Iglasti regulacijski zatvarač (“Johnsonov” zatvarač) se koristi kod lučnih brana, kada se mlaz želi točno usmjeriti, bez prskanja. Složene je konstrukcije i vrlo skup, osjetljiv na nanos i kavitaciju na “iglama”. Kontrakcija mlaza iznosi 0,60.
Hollow jet valves are a type of needle valve used for floodgate discharge. A cone and seat are inside a pipe. Water flows through an annular gap between the pipe and cone when it is moved downstream, away from the seat. Ribs support the bulb assembly and supply air for water jet stabilization.


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Note added at 1 hr (2019-11-06 11:10:29 GMT)
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For environment friendly discharge
In hydropower plants, irrigation dams, compensating reservoirs or detention reservoirs, hollow jet valves are used for environment friendly discharge of water downstream or into a tailwater pool while simultaneously enriching the water with oxygen. The hollow jet valves made from high quality steel in combination with a elastic/metal seal and integrated jet deflector facilitate cavitation- and vibration- free energy dissipation. Electric and hydraulic operation allow excellent control performance. Electric heaters for use in cold climates and other specific customer requirements can be implemented. The control system can be designed such that the control room can recognise the cylinder‘s position at all times. This allows a precise control of the discharge volume.

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Note added at 22 hrs (2019-11-07 07:46:36 GMT)
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https://www.eng-tips.com/viewthread.cfm?qid=179755#post-8622...
You may want to evaluate Larner Johnson needle valves or submerged discharge valves, they do come in smaller sizes (DN25 1" is the smallest I've seen) but would be relatively expensive vs conventional gate, ball type valves. Conventially Larner Johnson valves are not normally free dicharge applications, If you want a contact let me know or google Larner Johnson.