Exhaust Hood Spray In Steam Turbine

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ok sir i got my answer is that when exhaust temp rises then we will use hoood spray, but i have one doubt is that what is reason for raising exhaust temp at rolling time please explain sir

Dear Sir,

When suppose vaccume inside the condensor is more than 0.13 ata than your exhasut hood temp. will rise due to steam. like 80 deg C spary will start and 120 deg. C turbine trip is there,

low vaccume is the major reaseon.

sometime it may be false, if steam is leaking from seal body, becasue steam will directly hit the themocuples.

but sir i have seen that we pulled vaccum before starting rolling & is near about -0.9 then how we can say is due to low vaccum in condensor.

actually the reason behind temp. rise is that ,due to low vacuum in condenser the stem flow to the condenser decrease and the turbine blades start to agitate in the steam due to agitation of blades the temperature at last stages of turbine increases.

due to this temperature rise spray water injection is applied.

but sir we pulled vaccume before rolling & is near about -0.9 then?