Appropriate application of controlled turbine bypass valves

Feb 26, 2008

During recent years, operation requirements for controlled turbine bypass valves and bottom discharge valves have increased continuously. 10 or 15 years ago these valves might have operated „not neatly” under certain opening degrees, i.e., there were cavitation phenomena, vibrations or pulsating discharge of the water. Nowadays, this is not the state of the art any more. These valves must perform neat flow discharge across the whole range of opening, from „Open“ to „Closed“ position and vice versa, without any vibrations nor cavitation.

Requirement for Turbine Bypass Valves
The two main requirements for turbine bypass valves are essentially as follows:
  • Protection of the penstock against excessive pressure in case of turbine failure
  • Controlled water discharge in case of inspection of the turbine or ecological discharge.
 
For protecting the penstock against excessive pressure in case of turbine failure, it is often necessary to open the bypass valves as quickly as possible in order to break the pressure peaks involved.
The actual operating times (Closed-Open) heavily depends on the type of actuator chosen and on the size of the valve. Thus, roughly the following operating times can be achieved:
  • With weight-loaded hydraulic actuator (Figure 2a): more than 5 s
  • With electric actuator (Figure 2b): more than 20 s
  • With hydraulic actuator ( Figure 2c ): less than 5 s.
In case of the controlled water discharge, the required flow rates must be discharged safely and without cavitation. The upstream pressure is sometimes very high, the downstream pressure mostly very low (ambient pressure) and the flow velocities are extremely high (up to 25 m/s).
The following calculation shall give you an idea about the immense quantity of energy which a turbine bypass valve has to discharge – as far as possible without any damages. With reference to the calculation of the output of a hydraulic machine, the „output“ of a bypass valve can be calculated approximately as follows:

Let us assume a plant with a max. output at an opening degree of the valve of about 65 %. At this position, a water flow rate of 6.2 m 3 water per second will be discharged at an upstream pressure of 6 bar. The power is calculated from:

P = p ⋅ Q = 6,0 ⋅ 105 (N/m2) ⋅ 6,2(m3/s)

Which is approximately Pmax = 3720 kW. 
Some „5000 PS“ have to be controlled and converted in a regulated manner in order to assure safe operation of the plant.
For this case of application, the valves most frequently used are:
  • Needle Valves (RKS) (Figure 3a)
  • Fixed Cone Discharge Valves (KAS) (Figure 3b).
In rare cases, also butterfly valves and ball valves are used.
Operation without cavitation
Controlled turbine bypass valves can only operate without cavitation if a sufficient air rate is conducted immediately to the throttling gap of the flow control valve. In this way, the pressure in the throttle gap cannot get lower than the vapour pressure of water, thus avoiding cavitation.
Turbine Bypass Valve as End-of-Line Valve
The safest way of preventing cavitation consists in mounting the valve directly at the end of the pipeline (Figure 4). In this case, no part of the pipe, not even a very short one, is allowed downstream of the valve.
Turbine Bypass Valve for chamber installation
In areas with heavy frost, these valves are normally installed in the valve chamber. That means that nearly always an additional piece of pipe is necessary downstream of the valve as a passageway through the wall structure. Often, in this situation of installation, the water is fed into the stilling basin below the water level.
In these cases, it is absolutely necessary to install an additional air-admission device downstream of the flow control valve (Figure 5). The required air can then be sucked-in through this device. These air-admission devices often are sized one or two nominal sizes larger than the flow control valve. Thus, there will be sufficient space available for the air-water mixture.
Flow discharge characteristics of the Turbine Bypass Valves
The Needle Valves can be designed with different seat geometries. Their influence on the outflow characteristics is considerable.
  • Design with seat ring => hard core jet (Figure 6): the discharged water jet still contains a lot of energy
  • Design with vaned ring => soft, widely open jet (Figure 7) (depending on the jetguiding pipe): large part of the energy will be converted within the outflow jet, thus protecting the stilling basin.
The Fixed Cone Discharge Valves generally have a soft, widely opened jet, which may even be concentrated by a wall pipe or guiding pipe.
Model tests
Model tests may be carried out in order to show problematic operating conditions in terms of pulsations, cavitation, and vibrations, even in partial sections of the opening procedure. These tests make it possible to detect, eliminate or minimise in advance possible hydraulic problems of the valve, the air-admission device and the stilling basin.
Model laws
ERHARD Company uses the model laws of Froude and Euler when carrying out model tests. For the modelling of flow in pipes, the model law of Euler is used:

Eu = p/(ρ ⋅ v2)
That means that irrespective of the scale, the model characteristics can be transferred to the characteristics of the original valve if pressure and flow velocity correspond. This law proves to be extremely practical because it is relatively simple in terms of handling and transfer. The decisive argument for applying this model law, however, is the good compliance – approved over many years – of the results found in model tests with the later characteristics of the original plants.
If, apart from the characteristics of the valve, also the outflow characteristics in the stilling basin are modelled, it is an approved procedure to have two strings to our bow.
The hydraulic characteristics of the bypass valve will be modelled according to Euler. In the stilling basin, however, the forces of gravity and inertia affect the outflow characteristics considerably. Thus, here the model test will be carried out additionally according to the Froude model law.
The characteristic numbers for this model law are calculated according to the following formula:

Fr = [wM / √(LM ⋅ g)] = [wO / √(LO ⋅ g)] ⇒ = wM = wO ⋅ √ (LM/LO)
It is obvious that in this case the hydraulic parameters depend on the selected scale. The flow velocity varies by the factor “root of the scale“.
Experience showed that when applying the Froude model law to the complete system of pipe flow and stilling-basin flow, the pipe flow, particularly in terms of cavitation and the problems resulting thereof, could not be shown in a realistic manner. Therefore, the modelling of a complete plant now is effected in two steps:
  1. Modelling of the pipe flow with the valve, according to Euler.
  2. Modelling of the stilling basin according to Euler and Froude. 
The characteristics of the valves are shown rather realistically according to Euler. However, the outflow into the stilling basin is too intensive under the conditions according to Euler and it is comparatively “harmless” when applying the Froude law. Neither can the effect of air admission (air inflow) be judged in an appropriate manner, since the operating conditions to Froude are in most cases so harmless that neither cavitation nor air admission will occur.
The following approach holds good:

The real outflow into the stilling basin is somewhat between the outflow configuration according to Euler and the one according to Froude and as a tendency it is rather nearer to the modelling according to Euler.
Relatively reliable indications can be obtained concerning the flow rate to be expected as well as the characteristic curve of the valve and plant respectively. These can be well and rather realistically quantified. Likewise the operational characteristics concerning cavitation and air inflow can well be simulated in a model test. However, there are limits. Often indications are required concerning the air inflow rate. This rate cannot be determined in a model test and you can only make qualitative statements. The same applies to indications concerning vibrations and noises. They are often demanded, but in a model test they, too, can only be determined in a qualitative way. In terms of mechanical features, i.e. wall thickness and masses, model and original plants differ. Thus it is not possible to quantify this ratio and to transfer it.
However, if there are vibrations and/or noises during the model test, they will also occur in the original plant. In these cases, the intensity of the vibrations and/or noises will be considerably higher in the original plant than in the model test. Practical experience shows: The smaller the scale, the larger the difference in intensity. Thus, the model test can at least give information in this respect concerning possible occurrence of vibration and noise phenomena.
Comparisons: Model tests – original plants
Fixed Cone Discharge Valve (KAS) DN 800

A precondition for the transferability of the results is the geometrical similarity of model and original plant.
For this purpose it is necessary to form the hydraulically relevant components in the section to be considered in a manner which is true to scale. In the present case, the bypass valve with the adjacent stilling basin has been modelled completely (Figure 8). The model test’s objective consisted in proving the required flow rate in the model test. Furthermore, the outflow into the stilling basin was to be optimized in such a way that there was as little surge as possible during the outflow. This was achieved by changing the built-in components of the stilling basin in an appropriate manner. The air-admission characteristics of this plant have also been investigated and optimised. The results of this investigation are now being considered for any comparative project.
Needle Valve (RKS) DN 900: Feeding into an overflow pipe DN 1600
In this model test, we examined how a Needle Valve DN 900 would behave under the given operating conditions, feeding into an overflow line DN 1600 (open channel) at an angle of 45° (Figure 9). Under the projected arrangement, the Needle Valve cavitated only slightly and an air-admission device for the valve would possibly not have been necessary. However, due to the slight cavitation phenomena in the model test, the original valve was projected in such a way that, if needed, an air admission device for the valve could have been provided later on.
When putting the original plant into operation, the cavitation which was only slightly perceivable in the model test, occurred in a considerably more intensive manner and caused massive vibrations, noise and operational problems.
For that reason, this valve was retrofitted with the already projected and prepared air-admission device. By means of this device, the operational characteristics changed completely and the valve could then be operated without any problems.
All these examples prove that the test results can be transferred to the conditions in the original plants showing good conformity. The guaranteed discharge flow rates (head-loss coefficients) were completely confirmed. Also the qualitative forecasts concerning the operational and air-admission characteristics to be expected were confirmed in the corresponding original plants. These experiences will allow us also in the future to predetermine with good accuracy the operational characteristics of these valves, based on the results of the model tests.

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Dipl.-Ing. Jürgen Heugel (Executive Sales Engineer for Western Europe), ERHARD GmbH & Co. KG

89522 Heidenheim

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