Controlling flow using PID or other method?

flyers

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Join Date
Sep 2006
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Amk
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Hi guys,

I've come across an some application, but not too sure what is the suitable method to use.

Scenario 1
I'd like to control/maintain the flow in a pipeline. There is a flow meter (4-20mA) & a pump control by inverter (4-20mA). When we set the pipeline flow to (for example) 50 liter/hour, the pump must run 50%, when we set the pipeline to 80 l/h, the pump must run 80% automatically. For this scenario, it's more suitbale to use a PID or use some SCL/SCP instructions?
Current system design: They manually set the pump to run and monitor the flow (l/hr) at the RSView32.

Scenario 2
To control the temperature of a tank. There is a hot water pipeline feeding hot water into the tank, if we set 500 deg C for the tank, a control valve will open (for example) 50% to allow the hot water to flow into the tank. Once the tank reached the setpoint temperature, i'll close the control valve. When the tank temperature starts to drop, the control valve will open slighlty to allow the hot water to flow into the tank automatically. For this scenario, it's more suitbale to use a PID or use some SCL/SCP instructions?
Current system design: The system is using a digital valve (open or close), so the temperature in the tank will tend to overshoot/undershoot a lot (sometimes until temp high/low alarm is activated and shutdown their system).

Hope to hear from you guys who is more experience.
 
Your description of the flow scenario is a classic PID loop. Flow transmitter is the process variable, operator establishes a setpoint and the PID controller calculates the output that drives the inverter to produce that volume of flow. Some variable speed drives have PID built in for such control.

My tank heating experience is with heat exchangers or jacketed vessels, where the heated fluid doesn't mix with whatever is in the tank. It isn't clear to me how you deal with the changing volume of the tank that occurs by maintaining temperature. I'll let others address that.

Dan
 
If 50% gets you 50 l/min and 80% gets you 80 l/min then you have no need for PID control - just set the speed and forget.

As for the tank temperature, if you ever heat it to 500 deg. C with hot water, I will be truly amazed.

If, however, you have more modest ambitions - say, to heat the tank contents to something less than the temperature of your hot water - you will need to consider the mass and the specific heat of the tank contents plus the temperature of your hot water supply and you will need some method of controlling the mass of hot water added.
 
You can use PID for the heating application, assuming your heating water and vessel are pressurized so you don't flash to steam, and remember your thermodynamics. (Heat energy always flows from the hotter to the colder substance.)

The heating may be a challenge because of response time issues. You may be better off with a deadband temperature control (like a thermostat) but with tight deadband and appropriate delays to accommodate the response time.
 
If 50% gets you 50 l/min and 80% gets you 80 l/min then you have no need for PID control - just set the speed and forget.

This is true, if pump speed and pump speed only will cause the flow rate to change. If some other mechanical variable(s) may affect the flow rate, the PID is a very nice solution--It will adjust the pump speed to make up for other system variations.
 
Hellow,

In this case we can use PID controlled solenoid volve same as i have seen in the application of burners, where LPG gas quantity is controlled
by solenoid volve whose o/p is proportional to the PID control signal with respect to set zone Temperature.

REGARDS,

SAMEER.
 
flyers said:
Hi guys,

Scenario 1
It doesn't sound like you need a PID here unless there is something more we haven't been told like the back pressure or viscosity changes or the flow must be very accurate.

Scenario 2

To maintain temperature the
To control the temperature of a tank. There is a hot water pipeline feeding hot water into the tank, if we set 500 deg C for the tank, a control valve will open (for example) 50% to allow the hot water to flow into the tank.
That can't be good. Why not keep the valve open at whatever level is required to keep the temperature constant? This is just a simple energy in = energy out balance. Don't you have to worry about thermal shock? The only way to get 500 degree C is if the water is pressurized.

For this scenario, it's more suitbale to use a PID or use some SCL/SCP instructions?
This application requires a PID.

Current system design: The system is using a digital valve (open or close), so the temperature in the tank will tend to overshoot/undershoot a lot (sometimes until temp high/low alarm is activated and shutdown their system).
This isn't a good design unless the valve can pulse width modulated quickly relative to the time constant of the system. If so you must not only use the error but also the rate of error to determine when the valve is openned or closed. This way the temperature will not over shoot the set point.
 
Peter Nachtwey said:
It doesn't sound like you need a PID here unless there is something more we haven't been told like the back pressure or viscosity changes or the flow must be very accurate.


That can't be good. Why not keep the valve open at whatever level is required to keep the temperature constant? This is just a simple energy in = energy out balance. Don't you have to worry about thermal shock? The only way to get 500 degree C is if the water is pressurized.


This application requires a PID.


This isn't a good design unless the valve can pulse width modulated quickly relative to the time constant of the system. If so you must not only use the error but also the rate of error to determine when the valve is openned or closed. This way the temperature will not over shoot the set point.
Flow Control:Since there is no external disturbance to there is no need for PID controllers. The pump speed can be speed can be varied direcly according to the flow. However if VFD is being used to vary the speed of the pump, then the built in controller in the VFD can be utilised.

Temperature Control: Water can not be used for heating the tank content to a temperature of 500 C. However superheated steam can be considered. However in this case there external influences like mass of the material to be heated, capcitance/thermal inertia of the tag and lag(transportation) both on the proceess variabel side and on the correcting medium side, affecting the corrective action. Hence a PID Controller is to be considered.

R.Thiyagarajan
 
Flow Control:Since there is no external disturbance to there is no need for PID controllers. The pump speed can be speed can be varied direcly according to the flow. However if VFD is being used to vary the speed of the pump, then the built in controller in the VFD can be utilised.

Temperature Control: Water can not be used for heating the tank content to a temperature of 500 C. However superheated steam can be considered. However in this case there external influences like mass of the material to be heated, capcitance/thermal inertia of the tag and lag(transportation) both on the proceess variabel side and on the correcting medium side, affecting the corrective action. Hence a PID Controller is to be considered.

R.Thiyagarajan
 
Usually (never) is the flow=RPM/vfd control signal of the pump.

This sounds a little bit like a school example?

P-I-ID controls can be used for all kind of controls.
You just have to see if you need more than one PID to control the circuit.
In the other case you would use two pid controls to control the heat.
the second controller would control the valve the first one the temperature output to the second controller(PID)
 
1. There are many kinds of pumps - fan, gear, screw, vortex, piston.....
Each of them has a stabilization function. This stabilization function depends from type of pump.
You can contol, by speed of drive, desire parameter (flow or pressure) only upon appropriate choice of pump type.
You didn't wrote what type of pump used. So, to eliminate misunderstanding, you should clarify it.

2. PID-control is antique technique and can't be used for all kind of controls! Don't worry if you lose with it - there are quantity modern techniques.
 
dear all,


Thanks a lot and I'm really appreciate for all the replies. I'll be going to site to trouble shoot scenario 1 today without using any PID, as per replies.

For scenario 2, i'll have a further understanding on the systems and I'll come back to here again.

To be con't.....
 
Scenario 1: If the customer manually sets the flow, and the pump, pipeline, etc are sized correctly, just by the inverter can do the job.

Scenario 2: Don't really understand the application like, how fast can it heat up and how fast does it loses heat. If it is just a simple application where filling hot water to the tank (heat water to 500degC?), with temperature feedback and on/off valve can do the job. However, more complex job to maintain it at certain value, u may consider heating element and heat exchanger with PID control. Perhaps, 2 PID controls.
 
Hi guys,

I'm back again, sorry for the late reply, my apologize.

I've collected some data last at site last week, for scenario 1, the diesel pump frequency is not linear to the flow (l/h), so i think the simple scaling won't work, below is the data I've collected during their operation. The operator is managed to go as low as 18Hz & max. 27Hz only, beyond that might cause interruption to their system.

Diesel_Pump.JPG

The flow meter (4-20mA) & inverter (4-20mA) cabling are already in the SLC, so using the inverter to control the freq based on the flow meter is not a very good idea as they have to pull another cable to the inverter (AI).

With these information, is it enough to modify the program to PID control? As I do not have much knowledge on the PID settings.

Appreciate if anyone can provide me some hints, start with this simple pump speed control.

Thanks a lot.
 
Last edited:

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