Turbo-expanders Performance and Sizing

Turbo-expanders play a major role in many cryogenic processes. Their performance can affect plant production, energy efficiency, and product quality. It is very important that the process designer takes all the possible scenarios before selecting a turbo-expander to ensure acceptable performance at all process conditions. In this article, we will be examining the turbo-expander performance at design and off-design conditions.

Turbo-expanders Design vs. Off-design Operation

In all turbo-expander applications the inlet stream conditions i.e. pressure, temperature, flow rate, molecular weight....etc. Turbo-expanders, like all turbomachinery, get negatively affected by deviations from the design conditions.

The turbo-expander off-design efficiency can be predicted using efficiency correction factors. Using the following equation:

where CQ, CP, CT, and CMW are correction factors due to deviation of flow, pressure, temperature, and molecular weight with reference to design values respectively.

Typical turbo-expander efficiency correction factors are given in the below diagrams for constant speed operation of turboexpander (such as electrical power generation application). Similar curves exist for turbo-expander variable speed operation (like compressor drive application).

Turbo-expander Sizing

Factors required for proper sizing of a turbo-expander:

1- Inlet Pressure.

2- Inlet Temperature.

3- Flow rate.

4- Gas Composition.

5- Outlet Temperature or Pressure.

Inlet Pressure

It can be seen from the diagram that the reduction in turbo-expander efficiency is negligible with an increase of pressure relative to design pressure. This leads to the conclusion that if the lowest inlet gas pressure is selected as design pressure, then turboexpander will operate with maximum efficiency (from an inlet pressure point of view) at different operation scenarios.

Inlet Temperature

The turbo-expander efficiency is sensitive to the inlet gas temperature in similar to its sensitivity to the flow rate. This perfectly makes sense, because the gas temperature affects the gas density which in turn affects the volumetric flow rate.

In some cases, the inlet gas temperature must be controlled especially in cold climates. The natural gas pressure reduction stations are typically equipped with an inlet preheater, to prevent the outlet gas stream from getting chilled below its allowable temperature. Excessive chilling of the gas stream can lead to droplet formation or even a catastrophic metal failure. Turbo-expanders in pressure reduction stations are usually loaded with an electric generator.

So turboexpander inlet gas temperature is determined (by calculation) considering its discharge temperature to be the minimum allowable gas temperature. This temperature then should be maintained by controlling gas pre-heater.

Flow

Although the flow correction curve is nearly symmetrical and either lower or higher flow causes the same reduction in turboexpander efficiency but lower flow means that lower power can be produced at the turbo-expander shaft. So turboexpander design flow should be determined such that the maximum possible flow can be passed through it during the year.

If prolonged low flow operation is expected, that can lead to low efficiency and produced shaft power, then it will be more economical to select two turbo-expanders and operate only one in the low flow situation.

Molecular Weight

Molecular weight is a function of gas composition and its design value should be based on average gas components mole percent. The molecular weight affects both the turbine and the compressor (in compressor loaded machines) performance. Moderate molecular weight variations have a negligible effect on turbo-expander efficiency. If the gas composition is to be varied greatly, then a redesign should be considered.

To conclude, the energy-efficient operation of turbo-expanders starts with the correct sizing and taking all the possible process variations into consideration. The Design point should be selected carefully to ensure high efficiency most of the time and reasonable efficiency at off-design operation.

References

1-  Bloch, Soares, 2001, "Turboexpanders and Process Applications", Butterworth-Heinemann.