Multistage Cryogenic Pumping for Low-NPSH Tank Offloading
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Solution Overview
Problem
Existing pumping systems struggle to efficiently and reliably offload cryogenic and low-density liquids like liquid hydrogen from containers, especially when the liquid level drops, leading to inefficiencies and potential loss due to boil-off, and conventional pumps face challenges with seal leakage, high maintenance, and reduced efficiency at low liquid levels.
Innovation Solution
A multistage, submersible centrifugal pumping system with independently controlled pumping modules that adjust rotation speeds based on liquid level, maintaining optimal output pressure and flow throughout the offloading process by compensating with varying motor speeds.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If a conventional centrifugal pump is used to offload liquid from a container, then the pump can operate at high flow rates when the liquid level is high, but the pump efficiency drops significantly when the liquid level drops, leading to incomplete emptying and boil-off losses
Solution Approach 1:
The pump is divided into multiple independently controllable stages, each with its own motor and impeller. This segmentation allows each stage to operate independently based on the liquid level, ensuring that pumping continues efficiently even as the liquid level drops and maintaining the ability to completely empty the container.
Solution Approach 2:
The pump system dynamically adjusts the operation of individual stages based on real-time liquid level conditions. As the liquid level drops, lower stages are deactivated while upper stages continue operating, allowing the pump to adapt its configuration to maintain optimal efficiency throughout the entire emptying process.
2Productivity
If a vertical turbine pump with shaft seals is used, then the pump can achieve high pumping capacity, but seal leakage and high maintenance costs occur, especially with cryogenic liquids
Solution Approach 1:
The harmful shaft seals and lubrication systems are completely removed from the pump design. Each stage operates as a sealed unit with the motor and impeller integrated, eliminating the need for shaft penetration and associated seals that would leak with cryogenic liquids and require maintenance.
Solution Approach 2:
Each pump stage is designed as a self-contained unit with integrated motor and impeller, requiring no external lubrication or sealing systems. The stages operate independently without mechanical connections to the exterior, making them maintenance-free and particularly suitable for cryogenic applications.
3Productivity
If the pump operates at high speed to achieve fast offloading, then productivity increases, but the available net positive suction head becomes insufficient when liquid level drops, causing cavitation
Solution Approach 1:
The pump system dynamically adjusts the operational configuration based on liquid level. As the liquid level drops and available NPSH decreases, the system automatically deactivates lower stages and activates upper stages, which have better access to the remaining liquid head. This dynamic reconfiguration maintains adequate NPSH margins and prevents cavitation while preserving high offloading speeds.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The system ensures faster and more efficient offloading of low-density liquids by maintaining desired output pressure and flow until the container is nearly empty, reducing boil-off losses and minimizing maintenance needs.
Implementation Method 1
multistage, submersible centrifugal pumping system
Data Source
AI summary
A pumping system includes a plurality of interconnected integrated motor/pump modules (IMPs) submerged in a process liquid, such as liquid hydrogen (LH2), within a container, the IMPs being separately controlled by adjustable speed drives (ASDs). The rotation speeds of the IMP impellers are controlled such that the NPSH_A for each IMP remains above a minimum, critical suction head NPSH_c of the IMP, while the outlet pressure and flow of the last IMP is maintained at a specified level unless its NPSH_A falls substantially to its NPSH_c, or until the container is substantially empty. The IMPs can be identical, initially operating at the same speeds, or the first IMP can be an inducer IMP having a reduced NPSH_c. The IMPs can comprise permanent magnets or induction coils attached to their impellers that pass in proximate radial or axial alignment with stator coils. The ASDs can be variable frequency drives (VFDs).