Hydrotopics

April 17, 2009

GATE SLOTS FOR HIGH HEAD CONDUITS: PHASES OF DEVELOPMENT

Filed under: Uncategorized — Rajnikant Khatsuria @ 4:04 am

  The most practical application of slots is in high head outlet works. A slot configuration, in general, is shown in figure 1.If the downstream boundary of the slot is sharply cornered, flow separation at the corner creates a negative pressure zone. The pressures in the slot can also be negative if a clockwise vortex is caused in the slot. This is particularly so in the case with gates having skin plate and seal on the upstream side. The implications of these are quite obvious; the pressures can be as low as to cause cavitation damage to slot boundaries and negative pressure zone inside the slot can induce a downward force on the bottom of the partly open gate, viz. downpull.

  flow-around-slot8

  Improvements in slot configurations were aimed at modifying downstream corner so as to eliminate flow separation. Rounding the corners of the slot together with recessing the downstream corner and some part of the conduit wall in continuation of the slot or providing tapered transition there, was effective in reducing magnitude of negative pressures and intensity of slot vortex. Other modification consisted of providing a 5:1 quarter ellipse in continuation of the downstream corner. The improved configurations are shown in figure 2.

improvement-slot-configure4

While the above improvements were adequate for the outlets operating in relatively silt free waters, special modifications were required for the outlets handling sediment laden flows. Slot vortices can cause silt to undergo a circulatory motion in the slot area thereby inducing abrasion damage to slot boundaries. Deposition of silt into the slot can also take place and hamper smooth operation of the gate. Modifications were directed to accomplish slot configurations that would deflect sediment flow away from the slots. This indicated a need to attach some sort of a device that can deflect the flow towards centre of the outlet so that the area in the vicinity of slots remains relatively calm. Two designs were evolved; a configuration involving a slot flow deflector at the upstream corner of the slot, without projecting into the flow, and a simpler arrangement of a deflector projecting into the flow. These are shown in figures 3 and 4.

 

slot-with-deflectors3

offset-projecting-into-the-flow2

The flow conditions in the vicinity of the slots were improved to such an extent that downpull on the gates was reduced to nearly 60% of that without the modifications. However, there still remains a possibility of slight negative pressures on the slot boundaries and some abrasion damage. Therefore, such boundaries are generally provided with steel lining.

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