Triple benefits for novel valve design

After three years of thorough testing and R&D, a British company has developed a triple offset butterfly valve that, according to the firm, outperforms most other conventional designs on a range of performance factors. Coventry-based firm Dunlop Equipment, a company normally associated with the manufacture of air bleed valves for the aero engine industry, has developed a novel, triple offset butterfly valve which it hopes will be used in a variety of other industries including power generation, gas turbines, petrochemicals, water, oil and gas. The triple offset valve concept itself is not new; the idea was first used in the early 1970s and basically consists of using a valve disc suspended relative to the valve seat interface via three separate offsets relative to the valve axis. But what is new about Dunlop’s valve is that it has excellent sealing, low operating torques, fewer components, is more resilient to thermal shocks than most other valves and is therefore suitable for aggressive environments with temperatures in the region of 450°C. For those not familiar with butterfly valves, the standard design utilises a disc element suspended inside a tubular housing to form the desired obstruction in the flow of the fluid. Control is often achieved by adjusting the orientation of the disc in the valve housing relative to the axis of discharge, either by a rotary motion, or actuated pneumatically, hydraulically, electrically, or manually. Hardev Singh Phull, engineering group leader at Dunlop, told Eureka: “We basically captured 30 years of valve design knowledge within the company and used it to develop a new valve for harsh, contaminated environments.” The challenge often encountered at the design phase of a typical butterfly valve is how to operate the unit with minimal torque from the actuator whilst enabling effective sealing to contain the fluid within the desired parameters. Hardev explained: “The triple offset and sealing arrangement we’ve used on the valve offer excellent sealing capability with minimal friction at the interfaces during engagement and disengagement, rather like a ‘sink plug’.” He said the firm optimised the triple offsets by using in-house developed mathematical software codes to simulate the valve characteristics for a given set of operating parameters and the selected valve geometry. “The traditional method of designing offsets is to use a flow test rig or station to do trial and error tests, but this requires specialist skills and can be very expensive, as much as £300 per hour. “And trial and error doesn’t allow you to get all the information you need to optimise the design. You can’t measure the forces in isolation,” he added. “Once the model of the valve was built up, our mathematical software enabled the design engineers to simulate the fluid flow and focus on certain parts of the valve to make them more effective. It’s all about having the ability to refine your designs quickly and make design iterations on the fly. You might say we’ve taken out the guesswork.” There are several factors at the design stage that posed challenges to Dunlop. First, the flow characteristics had to be predicted, which, for gases or fluids, can be very difficult using conventional test methods. The company achieved this by using CFD analysis software. This provided the flexibility to refine the profile of the disc assembly for optimum flow rates. It also enabled the design team to determine the 3D aerodynamic forces generated at the disc assembly by the flow, which in turn helped them predict the torque generated at the drive shaft with reasonable accuracy. The next challenge to be faced was the leakage problems. Butterfly valves normally exhibit two forms of leakage: ‘internal’ leakage through the unit around the disc periphery at the fully shut position; and ‘external’ leakage developed at the interfaces and protrusions emerging from the pressure vessel forming the valve body, such as the drive shaft or flanges. According to Hardev, Dunlop’s triple offset design minimises internal leakages, particularly in those applications involving elevated temperatures or corrosive fluids where the conventional sealing materials, usually variants of rubber or plastic, cannot be readily used. The triple offset design offers virtually frictionless engagement and disengagement at the disc periphery and the seat. Dunlop used a high-chrome steel seal made of Inconel. As Harden explained: “This means we’ve eliminated the rubbing action normally associated with typical seals.” Hardev went on to say that material selection for the bearings was also critical to attain a comparatively low operating torque. “We undertook very detailed research to gain a clearer understanding of the dynamic behaviours exhibited by various bearing materials when used in applications similar to butterfly valves at elevated temperatures. The study revealed that, contrary to the traditional belief that material ‘friction’ was considered to be the key factor influencing the torque requirements, it was in fact the ‘stiction’ rating for the material. So we ended up using a carbon-impregnated bearing which had a very low stiction rating.” ‘Stiction’ is similar to friction but whereas friction occurs as two surfaces move relative to each other, stiction is the spike force before motion actually occurs.