Coal Fired, Renewables

Valves & actuators: risks and opportunities

Issue 2 and Volume 25.

To keep up with a rapidly changing energy landscape, valve and actuator manufacturers have seen their technology become more complex, incorporating new materials and processes within the context of an ever-growing demand to reduce costs. Tildy Bayar finds out how they’re responding

Modern power plant requirements have given rise to innovation

Credit: Auma

As more renewable power sources continue to be added to the world’s grids, the increasing need for flexibility in fossil-fuelled power plants is creating more demands on operational processes and greater stress on critical components. To keep up with this rapidly changing environment, valve and actuator manufacturers have seen their technology become more complex, incorporating new materials and processes within the context of an ever-growing demand to reduce costs.

We spoke with key manufacturers about their technologies, the risks and what they are doing in response to the new energy landscape.

The risks for valves

At its heart, valve and actuator technology is simple. The valve opens and closes, either allowing water or steam to pass through or stopping it from passing. The valve can control the degree of flow, the flow direction, and the pressure. The actuator directs the valve’s operation. This basic technology has remained unchanged for many decades, but there are still a number of issues that can cause failure.

Arvo Eilau, Marketing Manager Gas Power at Pentair Valves & Controls, says the number one failure mechanism for valves is simply time: “The valve will eventually wear out”. How quickly this happens, he says, depends partly on the medium that flows through it: for example, steam is abrasive and can damage the valve, while seawater intake valves wear out over time due to corrosion. “There is a limited lifetime to critical-duty valves,” he says.

In addition to normal wear and tear, though, the increasing temperatures and pressures that come with flexible operation are causing unprecedented stress on power plant components. “Twenty years ago, when plants ran at lower pressures and temperatures, a valve could easily last for 30+ years,” Eilau says. But with today’s requirements for higher efficiency and lower emissions, “we’re running right on the edge of our material technology and the material will fail much faster”.

One such materials issue is so-called hard facing failures. The hard facing is a layer of material applied to the internal area of the valve which is in contact with steam or water. It is designed to prevent erosion, corrosion and mechanical wear. Because the hard facing is welded to the valve, “it’s critical that we put it on correctly,” Eilau says. “The proper welding, heat treating and testing is required to avoid potential failures … and it does require additional work when applied properly”.

“Damage to power plants could become exacerbated in the future” due to fast ramping, he says. “Valves could potentially have issues in the coming years. But we know these components are going to fail, so we’re going to extreme engineering efforts to identify how to weld, manufacture and maintain them, and to use the right materials.”

For Pentair, the right materials means two steel alloys known as P91 and P92. These materials can cope with today’s higher pressures and temperatures, but Eilau says their manufacture is critical: “If you get several molecules wrong in the material, its quality can be affected.” And it is crucial that welding and heat treating are done correctly: “If you’re off by a fraction of a percentage it could affect the strength of the material.”

Electric valve actuators

Credit: Auma

Quality control is key, Eilau says. “Manufacturing the same valve as 10 years ago, with today’s materials, takes an additional four to five steps in the factory. Engineers have to provide a process to welders to know what temperature to heat the material to, and how to cool it down properly. If the processes are done correctly and the engineering data sheets are correct, it will be ok. Miss one step, or be off by a small percentage, and the part will fail much sooner.”

The water in desuperheaters (DSH), which control the steam temperature inside the heat recovery steam generator (HRSG), can also cause damage to equipment, Eilau notes. Water damage to downstream equipment is “one of the major failure mechanisms power plant operators must deal with”.

“Because we’re running power plants up to 600-700oC, and the equipment is designed to run at the design condition, temperature control elements have to be very consistent and make sure they hold the temperature where it’s supposed to be,” he explains. “Fifty years ago we made steam in a boiler, sent it to the turbine, the turbine spun and made electricity, and steam temperature control was much simpler. Now that we’re running at such high temperatures and pressures, adding water in to cool down the steam has to be done perfectly so it mixes properly. Water droplets can transfer through the steam and are very erosive to the steam turbine and piping if they don’t evaporate right away. We don’t have the wide allowable variances that we had a long time ago, so that the critical temperature is maintained and water evaporates immediately to prevent damage to the equipment.”

Avoiding the risks is a plant-by-plant process, Eilau says, as each plant will have different requirements: “steam flow, temperature, pressure, where the pipe is located, how long it is.” While manufacturers continually evaluate their designs to meet increasingly stringent operational requirements, power plant developers need to be aware too: “Companies building power plants need to be much more vigilant that what they buy is meeting the needs of today.”

Another issue is the ongoing debate between cast and forged materials for high-pressure steam valves (see PEi, volume 24, issue 9). Cast valves have been in wide use for many years, but Eilau says that “up to a certain point – and I don’t think any company knows exactly what that point is – in today’s extreme high pressures and temperatures, a forged product is much more suitable.”

However, cast valves are more cost-effective to manufacture, and the industry does not currently have a standard to define the operational pressure and temperature at which a switch to a forged product would be recommended. Eilau says Pentair offers both cast and forged valves, recommending different products depending on plant age, fuel type or different customer priorities, such as bottom-line cost vs performance and long-term reliability.

Regional energy mixes will also have an effect, so a cast valve “might last for 30 years in the Chinese market” due to China’s greater reliance on fossil fuel power and lessened need for cycling, but the same valve may not last as long in the North American market given its larger share of variable renewables and thus a greater need for flexible fossil fuel-fired plant operation. However, the same valve could last longer than 30 years in Germany because of its even higher renewables penetration, with reduced use of fossil fuels as renewables provide more of the day-to-day power.

Francis Bouziden, sales and marketing manager at large-frame gas turbine valve and actuator manufacturer Young & Franklin (Y&F), points to a turbine-specific issue: he says the hydraulic system is the number one risk to any valve or actuator, and its care and maintenance represent an “ongoing source of many owners’ headaches”.

“One leaky fitting or one stuck servo can cause a turbine trip or failed start,” he explains, and for a plant that needs to ramp up quickly, “both of these issues can be extremely costly. Even worse, hydraulic leaks cause environmental issues and there have been reported fires as a result of hydraulic oil spraying on the turbine.”

Bouziden notes that turbine technicians have traditionally focused on managing and treating the effects of hydraulic system issues such as deposits on the valve caused by lubricant degradation, called varnish and sludge. Steps such as filtration, flushing and specialized oils “are all trying to address one part of a system that has many opportunities for failure,” he says. Instead, “the primary cause of the issue is that lube oil is being tasked with double duty: both lubrication and control. While this may have been an acceptable solution for early turbine designs, it is clearly not acceptable for the modern gas turbine.”

The risks for actuators

Felix Metzenthin, sales manager at actuator manufacturer AUMA, agrees that a component’s lifetime is affected by operating temperatures, but also by other issues such as vibration and corrosive atmospheres. Most problems, he says, arise when actuators operate under conditions they were not specified for. “It is crucial that a plant designer sufficiently accounts for ambient conditions when choosing actuators,” he warns, offering an example: “In a corrosive environment, actuators with an advanced powder coating that meets the highest requirements for protection (such as category C5-I or C5-M from EN ISO 12944-2) will normally last longer than actuators with traditional wet painting.”

Where actuators are exposed to high temperatures or strong vibration, or both, Metzenthin says it can also help to choose a modular design that allows the actuator controls, with their sensitive electronics, to be installed at some distance from the actuator. In plants where actuator controls for steam valves are installed in a cooler location away from the steam lines, he says, “we have a strongly reduced fault probability.” So “engineering is really what makes the difference. If an actuator is selected according to the specifications, normally nothing can go wrong, so it can last for 20 or even 30 years.”

And he points out that other issues can arise due to human error. For example, if installation and commissioning are performed by personnel with insufficient training, problems can occur due to incorrect electrical connections or erroneous parameter settings within the actuator.

Opportunities: the digital future

Thanks to advances in electronics, actuators now do much more than just open and close valves. Modern ‘intelligent actuators’ include comprehensive datalogging and autodiagnostics functions, Metzenthin explains, allowing them to monitor operating parameters such as torque profiles, temperatures, vibrations and number of starts. This allows maintenance engineers to check that an actuator is not operating out of specification. Actuators can also be set to automatically issue warnings or alarms when limit values are exceeded.

Electric guide vane actuator

Credit: Young & Franklin

As access to, and exchange of, an increasing amount of data becomes more and more important for the power sector, Metzenthin says AUMA is working on a move to cloud-based solutions, including an app for mobile phones. With this app “you can scan a QR code on your actuator and get the right information directly on your smartphone, get the connection plan, get the user manuals, get information on how old the actuator is, what values for torque, speed, etc were set at delivery, and so on,” he says.

Y&F’s Bouziden is also enthusiastic about developments in digital communication which allow actuators to collect “a vast amount of performance and health data that can aid in predictive maintenance activities” and transmit it to the control system. This data is useful for maintenance purposes and, according to Bouziden, “will further increase the precision and repeatability of valves and actuators”.

Eilau is also excited about the possibilities presented by increasing connectivity, although he notes that there is still a ways to go. “Actuators and sensors are starting to have the ability to inform the operator about the condition of the valve. This can include the number of cycles, the force required to operate the valve and the life of the valve,” he says. “There are great things that can happen, but I don’t think the industry is 100 per cent there yet.” This kind of monitoring is “definitely done for gas turbines in the power industry, but we’re talking a $100 million piece of equipment; the rest of the power plant parts companies haven’t spent the money yet.” This is because “the cost to implement some of these features is pretty expensive, and customers aren’t buying it yet. But as the price comes down for sensors, there is a huge future.”

But as actuators are required to transmit growing amounts of operational data, could cybersecurity risks arise? Our manufacturers say probably not, although the industry is aware of the issue.

“The actuator itself has proprietary firmware,” says AUMA’s Metzenthin. “This makes access impossible except through a defined and authorized programming interface.”

He points out that devices within power plants communicate through either point-to-point signaling or an industrial fieldbus controlled by a distributed control system (DCS). A typical DCS can handle up to 1000 actuators, all of which are connected to a central computer which controls the complete plan and initiates up to 80,000 operations per second.

“Normally we have only one line from an actuator to the control room,” Metzenthin says. “This line is protected, so there should be no possibility of cyberattack.” And while service personnel can connect to the actuator directly, normally via Bluetooth or with a USB cable, Metzenthin says access is password-protected and the actuator is programmed to “only accept reasonable data” when anyone tries to change its parameters.

For the future, he says, his company is working on a solution to protect actuators’ firmware against industrial espionage, so that their designs cannot be transmitted or copied. For the present, he believes actuators are not vulnerable “because no cyberattacker knows how the data inside the actuator is handled, and thus cannot disturb the system.”

Electric stop ratio valve

Credit: Young & Franklin

Pentair’s Eilau says cyber protection is something the sector is “working on”, but that actuators are safe for the moment because their data “is not typically on the public web, it is internal only”. However, he says cybersecurity is something that “should be addressed in the industry”.

Flexibility, costs, innovation

With the growing need for flexible plant operation comes a corresponding need for flexibility in component design, operation and configurability. Y&F’s Bouziden says valve response and control system communications are areas where components can be easily configured to meet a wide range of potential demands. Among his examples of this flexibility are:

  • A flexible interface to the turbine control system that allows for various control logic paradigms;
  • Ingress protection rated valves, actuators and electronics that allow for indoor or outdoor installation;
  • Custom stroke lengths and speeds/responsiveness of valves and actuators;
  • Monitoring tools that can perform health assessments of the valves and actuators without uninstalling the actuator; and
  • A wide range of both analog and digital feedback for performance and health monitoring.

Valve and actuator combination

Credit: Pentair

 

Low pressure gate valve

Credit: Pentair

However, the constant drive to increase flexibility and reduce costs is necessarily balanced by environmental concerns. On the environmental side, Bouziden says electric actuators have eliminated the need for hydraulic oil systems that can cause environmental hazards. To further aid in emissions reduction, turbine operators can look to precision gas valves and electric, rather than hydraulic (oil-fuelled) guide vane actuators.

On the cost side, Bouziden says electric actuators “eliminate all maintenance activities associated with hydraulic systems and are extremely reliable. An average site typically recovers the initial investment after a few years.” And he adds that electric actuators can increase the maintenance interval to 96,000 run-hours, which “at most sites eliminates a full hydraulic overhaul cycle”.

Another kind of cost saving is produced with an upgrade to an electric system, he says, as this can eliminate almost 50 components within a turbine’s trip oil system, hydraulic supply system, gas fuel system and inlet guide vane system.

“Many end-users spend upwards of $100,000 a year maintaining their hydraulic system. This doesn’t even account for lost revenue due to one bad servo in the system causing a trip or a failed start. The cost of downtime during repairs, and any fines or loss of capacity payments can quickly add up.”

In terms of innovation in the actuator space, Bouziden says most will come from the introduction of microprocessor-based digital electronics, which allow for in-situ monitoring of key valve performance parameters. “This ability will minimize trips and forced shutdowns and allow maintenance to be performed during scheduled outages,” he says. In addition, distributed control protocols “allow for increased flexibility and data feedback not available with hydraulic systems.”

AUMA’s Metzenthin says his firm’s innovations are also largely IT-based rather than mechanical. “Actuators play a crucial role in the communication between the control system and the valve”, he says. “An actuator manufacturer has to ensure smooth integration with any kind of commonly used DCS, using any kind of communication type, be it parallel or fieldbus, hard-wired or wireless. We are constantly expanding the number of interfaces available for our actuators. At the moment these include Profibus DP, Modbus RTU, Foundation Fieldbus, DeviceNet, and HART, as well as the Industrial Ethernet standards Profinet and Modbus TCP/IP.”

He describes AUMA’s current actuator range as future-proof. “Of course we will have new models in the future,” he says, “but innovation can also lie in the method of production, not just in the product itself.” This includes new generations of electronics components and machining technologies.

Like Y&F and Pentair, AUMA recognizes that customer requirements will differ depending on the application. The company offers a range of actuator models in line with differing priorities: a lowest-cost model, a more-options model and a model designed for continuous modulation.

“Normally with a standard-duty actuator, there are times when it has to stand still to cool down,” Metzenthin says. “For example, the specification may say that it is only allowed to run for 15 minutes of each hour. Continuous modulation, on the other hand, can mean the actuator makes 3600 starts and stops per hour – it’s effectively moving all the time. For a properly-designed modulating actuator that’s no problem.”

In addition to modifying the technology, he says AUMA’s subsidiary SIPOS has worked to address what it sees as the biggest risk for actuators – human error – by redesigning the human-machine interface (HMI). Its new model includes a larger display and graphical service guides, which make it “nearly impossible to do something wrong”.

But despite the sector’s moves to meet evolving power plant requirements, Metzenthin does not see the basic design of valves and actuators changing in future, or at least in the next five years.

“I think the valve and actuator business is very traditional,” he says, “and if something is working well and reliably, most power plant business people say let’s keep it this way.”