Hydro Turbine – Crossflow – Ossberger

Our Crossflow turbines are always individually modified to local operating conditions (head/flow).

„Simplicity of the principle“, this was the motto for the development of the Crossflow turbine, which is designed for reliable permanent operation for several decades and can be operated without special maintenance means or tools. Even non-professionals are able to install it and put it into operation.

Operating turbine range

• Head height: H = 3… 200 m
• Flow: Q = 0,03… 13 m³/s
• Capacity: N = 5… 3 000 kW

Principle of Ossberger Crossflow Turbine

Crossflow turbines are radial, slightly overpressure turbines with tangential injection of the runner blades and with a horizontal shaft. They rank among low-speed turbines. The water flow comes through an inlet pipe, then it is regulated by guide vanes and finally enters the runner of the turbine. After passing through, the water leaves on the opposite side of the runner, providing so additional efficiency. Finally, it flows from the casing either freely or through a draft tube to a stilling basin under the turbine.

Picture 1: Horizontal inlet

Picture 2 Vertical inlet

In practice, the water flow in the runner provides a self-cleaning effect. Any impurities that are pushed between the blades when water enters the runner are also pulled out by centrifugal force. After a half revolution of the runner, the water takes the impurities out of the runner and flushes them away to the stilling basin.

If the water flow is variable, then the Crossflow turbine is designed with two cells. The standard division of the inlet cells is 1:2. The narrower cell processes small water flow and the wider cell processes medium flow. Both cells together process full flow. With such a division, the water flow is used from 100 to 17 % at optimum efficiency. Thus can Crossflow turbines use even very variable river flows, reaching the efficiency of 80%.

Level of turbine efficiency

The total efficiency of small Crossflow turbines with a small head is 84% throughout the flow. The maximum efficiency of medium and big turbines with a big head, is 87%.

In Picture 3, the advantages of a partially loaded Crossflow turbine are illustrated. River flows are very small for several months of the year. During those months, the ability of a turbine to produce electricity depends on the course of efficiency of the relevant turbine. Turbines reaching high efficiency under normal circumstances, but rather low efficiency during small water flow, achieve lower annual output in places with a variable water flow than turbines with a flat curve of efficiency.

Picture 3 Crossflow turbine curve of efficiency, if the flow is regulated by guide vanes in the proportion of 1:2, compared to a Francis turbine.

Guide vanes

In a split Crossflow turbine, the working water is directed by two force-balanced profile guide vanes. The vanes split the water beam, balance it and let it enter the runner smoothly – independently of the width of the cells. Both rotary guide vanes are set precisely in the turbine casing and they can serve as the closing device of the turbine if there is a lower head. Then it is not necessary to use a shutoff valve between the pressure piping and the turbine. Both guide vanes are fitted independently with extended arms, to which automatic or manual control is connected. Guide vanes are placed in self-lubricating casings and do not require any maintenance. The turbine will be able to close by gravitation in the event of its shutdown, by added weights to the arms ends.

Turbine casing

The casing of a Crossflow turbine is made of structural steel; it is robust, resistant to impacts and frost.

Picture 4 Design of a two-cell Crossflow turbine

Runner

The runner is the most important part of the turbine. It is equipped with blades which are made of polished drawn profile steel by a well-proven method. Both ends are fitted in end discs and welded with inner discs of the runner following a special procedure. Depending on the size, the runner has up to 37 blades. The linearly slanted blades create only slight axial force and therefore reinforced axial bearings with complex fitting and lubrication are not required. Blades of wider runners are supported by multiple discs. Runners are carefully balanced before final installation of the turbine and are subject to crack detection control.

Bearings

Crossflow turbines are equipped with normalized, self-aligning roller bearings having several advantages, such as low rolling resistance and simple maintenance. The design of the bearing housing prevents water leakage into bearings and contact of lubricants with working water. This is an important quality of the patent design of our Crossflow turbine bearing housing. Furthermore, the runner is centred in the turbine casing by means of the bearings. Such an inventive technical solution is completed with maintenance-free sealing elements. Apart from replacement of the lubricant every year, the bearings do not require any maintenance. Moreover, the used technical solution enables simple replacement of the runner without taking the entire turbine out of its position.

Draft Tube

According to the principle, the Crossflow turbine is a free stream turbine. However, in case of medium or low head, a draft tube is required. This ensures that the machine room is secured against flooding and simultaneously the entire head height is utilized. If there is a free-stream turbine with a wide scale of use, the water column in the draft tube must be controllable. This is ensured by a balancing air valve, affecting the under-pressure in the turbine casing. In such a manner, turbines with a suction height from 1 to 3m can be optimally used without any danger of cavitation. Besides, if there is a low head, making the draft tube as a collecting steel pipe reduces the costs of the lower construction considerably. That way many problematic projects can become much less financially demanding.

Operating characteristics

Thanks to the unique design of Crossflow turbines, there is no danger of cavitation and therefore the runner does not have to be placed below the level of ground water. The related costly construction and operating disadvantages are thus avoided.

For a head up to 90m, normal structural steel is used. For a head from 90 to 120m, the runner is made of structural stainless steel. For a head exceeding 120m, the entire turbine is made of stainless steel.

The maximum continuous speed of Crossflow turbines usually reaches a 2.3 multiple of the nominal speed. This fact allows to use mass-produced generators.

Economic advantages

The growing interest of people in environmental protection spurs efforts to search for the ideal use of natural forces, such as the production of electricity from renewable sources. The use of hydropower stations is unfortunately limited by the following significant factors – high installation costs, including design and planning, dimensioning, as well as the production of the required machinery.

Therefore, engineers as well as consultants and designers of turbines have tried to reduce the total costs by standardisation of water turbines. Such approach is feasible with big turbines. On the other hand, it may lead to problems with dimensioning of small turbines, if the projected head and variation of the water flow during the year is considered.

obr.5 Scope of use

Our Crossflow turbines are made of standardized components configured according to customer requirements – i.e. the quantity of water and the head height of the particular location as a whole. Such a modular system provides all designed functions and a good price at the same time.

Crossflow turbines have a long service life and they are maintenance-free. During operation, they do not require any costly or complex spare parts; repairing them is feasible on site. A specific advantage of Crossflow turbines is the possibility of using them in gravitation drinking water systems, even in very long conduits, not causing undesirable hydraulic impacts and thus not affecting the quality of drinking water during operation. This has been successfully tested by our company several times in numerous countries around the world.

For more information, please contact us