Fixed cone (hollow jet) Howell Bunger valves (or equal functioning valves) in smaller and cheaper sizes? Does this solution exist? I have searched the net in vain to try to cover following concept:

Application: bypass of dams and hydroelectrical generators to supply minimum required water to riverbeds, for smaller hydroelectrical plants. From dams/bypasses directly to air, valve as end of line.

Problem is price for smaller diameters and pressures. (See typical data below) Smaller hydroelectrical plants have to keep investments costs down.

The Howell Bunger valves are technically ideal with good capacity and regulating possibility, avoiding cavitation, but found only (?) from DN300 (12 inch ) and up, and at very high cost (adapted, single produced items).

Needle valves (vaned ring type or similar) exists from DN100 (4 inch), some down to DN50, and are generally cheaper (more serial produced) at same size as HBs', even better for regulating, but have lower (too low) capacities at low feeding pressure, and will have to be upsized. Again generally to high in price.

Orifice plates or similar single or in packages will not give necessary regulaing span or accuracy. Other solutions (series of parallell smaller ball valves for outlet, regulating valves) seems all to fall short on some aspect.  

Normal parameters required would be:
For smaller sizes DN150 (mm) (six inches) up to about DN500 (20 inches)
For lower pressure classes, European PN6 up to max PN25 (Equals 30% above ANSI 150)
Static head from say 5 meters (about 16 feet) up to 250 meters (roughly 800 feet).
Flow requirements : varying, from somewhat less than possible at a given diameter at low pressures, to be throtteled down to a reasonable pipeline outlet speed /amount at larger pressures, and able to regulate continously to shutdown.
Thight closure, protected sealing area (long standtime) and lifetime required.

Any ideas anyone? Do I have to start at scratch constructing the thing myself?... I already have some ideas but would be grateful for any input!

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.

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