Cryogenic liquefied gases include liquid hydrocarbons such as ethylene, liquefied petroleum gas (LPG), liquefied natural gas (LNG), liquid oxygen, and liquid nitrogen. These substances are typically stored and transported at temperatures ranging from -30°C to -196°C. Pumps used to handle these low-temperature fluids are known as cryogenic pumps or magnetic pumps.
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Now, let's take a closer look at the key characteristics of cryogenic pumps:
1. **High Inlet Pressure**
Liquefied gases are usually stored under specific pressure and low-temperature conditions. For example, in an ethylene plant, methane must be liquefied at around 3 MPa and -100°C, while ethylene requires about 2 MPa and -30°C. This means that the pump must operate with high inlet pressure to maintain the liquid state.
2. **Sensitivity to Temperature Changes**
The vapor pressure of cryogenic liquids changes dramatically with temperature. These liquids are often saturated or slightly subcooled, making their vapor pressure highly sensitive to even small temperature variations. A temperature change of ±25% can lead to a vapor pressure change of up to 100–200%. Alongside this, physical properties like density, specific heat, and latent heat also vary significantly. As temperature increases, vapor pressure rises sharply, density decreases, and the latent heat of vaporization drops, making the liquid more prone to evaporation. This affects both the pump’s structural integrity and its suction performance.
To prevent cavitation and ensure stable operation, the suction piping must be fully insulated. Any fluctuations in the tank temperature can lead to reduced net positive suction head (NPSH), which may cause cavitation. Additionally, throttling devices such as ball valves, orifice plates, and filters should be avoided to prevent local pressure drops that could cause vaporization.
3. **Strict Sealing Requirements**
Most cryogenic liquids are hazardous and potentially corrosive, so any leakage is unacceptable. If a leak occurs, it can lead to serious safety risks. Moreover, the low temperature of the fluid can cause ice formation on sealing components, further complicating the design and maintenance of the pump. Therefore, cryogenic pumps require extremely reliable and durable shaft seals to ensure safe and efficient operation.
In summary, cryogenic pumps are specially designed to handle extreme conditions, requiring precise engineering in terms of pressure control, thermal insulation, and sealing technology. Understanding these characteristics is essential for selecting and operating the right pump for cryogenic applications.
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