An impulse turbine has one or more fixed nozzles through which pressure is converted to kinetic energy as a liquid jet(s) – typically the liquid is water. The jet(s) impinge on the moving plates of the turbine runner that absorbs virtually all of the moving water's kinetic energy. Impulse turbines are best suited to high-head applications. One definition of an impulse turbine is that there is no change in pressure across the runner.

In practice, the most common impulse turbine is the Pelton wheel shown in the figure below. Its rotor consists of a circular disc with several “buckets” evenly spaced around its periphery. The splitter ridge in the centre of each bucket divides the incoming jet(s) into two equal parts that flow around the inner surface of the bucket. Flow partly fills the buckets and water remains in contact with the air at ambient (or atmospheric) pressure.

Once the free jet has been produced, the water is at atmospheric pressure throughout the turbine. This results in two isolated hydraulic systems: the runner and everything upstream of the nozzle (including the valve, penstock and conduit). Model the penstock independently using regular pipe(s), valve(s) and a valve to atmosphere for the nozzle. Transients occur whenever the valve opens or closes and the penstock must withstand the resulting pressures.