Wind turbines are designed to maximize power production based on the predicted wind speeds found at the plant site. However, excessive wind speeds are experienced at times, so it is crucial to limit power generation at those times to avoid spontaneous runaway turbine situations that may cause damage to the turbine rotor or other parts of the driveline or generator. Wind turbine brake systems are a vital safety system. A critical component in the hydraulic braking system is an accumulator, which provides a supplemental flow of hydraulic fluid during emergency braking operations. Diaphragm-style accumulators are standard, although catastrophic failures in the rubber diaphragm are not uncommon. An attractive and more reliable alternative is the piston-style accumulator that is not susceptible to this failure mode.
Download our white paper Improve Wind Turbine Safety with a Piston Accumulator Retrofit for a closer look into safety systems found in modern wind turbines, failure modes, and the benefits of having your wind turbine retrofitted with a piston accumulator.
Reliable operation and safety systems
Wind turbines are designed to operate reliably over their typically 20-year design life. That means a typical wind turbine may run for more than 130,000 hours over its lifespan. In addition to reliable operation, owners also demand that a wind turbine has robust safety systems that protect their investment and the technicians who perform routine maintenance on the equipment. This point is particularly important as each turbine normally operates without constant human supervision and the period between maintenance visits by technicians often extends to every six months. Safety systems are expected to provide overall protection of the equipment without operator supervision nor frequent maintenance technician visits.
During high wind conditions, a wind turbine may produce higher than rated power generation, so the rotational speed of the blade rotor must be controlled so that the rated power output of the grid-connected generator may be maintained. Should the grid connection be lost for whatever reason, the turbine rotor will immediately begin to overspeed within seconds. Every wind turbine must possess a robust speed control safety system to prevent high rotational speeds during high wind conditions. The turbine braking system is undoubtedly the essential safety system found in a modern wind turbine.
There are many safety systems found in modern wind turbines that provide continuous supervisory oversight without human interaction. For example, vibration sensors placed on the driveline alert operators to its condition and, if vibration reaches critical levels, the supervisory system will automatically shut down the turbine. As in other power generation equipment, sensors measure voltage, current, and frequency of the generator, as well as machinery temperatures. Should an overspeed condition or malfunction occur that allows the freewheeling of the blade rotor, overspeed sensors within the safety system will automatically call for aerodynamic braking to slow the blade rotor. Finally, an anemometer measures the wind speed, another critically important component of the wind turbine’s safety system.
All wind turbines use aerodynamic braking for two principal purposes. The first purpose relates to regular turbine operation. Aerodynamic braking controls the power produced by the generator by properly aligning the nacelle into the wind with yaw control and optimizing the blade longitudinal rotation, typically by full-span blade pitch control. The pitch control system, usually located within the turbine hub, rotates the three variable-pitch turbine blades in unison to precisely control the generator speed based on a feedback signal from the generator.
The second purpose of aerodynamic braking pertains to system safety. Should freewheeling in high winds occur, the aerodynamic braking system will turn each rotor blade 90 degrees on its longitudinal axis to minimize lift and maximize drag, effectively “stalling” the blades. The net result is to stop the rotation of the rotor, often within a few rotations. A mechanical brake is engaged when blade rotation stops to secure the rotor when the wind turbine is out of service, typically for maintenance.
Effective blade braking
The turbine safety system design is based on the requirement for redundancy. The sensor-based overspeed protection system, for example, is backstopped with a purely mechanical centrifugal brake release system. This concept is not unlike a typical engine generator set where a mechanical centrifugal governor backs up the electronic overspeed controls. On a wind turbine, the safety system backup to aerodynamic braking is a physical disc brake placed on the high-speed gearbox shaft. This approach to stopping the rotor minimizes stress to the blade rotor, tower, or other machinery.
The mechanical turbine brake is placed on the high-speed side of the gearbox before the generator. If the wind turbine rotor turns at 15 rpm, the gearbox will increase the output shaft to 1,500 rpm for the generator. The mechanical brake is actuated by a diaphragm-style accumulator that stores hydraulic fluid under pressure that can actuate a disk brake in the event of an overspeed event during a loss of power (Figure 2).
Potential failure modes
A diaphragm actuator is shown here of the manual brake actuator handle. The diaphragm accumulator stores a supplemental source of hydraulic fluid under pressure, which is used to actuate the caliper disk brake (shown in red) in the event of an overspeed of the rotor during a power outage. The silver “can” beneath the accumulator is part of the high-speed shaft hydraulic system. The gearbox is in the foreground, and the generator is in the background.
The design of a typical diaphragm accumulator is quite simple. The inlet side of the accumulator is exposed to the pressurized hydraulic oil system. A compressible inert gas, such as nitrogen, fills the top of the accumulator. Separating the two volumes is a flexible rubber diaphragm membrane that compresses the nitrogen to hydraulic system pressure. Should the hydraulic system lose pressure, such as during a power outage or electronics failure, the nitrogen will expand and release the stored hydraulic fluid, which is sufficient to actuate the mechanical brake system and stop the rotating turbine shaft.
Field experience has shown that diaphragm accumulators often catastrophically fail at unpredictable times. The failure mode is illustrated in the above graph. When a rubber diaphragm ruptures, the pressure in the accumulator immediately dissipates, and the wind turbine critical backup braking system is rendered useless. The rubber diaphragm surface area is gas permeable and therefore will require more frequent pre-charge maintenance. The blue region represents the pre-charge gas, typically nitrogen. The bottom of the accumulator is connected to the hydraulic system that will actuate the mechanical brake.
In a piston accumulator, the sealing surface is minimal compared to the cross-sectional area of a diaphragm. Much like the rings that seal a piston between oil and combustion gases, these seals separate the nitrogen charge from the hydraulic oil. Because the surface area of potential gas leakage is significantly less, the amount of pre-charge maintenance is greatly reduced.
The failure mode with the piston accumulator is also significantly less dramatic. Instead of an instantaneous failure, as in the case of the rubber diaphragm accumulator, the piston accumulator failure mode is very measured. The first scenario in the top right graph occurs when the fluid leaks past an accumulator piston, raising the nitrogen pre-charge pressure. The second scenario in the bottom right graph occurs when the pre-charge gas leaks past the piston, causing the pre-charge pressure to fall. Again, the failure mode is very gradual and never the source of a forced shutdown of the turbine.
The piston accumulator may experience two slowly developing failure modes. The first (top) occurs when the hydraulic fluid seeps through the piston rings and raises the pressure of the pre-charge gas. The second possible failure mode (bottom) occurs when the pre-charge gas seeps past the rings into the hydraulic fluid or otherwise escapes from the accumulator. The blue area represents the pre-charge gas, typically nitrogen. The bottom of the piston-cylinder connects to the hydraulic system of the mechanical brake.
Failure modes- piston vs. diaphragm accumulator
The failure mode of the piston accumulator seals occurs over a long period, over millions of cycles, allowing for a manual pre-charge adjustment until the equipment’s scheduled maintenance period. The slow leakage allows for increased machine uptime and prevents costly unscheduled maintenance outages. A ruptured rubber diaphragm, on the other hand, creates an instantaneous failure, resulting in a forced outage, unplanned repair costs, and more extended machine downtime. For this reason, diaphragm accumulators are not the best choice for wind turbine safety systems.
The pre-charge gas is separated from the fluid side of the accumulator by the very small cross-sectional area of the piston seal. The piston-accumulator design minimizes the effect of permeation, and thus requires less nitrogen pre-charge maintenance over time. The rubber diaphragm separates the pressurized gas side of the accumulator from the fluid side with a relatively large cross-sectional diaphragm. This design allows the entire diaphragm to be in contact with the pressurized gas, allowing for a larger area for the gas to permeate through. A typical 1-gallon piston accumulator has 97% less exposed seal surface area than an equivalent 1-gallon rubber diaphragm accumulator.
The cause of permeation is directly related to the rubber compound, the type of gas, the temperature of the gas, the pressure differential, its cyclic duty, and the volume of rubber used to form the diaphragm. The rubber diaphragm accumulators are typically limited to two standard diaphragm compounds, nitrile, and hydrin. The piston accumulator has six possible seal compounds with an available seal temperature range for –40F to 325F, so there is a seal for every type of hydraulic fluid and temperature requirement.
There are other application limitations for the rubber diaphragm accumulators. For example, this style of an accumulator is limited to operation in systems with a maximum compression ratio (the ratio of the maximum working pressure to pre-charge pressure) in the range of 4:1 to 8:1. In contrast, the compression ratio of a piston accumulator is unlimited. The technician merely adjusts the pre-charge so that the piston is allowed to ride at mid-stroke, not bottoming out on either end of the accumulator. Other application limitations of the rubber diaphragm accumulators include the limited number of port options and that the orifice limits the design flow rate. The standard piston accumulator allows for multiple sizes and allows for flow rates of eight to 10 times that of diaphragm accumulators. A piston accumulator replaced the failed diaphragm accumulator installed on the disk brake assembly.
The table below illustrates the general characteristics of the piston and rubber diaphragm accumulators.
Wind turbine maintenance evolutions are scheduled in advance and forced outages will disrupt that schedule. A rupture in a rubber diaphragm accumulator will signal an immediate turbine forced outage, and it will remain idle until a maintenance team is mobilized to replace the failed accumulator. Depending on maintenance team availability, it would not be out of the ordinary for the turbine to be out of service for five or more days before repairs are completed. Assuming this was a 1.5-MW turbine and that it was operating at the 2019 average capacity factor reported by the Energy Information Administration of 34.9%, then the turbine lost approximately 63 MWh of generation.
Depending on the region where the turbine resides, the lost revenue due to a single wind turbine forced outage would be between $1,200 and $2,000. The cost of emergency maintenance and other related expenses could quickly push the total outage expenses up to $5,000. Given that there are almost 13,000 General Electric 1.5-MW wind turbines in North America alone, the potential loss to the wind industry each year is likely in the tens of millions of dollars.
A wind turbine retrofitted with a piston accumulator will experience a completely different, non-emergency response. The piston accumulator is immune from a catastrophic diaphragm failure, but the piston-cylinder seals will slowly wear over time. Only at that time will the pre-charge pressure slowly begin to drop. Instruments will signal and track the pressure decrease so that a work order can be issued for a maintenance technician to merely “top-off” the pre-charge pressure until the piston accumulator can be replaced at the next regularly scheduled maintenance outage. No muss, no fuss.
Download our white paper Improve Wind Turbine Safety with a Piston Accumulator Retrofit for a closer look into safety systems found in modern wind turbines and the benefits of a wind turbine retrofitted with a piston accumulator.
Article contributed by Tom Ulery, business development manager of Renewable Energy
Jeff Sage, product sales manager, Accumulator & Cooler Division
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Electrification remains one of the primary trends in the automotive sector, as vehicle makers push hard to introduce cleaner technologies which result in lower emissions.
According to a recent report from global professional services company PwC, over 55% of all new car sales could be fully electrified by 2030. Cars of the future will be electrified, autonomous, shared, connected and yearly updated, it says, in what represents a new era of flexible mobility.
This trend towards electrification isn’t restricted to the passenger car market. Construction and mining vehicles, city buses and refuse trucks have all been developed with hybrid electric powertrains, as authorities look to reduce pollution by introducing more stringent environmental regulations.
But technological progression doesn’t come overnight. The shift to electrification needs to be viewed as an evolution rather than a revolution, delivered through the continued refinement of a broad range of on-board systems and components. These incremental achievements allow the industry to manufacture greener vehicles without having to compromise in areas such as performance and reliability.A high-power density motor for traction applications
Here at Parker, our global teams of scientists and engineers are supporting these environmental efforts, designing and developing new systems that accelerate the pace of electrification. For instance, we recently extended our Global Vehicle Motor family of high-power density, permanent magnet AC motors with the GVM310, which comes with a 310mm square frame. This new product provides a traction solution for a broad range of on-road and off-road commercial electric and hybrid electric vehicles.
So, let’s look at some of the benefits that GVM310 brings to the market. Primarily, when used in conjunction with Parker’s hydraulic pumps, the GVM family helps customers realise electro-hydraulic pump solutions that allow the electrification of formerly purely hydraulic applications.Higher performance motors for your electric or hybrid vehicles
The high efficiency / lower energy consumption of the motor helps vehicle makers comply with stringent emerging energy legislation. It reduces CO2 footprints, is extremely quiet, and its high reliability results in reduced maintenance and downtime for operators. Options with peak power values ranging from 147 kW to 409 kW are available – with high power density meaning the size and weight of overall solutions can be minimised easing design-in for customers.
In addition to operating as a high-power motor, the GVM310 can also be run as a generator enabling effective battery management, longer duty cycles and energy savings of up to 30% compared to induction technologies. Availability as low-flux versions for high-speed applications, or high-flux derivatives for high torque applications enhances versatility.
Furthermore, the GVM family incorporates a wide range of technical features that improve performance. These include a new thinner lamination design to reduce losses, a patented cooling system and a clean, oil-free design.
The introduction of the GVM310 is an example of how Parker is providing the building blocks for electrification, developing turnkey technologies that cut time to market while reducing supply chain complexity. It offers the industry with an optimized solution for the on-road and off-road commercial electric and hybrid electric vehicles of tomorrow.
Article contributed by Bruno Jouffrey, market development manager - Mobile, Electromechanical and Drives Division Europe, Parker Hannifin Corporation.
Connectors might not always receive the attention they deserve. Often times the specification of fittings and tubing is secondary to the attention paid to larger components. Design engineers may not anticipate system leaks. Even if pneumatic fittings and tubing are the last components to be specified in typical food and beverage packaging equipment, they still merit close consideration. Improperly specified connectors contribute to early component deterioration causing leaky connections and even pressure drop. Properly specified fittings and tubing will help to ensure that food and beverage packaging systems perform at levels end-users expect.
In the food and beverage industry, there are a number of packaging processes driven by pneumatic power carried through tubing and fittings that connect pneumatic valves, actuators and FRLs (filters, regulators, lubricators). The components may power any number of processes, such as the filling and sealing of bags of tortilla chips; the folding, filling and sealing of milk cartons; or the packaging of hamburgers and steaks. Regardless of the application, when pneumatic connections aren’t specified correctly, systems can end up with untimely air leaks and pressure drops.
To reduce the chance of leaks and flow restrictions, here are some tips for specifying, plumbing and routing pneumatic connections in food and beverage packaging builds:
1. Select fittings carefully based on each application
While most pneumatically controlled food and beverage processes use push-to-connect fittings over other styles, such as compression and flare, push-to-connect fittings also come in different materials for specific reasons. There are higher-end FDA-compliant fittings, such as Parker’s Prestolok PLM electroless nickel-plated brass fittings and Prestolok PLS stainless steel fittings, made for applications where the fittings may come in contact with foods and beverages.
For processes where foods and beverages don’t come in contact with fittings, such as secondary packaging operations, OEMs can opt for more economical push-to-connect fittings, such as Prestolok PLP metal fittings and Prestolok PLP composite fittings.
Fitting material type becomes important for applications that receive high heat or caustic washdowns, which could quickly compromise fitting integrity depending on the material.
Say the fittings will be installed throughout a dairy filling application where they will receive frequent and potentially caustic washdowns. In this case, Michael points out, all-stainless steel fittings are made to withstand these harsh conditions and keep processes leak-free and running. That’s in contrast to a tortilla chip packaging process, where the fittings might come in contact with foods, but don’t receive frequent caustic washdowns. Here, OEMs might choose Parker’s FDA-compliant PLM fittings. General industrial-purpose fittings, meanwhile, are likely to fit the bill for fittings mounted on automated box folding machines erecting outer packaging containers.
2. Select the fittings and tubing based on how they will be routed
Fittings come in many configurations that allow for effective routing of pneumatic connections. In many food packaging applications, OEMs mount valve manifolds and actuators on machines and then determine what fitting configurations will work best for connecting those ports. It is the last piece of the puzzle and where a design engineer decides to use, for example, an elbow fitting instead of straight or tee fitting. It’s all dependent on where the line is going to be installed in relation to the pneumatic components.
Routing questions also arise in situations calling for tubing to take a tight bend, leading machinery designers to weigh the benefits of using fittings instead of tubing to accommodate the turns. Often a complex decision, this can depend on the tubing material too, as using tubing with a high bend radius can allow for more turns, but also might put side-load on fittings.
If tubing is bent too close to the fitting, it could pull the tubing away from the fitting seal, creating the potential for a leak.
Another factor is the tubing diameter tolerance, or how much its outer diameter could vary from one manufacturing run to another. Tubing manufactured to a looser tolerance level could cause fit issues allowing fittings to leak or to blow off of tubing.
Parker tubing and fittings are tested and designed to work together. Parker Parflex tubing holds the tubing to a certain tolerance range, which helps in terms of fitting performance because tubing tolerance is so critical to working well with a push-to-connect fitting.
Customers should reference Parker’s Tubing Compatibility Chart (found in Parker Hannifin catalog 3501E) to be sure they choose the proper tubing for each fitting type.
3. Avoid unnecessary fitting connections
Finally, another problem area is extra fitting connections installed where they don’t belong. Every fitting is a potential leak point, so if the number of connections can be reduced so to can the chance of leaks. Each fitting in a pneumatic circuit also adds a flow restriction, as compressed air is forced to move through another orifice, which can hamper motive power.
One of the more common issues is using multiple fittings in place of one or two, as a way to adapt one fitting or tube type to another. It can happen when the OEM or end-user doesn’t have the proper fitting shape or type on hand to adapt to a certain thread system or port size required by the valve or cylinder. While the adaptation may function, it can also restrict airflow and add the potential for leakage.
OEMs can avoid the problem entirely by choosing the appropriate adapter. Parker offers hundreds of tube fittings and adaptors made to join different tube sizes and thread types, such as NPT to BSPT, BSPP or metric. Rather than trying to build a makeshift adapter out of two or three pneumatic fittings, using a single adapter fitting allows technicians or engineers to make the connection in one step, preventing unnecessary flow restrictions and reducing the risk of leakage.
With these suggestions, many connector issues like adapting to different sizes or standards, or accommodating system designs, need not lead to system slowdowns. With the right pneumatic fittings, adaptors and tubing materials, OEMs and end-users will be equipped to keep airlines flowing.
To learn more about specifying these components, locate a distributor near you.
This article was contributed by John Duba and Michael Nick, product sales managers, Parker Hannifin's Fluid System Connectors Division.
Glenn O. Hawbaker, Inc. brings over 60 years of experience to the highway, commercial and residential fields throughout central Pennsylvania. They continue to grow their business by expanding their reputation for safety, quality, service and reliability. That's one of the reasons Glenn O. Hawbaker purchased their first PowerTilt Tilting Coupler and continued to purchase several more for their entire backhoe loader fleet. PowerTilt has changed the way they've approached their grading and excavating business while at the same time positively impacting their bottom line and overall customer satisfaction.Life before PowerTilt
Prior to using PowerTilt, Glenn O. Hawbaker faced two challenges on the job site. Genn O. Hawbaker was using large and expensive Gradall specialty machines to grade and slope and spent extra man-hours to swap these machines in and out of job sites.The large rubber tires on the specialty machines often caused the Gradalls to slide or operators to spin around when a rock was hooked, making for an unstable work environment. With PowerTilt, Hawbaker could keep just one machine on the job site without the expense or logistics involved in scheduling the Gradall excavators between the different job sites.
The Hawbaker crew was also having difficulty with the outriggers on their specialty excavator and backhoe fleet - they had to take their hands off of the controls to tilt the outriggers and move the machine at different angles around the job site. When the outriggers were tilted at an awkward angle, the operators felt uncomfortable and unsafe. When they added the PowerTilts to their existing CASE backhoes, they kept just one machine on the job site to tilt their bucket or attachment instead of moving the entire machine to get the right angle.
“Now with PowerTilt, we’re doing everything on the fly - we tilt and grade at the same time."
Paul Peters, backhoe operator for Glenn O. HawbakerMultiple benefits from a single attachment
By switching to PowerTilt, Glenn O. Hawbaker and its customers received a wide range of expected and unexpected benefits.
“We saved on labor, got tasks done faster and safer, and increased the appearance of the end product. What's more, we had the unexpected benefit of people asking us what tool we were using, and how we were getting more work done with less hand work," stated Peters.
The benefits of switching to PowerTilt were:
Glenn O. Hawbaker uses their PowerTilts on their entire fleet of backhoes to perform a wide range of tasks throughout the construction process, ranging from site preparation, earth excavation, sub-grade placement and grading, utility installation, site concrete, site clean-up and landscaping. Ninety-five percent of the time they use a grading bucket with PowerTilt, whereas five percent of the time they use other attachments.
Peters stated, "I hate to take a PowerTilt off the machine. I can perform a broad range of tasks with a PowerTilt, and it keeps me on the job all the time”.
The most common applications for PowerTilt include:
Glenn O. Hawbaker uses PowerTilt with a variety of attachments in addition to their commonly used five-foot grading buckets to improve their machine's versatility. They first learned about PowerTilt when they noticed a local municipality using a T bucket to dig around pipes. Since then, the Hawbaker crew has used PowerTilt for a variety of specialty applications.
They have used PowerTilt with ripper shanks to rip frozen soil in the winter, or to rip rocks and stumps in tough-to-get corners or ditches. Compactors work equally well with PowerTilt when soil needs to be compressed around utilities or on slopes. PowerTilt has even worked well with hydraulic hammers when they needed to dig footers where there's lots of lime stone in the foundation corners.Inside Parker’s Helac rotary actuator technology
PowerTilt uses Parker’s innovative sliding-spline operating technology to convert linear piston motion into powerful shaft rotation. Each actuator is composed of a housing and two moving parts — the central shaft and piston. As hydraulic pressure is applied, the piston is displaced axially, while the helical gearing on the piston OD and housing's ring gear cause the simultaneous rotation of the piston. PowerTilt's end caps, seals and bearings all work in tandem to keep debris and other contaminants out of the inner workings of the actuator, prolonging product life and reducing required maintenance.
To learn more about PowerTilt, visit http://solutions.parker.com/powertilt
This article was contributed by Jessica Howisey, marketing communications manager and Daniel Morgado, applications engineer, Helac Business Unit, Cylinder Division.
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