Expanding into LNG
Dr. Kevin Kaupert, Turbomachinery Engineer, Nikkiso Clean Energy & Industrial Gases Group, describes LNG liquid cryogenic expanders with an outlook towards increasing their refrigeration benefit using flow flashing.
19 May 2026
Cryogenic expanders with submerged generators used in LNG liquefaction facilities provide three benefits:
Expanders with submerged generators in LNG and MR can be subdivided into two groups: liquid expanders and flashing liquid expanders.
Liquid expanders with submerged generators in LNG liquefaction trains are now industry standard and have been in operation for more than 25 years 1. Today they remain quite similar to standard cryogenic liquid pumps with submerged motors in LNG. In fact, the very first LNG liquid expander tests were merely LNG pumps spinning backwards with the LNG flow direction reversed. The flow in the liquid expander utilizes the centrifugal field in radial inflow runners to perform work on the expander. The liquid expander has liquid at the inlet of the expander and liquid at the outlet of the expander.
Unlike the pumps, liquid expanders are custom designed for the main duty point case to be at exactly the expander’s best efficiency point (BEP) flow rate. This is because customers purchase pumps to first and foremost transport fluid, so it is acceptable if the pump selected operates with its duty in the range of (80 - 110%) BEP. Contrastingly, the expanders are not needed to transport the fluid. The pressure difference in the piping can accomplish this fluid transportation with a pressure drop across a valve. For expanders, the efficiency is of prime importance to remove the maximum amount of heat from the flow which maximizes the refrigeration benefit discussed previously in benefit 1.
Figure 1 shows a sample expander dimensionless performance curve measured in LNG on the Nikkiso CE&IG test stand. Three speed curves are shown for the expander head at speeds N1, N2, and N3. The dimensionless power (Lr) and efficiency (eff_r) curves are shown only for the N1 speed. The measured zero torque curve is also shown. The dotted line shows the computational fluid dynamics (CFD) predicted head vs flow for this expander at N1 showing reasonable agreement with the measurements.
During a load rejection event, the generator will be accidentally knocked offline. This event causes the speed of the expander to rapidly increase to the runaway speed which is located on the zero torque curve. Liquid expanders are designed to run at their runaway speed for short durations as a requirement of the design including the generator overspeed. With regards to the power levels of the generator, this tends to be limited to just below 3 MW.
This limit comes not from the generator manufacturers but rather the need for sufficient rotordynamic margin between the runaway speed and the critical vibration speed of the generator. The generator is supported by ball bearings and hence there is little damping along the generator rotor-stator span, meaning that running the expander at the generator critical speed will cause equipment damage. A generator size larger than 3 MW leads to a larger span between the ball bearings and a lower critical speed of the generator which could intersect the expander runaway speed.
Figure 2 performance of a three-stage centrifugal pump operated as a radial inflow centrifugal expander in water with changing amounts of air content in the water at the expander inlet.
The industrial demand for flashing liquid expanders is not new. Over time many lessons have been learned on ‘how to’ and ‘how not to’ design flashing liquid expanders.
Historical development of flashing liquid expanders
The most obvious path for the development of a flashing liquid expander was to try and adapt exiting centrifugal expanders to handle a flashing liquid. This was attempted initially by both NASA of the US and NPO Energomash of the USSR in the 1960s, using radial inflow centrifugal expanders with unsatisfactory results in terms of efficiency and reliability. Later in the 1980s, several companies again retried radial inflow centrifugal expanders for handling flashing liquids, and they again found poor efficiency and poor reliability as the vapor volume fraction at the expander outlet rose 2. Figure 2 show results from the former study where the liquid was not actually flashing, but rather air was added to water in controlled measured amounts to have two-phase flow. The expander was a three-stage centrifugal pump run in reverse. The x is the mass fraction of vapor in the flow at the expander inlet. Note that as the mass vapor fraction reached 0.002 (a vapor volume fraction of near 30%), the efficiency dropped by more than 20 points.
Figure 3 a typical P vs h diagram for a single stage radical inflow centrifugal expander during liquid to vapor flashing with a hydrocarbon liquid (degree of reaction conservatively selected was 0.5)
The centrifugal field as a flow phase separator
The poor performance of these radial inflow centrifugal expanders in flashing fluids was correctly reasoned to be caused by the centrifugal field, which is the entire functioning basis for radial inflow centrifugal expanders. The centrifugal field acts as a centrifugal separator between liquid and vapor phases and hence leads to poor efficiency as the vapor volume fraction increases in the flow. An upper limit of near zero is set by physics on the amount of vapor that can be present before the flow enters the centrifugal runner. From a design perspective this can be reviewed in the example P vs h diagram of Figure 3 3. For this example, a degree of reaction of 0.5 is assumed for the centrifugal expander. If vapor forms in the nozzle before the runner is entered, then efficiency deteriorates and vibration levels rise. This result is due to the centrifugal separator effect as the vapor and liquid have different densities and the radial pressure gradient acts on each phase with: dP/dr = ρVθ2/r
Figure 4 two-phase expansion in a linear nozzle, titanium axial impulse runner being machined, and their interaction showing the two-phase jet flow from the nozzle
Where P is the pressure, r is the radius, ρ is the density, and Vθ is the tangential velocity. The heavier density liquid is therefore slung radially outward while the lighter density vapor moves radially inward, effectively having the pressure gradient separate the liquid-vapor phases.
The poor performance and high vibrations caused by flashing liquids in radial inflow centrifugal expanders were motivation for NASA and NPO Energomash, to embark on programs to develop a new means of expanding flashing liquids in the 1960s. The driving applications were magnetohydrodynamic power system projects. The flashing liquid expander methodology applied was a controlled linear nozzle for expansion of the flashing liquid flow, avoiding curvature and ensuring close coupling between the expanding vapor and liquid droplets. This method proved successful, producing a good conversion of the available enthalpy drop to nozzle outlet kinetic energy. The successful nozzle design was then coupled to a pure axial impulse expander runner as seen in Figure 4 4.
Avoiding centrifugal fields and curvature
The historical conclusions from these previous detailed design and test studies were that for a flashing liquid expander the design should:
A flashing liquid expander of an axial impulse construction style avoids the centrifugal field in the runner and separation of the vapor and liquid phases during the expansion in the expander. As a result, a properly designed flashing liquid expander can handle significantly large amounts of two-phase flow at both the expander inlet and outlet.
Commercial axial impulse expander designs
Hundreds of axial impulse style flashing liquid expanders have been in service for over 30 years. They are found mostly in refrigeration chillers 5. Their power levels are, however, small, up to 60 kW. Larger axial impulse style flashing liquid expanders have also been applied in geothermal applications including units in the 800kW to 1.6 MW power levels 6. Figure 5 shows a comparison of these flashing liquid expanders’ measured output power vs the predicted power based on design computations. The agreement is quite reasonable.
There are currently no flashing liquid expanders operating in LNG liquefaction plants. However, as seen in Figure 6, a 25 kW flashing liquid expander that was operated in LNG as part of a demonstration program in a test stand constructed for the flashing LNG. Motivation for larger size flashing liquid expanders in LNG liquefaction plants is documented 7.
Figure 4 measured vs predicted flashing liquid expander power in various fluids including serval refrigerants (R134a, R245fa) LN2 geothermal brine, and LNG
Both the liquid and flashing liquid expanders use cryogenic submerged generators. This completely eliminates the need for a gearbox, shaft seal, seal gas and seal gas support system, lube oil system, shaft coupling (submerged generators are directly coupled to the expander runners), variable geometry (e.g., eliminates potential for nozzle vane sticking or surface galling), and axial thrust bearing (with submerged generators the axial thrust is balanced to zero). And lastly it needs to be emphasized that the axial impulse style flashing liquid expander is only a single stage expander which leads to a simpler design.
As marine engine injection pressures increase, the design and manufacture of high-pressure FGSS introduce a series of complex technical challenges. Operating at pressures of 380 bar and above increases mechanical loads on pumping components, intensifies sealing and efficiency requirements, and places greater emphasis on material selection, particularly for wearing components such as bearings, piston seals, and packing seals, as well as system components such as valves, flanges, and instrumentation.
Summary
Cryogen liquid expanders in LNG facilities are a well provenmature technology used to provide a refrigeration benefit.Contrarily flashing liquid expanders have not yet beenapplied to large scale LNG facilities despite the potential toprovide a substantial refrigeration benefit with little risksince the flashing liquid expander is operated in parallel withthe existing Joule-Thompson valve.
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(7) HAHN, P., et al, ‘Application of a Flashing Liquid Expander to Enhance LNG Production’, LNG-15 Conference Poster Presentation, Barcelona, (April 2007)
