Welding 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.
Stress relieved welding offers the best potential for a long repair life, but is complicated and time consuming. The procedure follows this outline: A lower preheat of about 300 degree F is used. A shielded metal arc or metal inert gas weld is performed with a matching metal content filler. The casing is then placed in a furnace and raised to a temperature of over 1,000 degrees F. The exact temperature depends on the alloy, the procedure and the application. Much higher temperatures may be required. There are no problems with differential expansion during turbine operation since the weld uses matching filler metal.
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 Generator Tip in the series discusses casing distortion and erosion problems. For more information on your particular application, please contact Mr. Turbine®.
Casing Repair – Part 3: Distortion & Erosion
/in Steam Turbine Tips /by Mike.LakeCasing Distortion
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. Distortion 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.
Erosion
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, diagrams, 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 contributes 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 Mr. Turbine®.
Casing Repair – Part 2: Welding Considerations
/in Steam Turbine Tips /by Mike.LakeNon-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.
Stress relieved welding offers the best potential for a long repair life, but is complicated and time consuming. The procedure follows this outline: A lower preheat of about 300 degree F is used. A shielded metal arc or metal inert gas weld is performed with a matching metal content filler. The casing is then placed in a furnace and raised to a temperature of over 1,000 degrees F. The exact temperature depends on the alloy, the procedure and the application. Much higher temperatures may be required. There are no problems with differential expansion during turbine operation since the weld uses matching filler metal.
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 Generator Tip in the series discusses casing distortion and erosion problems. For more information on your particular application, please contact Mr. Turbine®.
Knife Safety
/in Safety Tips /by Mike.LakeAll cuts should receive first aid. Even the smallest cut can become infected, so treat all cuts properly. Always use a knife only for what it is intended. Never use it as a screwdriver or pry bar. Never use a knife that is defective. Keep knives sharp and in good condition. A dull knife can cause you to put too much pressure on the object you are trying to cut. The blade could slip and slice you or someone nearby.
The principal hazard when using a knife, whether on or off the job, is that the user’s hand may slip from the handle onto the blade, causing a painful and serious injury. A handle guard will reduce this hazard. Another cause of injury is the knife striking the free hand or the user’s body.
Industrial knife safety principles remind us to always make a cutting stroke away from the body when possible. Adequate protection should be worn to protect the body and provisions made to hold the material steady. Steel-mesh gloves are available in select industries, such as meatpacking, where materials must be held in close proximity to the knife. TGM carries these steel mesh gloves in every tool set we own. We are in the process of getting Kevlar gloves as well.
When on the job, carry a knife in a sheath or holder over the right or left hip, pointing backwards. Otherwise, a fall could cause a serious leg injury. Storage of knives is also an important safety factor. Cutting edges should be covered and not exposed. Knives should be kept in their proper place and not left on benches or on the floor.
If you are using the right knife for the job, it should cut without great difficulty. When you have to resort to force to make a knife cut, then you are headed for trouble–it could result in an injury to you, damage to the knife, or damage to the material that you are attempting to cut. Remember this, “our patience will achieve more than our force.” That is a good point to remember when using a knife.
Forklift Safety
/in Safety Tips /by Mike.LakeAccording to OSHA, forklift overturns are the leading cause of fatalities related to the use of forklifts and result in 25% of all forklift deaths. Other incidents that are associated with using forklifts include falling from a forklift, loads falling on workers, not using a seat-belt and being ejected or not following the proper procedures while traveling on grades or ramps.
With the proper training forklift operators will gain the knowledge and skill required to create a safe environment. When operators do not have the proper training problems arise and often lead to accidents and deaths.
Ask yourself the following questions:
1. Have you received training to operate a forklift?
2. Do you know how to properly report damage or problems during your shift?
3. Are you aware of any mechanical issues before operating a forklift?
4. Do you know how to properly operate a forklift on grades and ramps?
5. Do you know what to do if your forklift is overturning?
6. Do you know how to determine load capacity?
7. Are you using seat-belts properly while operating?
8. Have you received training on proper fueling?
9. Do you know how to properly mount and dismount a forklift?
10. Do you have the necessary information needed to comply with OSHA regulations?
(Courtesy of Crane Tech)
Casing Repairs (Part 1: Cracking)
/in Steam Turbine Tips /by Mike.LakeCracking is the most common problem on utility units built before 1970. Cracking typically occurs at the steam inlet areas on the HP and IP sections, where transient thermal stresses can exceed the yield point of the casing material. Cracking may be found on the interior surfaces of steam chests, valve bodies, nozzle chambers, seal casings, diaphragm fits and bolt holes. In the low pressure section (LP) cracking can also occur at the inlet sections, inner casings, support struts, bolt holes and diaphragm fits. Computer modeling and advanced alloys have reduced the likelihood of cracking in more modern units, but cracks can develop in any unit, especially those experiencing more stop/start cycles.
Every crack must be fully analyzed before attempting repairs. NDE inspection must be performed at a minimum. Radiograph inspections may provide greater assurance by revealing the extent of the crack in relation to its location and the thickness of the surrounding area. Some OEM’s have a detailed customer letter on known areas of potential cracking, their particular process to map out these cracks, and the proposed corrective action and potential life expectancy.
Although grinding is a common repair method, it can increase the potential for new cracks if improperly applied. Cracks in steam chests can potentially expand, making repairs more costly. Grinding on cracks in older machines may open up hidden voids in the casing, making the condition much worse. Another problem is that even when an NDE shows that cracks have been removed by grinding, very small undetectable cracks may still be present and may lead to future larger cracks.
Welding of cracks is another common repair method. There are two distinct procedures for welding: stress relieved and non-stress relieved. Non-stress relieved weld repair has the advantage of shorter outage duration but can fail much sooner than a stress relieved weld. This complicated topic will be discussed in our next Turbine Generator Tip in the series.
Mechanical Repairs can be applied to cracks, but must be properly designed to redistribute tensile loading away from the crack area. One method is to apply stitches. Metal inserts are placed across or along the crack and drilled and pinned to the case (see picture). Another method is to place bars or dog bone shapes across previously ground out areas. A more effective version of this method uses precision machining and the application of a lobe-lock designed through finite element analysis. The material used must provide adequate load properties and must be ductile at all temperatures to prevent cracking of the lobe-lock.
Mechanical repairs have several advantages. The repairs can be performed in place, with no possibility of casing distortion because there is no heating or welding. Machining durations are shorter and easier to quantify. These repairs can also extend life to the area (vs. welding). Disadvantages are that the mechanical repair is conducted on a low cycle fatigue crack and concentrated in an area surrounded by non-cracked material.
The next Turbine Generator Tip in the series discusses stress relieved vs. non-stress relieved welding. For more information on your particular application, please contact Mr. Turbine®.
Crane Safety Revisited
/in Safety Tips /by Mike.LakeJust when you think you have all the bases covered, the “impossible” happens. During a lift on one of our recent outages, the idler pulley and mounting bracket for the drive on the overhead crane fell approximately 25 feet. Fortunately no one was injured. We have discussed the need for pre-outage crane inspections in previous Safety Tips, identifying the need for an OSHA-compliant inspection before the outage. The customer had conscientiously performed the inspection, and we had examined the report. In a post-incident report, the company’s preferred crane inspection vendor discovered the root cause was an alignment issue with the driven sprocket for the trolley. The company implemented two corrective actions: the crane vendor installed a safety cable to prevent the bracket from falling in the event of a failure, and an alignment protocol was added to the inspection checklist. Discussions are continuing on additional preventive measures.
For TGM’s part, we recognize that we have a responsibility to not only request the inspection report, but to read and analyze it. We have discovered that this crane vendor includes an Inspection Report Key along with the report which gives a Priority Code and a Condition code for each finding. These codes help identify the importance of the finding and the need for corrective action. These codes are specific to this particular vendor. Others may use a different key or not have one. We will now be asking for an Inspection Report Key in addition to the report so we can double check that all deficiencies have been corrected. We will also be asking for the report with enough lead time such that any corrections can be made before the outage.
AC & DC High Potential Testing Fact & Fiction
/in Generator Tips /by Mike.LakeWhich is better: AC or DC Testing? Will these tests hurt my generator? Here are the facts:
In conclusion, AC high potential testing is better suited for acceptance proof testing of new equipment (i.e. coils, bars, and stator windings). DC testing is better suited for in-process proof testing, maintenance proof testing, and controlled over-voltage testing of new and/or used equipment. AC or DC high potential testing should not be used as the sole source of diagnostic or acceptance data. An experienced and qualified generator testing specialist will recommend a testing protocol specifically suited to your machine’s life cycle and operating history.
* A one minute AC or DC high potential test equates to approximately 11 hours of useful life expended. Assuming that a stator winding has a nominal 30-year life expectancy (263K hours), this would equate to only a 0.0004% reduction in service life per test.
Hose Connector Failures
/in Safety Tips /by Mike.LakeOnce again we are inviting comments on a recent safety concern and TGM’s efforts to remedy it. A 1” air hose came loose from the connector fitting. The hose whipped across the floor, narrowly missing two mechanics, before a quick thinking mechanic shut off the supply.
Our hoses were equipped with the fitting shown on the left. After discussion with our supplier, we have chosen to replace this with the crimped sleeve connector shown on the right. The connector is affixed with a computer operated machine. The operator inputs the inside and outside diameters of the hose, the type of hose and the connector used. The machine calculates and applies the correct pressure for a good crimp.
According to our supplier, most customers prefer the old double banded style connector because when a hose gets soft (usually at the ends by the connector) they can cut it and band it back together and save the cost of a new hose.
What do you think? Are we being too cautious? Is this a problem in your shop? How do you handle it? Please click the link below and join the discussion.
Direct Current High Voltage (Potential) Testing
/in Generator Tips /by Mike.LakeProof testing qualifies an insulation system to hold a specific voltage. By definition, it is a pass-or-fail evaluation – there is no diagnostic value. Acceptance proof testing is performed on new stator winding as part of in-process and final acceptance testing. Maintenance proof testing is performed on existing equipment, typically at 75% of the Acceptance test voltage. Although proof testing is pass/fail, the microampere readings for each individual phase should be measured and recorded, and gross variations should be noted and investigated further. An initial failure may be the fault of the test setup, as high resistance leakage to the atmosphere or to ground can occur. The quality of the insulation system should be questioned only if corrections to the test setup do not result in improved test results.
In Controlled Over-Voltage testing, the applied voltage is either “stepped-up” manually and incrementally, or “ramped-up” automatically and fluidly, over time. This provides a degree of diagnostic value since the measured current can be graphed, phase-by-phase, for comparative analysis. Controlled Over-Voltage testing affords the ability to salvage a winding that might otherwise fail in Proof testing. In plotting the measured current in real time, the operator may witness an exponential increase in current (indicating an impending insulation breakdown) and terminate the test.
DC stepped voltage and ramped voltage test results can be evaluated on a pass/fail basis, but important results can be obtained by a more thorough examination. The associated plotting might show evidence of a weakness. The higher the voltage level that presents the indication, the better the quality of the dielectric. Comparison phase-to-phase might show that one or more are comparatively high in resultant current. Most importantly, routine testing and comparatives can provide insight into the degree of insulation aging, and help predict end of useful life.
All of these tests should be performed in accordance with IEEE Standard 95™-2002, IEEE Recommended Practice for Testing of AC Electric Machinery (2300 V and Above) With High Direct Current.
Exploding Sockets Revisited
/in Safety Tips /by Mike.LakeRecently, one of our sockets exploded while loosening a bolt using a HyTorc hydraulic wrench. No one was injured, but we take these incidents seriously, generating a “Near Miss” report and subsequent corrective action.
We discussed exploding sockets in a previous Safety Tip, which you can review in the May 8, 2012 post (see below). Since that post, we purchased TorcUp sockets for all our tool sets, and sprayed them yellow to make sure they were exclusively used with our HyTorc heads. One of these dedicated sockets broke under load. Please comment (below) on additional steps we can take for our corrective action. We need your experience and expertise.
Additional information: The broken socket was less than a year old. It was a 1” drive, 1 5/8” 12 point socket under an 8000 PSI load. We chose TorcUP sockets as they were the middle of the road in pricing of the three vendors we reviewed (one was HyTorc).
All of us face similar safety concerns in our operations. If you would like one of your concerns discussed in this forum, please contact us via Mr. Turbine (click here) and we will start the conversation in our next newsletter. We will not share your contact info but you can also post anonymously. Mr. Turbine will be happy to give an immediate response if you request it, but of course we must have your correct info.