Thursday, December 26, 2013

Sequential Throttle Valve - partial arc admission

How does sequential throttle valve governor work? -partial arc admission
- valves lift off their seats in sequence
- valves open in sequential manner
- valves have preset lift clearances between stem nuts and the lift bar so that the length of the clearances are increasing
- valves with smaller clearance will fully open faster than valves with longer lift clearance
- shorter valves are lighter (lesser in weight) so they are raised first and then the longer, heavier valves
- the net effect is that valves are progressively opened to allow a sequential flow from the valves opening in sequenced order beginning with the smallest clearance to the largest clearance
- this design is commonly used for steam chest governor valve

Partial Arc Admission vs. Full Arc Admission - advantages & disadvantages:
- in partial arc admission, the greater the load, more valves will open to admit steam
- in partial arc admission, the lesser the load, fewer valves will open to admit steam
- partial arc admission: disadvantages = uneven heating of high pressure areas of turbine
- partial arc admission: advantages = more control on throttling the valves

- in full arc admission, all valves open at the same time
- in full arc admission, as the load increases, all the valves are more fully opened
- in full arc admission, as the load decreases, all the valves are less opened
- full arc admission: disadvantages = throttling loss in the valves
- full arc admission: advantages = more uniform heating around the high pressure part of turbine

Balance design of throttle governor:
- start with full arc and then switch to partial arc later in the operation process

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