| 摘要 |
A very reliable, trouble-free, safety shut-off is provided by a poppit valve arranged so that it can be included in series in a flow line. The head of the poppit is mounted upstream from the seat. It is biased open so that fluid can flow through the line at any rate from zero up to some maximum flow velocity. Flow through the valve is arranged so that it impinges upon the valve head, or something fixed to the valve head, whereby kinetic energy is lost at the head. That kinetic energy is used to move the valve head toward the seat until the spacing between them becomes so small that flow velocity is restricted. At that time, velocity through the valve is increased to the point where a large pressure drop appears across the valve and the static head of the fluid is used to force the valve closed and to maintain it closed. A dash pot develops viscous friction which is used both to damp any tendency of the head to oscillate and to prevent valve operation in response to short term transients or perturbations. Constructing the valve so that it is symmetrical about a center line and constructing it so that approaching flow is directed to the outer peripheral areas of the flow path, and then re-directing flow back into the central area of the flow path through the valve, results in a flow configuration that minimizes criticality in the selection of mass and bias and damping rate values. By confining the dash pot and biasing members such that they are isolated from gas flow, the shut-off is made useful for controlling flow of gasses which carry along fine grains of sand and other particulate material.
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