How does the operating pressure of LNG transfer pumps affect the choice of pump type (submerged vs. centrifugal)?

The operating pressure required for LNG transfer significantly influences the choice between submerged and centrifugal pumps. Centrifugal pumps are generally more suitable for high-flow, low-to-medium pressure applications. They operate by imparting kinetic energy to the fluid through a rotating impeller, converting it to pressure energy. Submerged pumps (also known as submerged motor pumps or canned motor pumps), on the other hand, are often favored for applications requiring higher pressures and lower Net Positive Suction Head Required (NPSHr). NPSHr is the minimum pressure required at the pump suction to prevent cavitation, the formation of vapor bubbles in the liquid. Cavitation can damage the pump impeller and reduce its performance. Submerged pumps have the motor and pump impeller submerged within the LNG, eliminating the need for a shaft seal and reducing the risk of leaks. Because the pump is submerged, the liquid head above the impeller provides a high NPSHa (Net Positive Suction Head Available), minimizing the risk of cavitation, even at higher pressures. When high discharge pressures are needed for LNG transfer, such as for long-distance pipelines or when loading LNG carriers, submerged pumps are generally preferred because they can generate the required pressure while minimizing cavitation risk. However, submerged pumps are typically more expensive than centrifugal pumps and may have higher maintenance requirements. The choice between submerged and centrifugal pumps also depends on other factors, such as the size of the LNG tank, the flow rate requirements, and the overall system design. For example, in smaller LNG plants or for in-tank transfer applications, centrifugal pumps may be sufficient, even if the discharge pressure is relatively high, if the NPSHa is adequate. In summary, while centrifugal pumps excel in high-flow, lower-pressure scenarios, submerged pumps are often the preferred choice when higher discharge pressures and minimization of cavitation risk are critical, particularly in large-scale LNG transfer operations.