The total energy consumption of industrial plants is a decisive factor for a successful, economic operation. While the energy costs were once the main motivation to reduce energy consumption, environmental reasons have become more important. In addition to carefully using limited fossil energy resources, reducing CO2 generation is key to slowing the heating up of the atmosphere.

In technical processes and pipelines, excess pressure is often available and is broken down or reduced by throttling devices. However, instead of throttling and wasting, these energy potentials can be recovered and used by hydraulic turbines. In the majority of applications, the process data is constant. Therefore, the recovery turbines do not have to be adjusted, and standard reserve running turbines with fixed geometry can be used. In view of the low price compared to conventional turbines (e.g. Pelton turbines), the investment costs can be controlled.

Operating Range

While trimming pump impellers is a good procedure for adapting to the relevant operating data, this method can only be used with restrictions in turbine-operation (Figure 1). The range that includes the maximum and minimum diameters above the inflection point can be designated as the operating range of the turbine.

Figure 1

[[{"type":"media","view_mode":"media_large","fid":"358","attributes":{"alt":"To increase the working range without any expensive control equipment, while maintaining nearly the same efficiency, it is possible to use partial flow and high flow impellers and diffusers, similar to those in pump operation.","class":"media-image","id":"1","style":"float: left;","typeof":"foaf:Image"}}]]To increase the working range without any expensive control equipment, while maintaining nearly the same efficiency, it is possible to use partial flow and high flow impellers and diffusers, similar to those in pump operation. Especially axial split case pumps where additional diffusers give the opportunity to vary the operating range over a wide band, by changing both the impeller and diffuser (already proven in many pipeline applications).

The same procedure can be implemented for reverse running pumps, with diffusers having an advantage compared to standard volute pumps. In turbine operation, the flow angle at the leading edge of the impeller, which is now at the outer diameter, is a decisive factor for the turbine curve and the flow at the best efficiency points. While a standard volute pump has only one or two "vanes" (the cutwaters), the diffuser has a higher number of vanes. The higher the number of vanes, the better the flow angle can be adjusted and the curve can be adapted to meet the required duty.

Depending on the rotational speed and size, a head drop of up to 500-m and a flow up to 5,000-m3/h can be reliably achieved with an axial split case pump with a diffuser.

Design Flow

In many customer datasheets, two capacities are given: normal flow and rated flow. The rated flow is in most cases about 10 to 20 percent higher than the normal flow. For pump operation, this can ensure a later increase of the production. But for turbine operation, a higher rated (guarantee) flow than the real available capacity would not use the whole available energy potential. The turbine curve lies "below" the real available data, so additional throttling by a valve is necessary. Depending on the steepness of the turbine curve up to about 20 percent of the available pressure difference has to be throttled. In the same ratio, the possible power recovery is reduced. Therefore, the turbine should always be designed for the normal flow to achieve the best possible power recovery. If a future need requires a rise in output, then the impeller and/or diffuser can be changed to optimize this new power output.

Application Example

In gas scrubbing processes, the gas is cleaned at a high pressure that is generated by a booster pump. The fluid used for cleaning is continually re-used and runs in a closed circuit. However, before entering the booster pump again, the pressure has to be reduced. This pressure drop is often realized by throttling via a valve. This energy, in terms of high pressure, can be recovered economically by the use of a hydraulic turbine in the form of a reverse running pump.

Figure 2
[[{"type":"media","view_mode":"media_large","fid":"360","attributes":{"alt":"A picture of the train on the test bed is shown in Figure 3.","class":"media-image","id":"1","style":"float: left;","typeof":"foaf:Image"}}]]A new facility for extraction of nitrous oxide was planned in a German chemical plant. For that process, a booster pump with the operating data of 2,500-m3/h at 220-m was required. In view of the low NPSHA, the rotational speed of the booster pump was selected at 1,490-rpm. To reduce the electrical power consumption, a turbine should release the high pressure of the fluid after cleaning the gas. The aim was to reduce the power consumption by approximately 1,100-kW. For the power recovery, a pressure difference of approximately 21.5 bar and a flow of 2,400-m3/h were available.

Because of the high gas content of the fluid, the back pressure had to be selected carefully. Due to the pressure reduction across the hydraulic turbine, the gas volume, which was low at high pressure, increased as the pressure was reduced. If the back pressure was too low, the volume of gas could block the piping or the suction flange of the turbine. The turbine itself could handle up to 20 percent gas content at the suction flange.

Figure 2 depicts the schematic sketch of the train with the booster pump and the pump running as a turbine. The electrical motor has two shaft ends: one for the booster pump and one for the turbine. An overrunning clutch was installed between the motor and turbine. At the start of the gas scrubbing process, only the booster pump was running. The overrunning clutch free-wheeled and was not engaged. When the system was stabilized, the high pressure valve of the turbine was opened. The turbine began to run, and when the speed of the turbine had achieved the motor speed, the overrunning clutch engaged. Since that time, the turbine has assisted the motor and the electrical power has been reduced.

A picture of the train on the test bed is shown in Figure 3. The discharge of the two-stage booster pump (right) is connected to the discharge of the turbine (left). With that installation, the booster pump feeds the turbine for testing. The turbine is an axial split case pump with a diffuser. The impeller and diffuser were specially designed for this project. With a recovered power of 1,108-kW at the design flow, the aim was fully achieved.

Assuming energy cost of $0.07/ kWh and an operation time of 8,000 hours per year, the cost for the power of the train is reduced by $620,000 per year. The investment cost will be returned in a short time.

Figure 3

[[{"type":"media","view_mode":"media_large","fid":"361","attributes":{"alt":"Assuming energy cost of $0.07/kWh and an operation time of 8,000 hours per year, the cost for the power of the train is reduced by $620,000 per year.","class":"media-image","id":"1","style":"float: left;","typeof":"foaf:Image"}}]]However, besides the economical aspect, there is also a big environmental advantage. The generation of electrical power in coal fired power plants is linked to the high production of CO2. On average, these power plants generate about 900 g/kWh CO2. With the power reduction of the turbine the CO2, production is decreased by nearly 8,000,000 kg/year.

Summary

Reverse running standard-pumps are used for energy recovery with increasing success. In view of the lower investment costs, this is an economic alternative to conventional turbines.

By using different sets of impellers/diffusers, the operating range of reverse running pumps can be adapted to achieve high efficiency at the operating point.

Energy costs are a decisive point in industrial production, but the environmental impacts are also important. The application of reverse running standard-pumps to recover unused energy potentials helps to reduce environmental problems, and provide free energy for the users.

Pumps & Systems, April 2009