Posted by : sanjay swain Tuesday, October 8, 2013
BASED ON ABS RULES
NUMBER OF SAFETY VALVES FOR BOILER
Each boiler (including exhaust gas boiler) and steam generator is to be fitted with at least one safety valve and where the water-heating surface is more than 46.5 m2 (500 ft2), two or more safety valves are to be provided. The valves are to be of equal size as far as practicable and their aggregate relieving capacity is not to be less than the evaporating capacity of the boiler under maximum operating conditions.
SAFETY VALVES - SIZE RESTRICTIONS
In no case,
- is the inlet diameter of any safety valve for propulsion boiler and superheaters used to generate steam for main propulsion and other machinery to be less than 38 mm (1.5 in.) nor more than 102 mm (4 in.).
- For auxiliary boilers and exhaust gas economizers, the inlet diameter of the safety valve must not be less than 19 mm (3/4 in.) nor more than 102 mm (4 in.).
SAFETY VALVES - FOR SUPERHEATER & ECONOMIZER
Each superheater, regardless of whether it can be isolated from the boiler or not, is to be fitted with at least one safety valve on the superheater outlet.
Each economizer, where fitted with a bypass, is to be provided with a sentinel relief valve, unless the bypass arrangement will prevent a buildup of pressure in the economizer when it is
MINIMUM RELIEVING CAPACITY FOR BOILER SAFETY VALVE
In all cases, the safety-valve relieving capacity is to be determined on the basis of the boiler heating surface and water-wall heating surface along with the fuel-burning equipment, and is not to be less than that given in the table (see later).
Where certification by the boiler manufacturer of the evaporative capacity of the boiler under maximum operating conditions indicates a higher capacity, the higher capacity is to be used.
MINIMUM RELIEVING CAPACITY FOR SUPERHEATER SAFETY VALVE
Where a superheater is fitted as an integral part of a boiler with no intervening valve between the superheater and the boiler, the relieving capacity of the superheater safety valve, based on the reduced pressure, may be included in determining the total relieving capacity of the safety valves for the boiler as a whole.
In such a case, the relieving capacity of the superheater safety valve is not to be credited for more than 25% of the total capacity required.
The safety valves are to be so set and proportioned that, under any relieving condition, sufficient steam will pass through the superheater to prevent overheating the superheater.
Specially designed full-flow superheater valves, pilot-operated from the steam drum, may be used.
PRESSURE RISE DURING RELIEVING
For each boiler, the total capacity of the installed safety valves is to be such that the valves will discharge all steam that can be generated by the boiler without allowing the pressure to rise more than 6% above the maximum allowable working pressure.
PRESSURE SETTING - BOILER DRUMS
At least one safety valve on the boiler drum is to be set at or below the maximum allowable working pressure.
If more than one safety valve is installed, the highest setting among the safety valves is not to exceed the maximum allowable working pressure by more than 3%.
The range of pressure settings of all the drum safety valves is not to exceed 10% of the highest pressure to which any safety valve is set In no case is the relief pressure to be greater than the design pressure of the steam piping or that of the machinery connected to the boiler plus the pressure drop in the steam piping.
PRESSURE SETTING – SUPERHEATERS
Where a superheater is fitted, the superheater safety valve is to be set to relieve at a pressure no greater than the design pressure of the steam piping or the design pressure of the machinery connected to the superheater plus pressure drop in the steam piping.
In no case is the superheater safety valve to be set at a pressure greater than the design pressure of the superheater.
In connection with the superheater, the safety valves on the boiler drum are to be set at a pressure not less than the superheater-valve setting plus 0.34 bar (0.35 kgf/cm2, 5 psi), plus approximately the normal-load pressure drop through the superheater.
SAFETY VALVE - EASING GEAR
Each boiler and superheater safety valve is to be fitted with an efficient mechanical means by which the valve disc may be positively lifted from its seat.
This mechanism is to be so arranged that the valves may be safely operated from the boiler room or machinery space platforms, either by hand or by any approved power arrangement.
SAFETY VALVE - CONNECTION TO BOILER & SUPERHEATER
Safety valves are to be connected directly to the boiler, except that they may be mounted on a common fitting.
However, they are not to be mounted on the same fitting as that for the main or auxiliary steam outlet.
This does not apply to superheater safety valves, which may be mounted on the fitting for the superheater steam outlet.
SAFETY VALVE - ESCAPE PIPE
The area of the escape pipe is to be at least equal to the combined outlet area of all of the safety valves discharging into it.
The pipe is to be so routed as to prevent the accumulation of condensate and is to be so supported that the body of the safety valve is not subjected to undue load or moment.
SAFETY VALVE - DRAIN PIPE
Safety valve chests are to be fitted with drain pipes leading to the bilges or a suitable tank.
No valve or cock is to be fitted in the drain pipe.
SAFETY VALVE - PRESSURE ACCUMULATION TEST
Safety valves are to be set under steam and tested with pressure accumulation tests in the presence of the Surveyor.
The boiler pressure is not to rise more than 6% above the maximum allowable working pressure when the steam stop valve is closed under full firing condition for a duration of 15 minutes for firetube boilers and 7 minutes for watertube boilers.
During this test, no more feed water is to be supplied than that necessary to maintain a safe working water level.
The popping point of each safety valve is not to be more than 3% above its set pressure.
SAFETY VALVE - PRESSURE ACCUMULATION TEST (WAIVER)
Where such accumulation tests are impractical because of superheater design, an application to omit such tests may be approved, provided the following are complied with:
- All safety valves are to be set in the presence of the Surveyor.
- Capacity tests have been completed in the presence of the Surveyor on each valve type.
- The valve manufacturer supplies a certificate for each safety valve stating its capacity at the maximum allowable working pressure and temperature of the boiler.
- The boiler manufacturer supplies a certificate stating the maximum evaporation of the boiler.
- Due consideration is given to back pressure in the safety valve steam escape pipe.
CHANGES IN SAFETY VALVE SETTING
Where, for any reason, the maximum allowable working pressure is lower than that for which the boiler and safety valves were originally designed, the relieving capacity of the valves under lower pressure is to be checked against the evaporating capacity of the boiler.
For this purpose, a guarantee from the manufacturer that the valve capacity is sufficient for the new conditions is to be submitted for approval, or it is to be demonstrated by a pressure accumulation test, conducted in the presence of a Surveyor.