Weir : Choosing the most cost-effective isolation valve…

By Tom Newstead, Principal Engineer, Weir Flow Control

In the modern power industry, there has been a drive to develop plants that operate at higher pressures and temperatures. This is due to the efficiencies gained in power generation coupled with the reduction in environmental pollution. At the same time, there has been a motivation to reduce valve costs due to global competition between engineering procurement contractors (EPCs).

The isolation valve is an integral part of any power plant, but businesses need to choose the right valve type which can give them the right balance between performance (long term isolation) and cost.

In simple terms, an isolation valve consists of four fundamental design principles: globe valves, ball valves, butterfly valves and gate valves. At Weir, we work with major power customers across the world to ensure they have the optimal valve, performing throughout the asset lifespan, and reducing the risk to power plant processes. Our isolation valves operate in some of the most demanding environments.

Starting with globe valves - these types of valves are extensively used for isolation of relatively small bore process lines. Due to the flow path through the valve body, they generate a slightly higher pressure drop than the other designs, and they can experience high velocities and seat wear. At the same time, although large globe valves can be supplied, they would normally be fitted with high powered actuators or large gearboxes to overcome the fluid pressures acting on the plug head. In most cases, this means globe valves are not indicated in larger process lines.

A relatively recent trend for low pressure isolation is to use a butterfly valve. Historically these would have been high performance double offset designs but now most isolation designs are specified as a triple offset design butterfly valve. This type offers significant advantages for low pressure applications. Their footprint is smaller than any other isolation valve, which ultimately reduces costs. Additionally, a triple offset butterfly valve ensures longevity of the seat integrity due to the point contact of the seating face and therefore minimum seat erosion.

One consideration is that a butterfly valve cannot be serviced in-situ and must be removed from the line, which means they are usually offered with flanged ends. The triple offset butterfly valve is the perfect solution on the low pressure (LP) steam and water systems.

Hopkinsons® Parallel Side Gate Valve

Considering isolation valves on the higher pressure systems (HP) essentially there are two choices. The first one, the wedge gate valve, has risen in popularity due to the drive to reduce costs. It offers a 'one time' sealing solution due to the wedging action between the plug and the seat. However, these valves should be avoided on steam plants where temperature changes potentially result in thermal binding of the wedge and locking of the valve in the closed position.

The best solution for power plants is the parallel slide valve, invented by Hopkinson in 1881. Compared to the wedge gate valve; all the components in a parallel slide valve are free to move no matter what the temperature. This means that, as the valve heats and cools, seat integrity is maintained without any thermal locking. The valve has a rising stem but, unlike the wedge gate valve, it is positioned seated, meaning there is no additional force required to seat or un-seat the valve.

Within the parallel slide valve range, there are essentially two design options, the full bore and the venturi design. In simple terms, full bore means that the valve bore is the same as the pipe bore. On the other hand, the venturi parallel slide valve reduces the size of the internal valve components while generating a small pressure drop across the valve.

The parallel slide valve has additional benefits when considering maintenance. The disc and seat faces are effectively self-cleaning as they stay 'on lap', meaning that when the valve is opening or closing, any potential damage to the seat faces due to line debris, is minimised. Due to the fact that the valve is pressure seated, it is significantly easier to maintain than a wedge gate valve.

Historically, parallel slide valves were manufactured with cast material for their use on conventional plants with maximum steam temperature of 565°C. In order to improve efficiency while reducing the environmental impact, temperatures have risen through the specification of Supercritical and Ultrasupercritical plants. In this context, valves need to handle the higher pressures and temperatures while minimising the impact of thermal shock and therefore, material failure.

The main cast material applied for high temperatures is ASTM A216/A217 C12A, which has important limitations due to the casting process. The use of C12A material often results in significant amounts of weld repair which in turn results in several stages of heat treatment and X-ray. Using a forged material, such as ASTM A182 F91 and F92, ensures security of delivery.

Ultimately, the forged parallel slide gate valve offers a higher integrity material than a cast valve while eliminating unexpected delays in material delivery.

In conclusion, power plants need to choose the right type of isolation valve to ensure plant efficiency while minimising cost. We are proud to work closely with our customers to deliver the most cost-effective design and ensure excellent performance and safety throughout the isolation valve's lifespan.

For further information on isolation valves, please click here