- Connection size and type
- Operating pressure
- Operating Temperature
- Back pressure
- Required capacity
- Thermal compensation
- Blow down & Operating gap
- The area of the inlet nozzle should be equal to the the inlet area of safety valve & that nozzle should be short as possible
- Inlet nozzle corners must be rounded to a radius of not less than ¼ of the diameter opening
- Safety valve should be installed at least 8D to 10D of pipe diameter down stream from any bend in steam line.
- Safety valves should not be installed closer than 8D to 10D to pipe diameters either upstream or down stream from the diverging or converging “Y” fittings.
- The safety valve nozzle should never be installed in a steam line in a position directly opposite a branch line of equivalent size.
- Exhaust drain & cover plate vent piping must be installed so that they will not impose under pressure on the safety valve.
- Discharge pipe of the safety valve should not be supported on the valve body
- Clearance between the valve exhaust piping and the discharge stack should be sufficient to prevent contact when considering thermal expansion of the boiler valve
Steam flowing vertically out of the discharge elbow produces a downward reaction on the elbow, in proportion on the quantity of steam flowing & its velocity.
- In no case should discharge piping smaller than the outlet valve
- For optimum performance safety valves should be serviced regularly
- Valve assembly should be within 10 vertical alignments.
- Gaskets fitted should be of correct size, should not close the valve inlet opening
- Quantity & quality of the steam
- Discharge piping stress and back pressure
- Variation in ambient temperature
- Improper gagging
- Improper bolting of flanges
- Foreign material in the steam
- Improper method of assembly & disassembly
- Ensure calibrated pressure & temperature gauges are fitted.
- Gauges for each individual valves should be fitted
- Discharge piping has to be inspected for binding on the valves,supports and welds on piping.
- A rope appx. 6-7 meters with a hook one end should be attached to the valve lifting lever before starting the pressure rise. It will help in operating the lever to avoid chattering & over pressure
- Have the correct tooling available
- Establish the good communication system
- If the unit has Electromatic safety valve, this valve should be in operation firts for more safety of the unit.
Drum valves to should be tested first: Possibilities of valve part damage because of GIRL BLASTING are grater on superheated valves in contrast to the drum valves .If super heater valve is gagged after seat damage while testing other valves, the total valve damage will increase.
- Boiler temperature increases during the testing cycle of the Drum valves. Consequently higher temperature steam will be available for super heater steam valves and produce accurate results
- Keep water level low as possible, if drum level is high the safety valves may slugged with water causing long blow down & also may result damages to seat & disc.
- Maintain pressure rising in the range of 2-3 kg/cm2 per minute, slow pressure rising may result into simmering of the valve.
- If fuel feeding system fails at nearer set pressure, then reduce the boiler pressure at least 10% & raise again. Holding the boiler pressure nearer to set pressure for long time may result into simmering & valve lift erratically.
- If a valve has to be lifted several times, cooling off period is very must. Cooling period is around 20-30 minutes.
- If valves have not been tested with hydro test prior to the steam condition, it is recommended to hand lift before steam actuation.
- Normally the highest set pressure valve is the valve floated first. While setting this valve other safety valves are gagged.
- Start the boiler as per cold start up procedure by modulating the firing.
- When the drum pressure reaches about 60–70% of operating pressure gently tighten gage on other safety valve.
- Raise pressure slowly by throttling start up vent valve. When 80% of popping up pressure is reached manually operate the safety valve under test. This will blow off any debris or dust left over in the valve internals.
- Raise the boiler pressure by modulating the firing
- When the pressure reaches nearer to the set pressure close the start up vent. While the safety valve pops (lift), open the start up vent valve and note down the lifting/set pressure value.When the valve sits back, note down the reset pressure
- Control of drum level is important to avoid possibility of water carry over from drum to the super heater.
- The set pressure is adjusted by either tightening or loosening the adjusting nut. Tightening the nut increases the set pressure and vice versa
- Blow down is adjusted by upper rings adjustment.
- After setting the set pressure and blow down, bring down the boiler pressure to operating level.
If a Super heater safety valve lifts at 189.5 kg/cm2 & reseats at 180 kg/cm2 at the temperature of 400 deg c, then calculate the blowdown calculation at 540 deg c
Lift pressure = 189.5 kg/cm2
Reseat pressure =180 kg/cm2
Difference =9.5 kg/cm2
Difference in temperature =540-400 = 140 Deg C
Asper above condition for every 55.6 deg c rise in steam temperature blow down percentage increases by 0.5% of set pressure
140/55.6 X 0.5 X 189.5/100
Hence, blow down at 540 deg c =9.5-2.385 =7.115 kg/cm2
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