Resources: Joburg Water Choose Vosa
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Recent projects have included the construction of new pipelines to supplement capacity, and the installation of isolating, flow control and pressure reducing valves (PRVs) at strategic points along water supply routes to the Linbro Park, Marlboro, Morningside, Illovo and Bryanston reservoirs.
The Bryanston reservoir complex has for many years been the site of a particularly acute problem, with extremely high operating pressures resulting in frequent control valve breakdown.
To achieve the reduction in pressure that would enable the Bryanston reservoir to operate satisfactorily, Johannesburg Water decided to introduce a series of pressure reduction chambers along the pipeline which, after supplying Linbro Park and then Marlboro, branches to feed the Illovo reservoir complex off one branch, and the Morningside and Bryanston reservoirs off the other.
The first chamber is currently nearing completion at Linbro Park, just downstream of the Rand Water connection, where the pressure is reduced from 20 bar to 11,5 bar. Also nearing completion is a second chamber at Marlboro, downstream of both branches, where the pressure is further reduced to 9,5 bar, and a third chamber on the inlet line to the Bryanston reservoir complex, where the pressure is again reduced, this time down to 3,5 bar.
Valves supplied by DFC Water play a vital role in this pressure regulation. More than 100 valves have been installed at the three reservoirs, the majority of them VOSA wedge‑gate valves. A large number of Cla-Val pressure reducing valves, Cla-Val combination level and flow control valves, and Vent-O-Mat air relief valves, have also been supplied.
Consulting work on the project was managed as two separate but linked entities, with consulting engineers Civec overseeing the Linbro Park section, and Themba Consultants in charge of work at the Marlboro and Bryanston reservoir complexes.
Pipe specials were fabricated by Bambanani, with Insitu Pipelines carrying out all installation work.
The Linbro Park reservoir marks the point at which the 20-bar high pressure supply from Rand Water is received by Johannesburg Water, and incorporates a connection that is the largest on the Rand Water system, delivering a peak flow rate of 4560 l/sec (394 Ml/day) to an area ranging from Marlboro all the way to Dainfern.
Although this is the sole operational supply to the largest portion of the northern suburbs, there is notoriously little redundancy.
A maximum upstream pressure of 2200kPa needed to be reduced to 1000kPa downstream. However, design of the pressure reducing chamber was complicated by an unknown minimum required flow rate needing exceptionally responsive valve operation to prevent pressure surges upstream and hunting conditions downstream of the control valve station.
Consulting engineers Civec paid close attention to the large diameters of the supply pipelines at Linbro Park – 1500mm on the existing pipeline and 800mm on the new, parallel pipeline. Control valve regulation of the large volumes flowing through these pipes would only be feasible if demand remained within 25% to 80% of control valve capacity. Above 80%, valve response would become slow and cause inconsistent downstream pressures. Below 20%, there would tend to be overcompensation during opening and closing, causing hunting. Problems could therefore be expected during off-peak times.
Civec carried out a detailed economic comparison of the available valves, the installation and maintenance costs, valve efficiencies and spares availability.
The optimal solution was found to be a division of the two large supply pipelines into eight smaller pipelines of 400mm diameter each, allowing the installation of control valves with best possible flow-to-size characteristics, and flow resistance values (head losses) as low as possible. An added benefit of this solution would be flexible operation, with any one valve easy to isolate and refurbish without shutting down.
To ensure good control valve response, Civec’s design incorporated an automatic sequential selection function that reduced to one the number of valves allowed to function during low flow conditions.
Valve protection was provided by the installation of large capacity 2500 kPa pot strainers equipped with removable heavy duty stainless steel cartridges and manual backwashing facilities.
More than 40 VOSA wedge-gate isolating valves were installed at Linbro Park, either as replacements for existing units controlling the reservoir level, or as isolating valves associated with the PRV chamber.
Near the Marlboro reservoir, and at the Bryanston reservoir complex, similar work was carried out on a smaller scale, with pipelines being similarly isolated by VOSA valves for the installation of PRVs.
Themba Consultants were the consulting engineers for this part of the project.
The levels of both the Marlboro and Illovo Reservoir complexes are such that their supply pressures could not be less than that available once the Linbro Park PRV arrangement had been commissioned.
Themba Consultants therefore positioned the Marlboro pressure reduction chamber downstream of the branches leading off to the reservoirs, on the pipeline feeding the Morningside and Bryanston reservoir complexes.
The new PRV chamber was constructed within the existing roadway at Philo Road, with a height restricted to that required to accommodate the existing pipeline and new isolating and pressure reducing valves, below the existing road level.
The main challenge in achieving this was the height of the 400mm wedge-gate valves, the tops of which needed to be accommodated within the roof slab of the chamber.
Traffic flow was not affected during chamber construction by Insitu Pipelines.
Another challenge that had to be resolved was the need to ensure that, in the event of failure of the PRVs in this chamber, the pressure experienced at the downstream Bryanston reservoir would not suddenly exceed that which the PRVs there could handle. The solution to this was to fit the Marlboro PRVs in such a way as to ensure that any failure would close the valves.
During construction, shut-downs of the existing pipeline were carefully managed to prevent the complete draw-down of water in the downstream reservoirs.
At the Bryanston reservoir, Themba Consultants found the original inlet chamber to consist of a 620mm diameter inlet manifold feeding four sets of 250mm Rollseal PRVs, these in turn feeding into another 620mm diameter distribution manifold from which two 200mm diameter pipes supplied Reservoirs 1, 2 and 3, and two 250mm diameter pipes supplied Reservoirs 4 and 5 through level control valves. The elevated tower was supplied through a pump station, but could, in the case of emergency, be supplied through 150mm or 250mm diameter pipes connected to the inlet manifold.
The inlet pressure was found to average 15 bar, with the static pressure being as high as 20 bar.
The average inflow into the complex was estimated to be 708 l/sec, for which the original manifold had been designed. An existing peak of about 1200 l/sec was expected to increase to about 1300 l/sec as demand increased over time. An actual flow that exceeded the design flow was producing very high flow velocities in the 200mm and 250mm diameter pipes and control valves. These, coupled with the high pressure drop and the opening and closing of the level control valves, was causing excessive noise and vibration in the control chamber, and consequent cavitation damage to the isolating valves.
Themba Consultants designed the new system to overcome these difficulties and take account of both the current and future demand scenarios.
The old pipework within the existing inlet chamber was removed and the required facilities provided by means of four separate construction projects.
First, a new pressure reduction chamber was built to supply water to the reservoirs. This chamber houses three 400mm Cla-Val PRVs, one 700mm, six 400mm, and one 300mm VOSA wedge-gate valves.
Second, a new level control chamber was built to supply the water tower, housing one 300mm Cla-Val level control valve and again two 300mm and one 150mm VOSA wedge-gate valves.
Third, a new air valve chamber was built to house one 150mm Vent-O-Mat air release and vacuum break valve, and one 150mm VOSA wedge-gate valve as an air valve isolator.
Lastly, new pipework was installed within the existing inlet chamber, comprising four 400mm Cla-Val combination level and flow control valves, eight 400mm and one 300mm VOSA wedge-gate valves.
Insitu Pipelines carried out all work within the reservoir complex to avoid traffic congestion.
Limited water reserves dictated that pipe assembly and valve installation had to be completed within a twelve-hour shutdown window, Insitu Pipelines managing this constraint by pre-fabricating the pipework in three segments lifted into place by mobile crane for final assembly.
During chamber construction, Insitu encountered very poor foundation conditions necessitating the removal of an additional 500mm of bed material. This was replaced with compacted dump-rock to provide the necessary strength.
The VOSA wedge-gate valves selected by Johannesburg Water as preferred products have been proven in water reticulation installations over many decades.
They are metal-seated valves manufactured to SABS 664 and 665 and available with fitted seats in bronze, stainless steel or iron. There is a choice of rising or non-rising stems. The range is available in sizes DN80 to DN900 and for pressure ratings of PN16, PN25 and PN40.
A spokesman for Johannesburg Water cited competitive pricing, local manufacture to world class standards, and excellent service support as the reasons for awarding the contract to DFC Water.
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