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SAFETY VALVE:-

BOILER ARE TO BE FITTED WITH NOT LESS THAN 2 SAFETY VALVES EACH HAVING A MINIMUM INTERNAL DIAMETER OF 25MM. BUT THOSE HAVING HEATING SURFACE < 50 SQM. , MAY HAVE 1 SAFETY VALVE OF INTERNAL DIAMETER NOT LESS THAN 50MM.

THERE ARE 3 MOST COMMON TYPE OF SAFETY VALVES:-

ORDINARY                                 HIGH LIFT                         IMPROVED HIGH LIFT

1.       WINGED V/V                                WINGED V/V                            WINGLESS V/V
2.     NO STEAM WASTE PISTON   WASTE STEAM PISTON      NO STEAM WASTE PISTON
3.     NO FLOATING RING                   NO FLOATING RING            FLOATING RING
4.         LIFT= D/24                                        LIFT=D/16                              LIFT=D/12

NOTE:- 1. WINGLESS VALVES IMPROVES THE WATE STEAM FLOW AND REDUCES RISK OF SEIZURE.

2. FLOATING RING REDUCES RISK OF SEIZURE.

3. LIP GIVES INCREASED VALVE LIFT.

FOR SUPERHEATED STEAM 

                                           A(S)=A(1+(T(S)/555))

WHERE A(S)= AGGREGATE AREA THROUGH THE SETTING OF VALVES IN MM2 FOR                                     SUPERSATURATED STEAM.

               A= FOR SATURATED STEAM

               T(S)= DEGREE OF SUPERHEAT IN DEGREE CELCIUS.

 

A(S) >A BECAUSE SPECIFIC VOLUME OF STEAM HAS INCREASED WITH INCREASE IN TEMPERATURE AT CONSTANT PRESSURE AND MORE SPACE IS REQUIRED FOR ESCAPE.

AREA OF V/V CHEST CONNECTING NECK TO BOILER=   A/2

AREA OF WASTE STEAM PIPE= 1.1*AREA FOR ORDINARY, HIGH & IMPROVED SAFE V/V

                                                        =2* AREA OF FULL BORE SAFETY V/V

 

ACCUMULATION OF PRESSURE RULE:-

TO RELIEVE THE BOILER OF ALL THE STEAM THEY ARE CAPABLE OF GENERATING WHEN FIRED TO FULL CAPACITY. IN THIS TEST AL STEAM OUTLETS OF BOILER EXCEPT SUCH AS MAY BE NECESSARY TO OPERATE THE BOILER IS SHUT OFF. THE FIRE IS THEN FORCED TO MAXIMUM CAPACITY FOR 15 MIN. FOR FIRE TUBE BOILER AND 7 MIN. FOR WATER TUBE BOILER. DURING THE TEST STEAM PRESSURE SHOULD NOT EXCEED 10% OF MAX. ALLOWABLE WORKING PRESSURE.

BLOWDOWN:- IS THE DIFFERENCE B/W THE SET PRESSURE AND THE PRESSURE AT WHICH VALVE CLOSES AFTER RELEASING EXCESS PRESSURE. VALVE SHOULD BE SET TO LIFT AT 3% ABOVE DESIGNED PRESSURE AND BLOWDOWN % SHOULD BE 4%.

MAXIMUM LIFT OF SAFETY VALVE:-

(PIE* D^2)/4= PIE*D*L

HENCE L=D/4

IMPORTANT NOTE:-

LET US SUPPOSE THAT THE ALLOWABLE WORKING PRESURE OF BOILER BE 15 BAR.

THEN ACCORING TO RULE THE BOILER SHOULD BE SET AT 3% I.E 15.45 BAR MAXIMUM. SINCE THERE ARE 2 SAFETY VALVES NORMALLY HENCE ONE CAN BE SET AT 15.2 WHILE LATER CAN BE SET AT 15.3 LIKEWISE I.E THEY WILL LIFT AT 15.2 AND 15.3 BAR IN SUCH A WAY THAT IT DONOT ACCUMULATE 16.5 I.E 1.5 BAR PRESSURE MORE IN BOILER FOR 15 MIN IN FIRE TUBE AND 7 MIN IN WATER TUBE.

IN WATER TUBE BOILER TIME IS 7 MIN BECAUSE STEAM GENERATION IN WATER TUBE BOILER IS MUCH MORE THAN FIRE TUBE BOILER.