The exploration of subsea oil fields — those more than 3 000 m (9842 feet) below sea level — provides the oil and gas industry with a host of challenges. The mechanical sealing of subsea boosting pumps is not exempt from these challenges. For a new generation of subsea pumps, Sulzer and EagleBurgmann co-developed a mechanical seal that withstands the high absolute pressure and high temperatures (HP/HT) of subsea oil discoveries.
Sealing a hot process
The oil temperature of the subsea oil discoveries corresponds with the recovery depth and tectonic area of the oil field. It ranges from 70 °C to 180 °C (160 to 350 °F). The high temperature of the pumped media raises the temperature in the seal and can cause the pump barrier fluid to degrade or can cause deflection of the seal faces. In a joint project, the engineers of EagleBurgmann and Sulzer optimized a seal for HP/HT applications. Other industry sponsors backed the project as well. The goal was to develop an improved HP/HT solution and overcome the limitations of traditional mechanical seal design for subsea applications.
The following aspects were analyzed and enhanced:
- Pressurizing direction of the seal faces
- Secondary seal design
- Seal face material and mounting of the seal faces.
The new seal is a novel combination of an external pressurized seal, with a new secondary seal design, a cooling jacket, and a diamond coated seal face material. The new combination provides good results for high-pressure and high-temperature applications.
“Deep-sea conveying equipment on the seafloor extracts oil at depths down to 3 000 m. This oil is pumped directly onto ships or onto land via underwater pipelines. Sealing systems in multiphase pumps must reliably handle high pressures, high temperatures, and fluctuating compositions of the pumped media. Because recovering and repairing underwater pump units is extremely expensive, the seals also have to ensure maximum failsafe protection and extended maintenance intervals. The cooperation of EagleBurgmann and Sulzer engineers was inspiring, and it resulted in a smart sealing solution for industrial use.”, Senior Product Engineer, EagleBurgmann, Germany
Pressurizing direction of the seal faces
Traditional subsea seals are internal pressurized seals. These seals offer excellent fluid film generation under moderate temperature applications. Any process-born particulates are centrifugally flushed from the seal faces. In case the subsea oil discovery has a high temperature or high gas content, internal pressurized seals have limitations. With higher temperatures, a deflection of the seal faces occurs and the lubrication barrier fluid, which also cools the system, cannot enter the seal (Fig. 1).
The functioning principle of a cooling jacket is easy: near the seal faces, a cavity filled with barrier liquid enables the rapid heat transfer to the cooling system of the subsea pump. The temperature distribution graphic (Fig. 3) shows that the cooling is able to reduce the temperature at the seal faces.
During the project, many CFD and FEA calculations were performed to predict the seal face lift-off deflection and the interaction with the surrounding structure under all operational conditions. Based on these calculations, the engineers tailored the design of the seal geometry accordingly.
Secondary seal design
The function of the secondary seal is to allow relative axial movement between the stationary and rotating parts of the seal but still seal between the process and barrier fluid. The sealing not only has to work properly under normal pumping conditions but it also has to survive unusual reverse pressure situations. Elastomeric O-rings are excellent for sealing in both directions but have the tendency to shrink under high pressure and high temperature. For this HP/HT application, such O-rings are not a suitable solution for a secondary seal. The engineers developed a spring-energized non-elastomeric seal made from polytetrafluoroethylene (PTFE) material. The seal has a U-Cup form. Under standard operating conditions, the normal pressure of the barrier fluid generates enough force to open both lips of the secondary seal radially (Fig. 4).
Seal face materials and mounting of the seal faces
Grown diamond is a significantly harder material than any carbide. Mechanical seals with DiamondFace® have an increased lifetime that has been proven in numerous applications. They can tolerate accidental dry running conditions and an increased number of solid particles.
Conventional, internal-pressurized seals have the carbide seal faces shrunk into a carrier bandage. After fitting the face into the bandage, the seal faces are ground to the final geometry needed. Because of their extreme hardness, DiamondFace cannot be polished afterwards. Therefore, a new assembly method had to be developed for HP/HT seals.
The solution was to use floating rings where the seal faces are not constrained but rather loose fitting. To ensure longevity with a loose fit, the engineers examined the hydraulic balance of the seal faces in detail. The optimized seal design with minimized contact forces ensures an extended lifetime. The structure of the grown DiamondFace is shown in Fig. 6.
Qualified and tested
Following the analysis and concept studies, Sulzer and EagleBurgmann optimized and built the mechanical seal. It had to undergo an intensive bench test to be qualified for use in a subsea pump. The bench test was performed using both high-temperature liquid and gas on the process side of the seal at speeds up to 6 000 rpm and over 500 starts/stops (Figs. 7 and 8).
Award for TechnipFMCs Innovative Subsea TechnologyApril 05, 2017TechnipFMC, a longstanding engineering partner of Sulzer, has won the Hydraulic Institute Award for Innovation and Technology for its innovative products and solutions. The global leader in oil and gas projects, technologies, systems, and services was recognized during a ceremony at the Institute’s Centennial Celebration Gala in Florida, USA.
Sulzer Management AG