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.
- 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.
- 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.
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.
- Needle Valves (RKS) (Figure 3a)
- Fixed Cone Discharge Valves (KAS) (Figure 3b).
- 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.
Eu = p/(ρ ⋅ v2)
Fr = [wM / √(LM ⋅ g)] = [wO / √(LO ⋅ g)] ⇒ = wM = wO ⋅ √ (LM/LO)
- Modelling of the pipe flow with the valve, according to Euler.
- Modelling of the stilling basin according to Euler and Froude.
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.
A precondition for the transferability of the results is the geometrical similarity of model and original plant.
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