Cryogenic Storage Vessel Subcooling to Maintain NPSHA During Pumping
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Solution Overview
Problem
The challenge of minimizing boil-off losses and maintaining optimal net positive suction head available (NPSHA) during the pumping of liquid cryogens, particularly hydrogen, due to its low viscosity and high heat sensitivity, leading to inefficient pumping and significant hydrogen losses.
Innovation Solution
A closed insulated cryogenic storage vessel with a hollow metal conduit and pressure reducer that flashes liquid cryogen into a biphasic mixture, transferring heat from the bulk cryogen to the expanded cryogen, effectively subcooling the liquid cryogen without additional refrigeration sources.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If liquid cryogen is stored at super-atmospheric pressures to maintain NPSHA, then pumping efficiency improves, but density decreases significantly (e.g., ~40% over 150 psi for hydrogen), increasing vaporization risk
Solution Approach 1:
The patent changes the temperature parameter by subcooling the liquid cryogen to a temperature below its normal boiling point at the storage pressure. This allows the system to maintain lower pressure (higher density) while still achieving sufficient NPSHA for efficient pumping, resolving the contradiction between pumping efficiency and vaporization risk
Solution Approach 2:
The system performs subcooling of the liquid cryogen before it enters the pump. This preliminary cooling action ensures that the liquid has lower temperature and higher density prior to pumping, providing adequate NPSHA margin and reducing vaporization risk during the pumping process
2Productivity
If external refrigeration is used to subcool liquid cryogen, then NPSHA is maintained and pumping efficiency improves, but system complexity and cost increase
Solution Approach 1:
The patent uses the liquid cryogen itself to provide the cooling effect through a flash evaporation process. A portion of the liquid is expanded to a lower pressure, causing it to flash evaporate and cool the remaining liquid. This self-service approach eliminates the need for external refrigeration systems, reducing complexity while maintaining pumping efficiency
Solution Approach 2:
The system exploits the phase transition of a portion of the liquid cryogen from liquid to vapor during flash expansion. This phase change absorbs heat and subcools the bulk liquid, providing the necessary cooling effect without external refrigeration equipment
3Stress or pressure
If liquid hydrogen is pumped with standard piston pumps, then medium and high pressure delivery is achieved, but volumetric efficiency and yield are poor compared to liquid nitrogen pumping
Solution Approach 1:
The patent changes the temperature parameter by subcooling the liquid hydrogen, which increases its density and reduces its compressibility. This parameter change improves the volumetric efficiency of the pump by reducing vapor formation and improving liquid seal effectiveness, thereby increasing productivity while maintaining the required delivery pressure
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 solution reduces heat content and vapor pressure, maintaining NPSHA, thereby improving pumping efficiency and reducing hydrogen losses, while avoiding the costs and inefficiencies of external refrigeration.
Implementation Method 1
A pressure reducer is disposed either within the conduit or at the first end that is adapted and configured to flash the liquid cryogen entering into the conduit into a biphasic mixture of liquid cryogen and vaporized liquid cryogen
Implementation Method 2
The conduit is adapted and configured to transfer heat to the biphasic mixture from the liquid cryogen in the liquid cryogen space and vaporize at least some of the liquid cryogen in the biphasic mixture
Data Source
AI summary
A liquid cryogen stored in a liquid cryogen space of a closed insulated cryogenic storage vessel is subcooled by allowing it to enter into a conduit disposed in the liquid cryogen space where it is expanded by a pressure reducer in the conduit, thereby producing a cooled biphasic mixture of the cryogen in liquid and vaporized forms. The cooled biphasic mixture has a temperature lower than that of the liquid cryogen in the liquid cryogen space. Heat is transferred across the conduit from the liquid cryogen in the liquid cryogen space to the cooled biphasic mixture.