cut back speed

Hello forum pleas help me with this thread
What is the essence of cut back speed in gas turbine during startup i.e. from 25% speed to 15% speed for firing.
thanks for your usual cooperation.

Generally, this is done to lessen the time required to purge the gas turbine and exhaust (including the HRSG ("boiler") and exhaust stack) of combustible gases during starting and before enabling the ignitors ("spark plugs") and admitting fuel. The purging portion of the starting sequence is performed at a higher speed than firing, because there is more air movement when the axial compressor is spinning faster. (Purging is required by most insurance companies at a minimum, and by most original equipment manufacturers to prevent explosions due to the presence of combustible gases in the turbine and/or exhaust. The typical purging requirement is three to five complete exchanges of air in the axial compressor, turbine and exhaust (including the HSRG and exhaust stack. The amount of time is calculated, usually, by the turbine supplier based on the volumes of the exhaust (including the HRSG and exhaust stack).) Since this is usually a requirement (and very good safety practice) and not subject to modification or change purging at a higher speed to shorten the purge time can be very advantageous in getting the unit started and producing power faster.

After the purge sequence is completed, the unit is allowed to coast down to a lower speed level for firing. This is done to reduce the amount of fuel required for starting--the main intent of which is to reduce the thermal stresses in the combustor and turbine and exhaust when establishing flame, thereby prolonging the life of the hot gas path parts.

Hope this helps!

By the way, many sites have worked with the supplier of the exhaust system (including the HRSG and exhaust stack) and the turbine manufacturer to verify the purge time calculation in order to possibly reduce the time required to complete the purge and establish flame. Some sites have even eliminated the reduction in speed for firing--recognizing that there is probably some reduction of hot gas path parts life because of increased thermal stresses caused by the additional fuel required to establish flame at a higher speed. For some sites, the reduced parts life (and increased maintenance costs associated with reduced parts life) is acceptable (based on their spreadsheet analysis)--they can get on line faster and get more money for the power they are able to produce in a shorter period of time than previously possible, and maybe even faster than a nearby power plant--which means they might "win" the dispatch to start and earn money.

It should be said, also, that a couple of sites have learned their current purge time was insufficient and had to decide to extend their purge time or not to meet their insurance company's mininum purge requirement. So, it's not always good news--though it often is.