CoC Oral Exam Preparation (Part – 17): Boiler Safety Valve
Boiler Safety Valves protect the boiler from over pressurisation. As per the requirements, at least two safety valves should be fitted to the boiler and both are mounted on a common manifold with a single connection to the boiler. Boiler with super heater, normally three safety valves are fitted; two to the boiler drum and one to the superheater. The superheater must be set to lift first to ensure a flow of steam through the superheater.
Improved High Lift Boiler Safety Valve:
Fig: Improved Highlift Boiler Safety Valve
The sketch shown is improve high lift safety valve . The are usually mounted 2 Nos. on a single chest. Valve , seat , spindle , compression screw and bush are made of non-corroded metal and valve chest is made of cast steel.
This valve improve than other type as because:
- Using wingless valve to improve steam flow
- Floating cylinder arrangement can prevent piston seizure.
The special shaped valve and seat deflect steam toward the lips on the valve and increase valve lift.This action also achieves the valve to lift and shut smartly at the blow off pressure. It is set to open at 3 % above working pressure. The lift of valve is one twelfth of the valve diameter.
A ported guide plate fitting adequately guide the spindle itself and allow the waste steam to the under side of the piston with pressure and gives increased valve lift. Waste steam pressure also keeps the floating cylinder in place while the piston moves. So floating cylinder seizure risk is reduced.
A drainpipe is fitted to the lowest part of the valve chest on the waste steam discharge side and lead to clearly drain, no valve or cock fitted through its length. This drain is important to be checked regularly. If it is choked, there is a possibility of overload to valve , due to hydraulic head and damage results by water hammer.
Fig: Boiler High Lift Safety Valve
Close examination and attention during overhauling
- Check valve and seat for wear, cavity corrosion , pitting and any fault. They must be ground in properly not to excess maker’s limited dimensions and clearances.
- Valve chest must be cleaned condition and drain line clear
- Hammer test to spindle for any crack and check its straightness
- Hammer test to compression spring for any fracture and check for corrosion. Free length is limited to 0.5 % of original free length.
- Check the guide plates and bushes for uneven wear and have sufficient clearance to allow free movement of
- Check compression nut and cover bush’s threads and any sign of wear and tear.
- Check the easing gear , cable , pulley and connecting links.
- Connecting pin should be a free fit in the lid and no bending. Pinhole to be regular shape.
Pressure setting of safety valve
- Take standard pressure gauge
- Shut boiler main stop valve and feed check valves
- Check waster steam pipe and drain line clear
- Ensure the correct assemble of valves with no hood and easing gear
- Keep the boiler under full fire condition with adequate water level visible in the sight glass.
- Bring up the boiler pressure to the required blow off pressure, 3 % above the approved working pressure.
- Screw down the compression nuts of any lifting valves until all quiet.
- Then adjust each valve in turn by slacking of its compression nut until its valve lift.
- Subsequently screw down the compression nut while lightly tapping its Spindle , valve will return to its seat and remain properly seated.
- Measure gap between compression nut and spring casing cover. Make compression ring with sane thickness to this gap measurement and insert under the compression nut.
- Retest the valves lift and close together.
- Then , fit the cap , cotter pin and easing gear
- Cap and cotter to be pad locked to prevent accidental alternation of setting.
- Test the safety valve with easing gear to satisfaction for emergency .
Accumulating pressure test
The accumulating pressure test is done to limit the excessive pressure rising in boiler while the safety valve is open, cause further compression of spring due to increased loading. .
The test is carried out, on the new boilers or new safety valves , under full firing conditions, with feed water and steam main stop valve is closed. The test is continued for as long as the water in the boiler permits but it need 15 minutes for a tank type boiler and 7 minutes for a water tube boiler. With the safety valves operating, accumulation must not exceed 10% of the working pressure.