Exam Question
Sketch a high-lift safety-valve lid and seat detailing their special features. Describe how such a valve is overhauled and any clearances that should be measured and noted. (16)
Class 2 (2nd Engineer)
MEKG
📅 Dec 2024
Reference Answer
### Special Features of a High-Lift Safety Valve Lid and Seat
A high-lift safety valve is designed to achieve a rapid, full opening at the set pressure, allowing for a large discharge capacity and preventing a significant rise in boiler pressure. The special features of the valve lid (disc) and seat are critical to this function.
1. **Reaction Lip/Annular Ring**: The valve lid is designed with an extended lip or skirt which overhangs the valve seat. When the valve begins to open (lifts), a small amount of steam escapes.
2. **Huddling Chamber and Deflection**: The valve seat is profiled to deflect this escaping steam outwards. The steam then impinges on the underside of the valve lid's lip. This creates an immediate, large-area pressure field, resulting in a significant additional lifting force. This sudden increase in force causes the valve to 'pop' open to its full lift rapidly.
A high-lift safety valve is designed to achieve a rapid, full opening at the set pressure, allowing for a large discharge capacity and preventing a significant rise in boiler pressure. The special features of the valve lid (disc) and seat are critical to this function.
1. **Reaction Lip/Annular Ring**: The valve lid is designed with an extended lip or skirt which overhangs the valve seat. When the valve begins to open (lifts), a small amount of steam escapes.
2. **Huddling Chamber and Deflection**: The valve seat is profiled to deflect this escaping steam outwards. The steam then impinges on the underside of the valve lid's lip. This creates an immediate, large-area pressure field, resulting in a significant additional lifting force. This sudden increase in force causes the valve to 'pop' open to its full lift rapidly.
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