Pelton Turbine and Specific Speed

Is this about a Pelton-wheel flowmeter?

Yes, it is about Pelton turbine. I mean I tried to make the unit size smaller by increasing no. of units and the size becomes smaller as speed increases. But I ntoiced that the specific speed decreases when I increase my speed from 600 to 625 rpm. Is it OK?

Not sure that I understand your problem, so will just give some information to see if it helps.

Specific speed is the rpm at which an HPRT will run if the wheel diameter is such that at 1 ft differential head it will produce 1 BHP (Brake Horse Power).

Ns is a dimensionless number by which HPRT's (and or pumps) are classified so that you can find the best type of machine for your specific conditions.

Pelton Turbines are impulse type machines that are usually used for relatively high differential pressure and relatively low flow.

The formula for a HPRT (Hydraulic Power Recovery Turbine) is Ns = rpm x BHP^0.5/H^1.25. As you can see, all other things being equal your Ns will increase with increase in speed, not decrease.

Also, it is usual to choose a speed that suits your generator right at the start of the HPRT design.

Hi Kaisan, thanks. It is really informative.

But you know , the BHP will decrease if I make three units instead of two to have smaller sized machine. So I get less BHP, High speed for smaller machine. Instead of specific increasing, It tend decrease here. Does it mean my optimal no. of unit should be two and not three?

Possibly. It becomes an economic/reliability/availability decision. Pelton wheels are very suitable for low specific speeds (they really like the high head), so if you do indeed lower the specific speed of the individual machines by reducing the size and installing 3 instead of two you may gain in efficiency, but you may increase installed capital cost. From a production standpoint and reliability/availability point of view (the two terms not to be confused), losing 1 out of 3 during maintenance/breakdown may be attractive as compared to 1 out of 2 but this is offset by the fact that you have an extra machine to maintain.

You need to do the sums with good quotes and sound technical advice from a reputable OEM and base your decision on the outcome.

Kaisan, thank you so much for your help. That was very clear and informative answer!