Gas Turbine as a prime mover

THE GAS TURBINE AS A PRIME MOVER

FOR STANDBY POWER APPLICATIONS

Bernt Marcussen, Kongsberg Dresser Power A/S

P.O. Box 173

3601 Kongsberg,Norway

Introduction:

Many think that a gas turbine, as the expression implies, solely burns gaseous fuels. This is not correct. It is true that gaseous fuels of different qualities are excellent for a gas turbine; however, the machine runs equally welt on liquid fuel.

Gas turbines on standby duty in most cases operate on liquid fuel, either a light diesel fuel or kerosene.

Principle of operation:

Gas turbines are normally classified in two groups, single shaft and two shaft engines. The compressor and turbine sections can be either of the axial or radial type. The principle of a single shaft gas turbine is shown schematically in Figure 1. Air at atmospheric conditions is drawn into the compressor and delivered from the compressor to the combustion chamber at an elevated pressure. Fuel supplied to the engine supports the continuous burning in the combustion chamber. The hot combustion gases pass through the guide vanes to the turbine

wheel where energy is released from the hot gases. The turbine wheel drives the compressor which is positioned on the turbine wheel shaft (single shaft concept).

The net power which is the difference between the power generated by the turbine wheel and the power absorbed by the compressor is transmitted through a reduction gear to the output shaft where a generator may be connected.

Single shaft gas turbines typically have very high rotational speed stability. However, the torque transmitted is at its maximum at rated speed and decreases rapidly with decreasing speed. Single shaft turbines are therefore ideal in constant speed applications such as generator drives.

In applications where operation over a wide speed range is required a two shaft engine may be the best choice. A two shaft engine consists of a gas generator section and a power turbine section. The gas generator section is in principle the same as a single shaft turbine, where the turbine wheel is designed to develop just enough power to drive the compressor. The residual energy in the gas generator exhaust drives the power turbine positioned on a separate shaft which is also the output shaft. With the dual shaft concept, high torques may be transmitted at low speeds, making the dual shaft gas turbine ideal for compressor drives, pump drives etc.

Figure 2 shows the principle of a dual shaft gas turbine.

General features:

Weight and volume Gas turbines are much lighter and much smaller than comparable diesel engines. The reason is that in a gas turbine the compression, ignition and expansion are continuous processes and in addition the rotating speed is considerably higher. The rotating elements in a gas turbine are in complete balance and therefore only a High engine frame structure is required. In the lower horsepower range the

difference in weight and size between the two engine types is not very apparent, but in the higher range the difference is significant. The low weight of a gas turbine and the fact that vibrations are almost non-existent mean that the foundation requirements are minimal. The force transmitted to the foundation is for all practical purposes equal to the static weight of the machine and there are virtually no forces transmitted due to vibration. It is fully acceptable to install a gas turbine generating set on a building floor dimensioned for the static

weight of the set.

A typical 1500 kW generating set has the following weights (KDP KG2):

Gas turbine and gear

2.5 tons

Generator

5.0 tons

Common base frame and auxiliary equipment

1.0 tons

Total

8.5 tons

Length: approx. 4 meters (12 ft).

Cooling:

The cooling of a gas turbine engine is simple. Only the heat generated in the rotor bearings and in the reduction gear needs to be dissipated. This can easily be achieved in small oil to air cooler or oil to water cooler, depending on the facilities available at the installation site.

It is customary to choose oil to air cooler for a standby generating set, since experience has shown that the water supply is often cut off at the same time a blackout occurs. The heat which needs to be dissipated normally amounts to about 5% of the rated power of the generating set, which calls for an oil cooler quite moderate in size. The ejector principal may be applied to eliminate the need for motor driven fans. This increases the potential for maintaining a high level of reliability. In a diesel engine the heat to be dissipated is of the same order of magnitude as the rated power of the engine. In a gas turbine the principal part of the heat loss is concentrated in the exhaust gases, which leave the engine at a relatively high temperature and at a high rate of flow.

Rotational speed stability:

Single shaft gas turbines have very high rotational speed stability. This is the result of a large mass rotating at high speed. Speed variations due to load changes are suppressed by the substantial amount of kinetic energy stored in the rotating elements.

Fuel consumption:

The fuel consumption of small and medium size gas turbines is normally about double the consumption of comparable diesel engines. The fuel consumption may be reduced considerably through adopting  recuperate for preheating which preheats the air before it enters the combustion chamber. However, this is expensive and increases the installation cost significantly. Standby generators would normally accumulate very few hours of operation and the fuel consumption is of minor importance.

Noise:

Because of its cousin the jet engine, gas turbine engines have a quite adverse reputation for being potential noise generators. The design philosophy of aircraft jet engines is entirely different and there is a very significant difference in the sound pressure levels of the two engine types. This is not to say that the gas turbine engine needs no silencing - some is required for all types of rotating power machinery. The noise emitted from a gas turbine is air borne and at high frequency, and the silencing is therefore quite simple.

Start-up time:

Start-up of a gas turbine implies that the heavy rotor mass must be accelerated to a high speed level. A gas turbine would therefore normally require a longer starting time than a comparable diesel engine. On the other hand a gas turbine may be loaded to 100% immediately upon reaching rated speed, while diesel engines often require loading in steps. Experience shows that a start-up time of 40-50 seconds is fully acceptable for large standby generating sets provided full load can be accepted immediately, when the engine reaches 100% speed.

Controls and supervisory systems:

By-an-large gas turbines require the same supervisory system and controls as other prime movers. Normally this implies alarm and shutdown in case of excessive levels of rotating speed, lube oil temperature and exhaust gas temperature. Whether the design engineer specifies alarm or shutdown depends on the nature of the specific installation in question.

Emissions:

Gas turbines operate with an air/fuel ratio high above the stoichiometric level. This secures an almost complete combustion and results in an invisible exhaust containing very small quantities of undesirable components. Figure shows an exhaust gas analysis of a Kongsberg Dresser Power KG2 gas turbine operating at approx. 1500 kW.

The extremely low content of CO is a result of the high air/fuel ratio (approx.4:1) while the very low NOx-numbers is the result of a combustion at low pressure and rapid cool-down of the flame front.

Lube oil system:

We mentioned earlier that a gas turbine generating set requires a supply of lube oil to the rotor bearings and to the reduction gear. Common mineral oils are normally specified however synthetic oils are equally suitable. In the Kongsberg Dresser Power KG2 turbine the two rotor bearings of hydrodynamic type are both positioned in the cold section of the machine. This means that the oil cannot be contaminated by the exhaust gas, nor will there be any loss of oil to the combustion chamber. The consumption of lubricating oil in the KG2 gas turbine is therefore very small.

Auxiliary power requirement:

While on standby a gas turbine generating set has some need for the supply of auxiliary power. For a KG2 generating set in the 1300-1700 kW power range the power demand is limited to the following:

Charger for control system batteries (24V) : 2 kW

Charger for start batteries (48V) : 7 kW

Lube oil reservoir heater : 2 kW

Heater inside generating casing : 0.4 kW

Reliability:

The starting and operating reliability is probably the most important feature required in a standby generating set. The single shaft gas turbine with a single stage radial compressor and a single stage radial inflow turbine is probably the simplest prime mover in existence today and it has the potential for being the most reliable. The single shaft gas turbine is in principle identical to a turbocharger except that the gas turbine has its own combustion section and an output shaft.

Applications of the gas turbine:

In principle gas turbines may be installed wherever there is a need for standby power, except in cases where an extremely short start-up time is essential. Experience shows that gas turbines are more competitive in applications above 1000 kW than in the lower power range. This has to do with the pricing structure. The price of gas turbines per kW increases with decreasing engine size, while it is the opposite for diesel engines. The explanation lies in the difference in production volume for the respective prime movers. The larger the power requirement, the more competitive the gas turbine, particularly when the low installation and maintenance costs are taken into consideration. Gas turbines often represent the only alternative, particularly in cases where standby generating sets are engineered into existing facilities and where it is difficult to supply the necessary amount of cooling air for a diesel engine. For roof top installations the gas turbine is often the only realistic alternative.