Power Recovery Turbines (2013)

Topic last reviewed: 1 February 2014
Sectors: Downstream, Midstream, Upstream

Power recovery turbines (PRTs) are pumps that run in reverse used to energy from a process and improve overall system efficiency. A PRT is a single or multistage pump or turbine that is used in a process when a fluid is pressurized and must be reduced in pressure. Rather than throttle the fluid pressure though a pressure reducing valve (PRV) and waste the stored energy, a PRT can be used to drop the fluid pressure and turn the pump or turbine impeller and drive a generator or drive shaft to another process. This way most of the stored energy can be recovered minus the efficiency losses of the PRT. While the scope of the PRT technology is for energy recovery from high-pressure liquids, a similar technology, known as turbo-expanders, is used for energy recovery from high-temperature and high-pressure gases[http://www.ipieca.org/resources/energy-efficiency-solutions/turbo-expanders-2013].

Figure 1 shows a power recovery system in a high pressure water mill application.

Figure 1- Power recovery system of high pressure water [Reference 1]

Application of technology

A PRT application is typically used as a secondary driver inline to a primary drive motor and a process compressor and acts as trim power to the system. A PRT may be used to drive an electric generator but the application as a standalone driver is less common. The term HPRT is also commonly used to refer to a hydraulic power recovery turbine.

A train arrangement may contain a motor as primary driver, a primary pump, and the PRT system would add a PRT, an over running clutch, bypass PCV, and shaft. As supplemental power is available, the PRT and clutch would be engaged and allowed to drive the secondary equipment shaft, thereby reducing the output needed of the primary motor and lowering fuel or electrical costs.

The most common PRT application when used with a liquid is simply a standard centrifugal pump running in reverse. This application, however, will have a narrow best efficiency operating range. A PRT specifically designed for the power recovery application will be able to run efficiently over a wider flowrate range, and may include modifications to inlet and outlet nozzles, blade profiles, bearings, and seals [Reference 2]. The performance characteristics of a PRT are similar to but different from those of a centrifugal pump running in forward direction so the manufacturer should be contacted when using a pump in a PRT application. An example of Multistage designs are also available.

Figure 2 is an example of the performance of a centrifugal pump versus flow rate when the pump is operated as a pump and as a power recovery turbine.

Figure 2- H-Q curves for a centrifugal pump when operating as a pump and as a turbine when the pump is running at constant speed [Reference 3].

A PRT may operate in a fixed head or fixed speed format. Pump affinity laws are still applicable. Although PRTs are commercially available, they are far less common than typical pumps and turbines and the list of suppliers is limited.

Technology Maturity

Key Metrics

Decision drivers

Alternative technologies

HRTs are a secondary system and are a useful way to recover energy from a process rather than waste it by throttling pressure off. But if they are not used, the primary process will likely not be changed to reduce the stored energy.

Operational issues/risks

The application of a PRT should consider the full range of expected operation. There may be cases where the PRT is not utilized or available to the system. If a system has a wide range of fluid flows but the PRT is only effective over a narrow range, then it may be bypassed during those cases. A PRT may suffer loss of efficiency and mechanical problems when two phase flow is present. Fluid quality issues can also hamper PRT performance by either corroding or plugging the turbine blades [Reference 4].

Opportunities/business case

Goulds Pump Turbine Recovers Energy for Industrial Gas Supplier;
http://locator.gouldspumps.com/download_files/CaseHistories/Summer_2004_GPTurbineRecoversEnergy.pdf

In this case study, a PRT was installed at a compressed air and gas plant to throttle cooling water and produce power for a primary pump motor. The cooling water used was previously throttled or “let down” through a control valve from 70 psig to 15 psig (482 kPag to 103 kPag) before going to the plant’s cooling tower. A centrifugal water pump similar to the one being used on the pumping side of the system was installed in-line with the primary pump and its 100 hp motor. In PRT mode, the new pump contributes 93 bhp, produces about $24,000 savings per year, and has a payback in about 2.4 years.

References:

1. Global Warming Countermeasures: Japanese Technologies for Energy Savings/GHG Emissions Reduction, p.137.
2. Hydraulic Power Recovery Turbine; Centrifugal Pump.org (No longer available)
3. Optimizing Efficiency When Pumps are Operated as Both Pumps and Power Recovery Turbines
4. Power Recovery Turbines For The Process Industry; S. Gopalakrishnan