Sulfur leakage, causing housekeeping and environmental issues in a refinery, was stopped with an innovative seal configuration.
No other industry has come under such intense scrutiny as the oil refining industry. Still, no one can deny that processing crude oil into useful petroleum products such as gasoline, diesel fuel and heating oil is an important part of the global economy. The oil refining industry has strived to become more environmentally friendly both in its product line and process facilities.
Liquid molten sulfur is a by-product of clean fuel production. As environmental legislation mandates stricter controls on refined products, oil refineries must remove more sulfur from refined products, such as diesel fuel. The recovered sulfur is sold to other industrial companies for use in other products—such as fungicides, black gunpowder, detergents and phosphate fertilizers—and for rubber vulcanization.
Leaking Liquid Sulfur
For one major United States refinery, the sulfur recovery process created problems in its process line. Sulfur has a high melting temperature of 250 deg F and must be constantly heated at or above this temperature to maintain a liquid state for pipeline transportation. However, molten sulfur also has an upper temperature limit of 300 deg F, at which point the viscosity increases, and it begins to re-solidify. Trying to control this narrow temperature range and maintain the molten liquid state can be difficult. As a result, the refinery experienced reliability issues with its pumps and for the mechanical seals.
Immediately after installation and start-up, the pump's mechanical seal would begin to leak. Within weeks, a large pile of hardened sulfur formed around the pump base causing huge housekeeping issues along with environmental disposal problems. The plant would operate the pump for an extended period of time, while the hardened sulfur formed around the pump. When an opportunity arose, they would replace the seal and clean up the sulfur. This bad-actor pump and seal configuration was a never-ending problem for the refinery. Not only did the plant have to contend with continually cleaning up the leaking sulfur, it also had to make sure that the sulfur was disposed of in an environmentally safe manner.
In searching for a solution to its problem, the refinery tried several different sealing configurations, but the leaking still occurred. Since the standard seal designs were not providing a solution, the refinery looked for customized help from a mechanical seal manufacturer. After assessing the situation, the manufacturer's team realized that the typical seal configuration would not work for this application and a new approach was needed.
The Sealing Situation
The engineers at the refinery gave all the details of the application and process conditions to the seal company. The existing seal was a typical rotating bellows design with a carbon bushing outboard of the seal faces and a steam jacket around the bushing. However, no steam quench was being used between the bushing and seal faces. Although a traditional steam 5-psig quench had been employed in the past to prevent the sulfur from accumulating and solidifying around the seal faces, the quench line would become plugged with sulfur and tended to accelerate the formation of solid sulfur around the pump. Therefore, it was eliminated. Because of the barrier fluid contamination of the sulfur, a double seal was not a viable option.
The sulfur temperature in the pump was at 280 deg F and the pump speed was 3,600 rpm. The refinery engineers and incumbent seal manufacturer theorized that the heat generation in the seal gap was significant enough that the sulfur migrating across the seal faces was reaching its upper solidification temperature (300 deg F). A steam quench on the atmospheric side of the seal faces was keeping the sulfur at this upper temperature. Without the quench, the solidification still occurred but at a much lower rate.
“In either case, the result was a domino effect,” said Jeff Batinick, a representative of the seal company. “Sulfur leaking past the faces was accumulating and solidifying around the atmospheric side of the faces, causing them to hang up, and ultimately leading to additional and accelerated sulfur leakage.”
A Non-Traditional Approach
After examining the situation at the refinery, the seal company engineering department was asked to find the best solution. The engineers recommended a pusher seal instead of a metal bellows seal to eliminate the sulfur build-up. “We looked at the application and, although a bellows seal is the traditional approach, we knew that what was required here was ‘out-of-the-box' thinking,” Batinick said.
This non-traditional approach looked beyond standard product offerings. “Ocassionally, in mature industries such as refining, the industry gets hooked into canned solutions to problems,” commented Batinick. “We looked at it differently.”
The pusher seal is a slurry seal design. It features a stiff, single-coil, stationary spring that loads up the faces to resist hang-up. It also has a dynamic O-ring on the OD of the spring-loaded, stationary face with the spring on the atmospheric side, and it uses faces with large clearances between their ID and the sleeve OD to resist hang-up if sulfur begins to accumulate on the atmospheric side. The other unique feature is a segmented carbon (Espey-type) bushing on the atmospheric side of the faces that can be used for a high-pressure (30-40 psig) steam quench.
Steam at 35 psig has a saturation temperature of 260 deg F, which is near the lower solidification temperature for sulfur. Therefore, introducing a steam quench between the faces and the segmented carbon bushing at this pressure and temperature and controlling it with a needle valve on the flange drain line would: