Modelling of subsurface vortex vibrations

Transcription

1 Modelling of subsurface vortex vibrations S. J. van Vuuren University of Pretoria, Department Civil and Biosystems Engineering, South Apica Abstract During 2001 the Paris Dam (South Africa) was constructed in the Buvaan River to augment the flow for the extension of irrigation downstream from the dam. The dam has a height of 42 m, and an active storage capacity of 80 * 106 m3. The wall is a double arch concrete structure. During the first filling in February 2001, the water level was lowered by releasing water through the bottom outlets to allow access for the last construction work to be completed. Water was released through the two 1200 mm diameter outlets, controlled by a sleeve valve at the end of the line. In the outlet structure, water entered the outlet works through the bottom sluice gate (4 m2), while the gates at the upper two levels were closed. In the outlet pipe, just upstream from the sleeve valves, full-bore butterfly valves were installed as maintenance valves for the sleeve valves. Unfavourable inlet conditions resulted in subsurface vortices leading to vibrations in the outlet pipe with extensive pressure fluctuation across the discs of the butterfly valves. These valves failed after a short period of time and the outlet control chamber was flooded. The outlet structure was modeled to an undistorted scale of 1:15 (Froude uniformity) and the pressure fluctuation was measured with (absolute) pressure transducers at a number of positions along the outlet pipe. Changes to the outlet were incorporated and the reduction on the pressure fluctuations was measured. Within the constraints of the existing structure, alterations were implemented and modeled, succeeding in eliminating the creation of the subsurface vortex and unwanted pressure fluctuations. Model investigation provided a valuable contribution to quantify and solve the fluid structure problem associated with the creation of the subsurface vortex. The changed developed in the model study was incorporated in the prototype. Physical vibration measurements on the structure indicated that the improvements to the structure reduced the vibrations.

2 354 Fluid Structure Interaction 11 1 Introduction During the first filling (February 2001) of Paris Dam, constructed for Impala Irrigation Board in the Buvaan River near Vryheid - South Africa, water was released through all the bottom outlets with the objective to reduce the water level in the dam and hence minimize potential interference with the final construction works. The release of water through the four bottom outlets (2x1,2 m diameter; 1x600 mm diameter and 1x200 mm diameter) resulted in extensive vibrations that were felt on the bridge between the wall and the intake structure. This aspect was investigated [l] and it was concluded that the origin of the vibrations are at the bottom outlets and are of a "high frequency". On Thursday, 20 February 2001, it was reported that the butterfly valve that was installed as a service valve upstream of the sleeve valve, had failed and that the valve chamber had been flooded (Photograph l). 2 System layout Figure l provides the plan layout view of the pipe work and Figure 2 is a section through the outlet structure that reflects the different levels of the sluice gates and the outlet pipe. The positioning and layout of the outlet pipe are favourable for the creation of subsurface vortices due to the vertical step in front of the intake. Vischer and Hager have indicated the disadvantage of such a layout [2]. The creation of the subsurface vortices was identified as a contributing factor to the creation of unstable flow, which contributed to pressure fluctuations at the valve. These pressure fluctuations probably led to the excessive vibration in the structure. Figure 1 : Plan view and sections of the pipe layout for Paris Dam

3 Fluid Structure Interaction I1 355 Figure 2 : Section CC through the intake structure indicating the different levels of the eates 2WO 158W I l7800 ZOOMED SECTION C-C Figure 3: Clockwise flow rotation at the inlet pipe (Section through the Inlet Structure)

4 356 Fluid Structure Interaction 11 Vortex formation, resulting from the swirl in pump stations, has been investigated in detail [3]. Newman [4] has indicated that at a pump inlet, the recirculation flow of a pump operating at the minimum flow is defined by: with : dh~"", V V dv = -(- + -) dr g r dr dhtu& = Total energy variation V - Mean velocity g - gravitational acceleration and r - the radius from the centre of the rotational flow dr = change in the radius dv = change in velocity, V By assuming that the energy is constant along a concentric circumference of a rotational flow it results in: from which it can be shown that : Through integration it follows that: In V + In r = constant or Vr = c, with c = constant This relationship is similar to that of a free vortex [5]. Along a streamline the total energy is constant (Bernoulli's equation) and can be written as follows: v - + y + z = constant 2g

5 Fluid Structure Interaction by substituting C V = - in Bernoulli's equation it follows that r On the horizontal axis (Figure 3), dh,,,, is a constant and z = 0, hence as r decreases (eqn 5) the pressure, represented by y, has to drop. The sub atmospheric pressure that can result from the highly rotational flow in the vortex will be limited to vapour pressure. 3 Physical model investigation of the origin of the vibrations An undistorted model of the intake structure, based on Froude uniformity and to a scale of 1 : 15 was constructed with the purpose of evaluating the flow and the creation of subsurface vortices and to model alterations that could improve the flow conditions through the intake structure. The intake shaft and outlet pipes were constructed in Perspex to allow unrestricted visual inspection of the flow patterns. Photographs 2 and 3 reflect the layout of the model that was constructed of the intake tower and the bottom outlet at RL 702 m. The flowrate through the model and the upstream water level was controlled by throttling the flow into the "stilling basin tank" upstream from the intake tower or by releasing water from the "stilling basin tank" upstream of the intake tower. Provision was made to record the pressures variations downstream from the inlet on the outlet pipe. The pressure recordings were used in conjunction with the visual recordings of the flow patterns, to compare the performance of the alternative intake arrangements. The results from the alternative layouts were compared, with the performance of the original layout (as-build layout). Photograph 3 reflects the 5 positions where the pressures were recorded. The initial tests conducted on the original layout (as-build) indicated the formation of a strong clockwise horizontal vortex at the inlet, stretching from the outward wall of the intake structure all the way through the outlet pipe. The big downward step (2,O m) (Figure 2) on the upstream side of the inlet pipe created the breakaway flow, which initiates the vortex. The vortexes were prominent at all the tested water levels (708 to 732 m) that were tested and even occurred at low flow rates when water was released through the small outlet (600 mm diameter - low irrigation demand outlet). These observations made it apparent that the flow conditions at the inlet should be improved.

6 358 Fluid Structure Interaction 11 Alternatives that were identified to improve the inlet conditions were: The improvement of the flow pattern by the backward movement of the step (Impractical - not modelled). Changes to the form of the step (Impractical - not modelled). Inclusion of directional vaneslflow directions into the inlet pipe (not modelled). Changes to the inlet configuration of the inlet - implementing a boxed inlet for dissipating the vortex (not modelled). Introducing a deflectorbaffle plate that will force some of the flow upwards and hence create an anti-clockwise vortex - Alternative layout A2. Introducing a deflector block that will force some of the flow upwards and hence create an anti-clockwise vortex - Alternative layout A3 Changing the inlet from a horizontal intake to a vertical drop inlet - Alternative layout A4. Improvements to the vertical drop inlet, Alternative 4, by adding a deflector plate on the end wall - Alternative layout A5. The flow released through the different outlets and the water levels in the intake structure, were altered and tested for the alternative inlet configurations (A2 to A5) that were tested. In all the tests the following data was captured: Water level in the intake tower Flow rate and velocity in the outlet pipe Pressure fluctuations on the outlet pipe Visual recording of flow patterns in the intake tower and the outlet pipes. Values for the variables were recorded in the time domain for all the alternatives (A1 to A5) that were modelled. No detailed recordings were made in the frequency domain, although dominant frequencies (20 and 120 Hz) were recorded with a handheld seismic accelerometer. Due to space limitations, only the results from alternative A1 (original design) and A5 (final proposed alteration) for the water levels indicated in Table 1 are reflected below. Table 1 : Results that are included in this report l I I I Description of the intake layout 1 Original design (Al) I Not included- See I Vertical drop inlet with deflector plate on the end wall (A5) I Water levels analysed 730 I Photographs Figure 2

7 Fluid Structure Interaction I1 359 Table 2 provides the results of the different layouts (A1 and A5) for the water level at 7 16 and 730 m (see page 360). The pressure variance that has been measured in the model is a reflection of the turbulence that occurred. Table 3 reflects the calculated value for a (velocity factor in the Bernoulli equation) for layouts A1 and A5. Table 3 : The extend in the turbulence variance in the outlet pipe at the measuring Point 4 of the model for an approximate flow of 15,3 11s and a water level of 730 m Description of the intake layout Original design (Option A l) Vertical drop inlet with deflector plate on the end wall (Option 5) Table 3 indicates how the improved layout (A5) reduces the pressure fluctuations (swirl or vortex formation). 4 Conclusion Model Pressure variation (m) 0, ,04407 Extrapolated pressure variation in the prototype (m) 1,1043 0, Calculated value for a 1,1559 1,0933 The experimental investigation showed the presence of a strong subsurface vortex that propagated through the whole outlet pipe. The vortex swirled down the outlet line in a corkscrew form. The erratic position of the vortex created pressure fluctuations across the disc of the butterfly valve. These pressure fluctuations will create vibrations that probably led to the failure of the butterfly valve. Improving the inlet flow conditions reduced the measured pressure fluctuations and prevented the formation of a subsurface vortex. Pressure fluctuations were used as a measure of the vorticity in the outlet pipe. The improved layout led to lower pressure fluctuations and to a higher discharge rate.

8 Table 2: Results of the tests that were conducted for Layouts A1 and A5 - Model study : Paris Dam Oplm AI A5 Desrnpliin As bulld venlcal drop mletallh deflectorplate Wnlerlew (m) F~lennme L ParBO00 Par1000 BPNWO BPGOOO POW i X C Pneryre meuuredtresjuy in roll) MUIPIIIO Mmhm vsriaiiat (~t~rimam - ~ iimum) ~oiatz I point3 I ( points I P~MI f ~oin~f point$ I Point4 I point$ I PO~C t [ hint2 I PG~II~ I PQMJ 1 P&S ha1 pressure mclrngs (m nnler) t/'s l 1930 l06?2?? 9$'73% 1 W7C : 1r-6'3 l iw,i ( 989' 3 U SCbO I rlrrd G Y.,6il? 876, Xd2, 0 X '901 1% r O 02IOl 00'813? U 04G I Pant2 : 1.6 Bar Absolute presswe mnrduceron top ofpipeline Point 3 : 1.6 Bar Absolute brejsure uanedufer on other side Pressure = x delta Volt bar I Pressure = x delta Volt bar I Variation in pressure Water level 730 m Variation in pressure Water level 716 m

9 Fluid Structure Interaction I1 361 Paris Dam - Model Study Photographs Photograph 1 Failure pf the butterfly service valve Photograph 2 : General layout of the Model of the Intake structure of Paris dam

10 362 Fluid Structure Interaction 11 Photograph 3 : General layout of outlet and position where the pressures have been monitored. Photograph 4 : Adapted intake structure layout

11 References Fluid Structure Interaction I1 363 [l] Unpublished Report by Prof D Stephenson, Wits, South Afiica, June 2001 [2] Dam Hydraulics, Vischer D L and Hager W H. John Wiley & Sons. Chapter 7 - Intake Structures. ISBN [3] Predin A., Torsional vibrations at guide-vane shaft of pump-turbine model. Shock and Vibration, 1997 V014 Issue 3 pp [4] Neumann B., 1991 The interaction between Geometry and Performance of Centrifugal Pump, Mechanical Engineering Publications Limited, London. [5] NB Webber Fluid Mechanics for Civil Engineers Chapman and Hall Chapter 4, Paragraph 4.9.3