Are Turboexpanders Complicated?

The short answer to this question is a "No"! Not all turboexpander applications are equal and not all of them are complicated.

This article is discussing a design characteristic that MIGHT cause turboexpanders to be (regarded) a more complicated machine compared to other rotating machinery. On a deeper level, it is addressing how power generation by a turboexpander is different from other type of turbines and why turboexpanders FALSELY have not been extensively utilized for power generation in gas pressure reduction stations.

1. Turboexpander Power Generation Capacity

Complication in design and application of a turboexpander arises when a low flow rate and/or a high-pressure ratio is involved. For a low flow rate, picture values in the order of 1 Sm³/s and for a high pressure-ratio, imagine values around 8 to 10.

A detailed discussion of the reason for that complication is complicated itself! Here a simple explanation is presented.

Power generated by a turboexpander comes from energy (or more precisely enthalpy) difference between its inlet and outlet, which depends on inlet and discharge pressure and temperature.

From another perspective, power generation is proportional to flow rate (Q) and pressure difference across turboexpander (ΔP). A proof of this is presented in the next section by a Dimensional Analysis.

Power ∝ Q × ΔP

Pressure difference is the available potential. It is determined by the process and acts as a driving force, waiting to happen! It can be wasted, or can be recovered in the form of power. The means to recover it, is flow rate.

Flow is the means to take advantage of the driving force, pressure. It contains the energy associated with the pressure difference. By harnessing that energy, power can be generated. A low flow rate does not provide enough means to utilize the potential energy. It is not sufficient for turning potential energy to kinetic energy.

From rotating machinery engineering point of view, low flow rate means lower momentum available on each pressure reduction stage. Then to be able to absorb the available energy efficiently, the machinery needs to work at an increased speed, hence increasing the momentum with the help of speed factor rather than the mass factor. In other words, power is proportional to flow rate and (square of) speed. This is shown in the next section by Dimensional Analysis.

Power ∝ ṁ × V²

The higher the rotational speed, the more efficient is the turboexpander in generating power from available potential energy. By operating at a low speed, for example 3000 rpm associated with a 50 Hz generator, efficiency will be very low, resulting in poor or impossible financial viability of the plant.

Complicated turboexpanders are then limited to high speed, low flow, high pressure-ratio turboexpanders. Careful considerations are required when designing rotating machinery at high speeds, the most important one being aerodynamic stability during transient operation, such as start-up and shutdown, while the machinery is passing its critical speeds.

With the current technologies though, high speed operation is not a big deal. Numerous turboexpanders are successfully under very high speed operations all around the world. However, these come at higher costs. The question then is the classic one; are the capital (and operational) costs worth the revenue by power generation?

Having said that, there are many high flow rate applications suitable for utilizing turboexpanders, resulting in low or moderate rotational speed, high power generation capacity, and no complexity. Examples are industries requiring gas for their processes or furnaces, such as Steel and Aluminium, and city gate stations of relatively populated cities.

2. Dimensional Analysis:

3. Case Study

A case study verifies the qualitative discussion presented above. With reference to Table 1, it is shown that for the same pressure ratio (3:1), a low flow turboexpander needs to operate at above 35,000 rpm to have a satisfactory efficiency (of over 80%), while moderate flow and high flow turboexpanders give an even better performance at 20,000 rpm and 10,000 rpm respectively.

While operating a low flow turboexpander without a gearbox, i.e. at 3000 rpm or generator speed, generates nearly zero power, power generation capacity of a high flow turboexpander directly coupled to a generator is only 8% less than its highest efficiency condition.

This case study is made possible by effective utilization of TePS, Turboexpander Performance Simulator.