Liquefied petroleum gas (LPG) and other liquefied gases — including liquid ammonia, liquid chlorine and carbon dioxide — represent approximately 3% of pump applications. They are stored and transported as liquids under pressure or at low temperature; at atmospheric pressure they immediately vaporise. This places uniquely demanding requirements on pump design, shaft sealing and NPSH management that do not apply to conventional liquid service.
Product Classification
- LPG (propane, butane): Stored at 6–8 bar at ambient temperature; flash point –104 °C (propane), –60 °C (butane); primary domestic and industrial fuel gas
- Liquid ammonia (NH₃): Stored at 8.6 bar at 20 °C; boiling point –33 °C; highly toxic; refrigerant and fertiliser feedstock
- Liquid chlorine (Cl₂): Stored at 6.8 bar at 20 °C; boiling point –34 °C; extremely toxic and corrosive; water treatment and chemical synthesis
- Liquid CO₂: Stored at 57 bar at 20 °C or at –78 °C (dry ice); food-grade for carbonation; fire suppression; refrigeration
- Liquid oxygen, nitrogen, argon (cryogenic): Stored at cryogenic temperature (–183 to –196 °C) at low pressure — see the separate Cryogenic Liquids article for details
Key Physical Properties
- Vapour pressure: Equal to or exceeds atmospheric at normal storage temperature — any pressure reduction causes immediate vaporisation; system must be maintained under positive pressure at all times
- Density: LPG liquid 0.50–0.58 g/cm³; liquid NH₃ 0.68 g/cm³; liquid Cl₂ 1.40 g/cm³ — significantly affects head and power calculations
- Viscosity: Very low — 0.1–0.3 cSt; even lower than water; centrifugal pump efficiency is generally good
- Latent heat of vaporisation: High — any heat input to the pump (bearing losses, motor heat, friction) must not be enough to vaporise the liquid in the pump casing
- Flash point: Well below –50 °C for LPG — extreme fire and explosion hazard; ATEX Zone 1 or 2 applies to all LPG pump areas
NPSH and Vapour Lock
Managing NPSH is the most critical design challenge for LPG and liquefied gas pumps. Because the vapour pressure equals the storage pressure at operating temperature, any reduction in pressure at the pump suction — due to friction losses, heat gain or elevation — causes the liquid to flash to vapour, starving the pump of liquid and causing cavitation, vibration and flow interruption.
- Pump must be located as low as possible relative to the storage vessel to maximise hydrostatic head on the suction
- Suction lines must be short, straight and insulated to minimise heat gain and friction loss
- Submersed pump designs (pump inside the storage vessel) are preferred for many LPG applications — no suction line at all
- NPSHr must be significantly lower than available NPSH — include a safety margin of at least 1.0 m
Shaft Sealing
- Double mechanical seal with barrier fluid: Standard for LPG and ammonia — barrier fluid at higher pressure than the pumped liquid prevents product escaping at the seal face
- Sealless magnetic drive: Eliminates shaft seal entirely — preferred for toxic liquefied gases (Cl₂, NH₃) where any leakage is unacceptable
- Canned motor pump: Motor and pump share a common sealed housing — hermetically leak-free; used for chlorine and other extremely toxic products
Materials of Construction
- LPG (propane, butane): Carbon steel for pressure parts; Viton seals — propane and butane are compatible with standard steel
- Liquid ammonia: Carbon steel compatible; avoid copper and copper alloys — ammonia attacks copper rapidly; use PTFE or EPDM seals
- Liquid chlorine: Dry chlorine is compatible with carbon steel; wet chlorine attacks steel — must ensure no water contamination; PTFE gaskets throughout; sealless pump strongly preferred
- Liquid CO₂: Carbon steel at elevated pressure; stainless steel for food-grade purity requirements
Applications
- LPG terminals — unloading from rail tankers or ships, storage tank filling, truck loading
- Domestic and industrial LPG cylinder filling
- Ammonia refrigeration systems — compressor liquid injection and evaporator feed
- Fertiliser production — liquid ammonia as nitrogen source for urea and ammonium nitrate
- Chlorine dosing in water treatment — transfer from storage to day tanks
- CO₂ transfer and dosing for beverage carbonation
- Aerosol propellant filling — butane and propane in consumer products