Inadequate Oil Supply: Don’t Kill Your Turbine on Startup

This is Part Two of a three part series on steam turbine tips, discussing the challenge of inadequate oil supply.

Your 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.

Inadequate Oil Supply - Part TwoAs the rotor slows down, the oil needs to be more viscous to repel the force towards the bearing.

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

During the first half of 2017, PSG successfully performed a number of maintenance activities at combined cycle power plants with Alstom GT 24 Integrated Cycle System (ICS) turbine-generator equipment.  GT24 work executed by the PSG team for multiple customers throughout the continental U.S. and Latin America:

  • Generator field assembly and owner oversight
  • SSS clutch replacement
  • Bearing # 12 investigation and replacement
  • Owner Oversight during triple C-B-B-Inspection
  • Owner Oversight of HP and IP-LP rotor rework at OEM shop
  • GT 24 assembly supervision
  • Balancing and vibration engineering services

PSG’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

This is Part One of a three part series on steam turbine tips, discussing the challenge of inadequate oil supply.

The International Association of Engineering Insurers has found that the loss of oil pressure causes the highest frequency of failure in steam turbines worldwide. 

Inadequate Oil Supply - Part OneMost of these steam turbine failures are caused by an unreliable backup system to maintain oil pressure to the bearings should the primary AC-driven lube oil pumps fail. These AC motors are powered by either the turbine’s output or the grid—causing a failure if the turbine or generator trips—or if there is an external outage.

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

Power Services Group recently completed a 500 hour base bolt tensioning project for 122 Siemens 2.3 megawatt wind turbines for a customer in Kansas. Consider Power Services Group for all of your wind turbine bolt tensioning needs.

Alstom STF Steam Turbine

The Power Services Group, Inc. field service team recently completed a turnkey B-inspection on an Alstom STF steam turbine rated at 190 megawatts output. The work scope included disassembly, inspection and reassembly of the HP stop valve, HP control valve, IP stop valves, IP control valves, bearings #1 through 5, and the thrust bearing. PSG’s turnkey solution supplied outage management, technical direction, supervision, craft labor, outage tooling, consumables, non-destructive testing, and commissioning and start-up services. The project was aggressively scheduled for 10 days mechanical duration, with a two shift around the clock coverage. Completed safely and without rework, the project was one day ahead of schedule. Another great example of PSG’s expertise with Alstom steam turbines, we provide our clients with a proven and trusted alternative to the OEM.

Casing Repair – Distortion and Erosion

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.

Casing Repair – Distortion and Erosion Turbine Tips6Distortion 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.

Casing Repair – Welding Considerations

This is Part Two of a three part Turbine Tip series, discussing the most common steam turbine casing problems: cracking, distortion and erosion. 

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.

Casing Repair – Welding Considerations Turbine TipsStress 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 Tip in the series discusses Distortion and Erosion in casing repair.

Casing Repair – Cracking

This is Part One of a three part Turbine Tip series, discussing the most common steam turbine casing problems: cracking, distortion and erosion.

Most units can be repaired by grinding, welding or by pre-stressed mechanical methods.  Finite element calculations show that in many cases, repairs can overcome some of the original design weaknesses and extend useful life by up to 20 years. But before proceeding with a repair, understand the mechanisms of both the casing damage and the proposed repair.  Improper repair can be useless or worse.

Cracking 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. 

Casing Repair Cracking Turbine TipIn 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 and 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 reveal 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 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 across the crack.  Another method is to place bars or dog bone shapes across previously ground out areas. 

A more effective method uses precision machining and the application of a lobe-lock designed through finite element analysis. The material properties of the lobe-lock must be such that it provides maximum pre-load at a certain temperature and a reduced pre-load at that same temperature. The material must also 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).

A potential disadvantage is 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 Tip in the series discusses stress relieved vs. non-stress relieved welding.

Proper Bearing Loading Helps Reduce Turbine Equipment Failure

The main reasons for bearings to wipe are:

  1. Insufficient lube oil supply
  2. Low lube oil pressure
  3. Water in the lube oil.

Every once in a while a fourth cause appears: High bearing loading.

Proper bearing loading is calculated by the elevations of the bearings, component weights and shaft alignments (bending moments, lateral, torsional). These forces have a direct impact on the stress of the equipment and thus, need to be a part of the equation.

The OEM calculates the elevations and coupling alignments during the design process, based on the catenary curve (or sag chart). Calculations of bearing loadings and alignment are usually accurate based on the design engineers’ mathematical calculations and computer model for the rotor’s geometry, speed, weight, and bearing design.

The following Catenary Curve graphic depicts bearing loading:

Proper Bearing Loading

 

Most of the time, high bearing loading is caused by misalignment of the turbine power train from the original design. That is, some force has moved the components from their original alignments. This may include temperature, rotor vibration and other forces associated with the wear-and-tear of the operation.

The source of the bearing failure can be eliminated by carefully measuring and re-aligning to the original specifications. We have also seen examples where the original calculations were not accurate or after years of operation, the bearing pedestals had moved.

Recalculating bearing loading is an arduous and potentially expensive process, so all other contributing factors should be eliminated before attempting this course.

It is important to maintain proper bearing loading to avoid the worst-case-scenario of any impact damage, equipment fatigue, or even catastrophic malfunction. Preventative maintenance can help you be proactive and reduce any bearing challenges that can cause a more severe event in your operation.  On three bearing units, it is not uncommon to utilize a dynamometer to check bearing loading during alignment and their adjustments.

PSG can perform the recalculation and re-alignment without the participation of the OEM.  If you believe your turbine may be affected by any of the above mentioned reasons for bearing wipe, please contact us today for a free assessment of your current situation.

 

Power Services Group Announces the Addition of Orbital Energy Services and Turbine Generator Maintenance to its Portfolio of Service Companies

PSG presents an alternative to the OEM for plant maintenance services in the power generation and industrial process markets

ANDERSON, SC (PRWEB) March 22, 2017

Power Services Group, Inc. (PSG), headquartered in Anderson, SC, is pleased to announce the joining of the three existing Airco companies, headquartered in Pooler, GA, with Orbital Energy Services (OES), headquartered in Gainesville, GA, and Turbine Generator Maintenance, Inc. (TGM), headquartered in Cape Coral, FL as wholly-owned subsidiaries. These companies have a solid track record and reputation with over 75 years of combined operations.

The parent company for the combined entity will be known as PSG. Together, the companies will provide customers a broad maintenance, repair and overhaul (MRO) solution for all rotating equipment and other critical plant maintenance services. More specifically, PSG’s service offering includes field services, field machining, steam path repairs, valve repair and parts manufacturing, generator inspection and repair, balance of plant maintenance, and specialty code welding. Industries served include power generation (nuclear, fossil, hydro-electric, and wind), industrial, and petrochemical.

“PSG now operates as a turnkey solution provider, setting a new standard of service excellence with an attractive value proposition relative to our competitors. Our safety record, customer focus, proven team, broad service offering, flexibility, and engineered solutions allows us to partner with clients in a manner that has been missing in the marketplace for some time,” said Ronnie Onofry, CEO of Power Services Group.

“Our clients seek a customer-focused, flexible, one-stop-shop for engineering and maintenance services. PSG provides the technical competence required to deliver a more cost-efficient and user-friendly solution. We listen to our customers and work in partnership with them to address the many challenges they face in a very demanding operating environment,” according to Jim Miller, Board Chair of PSG.

PSG is proud to be one of the few remaining U.S.-owned, non-OEM companies in our space. We are confident you will find Power Services Group to be different from other industry players.

MEDIA CONTACT:
Don Dodds
Executive VP Sales and Marketing
Phone: (314) 540-6386

ABOUT POWER SERVICES GROUP

Power Services Group (PSG) is a U.S.-based provider of integrated turnkey solutions in the areas of maintenance, repair and overhaul of steam and gas turbine equipment. PSG also provides balance of plant (BoP) services and equipment to support a wide range of industries, including Power Generation, Oil & Gas, and industrial process. PSG leverages the combined experience and legacy knowledge of its affiliated companies—Airco Inc., Turbine Generator Maintenance, and Orbital Energy Services.

For questions or more information on PSG, please call or visit our website at http://www.powerservicesgroup.com.