Polarization Index (PI) Test

A Polarization Index (PI) test is generally performed at the same voltage as the Insulation Resistance (IR) test.  Where the IR test is performed for a period of one minute, the PI test is performed over a period of ten minutes. This gives the absorption (polarization) current ample time to decay, and reveals a more detailed indication of the total leakage and conduction current. As such, PI is a good indication of winding contamination, moisture ingress (leakage currents), and/or bulk insulation damage (conduction currents).

Polarization Index testing is generally performed with an Insulation Resistance (IR) test set (commonly known as a Megger), immediately after performing the IR test. However, the test can also be performed utilizing a DC high potential (hipot) test set. The readings produced by the two instruments are different. A Megger commonly gives readings in ohms of resistance. A hipot registers the amount of current (typically in microamps (mA). One microamp is equal to 1 x 10-6 amps, or 0.000001 amps.

The Polarization Index is derived by the ratio between the one minute reading and the ten minute reading. Recommended minimum PI results for suitability for service (or implementation of high voltage testing) is widely accepted as 2:1 or greater. Any reading lower than this minimum value is a concern. The windings would be presumed to be wet, contaminated, and/or compromised in some fashion. Conversely, vintage windings (varnish cambric, asphalt mica, etc.) may produce an unusually high Polarization Index ratio.  The insulation may be void of binder content, thus making it dry and brittle.  According to IEEE standards, if the insulation resistance reading after the voltage has been applied for one minute is greater than 5,000 megohms the resulting polarization index may or may not be indicative of the true insulation condition and is therefore not recommended as a means of assessment.

Polarization Index testing should be performed in accordance with IEEE Standard 43-2000(R2006), IEEE Recommended Practice for Testing Insulation Resistance of Rotating Machinery. A Megger brand model number BM25 (or its replacement or comparable) is recommended if a Megger is used. A High Voltage, Inc. PTS Series DC high potential test set is recommended if a hipot is used.

Proper Manual Lifting

Back pain constitutes about 10% of occupational injuries and is the most common reason to take leave from work.  Each year about $50 billion is spent on treatment in The U.S. alone, making it the third most expensive condition after heart disease and cancer. With this in mind, an ounce of prevention is worth a pound of cure (pun intended). Before lifting an object of unknown weight, perform several trial lifts using gradually increasing effort.  Do not attempt to lift an object that you cannot confidently handle. Always identify the path you will be taking with the load and clear away all obstacles. Use proper gear for every lift. Wear shoes with good traction and solid gripping gloves which will help you to hold the object for a longer period of time.

Avoid standing too far from the load as it might not provide you the needed grip to hold the object properly. Align yourself properly over the load with your feet and shoulders wide apart. This will give you the exact balance needed to hold the load while lifting it. Never bend at the waist and lift an object with your back. Keep your upper body straight and parallel with your lower legs. Grab the item and push up with your legs, not with your back. Never rotate or twist your body while lifting. Keep your head up when handling the load. Look ahead, not down at the load once it has been held securely. Make sure you lift with a slow, steady force. This will help you avoid muscle strains from having to counter an unbalanced load. Take smooth and small strides to avoid muscle strain from overcompensating for shifting loads. For heavier loads, try lifting with your full breath, and tighten your abdominal muscles for added support. For long lifts, such as from floor to shoulder height, consider resting the load mid-way on a table or bench to change your grip on it.  Always use a lifting belt or back brace if preforming multiple lifts. Don’t lift or handle more than you can easily manage. There’s a difference between what people can lift and what they can safely lift. If at all possible, get help.  Lift with a buddy or use a mechanical lifting device such as a crane, chain fall or a jack.

Insulation Resistance (IR) Test

The Insulation Resistance Test measures the integrity of the generator’s winding insulation, and therefore the likelihood of developing a ground.  A test voltage is applied to the generator and the current flow required to maintain that voltage is measured over a period of time (typically one minute).  In simplest terms, the less current flow, the higher the resistance value, and the better the insulation.

An IR test should be performed immediately following any type of event that is suspected of over-stressing an insulation system, prior to the generator being placed back into service. This is the first test that should be performed. The results will indicate the ability of the insulation system to withstand  any more searching and/or strenuous testing. The IR test also measures the effect of contamination from water, oil, carbon, and other such undesirables. If a separate high voltage proof test is performed, an IR test should be performed both before and after the proof test. This in order to assure that the proof test itself has not compromised the insulation.

The Insulation Resistance test must be performed by a well trained and experienced technician.  Incorrect procedures can materially affect the results. The test should be performed to IEEE Standard 43-2000 (R2006), using a late model Megger brand machine such as Model MIT1025, or comparable.  It is best practice to test at the main and neutral leads of the stator, as close to the windings as possible.  Stator slot RTD’s should be disconnected from the terminal board and grounded. Surge capacitors should be disconnected.  The water or oil should be drained and completely evacuated from liquid inner-cooled windings, typically by vacuum-processing. Stator windings should be tested one phase at a time, with the other two phases grounded. In this manner, the windings are stressed both phase-to-ground and phase-to-phase.

The results should be interpreted by an experienced technician. Your final report should reflect that environmental conditions and even the age and configuration of the machine were taken into consideration.

If you have any questions concerning the application of this test or the interpretation of the results, please contact Mr. Turbine.

Starting Over

We’re embarrassed. We had our first recordable accident after 909 days (over
500,000 hours). Just when everything was going so right. We had the toolbax safety meeting at start of shift to discuss the day’s hazards. We had the Job Safety Analysis for the task to be performed. We had top management buy-in and a dedicated and experienced Safety Director conducting random on-site inspections. But a mechanic was not wearing gloves when he knew he should have been, and he cut his finger. He may not have a scar on his finger from the incident, but the permanent scar on his soul gives evidence of the depth of the safety culture at TGM.

And we still had an accident. Take heed from our mistake:

Hands are hurt more often than any other part of the body. Your hands are your wage-earners. As talented as your hands are, they can’t think; they are your servants, and it is up to you to think and keep them out of trouble.

Be sure you wear the right kind of gloves for the particular kind of work you are doing. When you wear gloves, you aren’t trusting to luck and you’re not taking unnecessary chances. Wear gloves when you are doing a job that needs them, but not around moving machinery. Time spent in preparing your hands for the job will not only save trouble for you but will probably save time in doing the job.  Refer to your SDS (MSDS) sheets for gloves required for use with certain chemicals.

What is a Confined Space?

A confined space does not necessarily mean a small, enclosed space. It could be rather large, such as a ship’s hold, a fuel tank, or a pit.

One of the first defining features of a confined space is it’s large enough to allow an employee to enter and perform work. The second defining feature is it has limited means of entry or exit. Entry may be obtained through small or large openings and usually there is only one way in and out. The third defining feature is that confined spaces are not used for continuous or routine work.

All confined spaces are categorized into two main groups: non-permit and permit-required. Permit-required confined spaces must have signs posted outside stating that entry requires a permit. In general, these spaces contain serious health and safety threats including:

  • Oxygen-deficient atmospheres
  • Flammable atmospheres
  • Toxic atmospheres
  • Mechanical or physical hazards
  • Loose materials that can engulf or smother

Although a confined space is obviously dangerous, the type of danger is often hidden. For example, a confined space with sufficient oxygen might become an oxygen-deficient space once a worker begins welding or performing other tasks.

These are some of the reasons confined spaces are hazardous:

  • Lack of adequate ventilation can cause the atmosphere to become life threatening because of harmful gases.
  • The oxygen content of the air can drop below the level required for human life.
  • Sometimes a confined space is deliberately filled with nitrogen as a fire prevention technique. Nitrogen cannot sustain human life, so you must use respiratory protection.
  • Many gases are explosive and can be set off by a spark.
  • Even dust is an explosion hazard in a confined space. Finely-ground materials such as grain, fibers and plastics can explode upon ignition.
  • Confined spaces often have physical hazards, such as moving equipment and machinery.
  • Tanks and other enclosed confined spaces can suddenly be filled with materials unless the flow process for filling it is controlled.

Before entering any confined space, you must test the atmosphere to determine if any harmful gases are present. There must also be radio contact with an attendant outside the confined space and a rescue team at the ready in case of an emergency.

Don’t Kill Your Turbine on Startup

Your lube oil temperature needs to be lower at startup and shutdown than at full

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. As the rotor slows, 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 F.  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. The tubes must be clean for efficient transfer of heat. The bundles should be cleaned every two (2) years.  Lube oil coolers are the single most common area for contaminants to hide. These contaminants can also lead to bearing failures, as discussed in an earlier Turbine Tip.

Prevent Crushed Fingers

Each year, workers suffer approximately 125,000 injuries that occur when body parts get caught between two objects or entangled with machinery. These hazards are referred to as “pinch points”. If you have ever slammed your finger in a door, you can appreciate the pain associated with this common type of caught/crush injury. The physical forces applied to a body part caught in a pinch point can vary and cause injuries ranging from bruises and cuts to amputated body parts and even death.

To prevent these injuries, look for possible pinch points before you start a task. Take the time to plan out your actions and decide on the necessary steps to work safely. Give your work your full attention. Don’t joke around, daydream, or try to multi-task on the job – most accidents occur when workers are distracted. Read and follow warning signs posted on equipment. If you value all that your hands can do, THINK before you put them in a hazardous spot.

Also, dress appropriately for work with pants and sleeves that are not too long or too loose. Shirts should be fitted or tucked in. Do not wear any kind of jewelry. Tie back long hair and tuck braids and ponytails behind you or into your clothing. Wear the appropriate, well-fitting gloves for your job.

Machinery can pose a hazard with moving parts, conveyors, rollers, and rotating shafts; these are only a few of the vast number of hazards. Never reach into a moving machine. Properly maintain and always use the machine and tool guards provided with your equipment. Don’t reach around, under, or through a guard and always report missing or broken barriers to your supervisor. These guards act as a barrier between the moving parts and your body. Turn equipment off and use lockout/tagout procedures before adjusting, clearing a jam, repairing, or servicing a machine.

Vehicles, powered doors, and forklifts can pose a crush hazard unless they have been blocked or tagged out. Never place your body under or between powered equipment unless it is de-energized. Doors, file drawers, and heavy crates can pinch fingers and toes. Take care where you place your fingers. Test the weight before lifting, carrying, and placing boxes; an awkward or heavy load can slip and pinch your hands or feet. Get help or use tools to move large and/or heavy items.

Take the time to learn about the caught/crush hazards in your workplace so you don’t learn about the consequences first hand (no pun intended).

When a Backup Isn’t a Backup

The International Association of Engineering Insurers found that the highest frequency of steam turbine failures worldwide is due to loss of oil pressure. Most of these 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, and will fail 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. 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. These tests will confirm the pressures when the DC oil pump will engage after the AC oil pump is actually turned off. Backup batteries should also be verified. These tests should be performed 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. While these pumps are usually very reliable, they still must be manually tested on a regular basis and after an overhaul. Care must be taken to not overspeed the pump or it will 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.  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.

Bottom line, a backup is not a backup unless it is reliable. And it can only be reliable if it is tested.

Surviving Winter Weather

Winter is definitely here across the U.S. Here are some points on surviving the cold that you can tailor to the particular circumstances of your workplace:

Winter Driving

* Keep in mind that while black ice can form anywhere the temperature drops below zero, the condition is more prevalent in some parts of the country than others. Find out about the weather and road hazard patterns in the area you will be traveling.

* Prepare any of your mechanics and subcontractors driving for the possibility of being stranded in bad weather. Remind everyone to carry winter clothing, including boots, gloves, and hats in their vehicles.

* If your mechanics don’t live in the snowbelt, winter driving hazards can be a big concern. You need to emphasize to them the special driving hazards and risks associated with longer hours of darkness and weather such as rainstorms.

Winter Hazards in the Workplace

But it’s not just the road that’s slippery in winter. Loading docks, stairways, equipment yards, parking lots, and other areas of the plant or facility can also become icy.

Adapt safety meetings to the particular fall hazards that are common to your work crew. Do they have to get in and out of vehicles in icy weather? Must they walk along loading ramps to do their jobs? Steps, stairs, ramps, and ladders are all more dangerous in wet or icy weather, especially during the night shift. Remember that moist skin can stick to freezing surfaces. Check your work-site for areas that could be a particular hazard in the cold weather and discuss them in your morning toolbox meetings.

Effects of Darkness

The winter solstice around December 22 is the darkest time of the year. Outdoor workers are likely to be affected most by the increased darkness. But anyone who arrives or departs from work in the dark also needs reminding about special safety concerns, such as increased risk of slips, trips and falls, and personal security risks.

Be cautious using outdoor lighting around utility or construction jobs where flammable gas may accumulate.  It is importance to use light devices designed for decreased sparking.

The long dark winter can create or enhance symptoms of depression in employees, including lethargy, irritability, and forgetfulness. The depression can lead to increased use of alcohol and/or difficulty in relationships, which can further decrease employee productivity and awareness.

Early hours can also lead to dangerous fatigue; talk to the crew about getting enough sleep and coping safely with early shifts.

Make your safety talks address the particular risks cold weather creates on the specific tasks workers perform.

Cold Stress

Workers who are exposed to extreme cold or work in cold environments may be at risk of cold stress. Extreme cold weather is a dangerous situation that can bring on health emergencies in susceptible people, such as those without shelter, outdoor workers, and those who work in an area that is poorly insulated or without heat. What constitutes cold stress and its effects can vary across different areas of the country. In regions relatively unaccustomed to winter weather, near freezing temperatures are considered factors for “cold stress.”

Prolonged exposure to cold, even at temperatures well above freezing, can result in abnormally low body temperature (hypothermia). A body temperature that is too low affects the brain, making the victim unable to think clearly or move well. This makes hypothermia particularly dangerous because a person may not know it is happening and will not be able to do anything about it.

Cold stress is accelerated if a body part is exposed to water. Water conducts heat away from the body faster than air. Local cooling can shut down circulation in the capillaries under the skin, which can result in permanent damage such as chilblains, trench foot or frostbite. These effects can be experienced at temperatures as high as 60 degrees F.

Whenever temperatures drop decidedly below normal and as wind speed increases, heat can more rapidly leave your body. Wear appropriate clothing and protect sensitive areas such as hands, feet and face. Several layers of loose clothing will provide better insulation, especially if you alternate periods of exertion and rest. Wear a cap or rag under your hard hat. Stay in heated locations during work breaks, and limit outside exposure on extremely cold days.

Weather-related conditions may lead to serious health problems. So be prepared to cover up and stay warm.

High Bearing Loading

Past Turbine Tips have covered the main reasons for bearings to wipe: 1) Insufficient lube oil supply, 2) Low lube oil pressure, and 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). The OEM calculates the elevations and coupling alignments during the design process, based on the catenary curve (or sag chart). Calculations ofbearing 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 Catenary Curve

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. The source of the bearing failure can be eliminated by carefully measuring and re-aligning to the original specifications. But we have seen examples where the original calculations either were not accurate orover 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. If necessary, TGM can perform the recalculation and re-alignment without the participation of the OEM. On three bearing units, it is not uncommon to utilize a dynamometer to check bearing loading during alignment and their adjustments.