PV/MV units issue on PID controller?

[jkittrell]

modelling control loop which monitors flow indicator to control discharge valve from tank, but i'm getting no control response, valve closes due to zero PV. At steady-state, flow indicator appears accurate (1 L/hr) but current value on PID Control object (FI) reads 3.6E+06, almost like i've got a units of measure problem. Then when i run in dynamic mode, PV reads zero, valve shuts, and i get no control response. Any insight on where I may be going wrong with this? Attaching a screenprint and flowsheet. Thx

PID Flow Control.zip

[DanWBR]

@jkittrell, you don't need to select a unit for the PID, since you're getting the value from the indicator, which is already displaying it in L/h.

[jkittrell]

Thank you Daniel. I've removed units of measure from MV, also changed PID to direct acting (e.g. PV<SP; increase valve output to increase flow). Steady state solution shows 1L/hr output, MV from FI-01 with FCV-02 at 50% opening. When I move to dynamic mode, reduce setpoint at 0.75 L/hr, I see zero for the PV and it never moves even though valve is opening upwards of 75% and MV is reporting 76. Seem to still have issue with PV measurement, any other thoughts i might look at?
PID Image.pdf

[DanWBR]

@jkittrell change the PID minimum and maximum outputs to 0 and 100 (% opening of the valve)

[jkittrell]

sure, updated min/max outputs. Still no response on PV, puzzling. I'm reading this as Setpoint is set at 0.75, manipulated value (valve opening) is set at 75%, but process variable is reading zero?

[DanWBR]

@jkittrell why don't you take the value of volumetric flow directly from the stream?

[jkittrell]

i tried that too, "TkOut" stream volumetric flow rate. No change, i'm still getting zero on the PV.

[DanWBR]

@jkittrell I spotted some issues:

• the tank model is atmospheric, so water vaporizes once it enters the tank. you need to use a pressurized vessel instead;
• the PID is set as reverse acting when it shouldn't, as the liquid flow increases with the opening;

[jkittrell]

thanks, i had spotted the error with reverse acting and switch todays iterations to direct. Regarding atm tank, i guess i'd be surprised its water is flashing at 25c. I dropped the feed stream to 1bar and added G-L separator in place of the tank though. Generating zero on the PV. Also generating 0 L/hr vapor stream off separator.