How to Select the Right Cryogenic Pump for Different Applications

Cryogenic pumps may all be designed to handle low-temperature liquids, but they are not interchangeable. A pump used for high-pressure cylinder filling operates under very different conditions from one used for tanker unloading or LNG transfer.

The correct selection must begin with the actual application. The liquid medium, required pressure, flow rate, suction conditions, operating time and installation method must all be considered.

The following four pump systems represent several common cryogenic applications: industrial gas cylinder filling, CO₂ cylinder filling, cryogenic liquid transfer and LNG submerged service.

Industrial Gas Cylinder Filling Pump

Industrial gas cylinder filling commonly involves liquid oxygen, liquid nitrogen or liquid argon.

In a typical filling process, a cryogenic reciprocating pump raises the pressure of the liquid before it enters a vaporizer. The pressurized liquid is then converted into gas and delivered to the downstream cylinder filling system.

Reciprocating pumps use a piston or plunger to displace a controlled volume of liquid during each stroke. This positive-displacement principle makes them suitable for applications requiring high discharge pressure at a relatively low or moderate flow rate.

When selecting an industrial gas cylinder filling pump, several conditions should be confirmed:

• Cryogenic liquid type: LOX, LIN or LAR
• Required filling pressure
• Required filling capacity
• Number and size of cylinders
• Continuous or intermittent operation
• Available inlet pressure
• Suction-piping arrangement
• Material and cleanliness requirements

For liquid oxygen service, particular attention must be given to oxygen-compatible materials, component cleanliness and the prevention of oil or combustible contamination.

The pump is only one part of the filling system. The vaporizer, valves, piping, pressure protection and filling controls must also be matched to the required flow and operating pressure.

CO₂ Cylinder Filling Pump

Liquid CO₂ cylinder filling also commonly uses a reciprocating pump, but its operating conditions are different from those of liquid oxygen, nitrogen and argon.

CO₂ has a specific pressure-temperature relationship. If the pressure or temperature falls outside the required operating range, vapor formation or dry-ice blockage may occur. This can affect pump suction, flow stability and system operation.

A CO₂ cylinder filling pump should therefore be selected according to:

• Liquid CO₂ storage pressure and temperature
• Required filling pressure
• Required filling capacity
• Pump inlet conditions
• Suitable cold-end and sealing materials
• Bypass and pressure-relief arrangements
• Measures for preventing vapor or dry-ice formation

A pump should not automatically be considered suitable for CO₂ simply because it uses a reciprocating mechanism. Its operating range, cold-end structure, seals and supporting system must all be compatible with liquid CO₂ service.

Cryogenic Centrifugal Pump

Cryogenic centrifugal pumps are generally used when an application requires continuous liquid transfer and a higher flow rate rather than the high pressure required for cylinder filling.

A rotating impeller transfers energy to the liquid and produces a continuous flow. Depending on the design and materials, cryogenic centrifugal pumps can be used for LNG, liquid oxygen, liquid nitrogen, liquid argon, liquid CO₂ and other compatible low-temperature liquids.

Typical applications include:

• Cryogenic storage-tank transfer
• Road-tanker loading and unloading
• Process-liquid supply
• Liquid circulation
• Tank-to-tank transfer

Important selection factors include:

• Required flow rate
• Required differential head
• Liquid type and operating temperature
• Available inlet pressure
• Storage-tank liquid level
• Pipeline resistance
• Elevation difference
• Continuous or intermittent operation
• Motor and drive configuration

Suction conditions are especially important. If the pressure at the pump inlet becomes too low, part of the cryogenic liquid may vaporize. This can cause cavitation, unstable flow, excessive vibration or loss of pump performance.

For this reason, a centrifugal pump should not be selected according to flow rate and discharge head alone. The storage tank, inlet piping and available net positive suction head must also be evaluated.

LNG Submersible Pump

An LNG submersible pump operates with the pump and motor immersed in the cryogenic liquid.

This configuration is commonly used in LNG storage tanks, terminals and transfer systems where in-tank installation and stable suction conditions are required.

Because the unit operates inside the liquid, the design eliminates the need for a conventional external pump shaft passing through the tank. Immersion can also provide more stable inlet conditions, particularly when the pump is installed at an appropriate depth below the liquid level.

However, this arrangement introduces additional engineering requirements involving the cryogenic motor, electrical insulation, cables, bearings, installation structure and pump-removal system.

The main selection factors include:

• Required LNG flow rate
• Required discharge head
• Tank dimensions
• Installation depth
• Minimum and normal liquid levels
• Operating frequency
• Start-and-stop conditions
• Electrical specifications
• Hazardous-area requirements
• Pump removal and maintenance arrangement

Because LNG is flammable, the pump must be integrated into a system designed according to the applicable hazardous-area classification, control requirements and safety standards.

A submersible pump should therefore be selected as part of the complete LNG storage and transfer system, rather than as an isolated component.

What Is the Main Difference Between These Pump Types?

The basic selection direction can be summarized as follows:

• Industrial gas cylinder filling: A cryogenic reciprocating pump is generally suitable for producing the required high discharge pressure.
• Liquid CO₂ cylinder filling: A reciprocating pump specifically designed and configured for CO₂ service is required.
• Higher-flow cryogenic liquid transfer: A cryogenic centrifugal pump is generally more suitable.
• In-tank LNG transfer: An LNG submersible pump may provide the required installation and suction arrangement.

This is only an initial selection guide. Two projects handling the same liquid may still require different pump configurations because of differences in pressure, flow, tank layout, piping and operating schedule.

Information Required Before Selecting a Cryogenic Pump

Before confirming a pump model, the following operating information should be provided:

1. Liquid medium and composition
2. Inlet temperature and pressure
3. Required flow rate
4. Required discharge pressure or total head
5. Storage-tank type and liquid level
6. Suction and discharge piping arrangement
7. Continuous or intermittent operation
8. Motor power supply
9. Hazardous-area requirements
10. Installation environment
11. Applicable technical and safety standards

Complete operating data makes it possible to determine whether a reciprocating, centrifugal or submerged pump is the appropriate starting point.

Matching the Pump to the Complete System

A reliable cryogenic pumping solution depends on more than the pump itself.

The inlet piping, storage tank, valves, vaporizer, instruments, pressure-relief devices, controls and operating procedures can all affect system performance. The pump must therefore be selected and configured as part of the complete system.

Huzhou Baihui Cryogenic Equipment Co., Ltd. provides cryogenic pump solutions for industrial gas cylinder filling, liquid CO₂ filling, cryogenic liquid transfer and LNG applications.

Each pump configuration is evaluated according to the customer’s liquid medium, required flow, operating pressure and installation conditions.

If you are selecting a cryogenic pump for a new project, providing complete operating data is the best first step toward an appropriate technical proposal.