This is Part Three of a three part Turbine Tip series, discussing the most common steam turbine casing problems: cracking, distortion and erosion.
The final Turbine Tip in this series discusses two common steam turbine casing problems – Distortion and Erosion. The repair methods employed – grinding, mechanical repair, welding and stress relief – have their own set of considerations which were covered in previous portions of the series.
Casing Distortion becomes a strong likelihood when the units accumulate operating cycles. The most common causes of distortion are steady state and transient thermal stresses which can occur within all turbine sections (HP, IP, LP). Inner casings distort more easily than outer casings due to their thinner cross-section and higher temperature differentials across the casing walls.
6Distortion typically causes problems during disassembly and reassembly. Some examples of this are bolting interferences, gaps at the horizontal joint, galling of the fits and misalignment of the steam path seals. These problems can lead to steam leakage and rubbing. Internal leakage due to distortion reduces efficiency and power output, while leakage to atmosphere and internal rubbing can both cause a forced outage.
Water induction can cause extreme distortion of the inner cylinders. This can damage internal steam path components and lead to forced outages. Inner casings as well as valve bonnet covers can become severely warped and may require extreme measures to remove and replace.
Casing distortion can be corrected by welding, machining, localized heating and rounding discs inserted during stress relief. See previous Tips in the series for considerations in employing these methods.
Damage from erosion affects different designs at different locations, but both rotating and stationary components are vulnerable. Erosion typically takes place in the LP section where steam enthalpy drops below the saturation point. Crossover pipes and inlet areas to the LP section could increase in roughness as the surfaces wear unevenly. Support struts may thin or be cut through.
Moisture erosion can also take place in the exhaust ends of HP and IP sections if the turbine operates for long periods at low load or goes through frequent start-ups. Horizontal joints may erode and leak between stages and stationary blade support rings may erode as well as crack.
Casings, diaphragms, hoods and crossovers are usually made of carbon steel or cast iron. These materials erode approximately 20 times faster than blading material made out of 400 stainless steel.
Erosion can contribute to major damage. Repairs must be aimed at improving the erosion resistance of the steam path and support surfaces. Methods also must be examined for reducing steam moisture content and the size of droplets.
Eroded areas can be rebuilt. Stainless steel or other erosion resistant weld metal can be applied to eroded seal surfaces such as horizontal joints, flow guides and diaphragm inner and outer rings and joints. Fabricated stainless steel liners can be welded inside of crossovers, seal areas and inlet flow areas of casings. They may also be applied over support struts to protect the existing cast iron, steel or low alloy castings.
No stress relief is required in most welding applications. Epoxy or ceramic coatings may be suitable for surfaces that are not suitable for weld overlay.
For more information on your particular application, please contact us at (864) 671-1443 .
This concludes our Turbine Tip series, but we invite you to continue reading our PSG blog for more useful information.
Power Services Group Successfully Executes Gas Turbine Major Maintenance in Africa
/in News /by Mike.LakePSG has had a strong start to 2018, with over 70 members of our field service team in Africa performing major maintenance on four combustion turbine-driven compressor trains in a LNG processing facility. Contact us to discuss how Power Services Group can execute your combustion or steam turbine maintenance safely, on schedule, on budget, and with the highest quality.
Power Services Group Attends HRSG Forum 5-7 March 2018
/in News /by Mike.LakePower Services Group is attending this year’s HRSG Forum in Houston, Texas. If you’re attending please stop by booth #42. We look forward to seeing you.
Inadequate Oil Supply: Measure the Cause, Not the Symptom
/in Steam Turbine Tips /by Mike.LakeDirectly measuring bearing metal temperature is the most effective way to really determine if a bearing is running hot.
Bearing oil drain temperatures are still being utilized on older machines. By the time the bearing oil drain temperature has increased, the bearing may have already been compromised (wiped). PSG recommends that these older machines should have temperature probes (thermocouples or RTD’s) installed in the bearing Babbitt to properly monitor performance.
Keep in mind that if the temperature rises abruptly and unexpectedly, the bearing may have been compromised and immediate action needs to be taken. Gradual temperature changes which trigger the alarm may be the result of other factors but are still a concern and should be thoroughly investigated.
The second alarm should be set at the maximum operating temperature of the bearing material. Operators should manually trip the unit in a controlled manner as soon as possible after this second alarm sounds and determine the cause.
The critical temperatures for each of the two levels can be supplied by the manufacturer or recommended by PSG for your individual unit configuration. Different temperature ranges are recommended for Tilt Pad, Elliptical, Short Elliptical, and Thrust bearings.
Measuring drain oil temperature is too slow and too imprecise to effectively minimize your overall cost of maintenance. Taking all of this into consideration, the best practice is to retrofit your machine and save your bottom line.
Do you have questions about your steam turbine backup system? Contact PSG today to explore how we can provide support and maintenance options to help you avoid backup system problems.
Inadequate Oil Supply: Don’t Kill Your Turbine on Startup
/in Steam Turbine Tips /by Mike.LakeYour lube oil temperature needs to be lower at startup and shutdown than at full speed to reduce potential issues.
Your turbine’s rotor does not actually ride on the surfaces of its bearings. It rides on a thin film of oil between the rotor and the bearing. At high turbine speeds the rotor hydroplanes across the oil, eliminating contact with the Babbit of the bearing. The heat generated by the turbine decreases the viscosity of the oil and increases its “slipperiness”, which is important at high speeds.
Failure to lower the lube oil temperature (and therefore increase viscosity) can result in light bearing wipes or smearing. These conditions would occur during turning gear operation, unit startup and unit coast down during shutdown.
The ideal lube oil temperature at these lower speeds is 90 degrees Farenheit. Of course, oil temperature can also be too cold on startup—similar to trying to start your car on a cold winter day. Operational personnel are ultimately responsible for maintaining this lower lube oil temperature by regulating water through the lube oil coolers.
Maintaining lube oil cooler cleanliness is also very important for turbine startups. The tubes must be clean to allow the efficient transfer of heat. Also, as a best practice the bundles should be cleaned every two (2) years. Lube oil coolers are the single most common area for contaminants to hide.
By following these tips, you can ensure the efficient startup of your turbine, as well as greatly reduce any potential operational issues or challenges.
Do you have questions about your steam turbine backup system? Contact PSG today to explore how we can provide support and maintenance options to help you avoid backup system problems.
Power Services Group (PSG) – the Best Alternative to the OEM for Alstom GT24 Gas Turbine Maintenance
/in Combustion Turbine Tips /by Mike.LakePSG’s leading Subject Matter Expert on Alstom GT 24 ICS equipment, Bob Fischer, comments “The demand for GT24 maintenance services is dramatically increasing as the OEM experiences a shortage of skilled and experienced resources and customers are looking for cost effective alternatives. With the GT24 fleet reaching maturity, more owners are searching to reduce their OEM dependency.”
If you are interested in starting a discussion on how PSG can help you be successful with our gas turbine maintenance options, please visit our website to request more information, or email us direct at inquiries@powerservicesgroup.com.
Inadequate Oil Supply: When a Backup isn’t a Backup
/in Steam Turbine Tips /by Mike.LakeThe International Association of Engineering Insurers has found that the loss of oil pressure causes the highest frequency of failure in steam turbines worldwide.
Modern turbines have backup powered DC oil pumps mounted on the oil tank, which are triggered by a pressure switch in the event of a loss in oil pressure. With this in mind, it is very important to conduct tests with the AC and DC oil pumps during scheduled maintenance inspections to ensure that the DC pump engages as required.
Such tests can be referred to as cascade pump pressure inspections. In addition, the tests will confirm the pressures when the DC oil pump will engage after the AC oil pump is actually turned off.
Another best practice is to verify backup batteries on a regular basis, when the unit is down, and mandatory tests should be performed before the unit is placed in operation after an overhaul.
Older turbines can use steam-driven pumps as backup. On these designs, a pressure regulator will sense the drop-in bearing oil pressure and turn on the steam supply to the blade wheel of the pump. But while these pumps are usually very reliable, they still must be manually tested on a regular basis and after an overhaul.
Care must also be taken to not overspeed the pump or it will potentially cause internal component damage and may even completely destroy the pump.
Some older turbines use gravity lube oil tanks. These tanks are mounted above the unit on stands and are controlled by a check valve type of arrangement. In such cases, there are no pumps involved—gravity provides the bearings with sufficient lubrication in an emergency situation. While less complicated than DC or steam powered backups, their operation must still be routinely checked.
The bottom line is, that a backup is not a backup unless it is reliable. And it can only be reliable if it is tested.
Do you have questions about your steam turbine backup system? Contact PSG today to explore how we can provide support and maintenance options to help you avoid backup system problems.
Power Services Group recently completed a 500 hour base bolt tensioning project
/in News /by Mike.LakeAlstom STF Steam Turbine
/in Steam Turbine Tips /by Mike.LakeCasing Repair – Distortion and Erosion
/in Turbine Tips /by Mike.LakeThe final Turbine Tip in this series discusses two common steam turbine casing problems – Distortion and Erosion. The repair methods employed – grinding, mechanical repair, welding and stress relief – have their own set of considerations which were covered in previous portions of the series.
Casing Distortion becomes a strong likelihood when the units accumulate operating cycles. The most common causes of distortion are steady state and transient thermal stresses which can occur within all turbine sections (HP, IP, LP). Inner casings distort more easily than outer casings due to their thinner cross-section and higher temperature differentials across the casing walls.
Water induction can cause extreme distortion of the inner cylinders. This can damage internal steam path components and lead to forced outages. Inner casings as well as valve bonnet covers can become severely warped and may require extreme measures to remove and replace.
Casing distortion can be corrected by welding, machining, localized heating and rounding discs inserted during stress relief. See previous Tips in the series for considerations in employing these methods.
Damage from erosion affects different designs at different locations, but both rotating and stationary components are vulnerable. Erosion typically takes place in the LP section where steam enthalpy drops below the saturation point. Crossover pipes and inlet areas to the LP section could increase in roughness as the surfaces wear unevenly. Support struts may thin or be cut through.
Moisture erosion can also take place in the exhaust ends of HP and IP sections if the turbine operates for long periods at low load or goes through frequent start-ups. Horizontal joints may erode and leak between stages and stationary blade support rings may erode as well as crack.
Casings, diaphragms, hoods and crossovers are usually made of carbon steel or cast iron. These materials erode approximately 20 times faster than blading material made out of 400 stainless steel.
Erosion can contribute to major damage. Repairs must be aimed at improving the erosion resistance of the steam path and support surfaces. Methods also must be examined for reducing steam moisture content and the size of droplets.
Eroded areas can be rebuilt. Stainless steel or other erosion resistant weld metal can be applied to eroded seal surfaces such as horizontal joints, flow guides and diaphragm inner and outer rings and joints. Fabricated stainless steel liners can be welded inside of crossovers, seal areas and inlet flow areas of casings. They may also be applied over support struts to protect the existing cast iron, steel or low alloy castings.
No stress relief is required in most welding applications. Epoxy or ceramic coatings may be suitable for surfaces that are not suitable for weld overlay.
This concludes our Turbine Tip series, but we invite you to continue reading our PSG blog for more useful information.
Casing Repair – Welding Considerations
/in Turbine Tips /by Mike.LakeWelding is a common method to repair turbine casing cracks, but it must be applied with consideration. Most turbine casing alloys can be welded using either of two distinct procedures: stress relieved and non-stress relieved. The procedure selected is often dictated by time and cost restraints.
Non-stress relieved welds have the advantage of lower cost and shorter outage time. The disadvantage is that the weld can be short lived. The procedure follows this outline: A preheat of about 500 degree F or greater is used. A shielded metal arc weld is performed with a non-matching high nickel content filler. This use of dissimilar metals as filler can lead to low cycle metal fatigue. No post-weld stress relief is performed but the preheat conditions are maintained throughout the process.
The pre-weld residual stress levels in the casing must be carefully assessed to increase the probability of a successful weld. The high levels of residual stresses in the casing can combine with the added stresses of welding to cause uncontrolled distortion and hot cracking during the stress relief phase. Residual stresses generated by the weld passes can be reduced through techniques such as grinding, peening between passes, and peening and grinding. Therefore, the welding procedure must be performed by a skilled contractor.
The best way to control distortion during stress relief is to bolt the casing halves together and place the assembly in the furnace. This would be most applicable to an inner casing that can be easily removed from its outer casing. If only the upper half of the casing is going to be repaired, a thick plate can be bolted onto the horizontal joint as a substitute for the lower case. Distortion can be further controlled by inserting custom fabricated rounding rings or disks into the assembly before thoroughly bolting it together.
If the facility has ample room, a portable furnace can be built on-site. Otherwise, the assembly must be sent out for this process. If the assembly is too large for the furnace, stress relief can be done on a local area of the case, allowing suitable temperature gradients away from the weld areas. Whatever the location, the temperature of the furnace and the assembly must be stringently monitored during the entire stress relief process.
Multiple heat cycles and possible re-tightening of the joint bolting between cycles may be necessary. This is a process which has been refined over the years and continues to get better. Again, it is always a good practice to perform an assessment prior to performing any of the above procedures.
The next Turbine Tip in the series discusses Distortion and Erosion in casing repair.