8 replies to this topic

[Tee2012]

Hello all,

 

I would like some advice on a suitable control loop for the following scheme (attached sketch to better understand the system). I have three scenarios:

 

1) if the feed flowrate is less than 450 m3/h, FCV-002 is in closed position and all the flow is directed to the process.

 

2) if the feed flowrate is between 450 and 550 m3/h, FCV-002 is in closed position and all the flow is directed to the process (this extra 100 m3/h is acceptable as per the design of the downstream processes, subject to some quality parameters).

 

3) if the feed flowrate is more than 550 m3/h, FCV-002 and FCV-001 need to modulate to achieve 450 m3/h going to the process and the rest going through the bypass line. When we did the hydraulic analysis, FCV-002 will be in full open position, and FCV-001 needs to exert 0.5 bar pressure drop to achieve 450 m3/h going to process. However, we are assuming in real scenario both might need to modulate. Note: feed flowrate is maximum 600 m3/h.

 

I have two flowmeters, FE-001 will read for scenario 1 and 2 and initiate the opening of FCV-002 for scenario 3. While FE-002 will control FCV-001 to achieve 450 m3/h in scenario 3. If flowrate drops back to less than 550 m3/h, FE-001 will close FCV-002.

 

I am not very experienced in process control and any help would be greatly appreciate.

Attached Files

•  Sketch.png   12.56KB   3 downloads

Edited by Tee2012, 20 August 2024 - 09:54 AM.

[Dacs]

What's the fluid source? Is it possible to monitor the source vessel for pressure (or level)?

[breizh]

Hi,

In the same vein, will it be possible to install load cells on the tank to replace the FIC (s) ?

Breizh

[Tee2012]

The source of the flow is an underflow from a thickener. The underflow extraction is controlled by a density meter controlling the underflow pumps. These pumps are connected to the feed line of the sketch I have provided. Unfortunately, there is no possibility to control the flowrate from the feed side, that is why we are proposing a bypass line to dump the extra flow we cant handle.

[Pilesar]

Add a controller for bypass FCV002 so that FCV002 is direct acting and proportionally opens above the 550 m3/h setpoint and closes below that setpoint. The controller on FCV001 will be reverse acting so that FCV001 opens below 550 m3/h setpoint and proportionally closes above that setpoint. This is called split range control. These two control loops will need to be tuned so they behave well together. You may need a deadband area or overlap area between the two controllers by giving them slightly different setpoints so that the valves do not fight each other when the flowrate is exactly at 550 m3/h. Size the FCV001 valve so that it is less than 100% open at the setpoint to get smoother response when it needs to close.

[breizh]

hi
Instead of a densimeter, you could have mass flow meter and get the mass flow, the density thus the volumetric flow rate and solid content using a DCS, PLC) for the volumetric flow and solid content. I did that on the feeding line of a centrifuge at the discharge of thickener for Nahco3 production.

Note: The pump was a cavity pump (pig tail type) and I used a three-way valve to feed the centrifuge or to return to the thickener if needed.

Breizh

[Tee2012]

 

Add a controller for bypass FCV002 so that FCV002 is direct acting and proportionally opens above the 550 m3/h setpoint and closes below that setpoint. The controller on FCV001 will be reverse acting so that FCV001 opens below 550 m3/h setpoint and proportionally closes above that setpoint. This is called split range control. These two control loops will need to be tuned so they behave well together. You may need a deadband area or overlap area between the two controllers by giving them slightly different setpoints so that the valves do not fight each other when the flowrate is exactly at 550 m3/h. Size the FCV001 valve so that it is less than 100% open at the setpoint to get smoother response when it needs to close.

 

 

Hello Pilesar, let me try to see if I fully understand you suggestion on a P&ID notation:

 

FE-001 --> FQIT-001 --> FQIC-001 A --> direct acting on FCV-002

                                  --> FQIC-001 B --> reverse acting on FCV-001

 

Is this correct? both FQIC's are connected to the same FQIT?

 

How will controller FQIC-001 B, control the flow at FCV-001 to 450? would this be possible since the setpoint is 550?

 

The reason we are only taking 450 and dumping 150 through the bypass is that we need to maintain high velocity in the bypass line to limit solid deposition in the pipeline.

 

hi
Instead of a densimeter, you could have mass flow meter and get the mass flow, the density thus the volumetric flow rate and solid content using a DCS, PLC) for the volumetric flow and solid content. I did that on the feeding line of a centrifuge at the discharge of thickener for Nahco3 production.

Note: The pump was a cavity pump (pig tail type) and I used a three-way valve to feed the centrifuge or to return to the thickener if needed.

Breizh

 

They are using density meters to measure the solid bed/content in the thickener underflow. If it gets too high, they have major issues with the thickener operation plus their associated underflow pumps. If it gets too low, they are losing water. The feed content to this thickener is highly variable and I dont see any benefit of measuring the volumetric flowrate instead of the density. Actually, there is a slurry magnetic flowmeter on the discharge of the underflow pumps but it is only for monitoring.

 

The thickener operator is against returning any excess flow to the thickener due to solid accumulation in the thickener. Unfortunately, we are unable to modify any of their current systems.

[breizh]

Hi,

My point was the following, with a single equipment (mass flowmeter) you have access to multiple information without having to buy multiple equipment (densimeter, volumetric flow meter). 

For reference, the mass flowmeter was a straight tube promass from E&H.

Anyway, it is already purchased.

Note: Better control of the upstream (feed) will certainly help.

BTW :Can you share info about your process? 

Breizh

[Pilesar]

FCV-001 is not controlling the flowrate to 450 m3/h according to your problem statement: 

"if the feed flowrate is between 450 and 550 m3/h, FCV-002 is in closed position and all the flow is directed to the process"

If you want to open the bypass valve at 550 m3/h for a high-velocity purge, then have FCV-002 open fully at 550 m3/h and close fully at 450 m3/h with no modulating in between.