Guidelines For Fuel Gas Supply To Gas Turbines

Gas turbines (GT) are expensive and a major investment in any oil & gas, refinery, petrochemical or power generation unit. Any breakdown in gas turbines can lead to plant / unit production deferment or loss of power to the power grid if gas turbines are used for electricity genaeration, and spending large sums of money in repairs and overhauling.
 
The basic requirement for any gas turbine is the quality of fuel gas supplied within the design parameters specified for fuel gas for the given gas turbine. Most gas turbine breakdown problems during normal running are associated with large variations in fuel gas parameters from the design parameters as specifiied by the gas turbine manufacturer.
 
Let us come to specifics on what are the basic fuel gas quality parameters which need to be controlled or maintained and what are the means to exercise such control over fuel gas quality. These are listed pointwise below:
 
1. Gas turbines designed for high btu / joule gas (natural gas or high pressure process gas) provide an optimum performance when the calorific value of the gas is high. The efficiency and performance of such turbines is directly affected by the calorific value of the gas.
 
2. Gas turbine manufacturer's specify a minimum fuel gas pressure at their battery limit. In case, the available gas pressure is not sufficient to meet the requirements specified by the GT manufacturer, pressure of the available gas may require to be boosted up using a booster compressor to meet the requirements.
 
3. Fluctuations in the fuel gas Wobbe Index as an indicator of heating value should be kept at a minimum (typically ±5% is acceptable for streams supplying gas turbine fuel). Gas turbine fuel is controlled on a volumetric basis and performance problems can occur when the Wobbe Index fluctuates widely. The gas turbine manufacturer will provide specific guidelines on acceptable Wobbe Index fluctuations. Wider Wobbe Index fluctuations may require fuel gas blending facilities or separate fuel gas manifolds and associated modification to the combustor nozzles.
For definition and values of Wobbe Index refer the link below:
 
http://en.wikipedia....iki/Wobbe_index
 
http://www.engineeri...ndex-d_421.html
 
4. In cases where the fuel gas contains, sulfur compounds (H₂S, mercaptans etc), metals and particulates, extensive gas pre-treatment may be required to reduce them to low-ppm and / or sub-ppm values before allowing them to be used for GTs. The GT manufacturer will specify the maximum allowable values of these unwanted contaminants in the fuel gas.
 
5. Hydrogen content in the fuel can also impact system design. Typically, if the hydrogen content is less than 5% by volume, no special precautions are necessary. For higher values, the GT manufacturer may require an alternate starting fuel as well as design modifications and additional safety devices specific to hydrogen use. Potential swings in hydrogen content in the fuel gas can also impact the design of the combustion controls.
 
6. Fuel gas streams containing olefins should be avoided if possible, especially when specifying gas turbines with Dry Low NOx combustors. Olefins may polymerize and form deposits in small diameter orifices. If fuel gas with olefins cannot be avoided, the manufacturer can provide a design to accommodate the olefins provided the concentration is stable.
 
7. Any liquid and / or solid carryover with fuel gas to GTs can cause flashback and / or plugging of combustor nozzles This is particularly true with dry low NOx combustors.Modern installations for GTs have a Fuel Gas treatment system which is available as a skid mounted package and has several operations to prevent liquid / solid carryover. A typical fuel gas treatment skid would have the following equipment for gas treatment in the order of sequence described below:
 
a.  Provide a Knockout drum (KOD) to remove bulk liquid in the gas as a first step
 
b. Downstream of the KOD provide high efficiency filter-separator to eliminate fine liquid droplets/mist and fine solid particles. Typically the filter-separator can be designed to remove 100% of all liquid droplets above 3 micron & 99.8% of all solid particles less than 3 microns
 
c. Downstream of the filter-separator a gas superheater (electric / water bath etc.) should be provided to ensure that the fuel gas temperature is at least 20°C (36°F) above its hydrocarbon dewpoint. Often, a temperature between 50 to 55°C is maintained which provides an adequate margin over the hydrocarbon dewpoint of the fuel gas.
 
d. Avoid loops or low points on the fuel gas piping running to the GT skid. If loops or low points are unavoidable provide low point drains with traps to prevent accumulation of any liquids.
 
e. Heat tracing of fuel gas piping downstream of the gas superheater may be required to prevent any condensation especially during winter.
 
f. It is a good engineering practice to change the material of construction of the fuel gas piping from carbon steel to stainless steel downstream of the filter-separator to prevent solids (rust) carryover to the GT.
 
The adherence to above guidelines will ensure a long and trouble-free operation of your gas turbine in terms of the fuel gas parameters as provided by the GT manufacturer.
 
Hope readers of my blog find this informative and I look forward to their comments and observations.
 
Regards,
Ankur