High Hot PA Pressure in XRP 783/803 Coal Mill

Dear all,

Greetings !

We have XRP 783/803 coal mills in our power plant. Those mills are fitted with rotary air vane. Presently we have overhauled 8 mills and converted those into HPMS system where sinter cast inserted grinding elements are in use. Also vanes fitted in rotary air vane segments are backward curved with an angle 45 deg in respect to direction of rotation. Total 8 such mills are converted.Among those 8 mills, in three mills we are facing Hot PA Pressure are running at its limiting zone with 55 % loading and causes air flow restriction. The mill internal measurements are as follows:-

Throat annular opening= 80 mm (previously it was 65mm)

Roll to bullring grinding clearance= 3 mm

Pressure spring preloading= 8 Ton

Outlet venturi collar height= 370 mm

Classifier deflector blade pitch= 30 mm(classifier closed)/ 150 mm(classifier open)

Present operating parameters are as follows:-

Current drawing= 31.5 amp (trip setting 35.6 amp)

Coal loading= 25 Tons/hour-55% loading (max 36.7 Ton/hr)

Primary Air Flow= 48 Ton/hr (max 52.3 Ton/hr)

Hot PA Pressure= 643 mmwc (max 650 mmwc)

Across mill Diff. Pr.= 438 mmwc (max 450 mmwc)

Inlet and outlet temp.= 185/85 deg C

All other HPMS converted mills are working well within parameters. In recent maintenance in those problematic mills we have kept those internal measurements as stated above. Nothing abnormal found in the air duct, its venturi section, mill wind box inlet guide vanes. What are the cause for this problem ? Please suggest.