Small gas turbines have been used for years as electricity generators —efficient enough to provide peak power as well as standby. Now, bargain natural gas prices are boosting sales. And the latest machines, boasting lower emissions, multi-fuel capabilities and reduced maintenance, have broadened their appeal in small-scale cogeneration schemes and mechanical drive applications.

One of the most challenging aspects when competing in this power segment is the variety of applications and geographical sites. “This calls for more flexibility than for larger GT’s,” said Per Johansson, Product Manager in Siemens Energy’s Oil and Gas Division. “The extremes spread from ship propulsion through mechanical drive part load operation to full load/peak load operation in combined cycle plants, from installation in desert areas with temperatures around 50°C through offshore installations to remote sites in Russia and Alaska where the temperature could be as low as -60°C, which demand fuel flexibility requirements.”

While they may be of less impressive dimensions than heavy frame machines, turbines below 20 MW are responsible for close to 60% of unit production, according to Forecast International (Figure 1). Smaller turbines, produced by Solar Turbines, Man Diesel & Turbo, Opra, Siemens, Kawasaki and others serve a variety of markets, such as Combined Heat and Power (CHP), decentralized power and growing opportunities in oil & gas applications.

By 2020, some 12,508 gas turbine machines will be built for electrical power generation, according to Forecast International. Their production value will exceed $150.8 billion (in current U.S. dollars). General Electric may be the projected leader in terms of the overall value of machines produced, but Solar Turbines is expected to be the leading player with regard to number of units, manufacturing over 30% of the machines, said Bill Schmalzer, analyst at Forecast International (Figure 2).

With customers in 98 countries, Solar has installed around 14,200 gas turbines with over 1.8 billion operating hours of service. The company offers industrial gas turbines in the 1-to-22 MW range (1,590 hp to 30,000 hp). However, some sites have combined several of Solar’s larger units, extending capacity as high as 100 MW (Figure 3). Solar provides continuous heavy duty turbines whereas their main competitors sell aeroderivative units, said Uwe Schmiemann, Solar’s marketing manager for power generation. The bulk of sales come in the 3 MWto- 10 MW range, but demand is increasing for units above 10 MW. Users need more power than before, such as for larger CHP plants, said Schmiemann.

Accordingly, the company introduced the Titan 250 two years ago. This 22 MW machine, said Schmiemann, has the highest simple-cycle efficiency (39%) in its class, as well as NOx emissions of 15 ppm or less (Figure 4). “The Titan 250 is gaining rapid acceptance for gas compression and power generation, and we expect this will become one of our top selling units in the future,” he said.

During overhauls, Solar offers an engine exchange program, delivering a zero runtime engine to the customer’s site to coincidewith the scheduled engine overhaul. This programallows the customer to slash downtime during overhaul.

Man Diesel & Turbo’s portfolio consists of gas turbines ranging from 6.9 MW to almost 26 MW. The company has observed trends, such as more demand for decentralized power generation as well as CHP. That led Man to extend its product line downwards with the development of new, singleand two-shaft gas turbines in the 6-to-8MW class (Figure 5). This aligns well with the projection by Forecast International.

“A detailed market survey in regards to market development and future demands revealed the gas turbine power class between 6MWand 10 MW to be the most promising range in terms of machine numbers to be sold,” said Volker Langusch, Man’s Senior Project Manager Gas Turbines (Figure 6). “Thus,we decided to develop not only a single engine but a brand new gas turbine family consisting of a range of models dedicated for different markets.”

Using the background of its THM GT (10 MW power class) in conjunction with experience gained in aeroderivatives (such as the FT8 from Pratt & Whitney), Man has released a twin-shaft model in the 6MW power class (6.9MW mechanical drive; 6.6 MWelectrical). Single-shaft versions are for power generation, while twin-shaft versions are used for mechanical drive. “Due to it having the highest efficiency in its class, the fuel consumption and thus CO2 emissions are the lowest in class,” said Langusch.

The company is installing one of these models at a CHP cogeneration unit at the Rheinberg chemical plant of Solvin GmbH & Co KG. The plant will supply 6 MW of electrical and 11 MWof thermal power and achieve a fuel efficiency of 80% by using waste heat of the gas turbine in addition to the 34% efficiency of GT itself.

Opra specializes in radial gas turbines, applicable for engines below 4 MW(Figure 7). The company’s low-emission generating sets using theOP16 can be combined to provide anywhere from 1.5 MWto 10 MWfor the oil & gas, marine, industrial and commercial markets. It achieves NOx exhaust levels of 25 ppm or less.

“The OP16 is a single-shaft industrial turbine aimed at power generation applications in the 1.5 MW to 2 MW range designed for lowmaintenance and long life,” said RegineMowill, Opra’s marketing manager. “It possesses an all-radial rotor construction, making it the simplest turbine engine available; and it is more robust than conventional axial turbines and reciprocating engines in its power range.”

The centrifugal compressor and radial turbine components provide high thermal efficiency at a relatively moderate pressure ratio of 6.7:1,meaning additional pressurization of the gas fuel is unnecessary.TheOP16 has four can combustors that, in addition to being able to burn different types of fuel, are accessible for inspection and maintenance. The hot exhaust gases of the OP16 gas turbine sets can be used for process and heating in oil & gas applications.

“Depending on the application, an OP16 turbine-based cogeneration plant can reach an overall fuel utilization of 90%,” said Mowill. “At full load, approximately 5 MW of heat is available in the clean exhaust gases.” This makes it suitable for industries needing high temperature heat, such as direct drying, steam production, absorption cooling and district heating, as well for the commercial and tertiary sector, including hotels, hospitals and datacenters. In addition, OP16 is often used in remote oil & gas fields, running on associated gases (Figure 8).

Siemens small machines

Siemens has introduced a 37MWmachine known as the SGT-750 (Jan/Feb12, p.22). But the company’s portfolio includesmany turbines smaller than that.

“The markets for 5 MW to 20 MW turbines are oil & gas for power generation and mechanical drive, and industrial customers for combined heat and power application, often in steam driven island mode,” said Christian Engelbert, Head of Gas Turbine ProductManagement at the Siemens Energy Oil and Gas Division.

The line begins with the SGT-100 gas turbine. The 5.4 MW single-shaft version serves power generation while the 5.7 MW twin-shaft model is available for mechanical drive.Twin shafts can operate over a range of speed and load demands for pumping applications, including crude oil, other refinery product transmission and water injection, as well as for compressors used in gas injection, pipeline transmission and boosting, gas processing and similar applications, said Engelbert. He lays out further members of the Siemens small turbine group such as the 7MWSGT-200, the 8MWSGT-300,which has recently been released as a twin-shaft (Figure 9) and the 13- to 15MWSGT-400.

“The SGT-300 twin-shaft is equipped with a Dry Low Emissions (DLE) combustion system, achieving low-NOx levels with gas and liquid fuels over awide load and fuel quality range” said Engelbert. “It has a mechanical efficiency of 34.6 % and has specifically been designed to meet the requirements in the oil & gas industry.”

He added thatwhen they are used to drive pumps or gas compressors in pipelines, they often see duties off the rated power at ISO. The reason is that the pipeline flow is fixed and independent fromthe ambient conditions. Gas turbines, however, are machines of fairly constant volumetric flow, which manifests itself in a power rating curve that is dependent on the ambient temperature for a certain installation — lower outside temperature leads to higher power output and vice versa.

“Users specify mechanical driver gas turbines based on a duty point representative for the flow through the pipeline plus power degradation margin,” said Engelbert. “This shows the importance of part load efficiency and emissions and a rather flat high ambient power curve.”

In the 15-20 MW range Siemens offers the SGT-500. It is a three-shaft machine that reliably handles a variety of fuels, including heavy crude, heavy fuel oil (IF380) and hydrogen rich gases.

The Turbomachinery Handbook 2012 provides a comprehensive listing of gas turbines and their specs. This article highlighted machines manufactured by Solar Turbines, Man Diesel & Turbo, Opra and Siemens. In addition, p. 20 provides an overview of the 8 MW Kawasaki M7A-O gas turbine and p. 22 discusses Capstone microturbines, now finding a growing volume of deployments greater than 1 MW.