Steamblow Out

STEAM BLOW OUTSteam blowing of MS lines/CRH/HRH/SH/RH/Transfer pipe lines of turbine will be carried out in order to remove welding slags, loose foreign materials, iron pieces, rust etc., from the system.

If these materials are not removed, it will cause damage to turbine- chemical regime of steam/water circuit caused damage of tubes- malfunctioning of valves or its failures.

Effect of blowing depends on

1) Thermal shock2) Removal force of steam3) Cleaning force of steam

Removal force of steam-

Blowing mass flow Q flow= 1.25 * Qmcr Vmcr/Vblow Qmc= mass flow at MCR Vmc= velocity at MCRVblow=blowing velocity

Cleaning force of steam

The necessasity to create in the system, the steam velocity greater than that is possible at MCR condition is obvious. These two velocities are expressed as a ratio Cleaning factor or Distribution factor denoted by X.

DISTRIBUTION FACTORa) X=Velocity of steam during blowing/Velocity of steam at MCRb) X=(Steam flow during blowing / Steam flow at MCR) * ( Sp.vol of steam at blowing / Sp.vol of steam at MCR)The recommended value X>1.25 ( between 1.25 and 1.4)

MCR parameters

Steam flow= 690 T/HrMS pressure= 158 KscMST/RH temp= 540 C

Recommended blowing parameters

Dynamic steam pressure= 20 KscMS temp= 380(not to exceed)HRH temp= 450( not to exceed)Steam flow= 250 T/HrCorresponding Drum pr.= 40 KscFurnace load 39%

METHOD:The technique employed is puffing method of steam blowing is to give thermal shock to the contour being purged , to dislodge scale etc., which will subsequently cleared by the expansion of steam.

METHOD: 1) Light up the boiler- Raise the pr. 30 to 35 Ksc.2) Shut off firing.3) Open Electrically Operated Temporary Valve(EOTV) in short time ( 10 sec).4) Thus allowing steam to escape to atmosphere at high velocity carrying all loose debris.

METHOD:5) To prevent undue stress due to sudden temp. variation, pressure drop allowed in drum is limited to ts>40 C max, accordingly EOTV operated.6) Effective blowing is only 15 to 25 min. with an interval of 18 Hrs.7) 6 to 8 blows/ day at interval of 1.5 Hrs.

8) Only one blow out / day follows overnight cooling and recharging the system every day till the complete cleaning ensure effective dislodging of scales and dedris. 9) Roughly around 15 blow outs carried out. METHOD:

TARGET PLATES: Generally SS plates Copper and Aluminium strips also used. Kept perpendicular to the flow of steam. 30*309 mm, thichness 9 mm. Brinell hardness < 90 Steam velocity 258 m/sec Target plates to be introduced just before steam blowing / light upto be removed soon after blowing is completed.

ACCEPTANCE: Completion of blow out is verified with the indendation on target plates provided in temporary piping i.e after ESVs and after Interceptor Valves.At least 3 continuous Target plates should not have indendation of dia.>0.5 mm.Dia. < 0.3 mm --- neglectBetween 0.3 mm and 0.5 mm not more than 10.

SAFETY VALVES

1) Fail safe system- Operating high pressures and safety against pressure rise above the designed limits.2) Safety valves mounted on the boiler pressure parts to safegaurd from being unduly stressed in extream cases of mal operation.3) Increase in pressure will be a possible hazardous explosion.

RELIEF VALVE: The relief valve is a form of safety valve but usually intended for less severe service and of less importance from the safety view point. Relief valve is the automatic relieving device actuated by the static pressure upstream of the valve which opens further with an increase in the pressure over the opening pressure.

SAFETY VALVEThus a safety valve is an automatic pressure relieving device actuated by static upstream of the valve and characterised by the full opening pop action.

POP ACTION:

Pop action is quick opening of the valve.

The area on which steam exerts pressure increases as soon as valve leaves the seat.

TYPES OF SAFETY VALVE:

1) Dead weight SV.2) Lever with weight SV.3) Lever with spring SV.4) Spring loaded SV.

CHATTERING OF A SV:

The rapid opening and closing of the SV is known as chattering.

BLOW DOWN:

The blow down of a SV is the difference between opening pressure and the closing pressure of the SV. The reason for allowing blow down on a SV is to prevent chattering of the valve that would take place if the steam were allowed to escape freely and there were no upward pressure on the spring other than that exerted on the bottom of the valve disc.Blow down ratio = (opening pr.--- closing pr.) / opening pr.Blow down ratio 3.5%

CODES AND ACTS:

1)ASME governing the design, mounting and operation of SV.2)IBR act.

ESSENTIAL REQUIREMENTS: 1)Boiler having more than 500 sq. ft heating area should have atleast two SVs.2)SV should be at the end of SH-O/L header.3)The valve will not obstruct free and full discharge of steam from boiler.4) Relieving capacity should not be less than 100% of max. designed capacity of the boiler. Max peak load evaporation of boiler will be completely discharged with a rise in pr. Of not more than 10% of SV blow off pr.

ESSENTIAL REQUIREMENTS(contd.)5) SV should be without chattering and attain full lift at a pr. not more than 3% above the set blow off pr.6) SV shall shut at a pr. Of a) not more than 5%, b) not less than 2.5% below the SV blow off pr.7) No valve shall be placed between SV and boiler or on discharge pipe between the SV and the atmosphere.

RELIABLE RUN OF A BOILER(OPTIMISATION)1) Heat rate improvememt2) Exit flue gas reduction3) Nox reducton4) Sox reduction5) Reduction in loss of ignition6) CO reduction7) Improved unit availability- less forced outages

RELIABLE RUN OF A BOILER(OPTIMISATION) contd.,8) Clean furnace walls- less slagging9) Reduced SH and RH sprays10) Reduced localised tube over heat, damage and tube failures11) Reduced soot blowing12) Reduction in RSD cleaning13) Reduced Air heater choke.