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.

A Note on Pump Selection (2)

Pitot Pump

In the previous post, low-flow high head application was introduced as one of the situations to choose reciprocating pumps. There was also a discussion on the low-flow high-head situations when selecting a high speed integrally geared centrifugal pump is preferred. Here another option for such an application is addressed.

Pitot pumps are considered to be a competitive option by many engineers in industry. Pitot pump is a specially designed centrifugal pump which extends the specific speeds of centrifugal pumps down to 50 o 350 (US units), namely low-flow high-head applications. Single stage pitot pumps with capability of delivering flows up to 800 gpm with a head up to 5500 ft have been successfully utilized.

1. Pitot Pump Design

An example of a pitot pump is shown in below figure. It comprises a closed rotating casing with a stationary pitot tube that extends into the rotating case along the axis of rotation with its inlet positioned near the maximum inner diameter of the casing. The liquid enters casing along the axis of rotation and picks up momentum as it passes through the enclosed radial vanes of the impeller into the rotating casing. The liquid is spun up to approximately the full rotational speed of the casing. It then impacts the inlet orifice of the pitot tube near the periphery of the rotating casing (where the pressure and rotational velocity of the liquid mass are the greatest). The liquid is discharged through the inner passageway of the pitot tube and out of the pump.

Pitot pump [Source: Roto-Jet High Pressure Pitot Pumps, Weir Specialty Pumps, www.weirsp.com]

2. Pitot Pump Performance

The head developed by a pitot pump is the sum of two components; the static pressure head created by centrifugal force and the velocity head. This sum will be approximately 1.6 times the head produced from a conventional centrifugal pump of the same size and speed.

Total developed head by a pitot pump can be calculated from the below equation:

where r is centerline radius of the pitot tubr inlet (in) and N is rotational speed (rpm). This equation is not significantly different from the theoretical head produced by a conventional centrifugal pump. It is the combination of more effective conversion of centrifugal and velocity heads developed by the pitot pump and minimal friction that allows this type of pump to develop high heads at relatively modest speeds and at good overall efficiencies.

There are two methods of changing the performance characteristics of the pitot pump; changing the size of the pitot tube and changing the speed at which the pump is operated. In order to represent both of these methods, a typical performance curve is generated for each pitot tube size with a series of head and power curves for each of the common operating speeds (see below figure).

Pitot pump typical performance curve [Source: Angle, Roudnev, Application of the Pitot Pump, Tutorial on Special Purpose Pump, Proceedings of the 14th International Pump Users Symposium, pp144-149]

Pitot pump is suitable for operation over a wide flow range. It can be safely operated at any point on its curve from full flow to shutoff.  It can be operated at its minimum flow indefinitely without any damage.

The head produced by the pitot pump can be easily adjusted by changing the speed at which the pump is operated. Pitot pumps are commonly driven either by electric motors controlled by a VFD, through a gear box used as a speed increaser or by V-belts and sheaves.

Reference: Angle, Roudnev, Application of the Pitot Pump, Tutorial on Special Purpose Pump, Proceedings of the 14th International Pump Users Symposium, pp144-149