Flow control at reservoir (water storage facility) inlets

Since water is a LIFE requirement, and our constitution allows for us to receive water as a right, our Governments over the years started providing storage reservoirs that accumulates water for re-distribution amongst people of South Africa.

However simple the concept may be, it is a problem application and maintenance intensive. This is so for one reason only; Damage as a result of cavitation; Since one of the basic rules of Mechanics of Fluids states that flow through pipelines will create a friction loss, where this differential pressure is directly related to the rate of flow through the pipeline.

Now when we consider the need of pipe fittings, valves and any form of restriction of flow, the differential pressure will increase as a result. There are limits to the allowable differential pressure that occurs over restrictions or valves, as when this becomes too high, the outlet pressure of the restriction may reduce to our vapour pressure line, and this will cause the effect of cavitation.

Cavitation is an effect of VAPOURISATION, which is a similar effect as water boiling at cold water temperatures. These vapour bubbles fly around at very high velocities, and they impact with the sidewalls of the pipe, fitting, or valve that will cause the vapour bubble to collapse (implode), while this causes damage. Should this process be continuous or frequent, the side walls will also collapse. This damage is detrimental to your equipment and has to be avoided by limiting the differential pressures, and this is achieved by limiting the rate of flow in the pipeline.

What is important is to have a level control valve that will close when the reservoir reaches full capacity. If this valve fails to close, we shall have an overflow situation. All reservoirs in cities & suburban areas, are designed to send the overflow to storm water drainage points.

If the storm water drains do not have clear unrestricted flow areas, which has happened historically, flooding of the streets will occur, and this can potentially cause homes and private properties to flood. NOT a pleasant experience!

To prevent failures to occur by your level control valve, we have to limit the flow through the level control valve, which will limit the differential pressure across the valve, and thus protect it from damage as a result of cavitation. This flow limiting action must not be achieved by using the level control valve as the flow limiter, therefor never use a throttling level control valve, but only on – off control valves. The flow limiting must be achieved by using another device to limit the flow. An orifice plate is not sufficient, as it will only limit the flow at the design flow rate, and as your level control valve starts to close, it will take the flow limiting function over from the orifice plate.

One should use a flow control device that will react to changes in flow rate, and therefor keep protecting the level control valve. At Floquip we use a combination of the DURA Alpine Level control (on – off), and the Dura-Maric flow control valve, obviously in series, and it does not matter which valve is first in the series, but I personally prefer to have the level control valve first, with the flow control valve on the outlet side.

Reason being is that the flow control unit throttles, causing a back- pressure on the level control valve and limiting the differential pressure across the level control valve, while outlet side of the flow control valve faces the atmospheric pressure line, and the outlet pressure of the flow limiter is in danger of reaching vapour pressure status. This will cause the cavitation to occur. The main advantage of the Dura-Maric flow control unit is it’s very short face-to-face dimension, which causes the cavitation to occur downstream of the flow limiter, only damaging pipe walls instead.

Did you know?

Floquip is a large stockist of the DURA Range of valves; Resilient seal gate valves, High Performance Wafer Butterfly valves, Double acting air valves with non-slam feature, Maric flow control valves, Alpine Control valves and also ARI double and single acting air valves. Our MOTTO; Supply high quality valves at very competitive prices.

Floquip (Pty) Ltd,

+27 (0) 82 677 5374,

hannes@floquip.co.za,

www.floquip.co.za