Turboexpander Performance (1)

Turboexpander Design and Off-Design Performance

Turboexpanders are being widely applied in cryogenic and power recovery cycles. In both cases there exists a fluid that has to be expanded to meet the process requirements. This is based on high chilling effect of expansion process in a cryogenic cycle. An example of such an application is medium-sized natural gas liquefaction (or LNG) plants. In a power recovery cycle, there is an end user with a specific fluid pressure requirement and turboexpander duty is to reduce the pressure to match the demand. Gas pressure reduction stations at the inlet of cities or power plants are examples of such an application.

For both cycles, power generated by turboexpander shaft (either driving a compressor or generating electrical power through a generator) is considered as a by-product intending to increase the overall efficiency.

In most turboexpanders applications, inlet gas conditions (flow, temperature, pressure and molecular weight) are varying. This variation can be intense for gas pressure reduction stations supplying gas to household sector as natural gas demands is much higher in cold seasons of the year. As turboexpander efficiency (same as any other turbomachinery) is highly affected negatively when operating in off-design conditions it won't operate efficiently throughout a year if the process designer fails to select the optimum design conditions for turboexpander operation. This can be done only if the process designer is familiar enough with turboexpander off-design performance.

Turboexpander off-design performance can be predicted using efficiency correction factors:

In which C_Q, C_P, C_T and C_MW are correction factors due to deviation of flow, pressure, temperature and molecular weight with reference to design values respectively. Typical turboexpander 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 turboexpander variable speed operation (like compressor drive application). This factors and how they affect determining turboexpander design point are briefly discussed below.

 

Turboexpander efficiency correction curves

 [Source: Bloch, Soares, 2001, "Turboexpanders and Process Applications", Butterworth-Heinemann]

Flow:

Although 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 turboexpander shaft. So turboexpander design flow should be determined such that as much as possible flow can be passed through it during the year.

If flow variation is too high that causes the turboexpander to operate at very low flows (and hence very low efficiency) in some months of the year, then it might be economically feasible to select two turboexpanders and operate only one (and keep one stand-by) in low-flow situation.

Pressure:

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

Temperature:

Unlike flow and pressure, temperature is an inlet gas parameter that can be controlled. That is done via a pre-heater normally considered upstream of the turboexpander. The need for pre-heating arises from the above-mentioned chilling effect of gas expansion process in a turboexpander; this can cause gas to be chilled to lower than its allowable temperature and may result in droplet formation. So turboexpander inlet gas temperature is determined (by calculation) considering its discharge temperature to be minimum allowable gas temperature. This temperature then should be maintained by controlling gas pre-heater.

Molecular Weight:

Molecular weight is a function of gas composition and its design value should be based on average gas components mole percent.

It should be noted that the exact design conditions can be determined based on these guidelines with the help of an economic analysis considering turboexpander power generation and pre-heater required energy for a 1-year operation. This requires the simulation of turboexpander performance with inlet gas parameters variations taken into consideration.