Blow hot and cold1 May 2019
Energy and industrial processes have never been so high, deep, hot or cold. The commercialisation of more remote resources has led to an increase in demand for valves that can withstand extreme environments. Mike Semens-Flanagan, chief technology officer, and Brian Metz, senior design engineering manager at IMI Critical Engineering, examine how valves can be designed to have a long service life in extreme temperatures.
Valves intended for service in cold ambient environments below -40°C or even -50°C present a unique set of challenges. Material selection, control accessory selection and meticulous testing are critical factors in ensuring valves and actuators can operate at extremely low temperatures.
Materials used for the valve body and bonnet must be selected carefully to compensate for the effects of very low temperatures on the ductility of metal. If the metal chosen isn’t designed for performance in low temperatures, there is a high risk of brittle failure. This risk can be mitigated by choosing special carbon or stainless steels, which are specifically designed to remain ductile in low-temperature environments. In some cases, special mechanical tests can be carried out to verify the metal used will maintain the specified ductility at low temperatures.
The valve actuator and control accessories also require special attention, mostly due to the extensive use of elastomers. Normal elastomer seal materials, typically used in an actuator, become hard and brittle at low temperatures. When a seal becomes hard, it loses the ability to conform to the mating surfaces, potentially causing the seal to break. On top of this, the hardening of dynamic seals (like the piston seal in an actuator) can cause erratic operation or complete failure to operate due to increased friction between the seal and the actuator cylinder. Selecting special low-temperature elastomers will ensure the seals remain flexible and maintain their functionality as designed.
Any accessories used to control the flow of air to and from the actuator, such as trip valves, air pressure regulators or boosters, must also be rated for low-temperature service for the same reasons. Control accessories must also be completely watertight, and any air exhausts must be aimed downward, and protected from water, snow and ice ingress. The air that feeds the actuator and control accessories must also be free of moisture when it leaves the plant air compressor, as excess moisture in the air lines can collect and freeze inside control accessories.
Lubricants used for actuator seals must be also suitable for low temperature service. Some lubricants become too viscous at low-temperatures, which can prevent the actuator from operating smoothly. Any lubricants carried over from the air compressor must also be filtered out, as this hydrocarbon oil can cause erratic operation or failure of the control accessories and actuator when exposed to low temperatures.
Other mitigation measures such as heat tracing or a heated enclosure may also be used in lowtemperature environments. Heat tracing involves wrapping the valve components and actuator in a special resistance heating element connected to a power source and a temperature controller. The resistance heating element warms the valve components, making sure the temperature never drops below a specified point. Alternatively, a heated enclosure can be provided. This is typically a small, insulated aluminium housing that completely surrounds the valve and uses a built-in resistance heater to maintain a local ambient temperature within a specified range. The heated enclosure provides a uniform temperature for the entire valve and actuator assembly, as well as providing shelter from snow, ice and wind.
Arctic conditions in western Siberia
The ZapSib-2 facility at the Tobolsk Petrochemical Complex in western Siberia illustrates the challenges of valve and actuator design in extremely low ambient temperatures. The new facility will be the biggest integrated complex for the production of polymers in Russia when it comes online.
As well as meeting the challenges of the extreme processing environment for polymers, which includes a highly corrosive atmosphere, suppliers have the additional challenge of ensuring their equipment can withstand the stress of extreme climate conditions, in this case, the potential for temperatures to fall as low as -52°C.
IMI Critical designs bespoke valves and actuators that will allow processors to operate safely, efficiently and reliably in some of the harshest environments on the planet. For this project, two IMI Critical companies worked together to produce anti-surge valves and actuators. The anti-surge valves required actuators ranging in size and stroke from ND200 with a stroke of 70mm, to an ND500 with a stroke of 300mm.
Another challenge was to tailor the actuators to the highly corrosive atmosphere and very low temperatures at Tobolsk. To withstand the corrosive environment, most components for the actuators were made in grade 316 stainless steel, as was the FasTrak positioner, which is TR CU-certified for -55°C. Grade 316 stainless steel includes molybdenum, which gives it higher resistance to corrosion.
Ensuring the actuators could work in very low temperatures was a bigger challenge. Actuators are typically required to work in temperatures of between -20°C to 70°C although our specialist actuation division, IMI STI, has previously supplied actuators to operate in -60°C.
The solution developed by IMI STI was to design a bespoke cabinet, also constructed in grade 316 stainless steel, to house the actuators. The cabinet contains heater cables to maintain a higher temperature inside. In addition, the design of the actuators incorporated a number of other common features to cope with the combination of a corrosive atmosphere and very low temperatures. The vent port faces inwards to avoid water entry or ice build-up or formation.
In addition, the stroking times specified were that all actuators must open in less than two seconds and close in less than three seconds, without overshoots, and while also guaranteeing the high dynamic performances required by anti-surge applications. IMI STI designed a purpose-built test sequence to simulate the actuators’ performance in the harsh conditions they were designed for. With correct use and periodic maintenance, the actuators are expected to have a lifespan of some 20 years.
Some like it hot
Valves in high-temperature service face a different set of challenges. Combined cycle power plants are operating at higher and higher steam temperatures in order to increase overall plant efficiency. Temperatures of 56°C are now commonplace and would have been impossible to design if materials technology had not advanced accordingly.
When selecting materials for the valve body and bonnet in high-temperature service, special alloy steels are preferred for their ability to maintain sufficient mechanical strength at high temperatures. This ensures that the valve will maintain its pressure boundary integrity and functionality when confronted with high steam temperatures.
High ambient temperatures can also spell disaster for actuation and control accessories. The elastomers that become hard and brittle at low temperatures can become soft and gummy at high ones. As elastomers are heated, they become soft and can extrude through gaps between sealing surfaces, leading to a failure of the seal. Additionally, elastomers in dynamic service (like the actuator piston seal) can soften, causing erratic operation, or even failure, of the actuator. Special, high-temperature elastomers must be used for any actuator installed in a hightemperature environment.
Valves in high-temperature service also need to be insulated for the safety of plant workers, as well as for plant efficiency and to protect surrounding equipment from heat radiating from the valve itself. This insulation typically extends to the top of the valve bonnet and provides some degree of protection for the actuator and accessories. Additionally, the valve yoke locates the actuator up and away from the hot valve body and bonnet, further protecting the elastomers in the actuator and control accessories.
The internal components for high temperature service valves are also analysed for the effects of thermal expansion to ensure there is no binding or interference of components once the valve operating temperature is reached.
Couple high ambient temperatures of deserts with valves for LNG trains (-162°C), and it is clear why engineering knowledge and experience is critical.
The heat is on in Ohio
The Lordstown Energy Center combined cycle power plant in Ohio, Pennsylvania, exemplifies the challenges presented by high-temperature severe service applications. A high-pressure steam to cold reheat bypass control valve was needed that could withstand temperatures of 574°C and pressures of 36 MPa. Experts at IMI CCI employed special materials to withstand the high steam temperatures, as well as a high-performance actuation and controls system that would meet the system response demands caused by a special, reduced-size high-pressure steam drum located on the Heat Recovery Steam Generator.
The challenge of high-temperature steam operation was met with a combination of materials specifically designed for performance at extremely high temperatures. The body and bonnet pressure boundaries were made of F91 alloy steel, while the superalloy Inconel 718 was chosen for the body-tobonnet bolting, valve stem and DRAG disk stack trim. The alloy steel valve plug and seat ring were overlaid with Stellite 6 to ensure smooth, reliable operation and tight shut-off under these demanding conditions.
Due to the reduced-size high-pressure steam drum, especially strict control requirements were imposed for the valve and actuator. A smaller steam drum requires faster and more accurate temperature and pressure regulation response time in order to keep the system stable. The total inaccuracy between valve demand signal and actual valve position had to be less than 0.5%; several times higher than typical. The actuator had to have zero overshoot and a full stroke time of less than four seconds. Typically, these specifications would require a hydraulic actuator, but the IMI STI QuickTrak actuation and control system was able to meet these specifications, while easily achieving the stroke and trip speed requirement of less than four seconds, open or closed. The actuation and controls can perform to these strict requirements and still withstand the 66°C environmental temperature caused by the nearby piping and equipment.
Today, thanks to innovation in materials, bespoke accessory design and painstaking testing, valves can be designed to operate at extremes of temperature that would have been unachievable a generation ago. This allows processors to operate their plants safely, cleanly, reliably and efficiently.