Energy Recovery At Pressure Regulating Stations

June 2014, Vol. 241, No. 6

Tony Cleveland, President, Cleveland Engineering Services Ltd, Canada and Saeid Mokhatab, Contributing Editor

Pipeline transmission systems operate at high pressures which must be reduced (regulated) at off-takes and distribution points. Additionally, at compressor stations and power plants, the gas pressure to feed the fuel systems of gas turbines has to be reduced to a level acceptable to the fuel system controls.

It is general practice to regulate the delivery pressure with a valve which involves a loss of energy to the system. Some of this energy can be recovered using an expander to generate power in place of the valve.

The fundamental technology for this form of energy recovery is not new and has been exploited in Europe although it has been generally neglected in North America. With the emphasis on “green energy” it is believed that this technology and the benefits in energy utilization and economics need to be explored.

This article explores the work done in Canada on expanders in this application and compares it what has been done in Europe, looking at both the technical and economic aspects.

Surveys undertaken in the United Kingdom of the potential for energy recovery at regulating stations suggest there is a significant untapped resource of energy, much of which could be recovered and put to work. Also, recent developments in small, high-speed electric generators show how the economic benefits of emerging technologies could result in a radically new design of the turbo-expander, which will encourage wider exploitation of this form of pressure regulation and energy recovery.

Road To Recovery

At every point where gas pressure is reduced or regulated through a valve throughout transmission and distribution systems, energy is lost. These losses occur where transmission pipelines meet distribution networks, through the networks themselves up to the customer’s connection, at compressor stations and at every industrial installation that uses natural gas as a feedstock.

This has long been recognized as a problem, but for various reasons has generally been ignored. Estimates made for several major systems suggest that as much as 30% of the power put into gas transmission is rejected in the process of pressure reduction in distribution.

Gas distribution systems in Europe, particularly in Germany and Italy, have made serious efforts to recover and use the pressure energy at regulating stations to generate electricity. The number of expander units in service in this application in Europe is now over 70. The driving force for this has been primarily economics because electricity costs are high. Pressure from the green lobby has also been a contributing factor.

The technology of using a gas expander in place of a valve as a regulator is not new. In fact, it is merely an extension of using expanders in gas processing plants. In these plants, the expander reduces gas temperature and separates out the heavier fractions from natural gas streams. The power generated by the expander is used to drive a recompression compressor or absorbed in a hydraulic brake.

In the pressure-regulating station, the expander is coupled directly to a generator and placed in parallel with the regulating valve. Applying this technology has been under consideration for over 25 years but economics and politics have been obstacles to progress. In the United States, San Diego Gas & Electric installed an expander in San Diego, CA and some other installations were built at chemical plants, where the local electric utility was prepared to offer competitive rates for the power produced.

Case Study 1

Interest in the use of expanders in regulating stations in Canada was initiated in 1985 when Canadian Western Natural Gas (CWNG), now ATCO, sponsored a review of their distribution system to evaluate the energy production potential. Several industrial plants were found to be good candidates and a fertilizer plant was selected for a trial.

With an expander regulating the primary pressure reduction from the supply of gas to the plant, possible savings of 15% on electric power was shown. This project did not proceed beyond the study stage because the utilities and plant owners would have had to enter into a complex financial arrangement that never materialized.

However, a more detailed study of the CWNG distribution system showed the potential for energy recovery. It showed typical regulating stations could generate power between 250kW and 500kW.

A more recent study in the United Kingdom bears out these conclusions:

• Of almost 800 pressure reduction stations surveyed, 9.5% could produce in excess of 1 MW, 15% (500 kW to 1 MW), 25.5% (250kW to 500 kW), the remaining 50% (less than 250 kW). Total power capability was estimated at 390 MW.

• This data should be examined with caution because not all of the power would be recoverable as some stations have high ratios of expansion and others have very low flows and power outputs of less than 100 kW.

Energy Balance

Dropping the pressure through a valve also reduces the temperature, which is known as the Joule Thompson (JT) effect and takes place at constant entropy. The gas is preheated to ensure that the discharge temperature stays above the freezing point. This additional heat offsets the gain in energy from the expander since energy cannot be created.

It would, therefore, appear to be no benefit in using an expander in pressure regulation, but the benefit is derived from the “leverage,” using low-cost gas to provide an output of high-cost electricity per unit of energy.

It is essential that the preheaters are highly efficient to ensure maximum benefit. It follows that in order to ensure maximum gain, not only must the ratio of electricity price to gas cost be high, but the preheaters must also be highly effective.

Where sources of waste heat are available, such as at compressor stations and process plants, the full benefit of the expansion process can be realized.

Economics
We have shown that an expander can perform the functions of a valve in regulating gas pressure and generate electricity, though the process will require additional heat energy to compensate for the loss of temperature. The economics, therefore, are dictated by the ratio of electricity revenue to the cost of the additional preheat required.

In addition, the capital cost must be amortized and any maintenance added to the total. Using these parameters, some rules of thumb can be set to determine the viability of a given project.

In these calculations, it can readily be shown that the efficiency of the preheat is crucial. If preheat is readily available in the form of waste heat, as may be the case in petrochemical plants, power stations or compressor stations, the expander can benefit the bottom line.

When waste heat is not available, natural gas or environmental sources are needed. The water bath heater is commonly used throughout the gas industry. It is simple and inexpensive; however, it has a relatively low combustion efficiency. This effect can be exacerbated if the heater output is not matched to the expander needs. Therefore, it is important that when gas is used, the preheater should be a high-efficiency furnace, matching expander requirements.

Concerning other alternative heat sources for preheat, a number of concepts have been studied. These include geothermal heating and heat pumps as well as regulating stations where power and gas supply are needed at the same site.

A mixed-use commercial and industrial site closely linked to a gas regulating station and distribution network can have local power generation provided by a gas engine or gas turbine. The waste heat from this power source can then be used to provide preheat to the expansion turbine that is controlling the local distribution network gas supply.

The power generated can be used at the site or sold to the grid. This combination offers yet another benefit – the cooling effect of the discharge gas can be used for local refrigeration and process needs.

As analyses of gas distribution networks have shown, there is a need for economically priced expanders of less than 500kW to satisfy a large potential market in local gas distribution systems.

Case Study 2
It was in an attempt to fill this niche market that React Energy was formed in Canada. Several papers have been published covering the work undertaken with trial units in Canada and the United Kingdom. The objectives of the program were to demonstrate that useful power could be produced, and the distribution system would function without disturbance or interruption in the event of an unscheduled shutdown of the expander.

Prototype units were installed at CWNG in Calgary, with Consumers Gas in Ontario and with British Gas in Manchester, England. However, the React unit, as designed, was unreliable. Problems with the gearbox and seals required major redesign work to realize reliable performance.

Solutions to these problems are now available. For example, direct drive with a high-speed generator would eliminate the gearbox, and immersion in the gas stream would eliminate seal problems.

Selection Of Location
The selection of sites for expander installations has to take into account the expansion ratio, preheat availability, the access to the power grid or the plant electrical system, and the potential power available. Also, in the earliest stages, a record of flows and pressures must be analyzed to determine the optimum size of the expander unit.

Conclusions
Experience in Europe has effectively demonstrated the use of expanders in pressure regulation at offtakes in natural gas systems. Expanders of all types are in use on these systems: radial, axial turbines and reciprocating engines. There are also some examples of high-speed direct drive turbine units with electronic frequency conversion.

The few installation in Canada have shown that expanders can be used to control pressure without detriment or risk to system integrity and produce revenue from power sales.

The potential for power generation from sources such as distribution regulating stations, chemical and fertilizer plants, and power stations using natural gas is significant.

While all these applications need additional energy in the form of preheat, it should be noted that the economic leverage of the ratio of gas price to electricity price is the dominant factor, and where waste heat or another source of low-grade heat is available, the economic advantage is multiplied.

The large number of distribution stations, many of which have a power potential of less than 1 MW, can make a major contribution to the power needs of the local community. Recent progress with micro- turbines could form the basis for development of turbo-expanders to serve this market. More attention should be paid to this source of relatively green power, which can avoid waste of energy in gas transmission and distribution systems worldwide.

References
1. Cleveland, A., “Turboexpanders for Energy Recovery, Design and Installation of a 250kW Unit,” ASME Paper No. 88-GT-266 (1988).
2. Cleveland, A., and Wright, E.A., “Turbine and Reciprocating Engines Producing Power from Natural Gas pressure Reduction and Supply Systems,” IDGTE 550 (1998).

Authors: Tony Cleveland is president of Cleveland Engineering Services Ltd in Canada, involved in energy utilization and efficiency. His early industry experience includes Bristol aero engines (later Rolls Royce), and Solar in the U.S.

Saeid Mokhatab has been involved in several international pipeline/compressor station EPC projects and has published numerous technical papers and books. He is a frequent contributing editor to Pipeline & Gas Journal.

Find articles with similar topics