Control of flow in open channels. 1

[sow]

We have to control the flow to full treatment on a sewage plant, to a maximum of 3.25m3/s.

The existing channel is 3m wide, and due to various constraints we cant use a control valve (e.g. eccentric plug), therefore the control must be in an open channel.

Weir penstocks are not favoured due to the build up of solids. Flumes, hydrobrakes etc cause hydraulic problems.

Many engineers have reservations regarding the use of an actuated penstock gates (i.e. ones that close downwards), as it is believed that these can "hunt" excessively. This may be due to the non linearity of the equations governing flows through orrifices and/ or mechanical hysteresis in the valve. There are those who believe that you can not control large penstocks with the required accuracy due to the need to open the penstocks in large steps, I for one do not believe that this is the case, why cant you crack a valve this size in 10mm increments?

Does anybody have experience of using penstocks for automatic flow control, in particular something of this size?

I am also interested in penstocks that close horizontally, like lift doors. Does anybody know of any manufacturers?

Any other penstock types better suited to flow control?

Are weir penstocks really any better than the gate type - I can only think that the you can control the free discharge , unlike a penstock, therefore the linearity is better.

It is thought that hunting and its effects may be reduced by the following, any more suggestions?

1) Controller to be specifically suited to penstock open channel flow control.
2) Actuator to be "oversized" and /or high spec.
3) Using a large penstock to control the majority of the flow, with a smaller one for trimming to the desired value.
4) An appropriate control algorithm.
5) High spec penstock.

Thanks

Sow

 

[rcooper]

I have seen penstock - flume combinations being used successfully on flows from 8l/s up to 1200l/s being sent forward to treatment. If they are allowed to hunt, then they tend to continuously move over a small travel, which can cause the thread on the penstock to wear quickly. A control system which has been used successfully is;

1) The penstock is fully open and clear of the surface of the water, allowing scum and rags to pass freely through the flow control area.

2) When the flows equal or exceed the maximum flow to treatment, the penstock motors to a pre-determined position which should raise the upstream level to the overflow weir level.

3) If the flows forward exceed the flow to full treatment, the penstock motors closed by a pre-determined step (5,10mm).

4) If the flows forward are less than the flow to full treatment the penstock motors closed by a pre-determined step.

(Steps 3 and 4 take place at intervals, normally set at commissioning. This stops the penstock continuously "hunting")

5) When the storm subsides, the penstock motors to the fully open position. This may be detected either by flow meters or by the position of the penstock.

6) Send a warning if the penstock ever reaches fully closed position.

It is advisable to return any screenings captured by the storm screen downstream of the penstock so they don't build up upstream of the penstock and then pass through as a slug when the penstock opens.

This control system does allow excess flow through the works for a short period of time, but this is generally damped out by the primary tanks.