PID Output and Valves

plcnovel said:
I am sorry guys i got back to this thread very late. Actually i was in the process of getting some data about the valaves Bench settings and Transducers pressure settings. Here are the numbers:


Operating Range (I/P output) 6-30 Psi
Bench setting for the valve 14 - 30 psi
PLC card output (4-20 mA to I/P)

Which means that valve will not start to open until I/P has a pressure around 14 psi(about 9mA output from Output card, 33%). If the I/P operating range is not equal to Valve Bench Settings, experts say we need to add a positioner, What are your comments on it? Also if the positioner is needed, can we eliminate the need of it and work around in our plc logic or output card span to make these I/P/Valves working properly. Thanks a lot for your input
Click to expand...

Rescale your analog output so that the minimum value from your PID gives you the ma value to achieve 14psi from the I/P
 
Last edited:
Do you have model/ manufacturer info on the actuators and I/P's? This may be helpful.

I'm not sure why you would have a bench-set that doesn't match the I/P or vice-versa. I've seen some specialty apps that needed it, but if you don't, I would re-calibrate the positioner or the I/P so they match each other.
 
cntrlfrk said:
Do you have model/ manufacturer info on the actuators and I/P's? This may be helpful.

I'm not sure why you would have a bench-set that doesn't match the I/P or vice-versa. I've seen some specialty apps that needed it, but if you don't, I would re-calibrate the positioner or the I/P so they match each other.
Click to expand...

The maximum calibration i can do on I/P is from 9 to 30 psi, i still not able to go to 14 psi.

Secondly i was thinking that if the valve is with the bench set of 14 to 30 psi and I/P is from 6 to 30 PSI. The pressure from 6 to 14 psi is used to overcome the tension on the spring that has been set on the valve to make sure it does not leak when fully closed. So even if i change the zero on my I/P to 14 psi so that valve will start to move when the signal to I/P increases above 0%. This means the valve will always have 14psi applied to it even when the signal to the I/P is at 0%. Which is not Correct. What will happen is the valve will now leak when it is supposed to be fully closed because you do not have the full spring tension on the valve seat. Your comments highly be appreciated.
 
plcnovel said:
The maximum calibration i can do on I/P is from 9 to 30 psi, i still not able to go to 14 psi.

Secondly i was thinking that if the valve is with the bench set of 14 to 30 psi and I/P is from 6 to 30 PSI. The pressure from 6 to 14 psi is used to overcome the tension on the spring that has been set on the valve to make sure it does not leak when fully closed. So even if i change the zero on my I/P to 14 psi so that valve will start to move when the signal to I/P increases above 0%. This means the valve will always have 14psi applied to it even when the signal to the I/P is at 0%. Which is not Correct. What will happen is the valve will now leak when it is supposed to be fully closed because you do not have the full spring tension on the valve seat. Your comments highly be appreciated.
Click to expand...

I know this is from left field, but what we used to do in ddc was have a non linear scaled output, & if we didnt have one pre done was make one up to suit by plotting the curve. I assume u can do the same in clx.
just a thought.
 
Tripper said:
I know this is from left field, but what we used to do in ddc was have a non linear scaled output, & if we didnt have one pre done was make one up to suit by plotting the curve. I assume u can do the same in clx.
just a thought.
Click to expand...

Can u give some more insight on to this,i really do not understand what you mean by having a non linear scaled output and plotting the curves.........
Thanks in advance
 
Without a positioner, you would need to calibrate your I/P so that it begins to move the actuator at a pressure slightly higher than the bench set so that you can take advantage of the spring force at the 4ma range, or you can make the actual I/P calibration with a range of something like 12 to 32 so you can use any additional air pressure to make sure the valve is going full open in addition to allowing the spring to force the valve closed.

Otherwise you may end up adding some type of 'dump valve' as mentioned above to be sure all air is released from the actuator.

This is why positioners are so valuable, any 'stiction' in the valve or actuator may cause huge swings in your process, plus, without a positioner, you really have no idea or control over the 'deadband' of directional changing during mid-range control.
 
In clx u have a low signal eg 4ma and low engineering eg 0; a high sig 20ma and high eng of eg 32767. so is linear between 0-32767,
we would plot the counts if u like over the curve u need so in effect it is a lot of linear steps over different x/y axis. the more plots u do the better the curve,
hope that makes sense bit vague at moment(2:00 AM here)
 
cntrlfrk said:
Without a positioner, you would need to calibrate your I/P so that it begins to move the actuator at a pressure slightly higher than the bench set so that you can take advantage of the spring force at the 4ma range, or you can make the actual I/P calibration with a range of something like 12 to 32 so you can use any additional air pressure to make sure the valve is going full open in addition to allowing the spring to force the valve closed.

Otherwise you may end up adding some type of 'dump valve' as mentioned above to be sure all air is released from the actuator.

This is why positioners are so valuable, any 'stiction' in the valve or actuator may cause huge swings in your process, plus, without a positioner, you really have no idea or control over the 'deadband' of directional changing during mid-range control.
Click to expand...

So basically you are trying to suggest that calibrate the I/P from 12 to 32 psi, in this way even at 12 psi (0% of plc output), the valve will be fully closed (12 psi to 14psi, because Bench set of valve is 14 to 30 psi, will be there to overcome the spring tension that has been set on the valve to make sure it doesn not leak). Is it true, should this design work?
 
Bench sets a usually done to compensate for real world conditions. Pressures, temperature, flow rates any thing that could cause the valve to open or close under operating condition. So have you put your process in service yet? ( pressured up) This may change how the valve operates. If so then maybe the bench set was set incorrectly for your operating conditions.
 
Mickey said:
Bench sets a usually done to compensate for real world conditions. Pressures, temperature, flow rates any thing that could cause the valve to open or close under operating condition. So have you put your process in service yet? ( pressured up) This may change how the valve operates. If so then maybe the bench set was set incorrectly for your operating conditions.
Click to expand...

YES MICKEy the valves are in service and this is the what we observed that that valve will not open untill your plc output is about 9 mA (33%) while pressuring up. Ofcourse if the bench set is not set correctly then without the manufacturer we can not change the bench set, which reduces are options to stick with only actuator and/or plc logic.
 
Bench sets are usually a matter of spring tension adjustments. But as others have said you need a positioner.
 
plcnovel said:
i can scale my card output from 4ma to something higher than 4ma to lift the valve from its seat. But on it way back, when the output is 0%, then valve would still be open adding heat to the reboiler which we dont want.Regards
Click to expand...

I might be way off base. Is the valve installed backwards?
 
plcnovel said:
So basically you are trying to suggest that calibrate the I/P from 12 to 32 psi, in this way even at 12 psi (0% of plc output), the valve will be fully closed (12 psi to 14psi, because Bench set of valve is 14 to 30 psi, will be there to overcome the spring tension that has been set on the valve to make sure it doesn not leak). Is it true, should this design work?
Click to expand...

Absolutely. Either that, or if you want to look at it differently, you could set up your actuator so that you are lifting off the seat with a signal of 4.1-4.2mA, and coming away from the 'full-open' at a signal of 19.8-19.9mA. This won't help alot if you are having a sticking problem, since you don't have any feedback, but it will atleast get you set up the best you can with a standard I/P.

If you set it up this way and it's not repeatable after several open/close cycles, I would tear the valve down and possibly invest in a positioner.
 

Similar Topics

C
Limit HP from Torque AI and RPM PID Output
Hello, I have a motor that we are sending a RPM Speed Output from 0-100% to the VFD. However, the Motor HP needs to be limited to 6000 HP and the...
Replies
3
Views
88
Rob...
R
B
generate the PWM signal with variable duty cycle according to PID output.
Hello Friends, I am looking for a way to take my PID Output( it is the result of my PI Algorithm in Real format) and generate a PWM Signal in...
2
Replies
16
Views
2,503
L D[AR2P#0.0]
L
D
Contrologix PID output locks up
Hello, I have a flow control PID that keeps locking up. It seems to control fine but after a while the output no longer moves. For instance...
Replies
4
Views
963
diat150
D
W
PID Output To Electric Actuator With Duty Cycle
I have an electric actuator with a 75% duty cycle. I am sending an analog output from a PID to a controller board on the actuator. My concern is...
Replies
6
Views
2,415
Tom Jenkins
Tom Jenkins
P
Compact Logix 1769 PID Negative Output
Hello. I am trying to figure out how to setup a PID in Control Logix 1769 Controller in a way that it would control a drive speed by sending...
Replies
3
Views
1,614
plc_dog
P
Back
Top Bottom