Answer EKG Question 34

Q34. With reference to auxiliary boiler safety valve:
A. Describe, with aid of a Sketch the safety valves for an auxiliary boiler;
B. Identify, with reasons, the part that require particularly close attention during overhaul;

C. Describe how the safety valves are reset after an overhaul.

Answer:
A. Describe, with aid of a Sketch the safety valves for an auxiliary boiler.

 

Valve lift: - Applicable for the flat or mitered face 45° valve. Area of valve bore = area of escape i.e the valve circumference X lift; Neglecting area taken by the valve prongs. To give greater lift than D/4, is unnecessary, this gives the full equivalent area. Excessive lift results in shock or hammering thus the fatigue will take place. Also fracture can take place because of flattering if the lift is excess. the lift of improved high lift safety valve is D/12, while it is D/24 for high lift and D/36 for ordinary safety valve. Blow down of a safety valve is the difference between lifting pressure and seating pressure. Improved high lift safety valve:- It is a spring loaded valve. Spring which produces the load is fitted under the cover with a compression nut, so that any degree of compression can be given to suit the pressure to which valve is to be loaded. On top a cap or hood, through which a cotter is passing. It is locked with pad lock so that once surveyor has set the compression it cannot be tempered with. The cotter has a clearance under it equals to 1/4th of the valve diameter, giving the valve sufficient lift to pass the maximum area of the steam escaping.

A clearance on the top ensures the valve is bedding on the seat. A clearance between the hood and spindle top is provided more than 1/4th of valve diameter. In this type waste steam assists the main boiler pressure in compressing the spring. Giving the valve more lift. While in a simple spring loaded valve the waste steam pressure acts downward on the valve lid and retard the efficient operation by restricting the valve lift. The loose floating ring surrounds an extension of the bottom spring cover, which carried the spring so forming a gland. The ring is held down by the waste steam acting on the annular area. Any waste steam which does pass the floating ring will escape to the atmosphere through the relief port in the spring casing. Drains are provided so that any condensate form in the spring casing will drain into the valve chest leads to hot well. The valve lid is constructed with a blow down ring of greater diameter. Just above the seat which provide a greater area for the steam pressure to act upon. When valve open from its seat, the steam pressure acts on the extended area causing the valve to pop open. If the valve is allowed to close slowly, the final thin film of the steam blowing across the faces causes feathering condition. Also this steam flow at very high speed cause quick cutting of the faces also called wire drawing of the faces of seat lid or disc. Feathering will result in falling of boiler pressure well below the boiler blow off pressure before the valve finally closes. The shaped valve seat causes the valve seat to pulsate and this dribbling action enables the valve to close clearly and sharply with very little blow off effect. This shaped valve makes a form of lips. The clearance between is called lip clearance. With decrease in lip clearance the amount of blow down increases and vice versa. The width of bearing face of the valve and seat should not exceed 1.6mm. Lip clearance should maintain between 3.2-4.8mm. which de[pends on the boiler pressure.

B. Identify, with reasons, the part that require particularly close attention during overhaul; Overhauling The following part should be examined during overhauling or survey . Valves or seat maybe marked or warn valve should ground in by means of a jig (since valve lid is wingless) taking care of makers specified clearance. valve wings if fitted must be at least 0.8 mm clear in the seat and must not project below the seat. The seat must be a good fit and securely held by set pins. Inside of chest must be free from corrosion and any scale adhering removed. drain should be examined to see that it is clear. the spring should be hammer tested to make sure that it is not fractured and inspected for corrosion. The spring should length should be tested against a new spare. the end of coil should be good fit in the recessed collar. the valve spindle should be hammer tested ensure it is not cracked and check for alignment. clearance between the top of the cottor and top of the slot in the spindle to be checked after grinding a safety valve. All bushes must be secured in the housing and have sufficient clearance. compression screw should be checked for tightness on the threads. The easing gear must be checked to see that it is in good order. After carrying out above inspection and adjustments the parts are assembled and valve made ready for setting to the required. 

C. Describe how the safety valves are reset after an overhaul. pressure setting of safety valve: Boiler safety valves are set to the required pressure using a standard tested certified pressure gauge as to its accuracy. A surveyor is is present to see the test and issue certificate. Block one safety valve by gagging tool with little wrist tight. Ignite the burner and increase pressure with low flame until non block safety valve blow off. Shut the burner immediately adjust the blow off pressure and repeat until correct blow off pressure obtained. adjust the second safety valve as described above setting tolerance of +/- 3% allowed by classification society. The compression ring is then machined and fitted in place and locked by Surveyor seal.