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Improved High Lift Safety Valve (for pressure < 25 bar)

This design originated from the ordinary spring-loaded valve and has the following features indifference to the ordinary spring-loaded valve.

  1. The valve has no wings and the problem from correct maintenance of wing clearance and valve seizure is eliminated; as a matter of fact, the shape helps in a guided flow of waste steam which further improves the lift. Problems with uneven thermal distortion of the valve lid due to the irregularly shaped wing are also removed.
  2. The valve uses a waste steam piston. The piston connected to the valve spindle is acted upon by the waste steam pressure and aids the lift of the valve. The waste steam which is detrimental to the lift of an ordinary safety valve is made 1:0 to assist the lift here.
  3. A floating ring is used inside which the waste steam piston moves — the loose ring reduces the risk of seizure.

 

Procedure to Set an Improved High Lift Safety Valve 

  1. Consider that the valves have been assembled correctly without the top hood and the easing gear. Check the drains and waste steam pipes and make sure that those are clear.
  2. Steam pressure is raised and the boiler is put on light load if required to maintain steady pressure during the setting process.
  3. Screw down each compression nut a few more turns than the previous setting.
  4. Raise the boiler pressure and maintain the blow-off pressure at which the safety valves are required to be set.
  5. Adjust one valve at a time. Hold the slotted spindle in position by inserting the other end of a file into the slot which prevents possible turning of the spindle; now keep slackening the compression nut slowly till the valve on the test lifts under the set steam pressure.
  6. Stop firing immediately as soon as the valve lifts. Screw down the compression nut a little and keep tapping the spindle top softly for the valve to sit back smartly and remain seated in place. This pressure would be slightly less than the lifting pressure.
  7. Restore the water level if necessary and start raising the boiler pressure back.
  8. Try out again for lifting and see if the pressures at which the valve is lifting and sitting back are correct; if not, reset again with the same
  9. procedure. The valve disc should not be turned on the seat. So, the lifting pressure should be adjusted when the valve is blowing or remains within 5% of the actual popping pressure, that is, staying within the simmering range.
  10. The setting is done with a bit of trial and error procedure but with some practice can be achieved fairly quickly.
  11. The difference between the lifting boiler pressure and the pressure at which the valve closes is called the “blowdown”. Too much blowdown causes wastage of steam, too less would give an unstable con condition and it should be limited from a minimum of 1 to a maximum of 44 of the blow-off pressure. If the blowdown is not correct, it is possible that the valve is sticking somewhere, the spindle may be slightly bent, the clearances are incorrect or a weak spring is in use.
  12. If all parts are correctly assembled, the right spring is used and all the clearances are right, the valve should not have an unusual blowdown, and the minimum blowdown (ASME code) should be not less than 2 psi (14 K Pa).
  13. Once set, the valve adjusted, should be gagged (by means of a crossbar clamp, bearing puller, etc) and the other valve should now be set and adjusted with the same procedure as the 1st one. The gag should only be finger-tight on the spindle and should never be fitted when the boiler is cold to avoid any chance of the spindle bending due to thermal expansion as the temperature rises.
  14. The gag from the 1st valve is now removed.
  15. Distance from the bottom of the compression nut to the top of the safety valve cover bushes is measured by a caliper and a split ring is made and fitted in that space.
  16. Next comes the hood which is locked to the spindle with a cotter key the easing gear is now fitted in place.
  17. Safety valves are now manually lifted by means of the easing gear to ensure their correct operation.
  18. One end of the hood cotter key is then locked by a padlock.
  19. For safety valve setting, the surveyor brings along his pressure gauge this helps in checking whether the boiler-attached pressure gauge is defective or not.
  20. During the s/v setting under steam, the surveyor also checks the operation of the gauge—glass by blowing through to his satisfaction.

For Advanced Read, check out:

Boiler Operation and Maintenance

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