A switch in role for fossil fuel-fired power plants has implications for small but vital plant components, writes Penny Hitchin

A large power plant uses hundreds of valves to manage flows of water and steam Credit: AUMA Group

Concern about climate change is causing industrialized countries to try to decarbonize their electricity supplies. Intermittent renewable energy is being given priority, leading to a switch in role for fossil fuel plants: from supplying base load to providing flexible peak power. This change in the pattern of operations has implications for many of the smaller components in the power plant as well as the high profile, big-ticket items.

Valves and actuators make up a small part of a power station – perhaps only two or three per cent by capital cost – but they play a vital role. A large power plant uses hundreds of valves to manage flows of water and steam. Valves may be used to stop and start flow, reduce or increase flow, control the direction of flow, regulate a flow or process pressure or relieve a pipe system of a certain pressure. Applications include pollution control, feed water, cooling water, chemical treatment and steam turbine control systems.

Frequent switching on and off of plant imposes additional stresses on the equipment. Every stop start can cause wear and tear. While the fundamental design of the valves has not changed much in decades, improvements due to advances in material technology and manufacturing techniques are enabling them to operate at the increased pressure, temperature and frequency that operators require to provide higher thermal efficiency.

Each valve is controlled by its own actuator – basically a motor mounted to a gear box – which moves the valve at a specific time or speed or opening rate. Accurate and timely controls are increasingly important. The basic design of the mechanical part of the actuator is much the same as it was 30 years ago; however, the range of control and communication functions of the actuator have been transformed by continuing developments in electronics.

Power plants are often built with a design life of around 50 years. This means that the fleet of old plants remaining in operation must be adapted to the new conditions if their productive life is to continue.

Additional flexibility

Rapid developments in electronics have driven advances in control systems and state-of-the-art actuator/valve combinations are now installed in new plants. Refurbishment of existing power plants offers an opportunity to add functions to existing systems.

Incorporating additional flexibility in startup times is particularly useful as conventional power plants may be required to start up or power down several times a day to support the large increases, and decreases, in available power. Modern electric actuators can offer modulating performance which helps to achieve this, enabling power plants to become operational much more rapidly than in the past.

A refurbishment project also provides an opportunity to consider corrosion protection enhancements – important in the harsh environment of high-temperature and high-steam areas, such as cooling towers. For example, modern powder coating processes can enable actuators to better withstand such highly corrosive environments.

Felix Metzenthin, International Sales Manager Power of the AUMA Group, a major supplier of electric actuators, describes the process of determining a programme for upgrading and refurbishing valves and actuators in a power plant.

“We usually do a walkthrough with the customer and look at the actuators. We look at technical data for mechanical and electrical specifications and then propose exchanges which are often done in several stages during planned outages.”

An upgrade scheme will usually start with the most important actuators which influence plant performance more than on-off valves, which are used in cold start. Following inspection and discussion, the bespoke actuators are prepared in readiness for installation during planned plant outages.

Bespoke actuator refurbishment Credit: Weir Group

Actuators are designed for a product lifetime of 20 to 30 years, but the actual lifetime is defined by the conditions in the plant. Actuators operating in ambient temperatures of around 40 degrees will last rather longer than those operating in conditions of high temperature or pressure.

Where a valve is required to operate in conditions of high heat and/or steam, such as in a cooling tower, the controls can be designed so that the control electronics are connected to the mechanical part of the actuator by cables, which can be up to 120 metres long.

Depending on the operating conditions, special high temperature variants with special lubrication or sealing can be used, which can withstand the higher temperatures without de-rating.

Standardization is a key factor in enabling efficient refurbishment and asset management. Increasingly, operators look for standardization of components, combined with good record keeping and documentation.

One of the challenges of refurbishing old plant is that, when they were originally built, there was no standardization in mechanical adaptations for mounting valves onto their actuators. It may be that no documentation exists, and that the adaptations can only be seen once they are dismounted. If the old valve is to remain in service, a bespoke like-for-like replacement mounting is required to enable the new actuator to control the valve.

The manufacturer responsible for the replacement actuator dismounts the electrical equipment from the valve, makes a drawing of the coupling and comes up with a proposal for how to fit the new actuator to the old valve. This requires skilled and experienced workshop technicians.

Modern actuators rely on electronics for control and communications. Electronics is a fast-evolving field and small components may not be available in the long term. Good design practice means enabling new elements to replace the functionality of the old electronics if they are no longer available.

Upgrade efficiency can be achieved by transferring data sets from older systems to new replacements. In a current refurbishment scheme at a large European lignite plant, SIPOS, an actuator manufacturer within the AUMA Group, is upgrading 140 actuators. The actuators were originally installed in 1990 and the mechanical part of the actuator on the valve can be retained while the electronics are replaced. SIPOS plans to replace the electronics module and to download the data sets (such as operating parameters, speed etc) from the old actuator onto the new one, avoiding the need for new programming.

Refurbishment is not for everyone. Umberto Bianchi, Product Sales Specialist Actuation & Controls with Pentair Valves & Controls, finds that customers upgrading their plant prefer replacement rather than refurbishment.

“Customers don’t want to fix an old actuator – they want a new one with the new technology,” he says. In other words, they want the diagnostics, data logging and potential for predictive maintenance offered by new actuators.

Supercritical environments

The only way to increase fossil fuel-fired plant efficiency is to increase steam temperature and pressure. Supercritical boilers consume less fuel than traditional drum boilers, making them more efficient. They operate by burning pulverized coal at such high pressure that the fluid matrix becomes a ‘supercritical steam’ which turns the turbine. When it subsequently drops below the critical pressure point it becomes a mix of steam and water, passing into a condenser.

Advances in materials technology and manufacturing techniques contribute to the development of valves capable of operating in such conditions. Valve designers work closely with OEMs to understand their needs. Critical elements in the design and manufacture of valves for supercritical units, and units that cycle much more than in the past, include use of super-alloys, specialized welding techniques, heat treating and manufacturing techniques.

Every piece inside the boiler has to adhere to specific welding procedures and standards. One of the biggest issues is hard-facing failures. On a ‘severe service duty’ valve operating at high temperatures and pressures, a very thin hard-facing layer of stelite is welded onto the valve. The cycling duty of the valves and boilers can affect the durability of the hard-facing materials. The proper welding procedures and applications are very important to prevent premature delamination of the hard facing.

In-situ valve replacement Credit: Weir Group

Arvo Eilau, Global Marketing Manager Power at Pentair Valves & Controls, stresses the importance of welding procedure and standards, especially in materials used for supercritical plants. He points out that, within the last decade, some of the hard faces for the valve have become critical. “Some of the new materials can only be welded once, because heating it up again changes the molecular property of the molecule and it is no longer able to hold the temperature and pressure. The industry needs to understand how to manufacture this properly.”

Re-engineering systems

There are areas where upgrading old valves and actuators to newer designs is not appropriate. Examples include established coal-fired and nuclear plants which may have perhaps a decade of service life remaining, where purchasing new equipment is not viable. Control systems in some older power stations may not be suited to modern electronic systems and, in this case, the best course of action might be to overhaul and refurbish the original electro/mechanical valve actuators to maintain plant continuity. In the highly regulated environment of a nuclear power plant, it may be less problematic to replace like with like, rather than seek regulatory approval for a new system.

Modern actuators rely on electronics for control and communications Credit: AUMA Group

Valves and actuators in existing plants may be decades old, and documentation about their specifications may not be available. Refurbishing old valves and actuators is a specialist area, combining a problem-solving approach, new technology and traditional engineering skills.

Weir Group’s service hub in Alloa, Scotland specializes in repair and refurbishment of valves, pumps and actuators. The team works closely with Weir’s OEM businesses, such as the Hopkinsons team in Elland, to offer a “cradle to grave” solution for equipment integral to flow control within fossil, nuclear and hydro power generation.

Where possible, component parts are prepared in the workshop, ready for installation during a planned outage. Spares may be part of a service exchange scheme, they may be re-engineered, or possibly even recycled from a redundant power station and refurbished before being installed elsewhere.

The company has a contract with the operator, EDF, to deliver a major outage valve scheme across the UK’s advanced gas-cooled fleet of 14 nuclear reactors. These reactors are a unique British design built in the 1970s and ’80s. Although they are entering the last years of operation, EDF is committed to a programme of life extension and is working to gain regulatory approval to keep the reactors generating into the next decade. This will require major investment: in 2015 EDF announced plans to spend £150 million ($219 million) to extend the life of Dungeness B from its projected shutdown date of 2018 to 2028.

A typical coal-fired or AGR power station has as many as 1000 major system valves per unit, largely parallel slide valves. Safety valves are the most critical, and account for around five per cent of the total number. Non-return valves, control valves and butterfly valves make up the balance. In the course of a major outage, around 500 valves will be refurbished by a team of up to 150 engineers.

Steve Holman, Weir’s Business Development Manager, Valves, points out advantages that valve refurbishment can offer over commissioning new replacements.

“Lead time for new valves is generally around 26 weeks, and bespoke refurbishment can take half that time, including data capture if a valve has no documentation. Weir is able to offer a choice of either new equipment or lifetime extension, depending on the operating environment and individual customer needs,” he says. Advances in technology such as additive manufacture offer the prospect that this can be reduced in the future.

Actuator specialists at Weir’s workshop can manufacture replacement actuators to existing designs. General Manager Peter Boyle sees this as a growing market, offering substantial cost reductions over installing new designs, and enabling plant to operate over a 50-year life cycle.

Holman says the announcement that UK coal power will be phased out within 10 years has boosted interest in refurbishment schemes.

Weir’s team takes pride in being able to refurbish key components which may have a long operating history and no documentation. Each project demands a bespoke solution.

Some massive and ancient valves are brought to the workshop for refurbishment. A 74-tonne ball valve which had been operating in a hydropower plant for 40 years was recently brought in to be given a new lease on life. There was no documentation, so the team set about data capture to establish its specifications. An eight-month project saw it refurbished and sent back into service, where it should serve for another 40 years.

As technology evolves, reactive maintenance is being augmented by more proactive approaches. An example is the in-situ valve seat replacement service developed by Weir in alliance with Doosan Babcock and first offered in 1999, which has been used in fossil, combined-cycle and nuclear power plant. Replacement of high pressure parallel valve seats has traditionally involved removing pipework, cutting out the damaged valve and returning it to the workshop for repair and welding. Such valves can weigh tonnes, and cutting and moving them from their location is usually problematic. In-situ valve seat replacement offers considerable savings in cost, time and risk.

Holman estimates that the cost is halved and the downtime reduced to three or four days. The technique has become feasible as new welding techniques and advances in remote control and on-board camera feeds have enabled technicians to overcome access and dimension issues and work within a confined space to replace the valve seats without having to move the valves.

The operational life of a power station can be measured in decades, with thermal and nuclear stations remaining in production for as many as 50 years. Operators are keen to extend the life of existing thermal and nuclear stations where this is feasible.

Rapid developments in electronics have driven advances in control systems and state-of-the-art actuator/valve combinations are now installed in new plants. However, it may not be appropriate to retrofit these devices to an established plant. A lot of the control systems in older power stations may not be suited to modern electronic systems and the best course of action might be to overhaul and refurbish the original electro/mechanical valve actuators to maintain plant continuity.

Valve Workshop Manager Steve Sneddon reflects: “As power stations near the end of their working lives, operators are looking for innovative solutions. That’s what we are trying to provide.”

Penny Hitchin is a journalist specializing in energy matters