At a Glance of Francis Turbine

In today’s blog, we will discuss Components of Francis Turbine such as Penstock, Spiral casing, Guide vane/ blades, Stay vanes, Draft tube, Runner, Runner blades also the Working of Francis Turbine

Introduction to Francis Turbine

The Francis turbine was invented by James Bicheno Francis, who was a British American civil engineer.

Francis turbine is an inward mixed flow type that works on reaction force mainly for medium head and medium flow power plants.

The flow of water is inward that is going towards the turbine along the radius. In this type, the water flows in a circular manner inwards turbine. When the flow of water is perpendicular to the shaft and parallel, water flow is axial and tangential. In this case, mixed-flow turbines are used.

Francis turbine is a medium head turbine. The medium head is considered when the height of standing water is between 30 meters and 300 meters.

Mechanical power generated by the Francis turbine is due to the velocity of water and the difference in pressure acting on the back and forth of runner buckets.

The advantage of the Francis turbine is that it can be used vertically and horizontally and works the same for both conditions. Primarily we use an arrangement in which water enters radially and leaves axially. This means the input of the Francis turbine can be given radially as well as axially.

The specific speed of the Francis turbine is 200- 400, higher than the Pelton wheel turbine but lower than the Kaplan turbine.

Components of Francis Turbine

Penstock

Penstock is a pipe or a long channel that connects the reservoir to the turbine building. However, it also carries water. In the case of the Francis, turbine penstock is connected to the spiral casing.

Spiral casing

The spiral casing is the outer surface of the turbine whose primary function is to direct the water from penstock around a water turbine and into the rotor.

Construction of spiral casing is done so that the cross-sectional area of spiral casing decreases uniformly around the circumference. This arrangement is because the speed at which water hits the impeller blades should be of uniform velocity.

Now you all must think the logic behind all this flow rate will reduce along the length of casing. Therefore, with the reduced cross-sectional area, uniform water pressure will be maintained, which helps keep the velocity constant.

Guide vane/ blades.

Guide vanes are also known as guide mechanisms.

These are fixed vanes whose primary function is to guide the direction of flow towards the turbine.

It changes the angle between blades so that angle at which water strikes blades can be controlled. However, this regulates the flow of water entering on impeller blades to control turbine performance.

Stay vanes

The primary function of stay vanes is also to guide the water to the rotor blades. Stay vanes mechanism is also stationary.

You all must have thought that if we have the guide vanes, why do we need to stay vanes. As we discussed above, the input water flow for a Francis turbine is radial. However, this radial flow created a vortex( you all have seen the vortex caused by heavy wind flow; same happens in the case of water. In this condition, water flows around the axis line somewhat similar to tropical cyclones.)

To reduce this vortex problem, stay vanes are used. However, this causes the turbine to work more efficiently.

Draft tube

The draft tube is an outlet of the impeller turbine to the tailrace.

After rotating the turbine, the remaining water discharges from the draft tube; due to the draft tube arrangement, the turbine can be placed in a high area so that maintenance and handling can become more accessible.

Construction of the draft tube is such that the cross-sectional area is less at the start, but it starts increasing as we go down toward the tailrace. That means a cross-sectional area of the draft tube increase gradually.

As we know that water requires to flow turbine is of high velocity, and after discharging the water axially, this creases low vapor pressure due to which cavitational problem occurs; this problem is solved with the help of the draft tube

The draft tube has the diffuser’s shape. We all know that the diffuser decreases the velocity and increases water pressure. Therefore with increased pressure, cavitation reduces.

Runner

Runner, it is not a person who runs neither door runner. In the case of a hydroelectric power plant, the runner is of circular shape. The number of blades is mounted with equal spacing.

A runner is the essential part of the turbine, which provides motion when struck by water. It is a part of the turbine where the water energy is transferred into the rotational force that is mechanical energy that drives the generators.

Runner blades

The Francis turbine blades determine the performance of the turbine.

Francis turbine consists of two parts: upper blade and lower blade.

The upper blade rotates when the reaction force of flowing water is exerted. And lower blades are the buckets same as we have seen in the case of Pelton wheel turbine. These buckets rotate under the impulse flow of water. And the runner rotated through these two forces.

Working of Francis Turbine.

Image source: Google| Image By- hkdivedi

Water from the high head through the storage reservoir or dam is conducted through a penstock and provided to the water turbine. This water generally has a higher pressure.

This high-pressure water enters the spiral casing. As I explained its construction earlier, water pressure drops while flowing through the curls. Still, the water speed remains unchanged in this process.

Water passes through the stay vanes and guide mechanism due to the shifts in the angles of blades. Water crosses the blades precisely and strikes the runner through the optimum angle. And the rotor converted water energy into mechanical energy, which is an input for the generator.

The water crosses rotor blades, some of this water loses its whirl force. It falls axially into the draft tube where its velocity decreases due to its construction.

While water passes through the guide vanes and falls into the turbine runner, some water gets diverted. Water is diverted through the runner blades exerts a force that propels the blades to the opposite side, deflecting water. As per Newton’s third law of motion, this is known as reaction force that every action has an equal and opposite reaction. This causes power to be carried from water to the turbine shaft.

This process of altering direction decreases the pressure within the turbine. As the turbine works on reaction force, Francis turbine is also known as reaction turbine.