Gas Turbine Operating Principles

Learn Oil and Gas Oil and Gas Equipments Plant Utilities

A method of representing the gas turbine cycle is to show the basic gas turbine in a simplified graphic form. The triangle on the left represents the compressor and the triangle on the right the turbine section.

Gas Turbine Operating Principles  :

  • The gas Turbine Cycle
  • Cycle Efficiency
  • Single shift machines
  • Two Shift machines
  • Variable Inlet guide Vanes
  • Bleed Valve (Anti – Surge Device )

 

CHAPTER I

GAS TURBINE OPERATING PRINCIPLES 

  1. THE GAS TURBINE CYCLE

A method of representing the gas turbine cycle is to show the basic gas turbine in a simplified graphic form. The triangle on the left represents the compressor and the triangle on the right the turbine section.

The line connecting the two represents the shaft, which transmits the turbine power into the compressor to drive the compressor.

The same shaft would also extend out from the turbine and be used to drive the load equipment.

The combustion system is shown at the top of the schematic with a line connecting to the compressor and to the turbine, indicating the flow of air between the two elements.

Fuel is also indicated as being added at the combustion section.

The numbers on the schematic represent temperatures and pressures at the particular stages of the gas turbine cycle.

In this particular case, the inlet is assumed to be at sea level and an ambient inlet temperature of 20 degrees C.

As the air is compressed through the compressor, the discharge pressure is increased at a ratio of about 9 to 1 and an increase in temperature due to the compression process results in a discharge temperature of about 300 degrees C. The addition of fuel to the combustion system and burning of the fuel results in an increase in temperature level at the inlet to the turbine.

 

There is also a slight drop in gas pressure as the compressed gas flows through the combustion system.

High pressures high temperature gas (950 °C) expands through the turbine back to atmosphere with a resulting temperature and pressure drop. The discharge gas temperature in this example is in the order of magnitude of 490 degrees C.

The pressures and temperatures represented on this slide may vary with individual turbines, but are a relative indication of what would occur in a typical gas turbine cycle.

The total power output available from the gas turbine plant would also be a function of the total amount of air that is brought into the compressor inlet.

Thus as machine sizes change, the airflow changes and the relative useful output also increases with the increasing airflow.

In the example shown here with the gas turbine’s compressor and turbine being connected on the same shaft and the same output shaft being used to drive the load, the arrangement is referred to as a single shaft gas turbine design.

The cycle as represented here is also referred to as a simple cycle gas turbine. Other cycles can be developed utilizing the heat energy that remains in the exhaust of a simple cycle gas turbine.

The basic gas turbine cycle is referred to as the Brayton cycle This is represented on a pressure-volume diagram by the four stages associated with the basic cycle.

Ambient air is brought in at stage 1 to the inlet of the compressor.

The compressor compresses the air to stage 2 with a resulting increase of pressure and reduction in volume. From stage 2 to 3 fuel is added to the air and burned, thus heating the combustion gases, resulting in an increase in volume at the same pressure level.

Stage 3 to 4 represents the expansion of these high energy gases through the turbine section back to ambient pressure, resulting in the production of useful power in the gas turbine section.

 

The basic cycle is an open cycle and therefore there is no direct connection between stage 4 and 1, although the inlet and exhaust conditions are both ambient atmosphere.

 

An important difference between a gas turbine and a reciprocating engine is the fact that the gas turbine cycle is a continuous process. There is a continuous inlet, compression, combustion, expansion and exhaust process going on. This means that a gas turbine delivers a constant torque output at the output shaft which is an important advantage over the reciprocating engine.

Furthermore, the Brayton cycle can be realized in relatively large machines, making output powers up to 100 MW possible.

  1. THE CYCLE EFFICIENCY

With every heat engine the important question is: what is the efficiency of the process, in other words: what percentage of the fuel supdi’7 US available at the Output shaft as useful power? The efficiency of the process can be calculated with the available thermodynamic formulas. In the Brayton cycle we see three processes that can be expressed in thermodynamic formulas. Firstly, the Compression from 1 to 2, during which energy has to be Supplied to the air being compressed. This energy per second, or the power, is:

Wc = m . Cp . (T2 -T1)

Wc         = power, supplied for compression             (kw)

m          = mass flow of air                                      (kg/s)

Cp          =specific heat                                         (kj/kg.K)

T            = absolute temperature                                 (K)

Secondly, from 2 to 3, fuel is burnt in the combustion chamber, thereby adding energy to the air.

This energy per second, or power is:

Q = m . Cp . (T3 – T2)

Q = power, supplied in the fuel                                  (kw)

(other variables as in the first formula)

Thirdly, from 3 to 4, the hot gases expand in the turbine, thereby developing mechanical energy.

This energy per second, or again the power, is:

WT = m . Cp . (T3 – T4)

Generally, the efficiency of a process is expressed as

The useful Output

——————————    x 100%      =   h

What has to be supplied

In the Brayton cycle the efficiency will be

WT – Wc            (T3 – T4) –  (T2 – T1 )

h     =    ———–   =         ———————-

Q                      ( T 3 –  T2)

This can be rewritten as

                            T4   –   T1

h      =    1   –     ————–

T3  –   T2

 

From this efficiency expression it can be seen that the efficiency will be high if:

– The exhaust temperature is as low as possible, so that (T4 – T1) is small.

– The firing temperature is as high as possible so that   (T3 – T2) is large.

T1 is the ambient temperature which cannot be influenced.

T2 is the compressor discharge temperature which is influenced by the efficiency of the compressor. This is one reason why the compressor should be kept clean.

T3 is the turbine inlet temperature, often called the firing temperature Tf. Material temperature limits of the turbine hot parts limit the value of Tf to approx. 1000 C. Constant research is being done on new heat resistant materials and cooling of machine hot parts to enable the designer to increase Tf

T4 is the turbine exhaust temperature, often referred to as Tx. The value of Tx is given by the thermodynamic expansion process in the turbine, which ends at atmospheric pressure. Further expansion is not possible, so that Tx can not be lowered.

With the data, given in the schematic on page 1, we can calculate the efficiency of the simple cycle gas turbine process.

Remember that the absolute temperature T in Kelvin is the temperature in °C plus 273  :

T4  –  T1

  • h = 1   –     ————-

T3   –  T2

(490 + 273) – (20 + 273)

=     1     –     ———————————

(950 + 273) – (300 + 273)

 

763   –   293                      470

=     1    –        ———————     =    1  –    ——

1223    –     573                   650

=       1  –  0.72

=     0.28

h  = 0.28 x 100% = 28%

Cycle efficiencies can also be expressed in so called Sankey diagrams. These are diagrams in which the energy flows to and from the machine are expressed as beams of proportional width. The Sankey diagram for the simple cycle gas turbine would be as shown below.

Actually, this Sankey diagram is a simplification of the processes in the gas turbine. Not shown is the energy, used to drive the compressor.

The power, necessary to drive the Compressor is roughly twice the output power of the gas turbine. This is a considerable amount of energy, nut fortunately it is not lost and it plays no role in the efficiency consideration. The power, used to drive the compressor is won back in the turbine, where the compressed air expands again. The compressor power thus follows a loop that is illustrated in the Sankey diagram below.

3 . SINGLE SHAFT MACHINES

Better understanding of the energy (or power) streams in a gas turbine can be gained by studying the example given below. Rounded of figures have been used in a gas turbine with a 10 MW output to the load. With the fuel an input power (chemical energy) of 35 MW is flowing into the turbine. The compressor power is 20 MW and this power follows the closed loop from turbine to compressor and vice versa.

In the exhaust 25 MW of power are flowing to the atmosphere.

If the load (generator, compressor or pump) is driven from an output shaft on the gas turbine side, the arrangement is called “hot end drive”.

Some turbines have the output shaft on the compressor side, which is then called “cold end drive”.

Both systems have some advantages and disadvantages.

One disadvantage of cold end drive is that the shaft between the gas turbine and the compressor has to take the load of the compressor plus the output load. This will be three times the output load, vs. Twice this load for a hot end drive machine.

Cold end drive is not possible on two-shaft gas turbines, covered on the next pages.

 TWO SHAFT MACHINES .

Another version of the simple cycle gas turbine is the two shaft gas turbine.

In this particular case the output shaft is driven by a separate turbine element that is not connected mechanically to the turbine that would be driving the axial flow gas turbine air compressor.

As shown here we still have a simple cycle gas turbine.

The turbine that is used to drive the air compressor is referred to as the high pressure (H.P.) turbine. The compressor / high pressure turbine arrangement is frequently referred to as the high pressure set.

The turbine stage that is driving the output or load equipment is therefore usually referred to as the L.2. Or low pressure turbine.

In the example represented here the flow area of the second stage nozzle can be varied. And this is used as a means of Controlling the high pressure turbine speed independently from the output shaft speed.

An advantage of the two shaft gas turbine is that the axial flow compressor can be driven at its maximum operating speed and airflow by the high pressure turbine while the output shaft speed can be varied from its minimum governing speed to its normal rated speed.

The ability to change the speed of the high pressure turbine independent of the speed of the low pressure turbine allows considerable flexibility in other cycle arrangements than the simple cycle machine and also allows considerably flexibility in the speed range available to drive the load equipment.

The two shaft machine is most frequently used in applications requiring a varying output shaft speed such as ship propulsion units as well as variable speed mechanical drive units.

Output shaft speed variations of 50 to 100% of rated speed are possible. At low output shaft speed a considerable part of the output power is still available. This is illustrated in the diagram below, comparing single shaft and two shaft machines of the same rated output. At part load the efficiency of the gas turbine can be kept at the optimum level by controlling the turbine firing temperature with the use of variable area second stage nozzles. This is equally important when the gas turbine is connected to a waste heat recovery system that operates at high efficiency only when the exhaust temperature of the turbine is high.

Finally when starting up the two shaft gas turbine requires less starting power than the single shaft machine, since the output shaft of the two shaft machine only starts rolling after the H.2. Shaft has reached a high speed.

The effect of operating the variable area second stage nozzle of a two shaft gas turbine sometimes causes confusion. In the sketch below, two situations are illustrated: firstly the situation with 2nd stage nozzles wide open and secondly with the nozzles partly closed.

If the nozzles are widely open, the pressure drop over the nozzles is low. This results in a relatively low gas jet velocity downstream of the nozzle, so that little mechanical energy is developed on the turbine rotating buckets. Most of the gas expansion takes place in the first stage nozzle, so that nearly all the power, supplied in the fuel is used to drive the H.P. shaft.

If the 2nd stage nozzles are partly closed, we get a larger pressure drop over the nozzles, resulting in a higher gas jet velocity and thus in more mechanical energy output of the L.P. shaft. Now only a smaller part of the total gas expansion takes place in the first stage nozzle, so that relatively less power from the fuel is used to drive the H.P. shaft. Of course, to keep the H.P. shaft running at constant speed, more fuel must be supplied when the second stage nozzles are further closed.

In the graph below, the output and the turbine efficiency are given as a function of the position of the 2nd stage nozzles. The nozzle position is determined by the angle of the nozzle partitions (or vanes) relative to the axis of the rotor. This angle can be varied between -5° (fully open) and + 15 ° (maximum closed position).

5 . VARIABLE INLET GUIDE VANES .

Gas turbine plants with heat recovery steam generators, which have to run on part load for extended periods, will reach a higher part load efficiency when variable inlet guide vanes are used on the compressor. Variable inlet guide vanes allow step-less control of the mass flow of air entering the compressor.

With the turbine running at rated speed, but at part load and with fully open inlet guide vanes, the amount of air drawn into the compressor is too large, resulting in low firing temperatures and low efficiencies. If now the inlet guide vanes are throttling the inlet air flow, the firing temperature can be kept on the optimum (high)  level. This results in high efficiency both for the gas turbine and for the steam generator.

The higher efficiency of the steam generator can be explained, looking at the exhaust temperatures.

With smaller mass flow of exhaust gases, but at higher temperature levels, more steam can be generated than with larger flows at lower temperatures.

The following graph shows the difference between fixed (or two position) inlet guide vanes and variable inlet guide vanes. (Refer to graph on the next page).

From the graph it can be seen that at cart load, due to the use of variable inlet guide vanes:

  • The exhaust temperature is higher
  • The air flow is smaller
  • The exhaust energy is larger

The exhaust energy can be defined as the product of mass flow, temperature drop and specific heat of the gases (m. Cp. DT). This energy is recovered from the gases into the steam.

6 . BLEED VALVE (  ANTI-SURGE DEVICE )

THE MAIN CAUSES OF LOW SPEED SURGE (SOMETIMES REFERRED TO AS COMPRESSOR STALL) ARE :

– A restriction in the air inlet or exhaust system, either of which reduces the velocity of the air through the compressor and increases the required pressure ratio which it must maintain.

– A fouled compressor which reduces the ability of the compressor to develop the designed pressure ratio.

– A delay in ignition which causes a sudden increase in combustion pressure, forcing the compressor into surge. Once the compressor is in surge, it will not recover if conditions are restored to normal and, therefore, the gas turbine must be shut down .

– Excessive fuel which causes essentially the same conditions as a delayed ignition but with higher exhaust temperatures.

If low speed compressor surge is experienced, each of the above possible causes should be investigated and the problem corrected.

During the acceleration to full speed , the bleed valve will close if the unit is so equipped. This valve bleeds air from the compressor to keep the compressor out of the surge region during low speed operation. Not all gas turbines require this and there are other methods of performing the same function. However, due to their simplicity, bleed valves are normally used on the smaller gas turbines.

If the bleed valve closes too soon , it may cause high speed surge (50 to 80% speed ) which is characterized by a series of explosion -like reports and a sudden increase in EGT causing an over temperature shutdown . If on the other hand , the bleed valve does not close completely, it will cause abnormally high EGT readings at higher speeds.