris can inhibit this movement. An immobile vane changes the stress profile on the vane which can cause cracking and potential failure. Mechanics should check for proper movement at each inspection.
Removing locked-up stator vanes has been a challenge for service providers. The OEM has come up with material upgrades to address the corrosion problems in these areas. The compressor upgrades are designed to reduce the potential for compressor failures and lock-up of the stator vanes. Some providers use destructive methods to cut out the vanes, potentially damaging the compressor casing. The OEM recommends the use of their (expensive) special tooling to reduce and hopefully prevent casing damage. An alternate method that TGM®has used very successfully is a process of heating and quenching. Heating the locked-up stator vane segments with a torch and quickly quenching the vane with cold water will typically free the vane segment for removal without damaging the casing. The process of heating and quenching (applied by an experienced team) can save time and money while reducing and hopefully eliminating damage to the compressor casing. TGM® can provide its expertise in removing stubborn stator vane segments and, if needed, quickly replace damaged stator vanes to get the customer back on line as soon as possible.
TGM®‘s experienced combustion turbine Technical Directors and crews often delight customers by providing innovative methods that can efficiently and permanently solve issues and get the customer back on line to make power. Early detection of shim migration and stator vane problems can be performed by TGM®through borescope inspections and eddy current NDE.
TGM®‘s comprehensive borescope diagnostics can provide recommendations and solutions to address many other compressor issues. Contact Us to find out how we can help.
Safety and Hand Grenades
/in Safety Tips /by Mike.LakeOur mechanics were using a striking wrench to loosen bolts on a turbine. One mechanic held a rope attached to the end of the wrench in order to apply a slight torque to keep the wrench seated on the nut. Another mechanic struck the wrench with an 8 pound hammer. Naturally, the holder of the rope is standing along the path of the hammer swing. The rope is normally long enough to position the mechanic well outside the hammer’s path. In this recent case, space was tight and the rope was too short. No problem if every hammer blow strikes the wrench. The mechanic was confident, declaring “I never miss!”. Except he did, and struck his partner in his safety glasses. His partner suffered a slight cut from the glasses but was
otherwise OK. (See adjacent picture.)
Our solution is a specialized tool which allows a mechanic to stand perpendicular to the path of the hammer blow while holding the striking wrench. The picture at the top of the article shows a mechanic setting the wrench on a nut. He will get out of the way after another mechanic grabs the end of the tool. A second mechanic will strike the wrench. An added benefit is that the wrench will not go flying if it is dislodged from the nut.
Hydraulic wrenches are also used to remove nuts in close quarters. TGM® uses this tool where warranted. Hydraulic wrench manufacturers maintain explicit warnings regarding their use and require operators to have specialized training. The hydraulic sockets can shatter even when used properly. We have experienced several Near Miss incidents in their use, and have discussed the dangers in several other Turbine Tips. (See below).
Please Contact Us if you would like more information on procuring or using any of these tools.
Protect your turbine with a good coat
/in Combustion Turbine Tips /by Mike.LakeTBCs are designed to reduce the temperature of the buckets and stators while providing resistance to corrosion and reducing oxidation of the component. TBCs form aluminum-oxide and chromium-oxide scales and act as a physical barrier to reduce component temperatures, extending the life of the parts. These TBCs are subjected to mechanical stresses, and spallation (coating separation) can occur. If a significant amount of TBC has separated from the metal, the parent metal will be exposed to the hot combustion gases and component degradation will be accelerated.
Hot gas path components should be inspected for TBC spallation at every Hot Gas Path or Major outage. Streaks of brown lines that appear to be coming from the cooling holes are a good sign that the component is receiving adequate cooling air, and spallation is at a minimum. When receiving components from the repair shop it is important to carefully inspect the components for an even coat of TBC. Also inspect the cooling air passages for debris and proper sizing to ensure that proper air flow can pass through the tiny passages for designed cooling.
When a water wash fails to recover efficiency…
/in Combustion Turbine Tips /by Mike.LakeSome users experience no gain in gas turbine output or improvement in heat rate even after a Major outage. A likely suspect is the first row nozzles; refurbished or degraded nozzles may be contributing to the problem. In many large combustion turbine frames such as the GE 7FA, the amount of compressed air used for cooling has a significant impact on gas turbine efficiency. The first row nozzles in a 7FA can use up to 20% of the air developed by the compressor for cooling. Turbine efficiency can be greatly reduced if these cooling holes are enlarged and use too much air for cooling. First stage nozzle cooling holes may not have been checked for hole dimensions during the refurbishment process. This can also affect your ability to tune low NOx combustion systems. The amount of air being used by first row nozzles affects the fuel/air ratio, making it more difficult to tune the unit.
The only solution is to remove the first row nozzles and replace with new or refurbished first row nozzles with correct cooling hole dimensions. It is also worthwhile to check the first row nozzles when they come back from the repair shop to ensure the cooling holes are not blocked and are the correct dimension before installing them into the unit.
Your fuel bill (and possibly your EPA permit) will thank you for it.
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.”