Controlling RAS

[PEorl]

The design has 4 clarifiers - 2 @ 60 ft diam, 2 @ 90 ft diam. Think of two separate trains/systems where the RAS is drawn from the bottom of one of the 60 ft and one of the 90 ft clarifiers by pipes that are manifolded together that connect to a common suction header at the RAS pump station. The pipes for the RAS is the same diameter (10-inch) from the 60 ft and 90 ft clarifiers. A mirror image occurs with the other two clarifiers (mainfolded together and tie into a common suction header at the RAS pump station). Two separate trains, but utilizing the same RAS pumps station. What is the cheapest way to control the RAS? What about plug valves installed on each of the RAS lines, which would allow the operators to adjust the RAS return from each clarifier.

 

[Pulpeng]

Here's how my plant's set up. Each clarifier underflow runs to a sump. The inlet is gated and the height of the gate influences the flow rate from each clarifier. Since the system does not change rapidly we measure the clarifiers solids level with dipsticks and adjust. The sump uses some internal screw pumps (archimedes)to lift the underflow to a header - we pump out of the header the desired flow rate for WAS and the rest returns as RAS. The screw pumps keep the sump level low and always turn at the same rate. The flow from each clarifier is adjusted by the gates as mentioned.

I would suggest that if you need more control you're on the right track and adding control plug valves, flow meters and bypasses, isolation valves etc would be appropriate. Maybe pinch valves but I doubt it.

 

[bimr]

If you are asking whether adding plug valves on to existing sludge piping so that you can control the flow from two clarifiers connected in parallel, then the answer is yes.

If you have a new project, then you probably want to run the sludge piping separately to a common sump. At the common sump, you can use telescopic valves on each separate sludge line or a separate weir gate on each of the sludge lines. At startup, you adjust so that the flow is equalized.

Manifolding the pipes together before the sump is probably not a good solution since you have no guarantee on the flow from each separate pipe entering the manifold.

 

[PEorl]

bimr - yes, the sludge piping is existing. The piping goes from one of the clarifier then past the other clarifier line then to the RAS pump station.

 

[ch81pc]

If your inlet screens are letting rag into the system, throttling the valves will lead to blockages and eventually blanket spills. I have operated a system like this, but we installed a valve on both lines and actuated both so that once every 24 hours, one line woul dbe shut and the second opened fully to get maximum flow to flush out any deposits.

 

[PEorl]

I appreciate all the feedback. It is interesting to hear how different plants are set up and how they are operated. Any other examples of controlling RAS?