Heat Exhaustion

heat exhaustion

A team tries to predict how well a new refinery valve will stand up to its punishment.

This article was prepared by staff writers in collaboration with outside contributors.

In the hydrocarbon refining process known as fluidized catalytic cracking, a key part of the equipment is a slide valve that controls the catalyst flow. The valves are typically installed in refractory lined piping approximately 5 feet in diameter. Operating temperatures inside the valve range from 900°F to 1,400°F and, occasionally, go as high as 1,800°F. Replacements require a shutdown that can run into days just for cooling time and then reheating.

A major Houston-based manufacturer of slide valves, Tapco International, came up with a design that would eliminate bolts to make the valve last longer. The company asked BES Engineering of Houston to analyze the stresses due to steady-state and transient heat transfer, and to evaluate their effects.

According to Andre Koerner, project manager at Tapco International, "The purpose of this study was to verify the new valve design and compare it with proven bolted designs."

Bolts cause the biggest problems with slide control valves, according to Dana E. Petroni, BES principal engineer. "Over the years, severe heat stresses build up in the valves," he said. "Because of the accumulated heat stress, fatigue, and creep-fatigue, the bolts creep, eventually lose their preload, and cannot maintain a tight seal."

In concept, a slide control valve is simple, a modified cylinder with only two moving parts. The valve body is a cylinder with an internal cone that supports the orifice plate. The orifice plate holds a movable disc that adjusts the size of the opening through which the process catalyst flows. The disc is moved by a valve stem and actuator located outside the valve.

Illustration: Ansys model of Tapco's boltless slide control valve.

The valve design analyzed by BES was constructed of carbon steel, stainless steel, and refractory. The valve body was 1-inch-thick carbon steel. The designers and engineers were most concerned about thermal stress, fatigue, and creep-fatigue interaction in the stainless steel internal support cone. Mark Gray, BES engineering manager, said, "The ultimate issue was to improve the creep-fatigue life of the valve components."

The analysis focused on connections, namely those between the cone and body, and the cone and orifice plate. The bimetallic welds of stainless to carbon steel and two bolt locations were also analyzed. The team began with a CAD file in Pro/Engineer from PTC of Waltham, Mass. They carried out their analyses in Ansys/Professional Version 5.5.3 from Ansys Inc. of Canonsburg, Pa.

"Both steady state and transient thermal analysis, including modeling the steel and the refractory, determines the temperature distribution within the valve," Petroni said. "A linear stress analysis determines the stresses and strains within the valve due to the temperature gradients in addition to the mechanical loads."

The challenge was to transfer nodal temperatures to the stress model. A single model sufficed for both thermal and stress analyses. There were 210,000 elements in the thermal model. The stress model required only the valve's carbon and stainless steel elements. Without the refractory, the stress model was reduced to 125,000 elements.

Several design cases were analyzed, including operating, heat-up, and cool-down. Steady-state and transient heat-transfer analyses were used, depending on the design case. For transient, the worst case thermal gradients were determined and used for the stress analysis.

"The resulting files for the stress analysis were 250 mega-bytes per load step and the database for the model was 400 MB," said Petroni. "Time required to run the stress models was three hours per load step."

The stress analysis results were compared with ASME codes on allowable stresses and acceptance criteria.

The results suggested that, although strength varies with valve size, on average the welded valve has approximately four times the strength of a bolted design, and lasts more reliably, especially during high-temperature upsets.

Tapco has about two dozen of the boltless valves in the field. According to the company, the reliability of the new design can save hundreds of thousands of dollars by eliminating unscheduled shutdowns and unexpected maintenance.