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Can a control valve perform more than one function? 1
- Thread starter hirenyp
- Start date
hirenyp (Chemical)
I will try to be diplomatic.
Yes! a Control Valve does do more than one thing at the same time.
It, a TCV (Temperature Control Valve) controls both the temperature and the flow at the same time. It controls the Temperature BY controlling the flow.
All control valves control something (Temperature, Level, Pressure, etc.) by controlling the flow through the line.
That is basic.
I will try to be diplomatic.
Yes! a Control Valve does do more than one thing at the same time.
It, a TCV (Temperature Control Valve) controls both the temperature and the flow at the same time. It controls the Temperature BY controlling the flow.
All control valves control something (Temperature, Level, Pressure, etc.) by controlling the flow through the line.
That is basic.
A valve is a vave
It dosnt know if it controls "flow" or "temperature" - its just open in a certain positions.
Its the controller, the measurement and logic that "knows".
If you need two dependant variables at the same time - then you need to free - but if you need sometime to control flow and sometime temperature then one is fine.
You dont clearly describe your situation but say that you control the temperature of a process stream using a HX with an outlet control valve on the utility side (common set-up).
In this case you cannot control the flow of utility - because you want a stable temperature of your process medium - the controlled variable.
If you installed a bypass on the utility side and added a control valve here - then you could maintain a steady utility side flow and a steady process temperature.
But say that you need a minimum utility flow rate - and that this is MORE important than the process medium temperature - then you could make a controil logic that would normally control the temperature - but IF flow rate dropped below the treshhold value then it would choose valve position based on utility flow rate.
Best regards
Morten
It dosnt know if it controls "flow" or "temperature" - its just open in a certain positions.
Its the controller, the measurement and logic that "knows".
If you need two dependant variables at the same time - then you need to free - but if you need sometime to control flow and sometime temperature then one is fine.
You dont clearly describe your situation but say that you control the temperature of a process stream using a HX with an outlet control valve on the utility side (common set-up).
In this case you cannot control the flow of utility - because you want a stable temperature of your process medium - the controlled variable.
If you installed a bypass on the utility side and added a control valve here - then you could maintain a steady utility side flow and a steady process temperature.
But say that you need a minimum utility flow rate - and that this is MORE important than the process medium temperature - then you could make a controil logic that would normally control the temperature - but IF flow rate dropped below the treshhold value then it would choose valve position based on utility flow rate.
Best regards
Morten
Simple
The measured variable can be temperature with a temperature setpoint and an output to the control valve to control temperature. However, temperature control can be slow. An example might be controlling the fuel gas to a furnace. The heat of the fluid flowing through the tubes is directly affected by the fuel gas rate but the response can be slow. OTOH, reflux in a distillation column can be faster using a sub-cooled reflux stream like a shower that cools the upper trays.
Cascade
You could measure the outlet tubes from the furnace and the fuel gas flow and include a controller for each measurement. You can use the output of the slow temperature controller as a remote setpoint for the fast responding fuel gas flow controller.
Other possibilities exist too.
The measured variable can be temperature with a temperature setpoint and an output to the control valve to control temperature. However, temperature control can be slow. An example might be controlling the fuel gas to a furnace. The heat of the fluid flowing through the tubes is directly affected by the fuel gas rate but the response can be slow. OTOH, reflux in a distillation column can be faster using a sub-cooled reflux stream like a shower that cools the upper trays.
Cascade
You could measure the outlet tubes from the furnace and the fuel gas flow and include a controller for each measurement. You can use the output of the slow temperature controller as a remote setpoint for the fast responding fuel gas flow controller.
Other possibilities exist too.
I like to have valves with more than 1 setpoint from different controllers.
FOR EXAMPLE, we have a flow control taking gas from a point going to a compressor. Put if 1 out of 4 compresors goes down, the pressure will rise and overload the engine, so we use a low select relay that sends the output from a pressure controller to the valve. As the pressure is rising, the pressure controller sends a lower signal to shut the valve, the relay selects the lowest signal which is pressure control. The flow controller wants more gas, and sends a higher signal to try to open the valve.
So a high or low select can can have multiple set points. On liquid meters we have a 3 way system, Inlet to the meter pressure that can not be too low, outlet from the meter run, that won't let the outlet presure get to high, and flow control set point. all threen varibles met, one control valve.
FOR EXAMPLE, we have a flow control taking gas from a point going to a compressor. Put if 1 out of 4 compresors goes down, the pressure will rise and overload the engine, so we use a low select relay that sends the output from a pressure controller to the valve. As the pressure is rising, the pressure controller sends a lower signal to shut the valve, the relay selects the lowest signal which is pressure control. The flow controller wants more gas, and sends a higher signal to try to open the valve.
So a high or low select can can have multiple set points. On liquid meters we have a 3 way system, Inlet to the meter pressure that can not be too low, outlet from the meter run, that won't let the outlet presure get to high, and flow control set point. all threen varibles met, one control valve.
In the end however the control system must choose - because the valve only has one position (at any given time) 
djack77494
Chemical
One variable to manipulate (control valve position) = one process variable that can be controlled. The system has only one degree of freedom. Thinking you could simultaneously control more than one independent variable would be like believing you could violate the laws of physics - just ain't going to happen.
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1
- #8
A valve controls flow. That is what a valve does. What the flow does, its repercussion is the other thind that will be, by default, controlled by a valve.
e.g. if it is to feed a reactor, the valve will control the flow, the flow will effect the kinematics of the reaction.
Same with temperature. the valve controls the floe, the flow will effect the heat transfer.
<<A good friend will bail you out of jail, but a true friend
will be sitting beside you saying ” Damn that was fun!” - Unknown>>
e.g. if it is to feed a reactor, the valve will control the flow, the flow will effect the kinematics of the reaction.
Same with temperature. the valve controls the floe, the flow will effect the heat transfer.
<<A good friend will bail you out of jail, but a true friend
will be sitting beside you saying ” Damn that was fun!” - Unknown>>
moltenmetal
Chemical
IF you care about both flow AND temperature, you can use cascade control: a flowmeter and control valve in a loop controlling flow, receives setpoints from the temperature controller to control temperature. This approach is used when one process responds quickly to changes in valve position (ie. flow) and needs to be kept within limits for process reasons (ie. periods of no flow or 100% flow may not be tolerated by the process), or where other disturbances are purturbing the fast (flow) loop, whereas the other (temperature) loop responds more slowly to changes in valve position. But as unotec says, you're only varying one thing with the valve: the flow through the valve. You can't independently control two variables with one valve.
djack77494
Chemical
Maybe just semantics, but I'd modify what you can control to be valve stem position. Flow depends on that but also on other factors.
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