Subject: Article describes the challenges involved in selecting ball valve components and materials of construction for high temperature service.
By Jason Jablonski and Wade Helfer, Emerson
Jason Jablonski, Director of Rotary Engineering at Emerson Automation Solutions and member of the API subcommittee on Piping and Valves and Wade Helfer, Rotary Valve Specialist at Emerson Automation Solutions, recently published an article in the November 2019 issue of Hydrocarbon Processing magazine that describes the significant challenges associated with specifying ball valves for high temperature processes. Their article is titled Using Ball Valves in High-Temperature Applications and is summarized below.
The Heat is On
Ball valves (Figure 1) are often an economical solution for controlling flows in refinery high temperature (>400˚F) applications, but choosing the right valve design and components can be very difficult, as Jason and Wade explain:
Figure 1: Fisher Type Z500 Floating Ball Valve installed in a high-temperature application.
Although API RP 615 defines high temperature service for metal-seated valves as temperatures greater than 750˚F (400˚C), 400˚F (204˚C) is a natural transition temperature where most elastomers and polymers break down. Also, some softer metals, such as aluminum alloys, begin to weaken as temperatures increase.
In high temperature applications, poorly designed valves can fail quickly in multiple ways. A common mode of failure is the binding of drivetrain components. Depending on the severity of binding, one can expect accelerated wear on metal parts or a complete stall out of ball rotation. Actuator torque can exceed the capability of the drivetrain, resulting in sheared keys, a twisted shaft and/or a deformed ball. A coating failure of the ball to seat (Figure 2) may also occur.
Figure 2: Trunnion ball showing coating failure near the bore and trunnion bearing surface.
Drivetrain friction goes up with increases in temperature. During normal operation, torque may increase up to two times compared to what is experienced at ambient temperature, thus making actuator sizing critical. Factors influencing this increase in torque include the shifting of parts due to thermal expansion, thermal growth of complex geometries and the dissipation of assembly lubricants such as molybdenum disulfide. Metal bearings and graphite packing rings have higher friction than polymer equivalents, and the softening of load-bearing parts results in higher friction and the potential for galling or wear.
Valve Shutoff Challenges
Sizing the actuator and choosing the correct drivetrain materials and seals is only the start of the challenges. An even bigger problem is reliably achieving a tight shutoff under these extreme conditions. Jason and Wade describe the problem:
Since the ball and seats are in the flow stream, the only option is a metal-to-metal seal. Obtaining tight shutoff with metal seats provides a greater challenge than with soft seals. To maintain a leak-free joint between the ball and seats, the following parameters must be controlled: fit of parts, surface finishes and a contact stress that provides the desired shutoff while not damaging the coating. If the design cannot satisfy these requirements, excessive seat leakage will occur.
Hardening the outer surfaces of various trim components can extend usable life. Some of the more common hardening methods are:
Stem Seal Issues
The next area of engineering focus is the design of the valve stem seal. Unfortunately high temperature can also create a host of problems in this area, explain the authors:
The inability to use most polymers and elastomers above 400˚F (204˚C) presents a challenge in seal design. Graphite has become the status quo for most high temperature seals despite its limitations. Graphite stem packing can experience oxidation, consolidation and/or extrusion—leading to premature seal leakage.
To minimize oxidation, the temperature of the packing set should be limited to 850˚F (454˚C) in oxidizing environments, and to 1200˚F (649˚C) in non-oxidizing services such as steam. Keeping the packing rings below this limit can be accomplished by using bonnet and stem extensions and/or lantern rings, both of which serve as insulators. As a rule, any refining applications over 800˚F (426˚C) should include coordination with a packing ring manufacturer.
“Live loaded” packing uses springs to obtain a constant stress in the packing studs and packing rings to compensate for small amounts of oxidation, consolidation and extrusion. Springs can be placed over the packing studs and under the nuts, though larger springs that surround the stem (Figure 3) provide a more consistent load over time. These live-loaded packing sets benefit from occasional adjustment, with best performance occurring with this regular maintenance.
Figure 3 (was Figure 4 in article): Live loaded graphite packing arrangement for ball valves uses springs on the valve stem to provide a constant load.
Determining an appropriate torque for the packing studs is critical to the valve’s performance. Ball valves in high temperature applications experience flow induced vibration and thermal cycles as the ball rotates from a closed to an open position. If the bolt torque is too low while the valve is in service, the packing nuts may loosen and cause a packing leak. Excessive bolt torque leads to excessive valve torque, which may result in the valve failing to operate, or cause a “stick/slip” behavior in a control valve, leading to poor control of flow.
Lab Testing is Critical
The author stress that lab testing, including operation at expected temperature and pressure, is the only way to ensure the valve will operate reliably (Figure 4). Critical tests of thermal shock, leakage, and cyclical part wear must be executed under actual process conditions to guarantee performance when installed in a plant or facility.
Figure 4 (was Figure 7 in article): A valve wrapped in heat tape and insulation and instrumented with multiple thermocouples.
Tests normally use hot air, helium or methane as the process fluid. Steam testing, where the fluid heats the valve from the inside, may also be used to gauge the operation of the valve under thermal shock, as would be experienced in operation. Though this may better represent the temperature gradients in operation, the steam can act as a lubricating fluid, which may reduce measured torques.
Summary
The authors summarized their article as follows:
Many refinery processes call upon engineered ball valves to operate at high temperatures where elastomers and polymers cannot be used. These valves can operate successfully when a holistic approach is taken during the design, including the selection of materials, actuation and accessories. Even with attention to these details, the severity of these applications requires a program to test and verify performance