It’s that time of year again – summertime – hot and unbearable weather. This year is starting off in pretty much the same pattern as years gone by. All over the U.S we are seeing extremely high heat temperatures, especially in the western portion where temperatures are reaching an average of 112° to 128° degrees. Many people are exposed to heat on some jobs, outdoors, or in hot indoor environments. Operations involving high air temperatures, radiant heat sources, high humidity, direct physical contact with hot objects, or strenuous physical activities have a high potential for causing heat-related illness.
Why is heat a hazard to workers?
When a person works in a hot environment, the body must get rid of excess heat to maintain a stable internal temperature. It does this mainly through circulating blood to the skin and through sweating.
When the air temperature is close to or warmer than normal body temperature, cooling of the body becomes more difficult. Blood circulated to the skin cannot lose its heat. Sweating then becomes the main way the body cools off. But sweating is effective only if the humidity level is low enough to allow evaporation and if the fluids and salts that are lost are adequately replaced.
If the body cannot get rid of excess heat, it will store it. When this happens, the body’s core temperature rises and the heart rate increases. As the body continues to store heat, the person begins to lose concentration and has difficulty focusing on a task, may become irritable or sick, and often loses the desire to drink. The next stage is most often fainting and even death if the person is not cooled down.
Excessive exposure to heat can cause a range of heat-related illnesses, from heat rash and heat cramps to heat exhaustion and heat stroke. Heat stroke can result in death and requires immediate medical attention.
Exposure to heat can also increase the risk of injuries because of sweaty palms, fogged-up safety glasses, dizziness, and burns from hot surfaces or steam.
How can heat-related illness be prevented?
Heat-related illnesses can be prevented. Important ways to reduce heat exposure and the risk of heat-related illness include engineering controls, such as air conditioning and ventilation, that make the work environment cooler, and work practices such as work/rest cycles, drinking water often, and providing an opportunity for workers to build up a level of tolerance to working in the heat. Employers should include these prevention steps in worksite training and plans. Also, it’s important to know and look out for the symptoms of heat-related illness in yourself and others during hot weather. Plan for an emergency and know what to do – acting quickly can save lives!
Remember, refrain from alcohol intake the night prior and drink plenty of fluids during the shift.
Correct Gap Critical to Rotation Speed
/in Controls Tips /by Mike.LakeMaintaining the correct gap between the sensor and the rotor is critical to correctly measuring turbine rotor speed. Setting the gap can be problematic if the correct gap is unknown or if it cannot be accessed with a feeler gauge.
First a little background on why the sensor gap is critical: The sensor, or “pickup”, is mounted perpendicular to the shaft, facing a toothed gear fixed to the rotor. Pickups can be either “active” or “passive” (see below). In either case, the pickup counts the teeth by sensing the difference in height between the tip of the tooth and the valley between the tooth. If the sensor is too close, it can’t reliably distinguish between the tip and the valley. If it is too far away, it can’t reliably register the tip. The correct gap will register a voltage differential which can be counted. The electronic circuits determine shaft rotation speed by dividing the number of teeth on the gear into how fast the voltage changes over a period of time.
Always use the manufacturer’s specification data to determine the correct gap spacing for the pickup. If the specification is not available, an initial setting of 0.025″ (0.64 mm) will work in most cases. Once the unit is running, the controls engineer can determine if the output voltage or signal level is sufficient for the type of control used.
Sometimes the pickup gap cannot be accessed with a feeler gauge. If so, an accurate setting can be obtained with a little math and a technique called “counting the flats”. The two most often found pickup sizes are the 5/8″ – 18 and the 3/4″ – 20 thread sizes. If you do the math, the 18 threads per inch (TPI) device will move 1 inch in the mounting hole if it is rotated 18 times. Looking at it the other way, it will move in the hole 0.055 inches if it is rotated one time. Breaking it down further, it will move 0.009″ if it is rotated one “flat” of the hexagonal shaped body or hex nut. In the case of the larger 3/4″ inch device, it will move 0.050″ per one rotation and 0.008″ per “flat”.
To “count the flats”, line up the tooth of the gear as close as possible to the center of the mounting hole until it looks like the picture above. Once the gear tooth is aligned with the center of the hole, screw the pickup down BY HAND until the face of the pickup gently contacts the tooth. Set the gap by unscrewing the pickup while counting the flats from a fixed reference point (can even be a line made by a Sharpie pen). For a 0.025″ gap unscrew it by 2 ¾ flats. The math would be 0.025″ (gap) / 0.009″ (movement per flat) = 2.77 flats or approximately 2 ¾ flats. For the larger pickup size that would be 0.025″ / 0.008″ = 3.1 flats or just tad over three full flats. Tighten up the locknut and you’re done!
There are many different types of pickups out in use today but the most common types used on steam and gas turbines are the “passive” type (sometimes called inactive pickups) and the “active” type. They look very similar but operate quite differently. Very simply, the typical magnetic or “passive” pickup is simply a coil of fine wire wrapped around a magnetized iron core that self generates a voltage. When the tooth of a gear passes in front of the iron core, a small voltage is generated and when the valley between the teeth passes the iron core, the voltage falls off. The “active” type pickup receives power from an outside source instead of self generating it. There is a small transmitter and receiver inside of the device that sends out a signal from the end of the pickup. When a gear tooth passes this signal, it changes the characteristic of the signal that is reflected back to the receiver. The internal electronics then interpret this and send out a voltage pulse. Although the passive sensor generates a sine wave and the active sensor generates a square wave, both sensors count cycles over time, which represents teeth rotation speed.
Please contact Mr. Turbine® for answers to any issue with Steam or Combustion Turbine Controls, or Generator and Exciter Controls for any motive power system.
Heat Stress
/in Safety Tips /by Mike.LakeWhy is heat a hazard to workers?
When a person works in a hot environment, the body must get rid of excess heat to maintain a stable internal temperature. It does this mainly through circulating blood to the skin and through sweating.
When the air temperature is close to or warmer than normal body temperature, cooling of the body becomes more difficult. Blood circulated to the skin cannot lose its heat. Sweating then becomes the main way the body cools off. But sweating is effective only if the humidity level is low enough to allow evaporation and if the fluids and salts that are lost are adequately replaced.
If the body cannot get rid of excess heat, it will store it. When this happens, the body’s core temperature rises and the heart rate increases. As the body continues to store heat, the person begins to lose concentration and has difficulty focusing on a task, may become irritable or sick, and often loses the desire to drink. The next stage is most often fainting and even death if the person is not cooled down.
Excessive exposure to heat can cause a range of heat-related illnesses, from heat rash and heat cramps to heat exhaustion and heat stroke. Heat stroke can result in death and requires immediate medical attention.
Exposure to heat can also increase the risk of injuries because of sweaty palms, fogged-up safety glasses, dizziness, and burns from hot surfaces or steam.
How can heat-related illness be prevented?
Heat-related illnesses can be prevented. Important ways to reduce heat exposure and the risk of heat-related illness include engineering controls, such as air conditioning and ventilation, that make the work environment cooler, and work practices such as work/rest cycles, drinking water often, and providing an opportunity for workers to build up a level of tolerance to working in the heat. Employers should include these prevention steps in worksite training and plans. Also, it’s important to know and look out for the symptoms of heat-related illness in yourself and others during hot weather. Plan for an emergency and know what to do – acting quickly can save lives!
Remember, refrain from alcohol intake the night prior and drink plenty of fluids during the shift.
Common Sense Safety
/in Safety Tips /by Mike.LakeThe experts say at least 80% of industrial accidents are caused by unsafe acts on the part of employees–and not by unsafe conditions. Although employers are required by law to provide a safe and healthful workplace, it is up to you to be aware of your work environment and follow safe work practices. Statistically, most accidents are caused by unsafe acts, including:
Being In A Hurry – Sometimes there is more concern for completing a job quickly instead of safely. Take time to do a good job and a safe job.
Taking Chances – Daring behavior or blatant disregard for safe work practices can put the whole work team at risk. Follow all company safety rules and watch out for your fellow employees. Horseplay is never appropriate on the job and can lead to disciplinary action.
Being Preoccupied – Daydreaming, drifting off at work, thinking about the weekend, and not paying attention to your work can get you seriously hurt or even killed. Focus on the work you are paid to do. If your mind is troubled or distracted, you’re at risk for an accident.
Having A Negative Attitude – Being angry or in a bad mood can lead to severe accidents because anger nearly always rules over caution. Flying off the handle on an outage is potentially dangerous. Keep your bad moods in check, or more than one person may be hurt. Remember to stay cool and in charge of your emotions.
Failing To Look For Hidden Hazards – At many jobsites, work conditions are constantly changing. Sometimes new, unexpected hazards develop. Always be alert for changes in the environment. Hidden hazards include spilled liquids that could cause slips and falls; out-of-place objects that can be tripped over; unmarked floor openings one could step into; low overhead pipes that could mean a head injury; and other workers who don’t see you enter their hazardous work area.
Remember to stay alert for hazards, so you won’t become one more accident statistic: You can do a quality job without rushing. Maintain a positive attitude and keep your mind on your work. This is just common sense–something smart workers use!
Copper Resistance Testing
/in Generator Tips /by Mike.LakeTest Setup & Execution. The main and neutral lead connections should be broken and open. The lead ends should be free and clean of surface contamination so that the test probes make good contact. Copper resistance testing is performed with a Digital Low Resistance Ohm Meter (DLRO) test set. The DLRO instrument is extremely sensitive. Poor contact and circuit set-up can either produce erroneous readings or no readings at all.
One probe of the DLRO is connected to one lead of an individual phase, and the other probe connected to the other lead of the same phase. A reading (generally to the fourth decimal place) in ohms resistance is measured and recorded. This same process is repeated on the second and third phase. The ambient air temperature and humidity should be recorded as well.
Interpretation of Results. Temperature significantly influences the resistance of a dielectric as well as a conductor. For this reason, the copper resistance measurement should be corrected to standard (typically 40°C).
The original equipment manufacturer normally records and documents the as-built phase-by-phase copper resistance measurements. These are used as the baseline by which all future readings can be compared.
An increase in in copper resistance indicates the presence of some form of high resistance issue (i.e. broken conductors, cold braze joints, turn-to-turn shorting, incorrect connection, incorrect number of turns or stranding, open circuit). Additional testing will be required to determine the specific cause of the variant reading.
Standard. IEEE Standard 11 8TM-1 978, IEEE Standard Test Code for Resistance Measurement.
Test Equipment. A Megger, Model DLRO-10 or comparable is recommended. Kelvin and Wheatstone bridges are also used to measure resistance.
Slips, Trips, and Falls
/in Safety Tips /by Mike.LakeMost common locations for falls:
Falls can be prevented – Ladder Safety
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®.