AUTOMATIC SELF-ACTUATING DAM OUTLET VALVES

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1 AUTOMATIC SELF-ACTUATING DAM OUTLET VALVES PD Townshend Amanziflow Projects (Pty) Ltd, Johannesburg, South Africa ABSTRACT Many older dams have bottom outlet valves which are now malfunctioning and require replacement. For a large dam in Swaziland one of the bottom outlet control valves is now dysfunctional and has been replaced with an automatic control valve which is not operated by any electro-mechanical mechanisms nor reliant on sophisticated control systems. This removes the mechanical problems associated with the old control valves and makes the new operation simple and effective. This paper describes the design and fabrication of this large valve and its flow control mechanism as well as the installation under difficult shut down periods due to irrigation requirements. 1. INTRODUCTION The Mnjoli dam on the Mbuluzi River in Swaziland was constructed some 45 years ago to provide irrigation water to the large Royal Swaziland Sugar Corporation (RSSC). The dam owner is the Swaziland Ministry of Natural Resources and Environment (MONRE). The dam has a 40m high earth embankment with a rated capacity of 136 Mm 3. The dam was raised in 2007/8 using the TOPS automatic self-actuating spillway gates to add an extra 22Mm 3 capacity. The dam has a concrete lined service spillway on the RH flank and a 100m wide unlined auxiliary spillway on the LH flank. There are two outlets: A bottom outlet discharging into the river where the water is extracted from a river weir for irrigation. A raised level outlet serving a canal leading to a balancing dam to irrigate the Ngomani area of the estate. 2. THE EXISTING VALVES The valves are electrically activated control valves made by Larner Johnson in the United Kingdom. They are cone valves with a centre shaft moving a cone backwards and forwards to control the flow. They are large valves constructed in cast iron, in the order of 6m long by 2,5m diameter and weighing 22 tonnes each. There is one valve at the river outlet and two valves at the canal outlet. The flow rate is controlled from a control room. The flow rate varies almost daily depending on RSSC irrigation demand. The river outlet can discharge up to 7m 3 /sec and the two canal valves up to 6m 3 /sec combined. They had served their purpose well until about 5 to 8 years ago when the electrical control and the actuating motors malfunctioned and the valves had to be adjusted manually. However even the manual operation was difficult. Despite new gearboxes fitted the movement of the cone was extremely difficult to adjust the flow rate. 1

2 Figure 1. A large control valve to the canal outlet. In 2015 one of the two canal valves was removed and stripped down at the RSSC Mill workshop. The pieces are shown in figures 2&3. Figure 2. Stripped down LJ Valve showing the adjustable cone 2

3 Figure 3. Control valve cone showing cavitation and pitting of the surface The central shaft displayed movement which required new bearings. The cone was pitted from cavitation and impact erosion. The pipe delivery to the two canal valves bifurcates just before the valve house. The 2 45-degree bends in the pipework create uneven rotation in the water which impinges unevenly onto the cone and this caused wear on the bearings and subsequent movement and vibration of the shaft and cone. The manufacturer was requested to examine the valve and to repair it. However, it required shipping it back to the UK. The exorbitant cost made it impractical as well as time consuming. The valve was reassembled and installed back in position but with the valve fully open. The other canal valve was also stuck open and the flow to the canal was regulated by a 1500NB geared butterfly valve. This situation was clearly not acceptable and RSSC decided to install a locally designed and manufactured automatic diaphragm valve (DV). RSSC based their decision on other large DVs installed and operating at Maguga Dam regulating weir and the Orange Riet Irrigation Dam shown in photographs 3 and 4. The valves are also specified for the two outlets at the Ncora Dam in Eastern Cape. Figure 4. Large DV at Maguga regulating weir to control flow into the Komati River 3

4 Figure 5. Large DV at Orange Riet balancing dam to control flow into a canal 3. THE AUTOMATIC DV CONTROL VALVE This particular DV is 1300mm NB with a discharge capacity of 6m 3 /sec and static head of 25m. The DV is upward facing on a bottom pipe bend. It discharges radially under 3m depth of water in a 12 m deep canal forebay. The DV is controlled by the water level in the canal by means of a float valve positioned 30m away in the canal. The float valve controls the pressure in the diaphragm of the valve to regulate and maintain the set flow in the canal. Gauge posts in the canal are graduated in flow increments and vary from 1, 5 m 3 /sec to 7m 3 /sec. The float is set manually once a day to give a constant discharge. 4. HYDRAULIC DESIGN Because of the bends in the pipework, the water flow rotates and would be uneven when it reaches the DV. A series of flow straightening vanes were fitted in the straight pipework upstream of the DV to reduce the effect of two 45 degrees bends leading to the valve house. The bottom 90-degree bend under the DV causes rotational flow and these are overcome with 8 vertical vanes in the DV pipe. The flow regime in the DV was modelled by the Ansys Fluent Computational Fluid Dynamic (CDF) package by the University of Stellenbosch by Mr O Sawadogo and under the auspices of Professor Gerrit Basson. 1 This analysis was performed on the Ncora outlet valves but they are the same size and configuration as this Mnjoli dam outlet valve and hence are applicable to Mnjoli DV as well. The flow was modelled through the DV without vanes and the flow pattern is shown in figures 1 and 2 which clearly shows the uneven flow pattern on the DV. Figure 6. Flow without flow straightening vanes showing the uneven pattern. 4

5 Figure 7. Spiral flow through pipe bend without flow straightening vanes The DV was then modelled with 8 vanes in the pipe and the effect can be seen in figures 8&9. Figure 8. Flow with flow straightener vanes to give even pressure distribution The flow is straightened by the vanes and impinges evenly on the deflector plate of the DV. A downward facing skirt around the DV outlet also helps to centre the deflector plate. 5. INSTALLATION The installation proved to be awkward because the canal flow could not be closed off for more than 2 days at a time because of the heavy demand on irrigation. The installation comprised the following elements as shown in figures 9& A reducer piece from the existing 1500NB pipe down to 1200NB incorporating vanes. 5.2 A 1200NB butterfly isolating valve. 5.3 Straight pipe pieces which fit through a 1500NB puddle pipe in the wall leading into the wet chamber. 5.4 A 90-degree bend in the wet chamber incorporating vanes. 5.5 The 1300NB DV complete with vanes. 5

6 Figure 9. Plan on new control valve Figure 10. Elevation on new control valve 5.6 Control piping leading to the float control valve in the canal 5.7 An access bridge over the canal supporting the float valve 6

7 Figure 11. Upstream elevation of float valve in canal Figure 12: Side elevation of float valve in canal The installation took place in 4 shut downs of mostly 2 days each. The first installation was the removal of the existing LJ valve using an 80 tonne mobile crane. The new reducer pipe together with the 1200Nb butterfly valve was installed and the valve closed. A blank flange was fitted to the pipe protruding into the wet canal forebay chamber to prevent back flow. 7

8 The second installation involved fitting the pipework in the dry chamber as well as the rising bend in the wet chamber. It was not considered prudent to connect to the existing 1500mmm diameter pipe through the wall into the wet chamber as it was mild steel, corroded and had been submerged in place of 45 years. The new pipe is a 1200NB and fitted through the existing pipe opening. The gap was grouted with concrete to seal it. The third installation was to place the completed DV onto the rising bend and secure to the side walls. The forth installation is the connection of the control piping from the DV and setting up the float valve and access platform. At the time of this paper, it had not been completed because of the heavy irrigation demand and there is not enough down time to complete the installation and commissioning of the DV. The float has a travel of 1.7m from a low flow of 1.2 m³/sec to a high of 7 m³/sec. The float is adjusted manually using a handwheel to move the float to its required position. 6. FABRICATION The diaphragm valve and associated pipework was fabricated in Swaziland by Iron Engineering (Pty) Ltd. They also installed the equipment. The material was carbon mild steel grade 350WA and the platework was mostly 10 and 12mm thick. The diaphragm is rubber with antioxidants for ultra violet resistance and reinforced with 7 plies of high strength nylon chord. The diaphragm was pressure tested to 4 bar which is almost twice the maximum static pressure on the diaphragm. All fasteners are galvanised in the dry valve chamber and stainless steel in the wet canal forebay chamber. The control piping is 65 NB class 10 HPDE. The corrosion protection system used was a two component polyamine cured mastic coating. The non- immersed surfaces inside the dry valve chamber and out of sunlight was a total dry film thickness (TDFT) of 200 microns. This was mainly to the pipework inside the valve chamber in two coats The water immersed insides to the pipework as well as both internal and external surfaces to the wet canal forebay chamber was a TDFT of 330 microns in two coats The water immersed internal surface to the deflector plate of the diaphragm valve which has higher abrasion possibility has a TDFT of 500 microns in two coats The valve and piping took in the order of 3 months to fabricate. The completed valve together with control piping is shown in figure 13. Figure 13. Completed fabricated 1300 mm diameter Diaphragm Valve 8

9 7. CONCLUSION Electro mechanically operated dam outlet valves will, with time, become problematic and difficult to operate. These types of valves also require ongoing maintenance which may not always be undertaken particularly on dams at remote sites. Further, imported valves which have sophisticated control mechanisms should be avoided where possible because of the vulnerability to expensive spares and repairs. There are however locally produced automatic outlet valves which can serve the purpose well which are simple to use and more affordable than imported valves. Engineers need to consider the maintenance and operating problems of the valves when specifying dam outlet valves. 8. ACKNOWLEDGEMENT The author thanks the management of Royal Swaziland Sugar Corporation for their permission to publish this paper. The opinions and views presented in this paper are, however, those of the author and do not necessarily reflect those of RSSC. 9. REFERENCES 1 Sawadogo O, Townshend P, & Basson GR (2015) Computational Fluid Dynamic (CFD) Modelling of the outlet control valve of Ncora Dam. pp , Design of Hydraulic Structures, Stellenbosch, South Africa, 28 September -01 October