Nikolaus Necker from Burgmann Industries GmbH & Co. KG explains the advantages of using mechanical seal technology in heterogeneous pumping systems. Over the past two decades, several technologies have been developed for oil and gas production, but heterogeneous pumping has emerged as one of the most promising. This method offers significant reductions in offshore and onshore capital investment, along with improved productivity. As a result, the demands on pumping systems have increased, requiring more robust and reliable solutions.
One of the main challenges in heterogeneous pumping is the transition between liquid and gas phases. The process fluid can shift from 100% liquid to 100% gas, creating extreme operating conditions for the pump. This variation places additional stress on the mechanical seals, which are critical components of the system. Mechanical seals are designed for both liquid and gas applications, but in multi-phase environments, they face transient conditions such as pressure fluctuations and air locks. Therefore, specialized mechanical seals must be developed to handle these harsh conditions effectively.
There are two primary types of multi-phase pumps: rotary power pumps and positive displacement pumps. Rotary power pumps operate based on helical axial flow mechanics, allowing for multiple compression stages on a single shaft. These pumps can handle high gas volume ratios without prior separation, making them compact and efficient. They also allow for the presence of solid particles at high flow rates.
Positive displacement pumps, such as twin-screw pumps, are capable of handling up to 70% gas volume ratios. These pumps feature two counter-rotating screws that create a sealed chamber, allowing for dry running performance. Their reliability has been demonstrated in various multiphase booster applications.
Mechanical seals in multi-phase environments must deal with unpredictable media compositions, changes in density and viscosity, temperature variations, and erosion caused by sand. Additionally, they must manage high and low pressures, as well as ensure proper lubrication and cooling at the seal face.
The chemical composition of the process fluid can vary depending on the location, often including wax, salt, water, acid gases like H2S or CO2, and solid particles. These elements can cause corrosion and wear on the mechanical seal, affecting its performance and longevity.
The sealing principle involves reducing the pressure of the fluid radially across the seal face. Friction at the seal can increase the temperature, leading to vaporization of volatile components. This can result in insufficient lubrication and cooling, increasing the risk of failure. In optimized parallel clearance, the forces acting on the seal are balanced, ensuring stable operation.
Two common gap configurations are A-shaped and V-shaped. An A-shaped gap creates high contact force at the outer diameter, leading to unstable operation, while a V-shaped gap provides a gentle pressure gradient, resulting in steady performance.
For multi-phase applications, single-end mechanical seals (API 610, Plan 32) are used, relying on external flushing for lubrication and cooling. However, in cases where continuous external flushing is not feasible, pressurized double-ended seals (API 610, Plan 53 or 54) are preferred. These seals use an external closed-loop system for lubrication and are designed to remain sealed even under pressure fluctuations.
Special attention is given to material selection. Silicon carbide is commonly used for sealing faces due to its hardness and thermal conductivity, while duplex stainless steel is used for metal components to resist corrosion. The design of the seal is compact, rugged, and easy to maintain, making it suitable for heavy-duty applications.
In conclusion, mechanical seals have proven their reliability in sealing multiphase mixtures across a wide range of conditions. With the right materials and design, they can meet the technical demands of multi-phase pumping. Close collaboration between manufacturers and end users is essential to ensure optimal performance. Proper application, installation, and operation are key to achieving long mean time between failures (MTBF).
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