Hydraulics

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.

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.

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).

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. 

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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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.

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. Hawbaker

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:

These days, mobile machines used in a broad range of markets such as agriculture, construction, forestry, material handling and transportation are exceptionally sophisticated pieces of equipment. Industry megatrends such as automation, connectivity and electrification have converged to ensure that excavators, loaders and other hydraulic mobile machines have more functional capability than ever before.

In this digitally driven environment, the demand for innovation is relentless. That provides a real challenge for OEMs, who are expected to work faster and smarter to come up with new features that deliver a competitive advantage.  

For engineers, this means developing and prototyping new electrohydraulic systems in the shortest time possible. One means of achieving efficiency in product development is through the use of more high-level graphical design tools to design, simulate and deploy innovations at speed.

That was the inspiration for a collaboration between motion and control technologies specialist Parker Hannifin and computer software giant MathWorks® on IQANdesign 6TM – a new software package with MATLAB® Simulink® integration that has been developed to speed innovation in electronic controls for mobile machines. IQANdesign 6 features an IQAN toolbox for Simulink that can streamline the model-based design, simulation and deployment of electronic controls – a significant development which is expected to transform work that involves the creation or improvement of mobile machine functions. 

Historically, engineers and scientists around the globe have benefitted from the IQAN and Simulink product families, accelerating the pace of discovery, innovation and development in sectors that extend from automotive to electronics. Integrating the two software environments reduces development time with a more efficient and convenient tool chain, offering users an unprecedented way to rapidly deploy code on production-ready hardware in real-time applications. 

But what does that mean in terms of functionality for the design engineer faced with the competitive pressure of bringing new electronic controls for mobile machines to market?  

Firstly, the seamless integration of Simulink models within the IQAN ecosystem enables automatic generation of real-time applications from Simulink models targeting IQAN-MC4x controllers. Concurrent execution of multiple models with individual time bases and priorities are also supported.  

Also, Simulink models are executed in a dedicated real-time kernel, with Inports and Outports available in IQANdesign application logic. Using IQANdesign, users can view and navigate Simulink models, which are also included within system simulations provided by IQANsimulate. In addition, Simulink Testpoints are visible and measurable in IQANdesign/IQANrun. 

In short, the collaboration between Parker and MathWorks is about removing time and resource obstacles for research and development teams. It provides the perfect solution for businesses coming under increasing pressure to innovate at a time when mobile machines will get smarter than ever before.  Learn more about IQAN products, or contact us for more information.  

This article was contributed by Johan Lidén,  product manager IQAN Electronics,  Electronic Controls Business Unit, Parker Hannifin Manufacturing Sweden AB.

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We think so, because OEMs can reduce the cost of underground construction machines by using Parker's Overcenter Intelligent Flow Control (IFC) architecture.

Closed-loop hydraulic systems are commonly utilized for rotary functions. They provide a simple solution without additional valving but can be very expensive when compared to open-loop hydraulics. In certain applications, closed-loop systems are used, but may be over-specified for the application. In these applications, a partial overcenter solution may be worth considering.

In looking at drills and trencher machines there is an opportunity for OEMs to reduce the costs of small and medium-duty machines using the Parker's Overcenter IFC architecture. The applications where this system could be applied include:

• Trenchers – trencher chain drive
• Directional drill – drill rotate

To see why these machines are an attractive application, think about the typical duty cycles. Reversing applications are rare and typically do not require full flow. The reversals are typically only required when breaking or making pipe, or in the case of trenchers, cleaning out a trench.

During these reversals, other pumps may be available to provide reverse flow such as a mud pump or front implement. This architecture is based on applications where during a reversal, another system on the machine is idle or has capacity to provide flow for reversing the drill rotate or trencher mode. 

Parker's Overcenter IFC architecture uses our uniquely designed overcenter open-loop pump in conjunction with a reversing valve and one additional flow source. This architecture better optimizes the use of pumps on a machine and helps to reduce the need for expensive closed-loop components.  

When applied to a drill, for example, the hydraulic flow from a mud pump function can be diverted to the drill rotate function to provide reversal. During pipe break, the mud function should not be required. This diverter functionality can be added to an existing manifold valve for packaging.

The diagrams below compare a traditional system to Parker's Overcenter IFC system. First, we explore the drilling duty cycle. A traditional system may consist of a closed-loop pump that operates the drill in both directions. When adding a pipe to the drill string, wrenches hold the pipe while the pump reverses direction to unscrew from the pipe allowing a new pipe to be added.

Using the Overcenter IFC system, the duty cycle is the same, however, instead of reversing the drill by changing the direction of a closed-loop pump, flow is diverted from the mud pump function that then reversed the drill motor. The flow then drives the drill pump overcenter returning the flow back to tank.

Traditional System                                                        Overcenter IFC System

Because proportional control is provided by the pump, valves are needed simply to provide direction change, assuming directional control is needed. When designing these systems, pressure drop through a directional valve must be considered. High-pressure drop can lead to excess heat and may reduce system performance. Parker has designed custom low-pressure drop manifolds that can be integrated into existing manifolds or mounted as a standalone component. These directional cartridges and manifolds are designed for high flow while maintaining pressure drop. Our target is to be no more parasitic than a charge system.

The R08E3 cartridge provides simple reliable directional flow control at 300 LPM while maintaining pressure drop at or below 3 bar. The R08E3 is also pressure rated to 420 Bar for high-pressure systems.

There are three main reasons why the Overcenter IFC is superior. The first is cost, the closed-loop systems tend to be more expensive than open-loop systems. The second is that the system reduces complexity. There is no longer a need for a charge filter, and there is no need for a flushing valve in the system. And, finally, the system may provide superior efficiency, it eliminates charge losses, although valve pressure drop should be considered and minimized.

In many applications, we see cost and system simplification advantages to utilizing the Parker Overcenter IFC concept. While not all systems are applicable, it is probably time to consider the Parker Overcenter IFC system during your next machine architecture redesign.  

The Parker Global Mobile Systems engineering team and Hydraulic Pump and Power Systems Division's application engineering experts are available to assist our customers in designing and implementing new systems to meet your application needs.

This article was contributed by David Schulte, P.E., senior systems engineer, Parker Hannifin Corporation.

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Four Check Valves Functions that Keep Mobile Equipment Mobile

A common place integrated hydraulic circuits can be found are on mobile applications in the construction, forestry and material handling markets. To meet the essential feature of load controls in these applications, a counterbalance valve is often used.

The basic function of a counterbalance valve is to control a load, by providing a restriction through a differential area. They also help prevent cavitation when controlling an overrunning load. Load control is achieved when enough pressure is present at the work port of the valve to overcome the spring setting. Specifically, when an actuator is pressurized, pressure is developed at the work port. This pressure builds and acts on the differential area until it overcomes the spring setting, shifting the poppet and allowing fluid to pass through to tank.

In addition, there is also a pilot signal (usually the opposite side of the actuator) that connects to the counterbalance valve. This signal enables the valve to shift with much less load pressure. If the load attempts to “run away” (when it moves faster than the pumps can supply flow), the pilot signal is diminished, resulting in the piston to restrict flow to tank thus controlling the load.

An added feature of a counterbalance valve is a built-in thermal relief. Because oil expands under intense heat conditions, actuators of load holding applications sometimes have unintended movement. However, built-in thermal relief allows drops of oil to pass when work port pressures reach the spring setting eliminating unintended movement in load holding applications.

There are many factors to consider in selecting the right counterbalance valve for a specific application. From flow selection, vented or non-vented, to adjustment types and setting selections, there is a lot to consider. However, the pilot ratio is one of the most important features when selecting a counterbalance valve.

Defined, a pilot ratio is the ratio of pilot area to differential area. This means the higher the ratio, the less amount of pressure needed to assist the valve and unseat the poppet. However, a caveat for selecting a higher pilot ratio is less restriction, less control, and less horsepower required.

The decision of pilot ratio is highly dependent on application. For example, the most popular counterbalance valve ratio is 3:1, often used in position-critical applications such as pick and place applications, where control is essential. On the flipside, a 10:1 ratio is common amongst high speed and motor control applications where positioning is not critical, and required horsepower is reduced. At the end of the day, it is important to remember the following:

• Higher pilot ratio = less restriction, less control, and less horsepower required
• Lower pilot ratio = more restriction, more control, and more horsepower required

Other functions a counterbalance valve performs include holding a load, protecting against hose failures, and offering control in critical metering applications. However, it is important to note that counterbalance valves are only needed if the application has varying loads or speeds. If they are fixed, Parker Hydraulic Cartridge System Division’s flow control valves and pilot operated check valves should be used. For example, using a counterbalance valve on outriggers may result in the scenario pictured below. Here, back pressure developed assisting the counterbalance valve through the pilot signal, causing the cylinders on the construction equipment to give out.

As one might realize, picking the right features for a counterbalance valve might seem harder than expected. With Parker’s Hydraulic Cartridge Systems Division’s Application Engineers, finding the right products for a specific application is easy. For more information on Parker’s cartridge valves, and how to select the right counter balance valve, contact us.

Article contributed by Nate Borries, technical sales associate, Parker Hannifin's Hydraulic Cartridge Systems Division

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Today’s off-highway mobile construction equipment is more complex than many would have ever imagined a mere 20 years ago. At the turn of the century, mechanical advancements and machinery upgrades rarely involved electronic controls or computerized enhancements. Yet in today’s world of off-highway equipment, special devices called “gateways” are utilized for security, safety and increased efficiency. These gateways serve as central hubs that allow for secure and reliable interconnections that process data across a wide range of vehicle networks and electronic control units (ECUs). 

Because of the advancing technologies in the off-highway machinery space, it is more important than ever to stay prepared for future changes. As large mobile equipment networks continue to become more complex, the existing communication strategies will result in system bottlenecks down the road. A gateway device, such as the PVSG, solves these challenges by enabling future communication network architectures which allow for multiple network buses and managed communication between networks.

Ideally, such a wireless gateway provides routing support for different CAN network protocols, such as 11 bit CAN, 29 bit CAN and J1939. In order to support a multitude of CAN network protocols, the PVSG was developed with a PC configuration tool that enables OEMs to easily set the routing table without custom applications or detailed software competency - taking the challenge out of the process so OEMs can focus on reliability, efficiency and safety.

This article was contributed by Sharlette Carey, marketing specialist and Curtis Rebizant, marketing manager, Electronic Controls Division.