
Limiting max output of PID
Has anyone found it necessary to limit the maximum output of a PID when replacing valves with new ones that have higher a Cv? Especially with steam fin tube. I have been doing a job with just this and 100% output seems to be way too much for spaces that have since had improved insulation (and thus reduced heating loads) but the terminal units remain the same. I have to manually tune these PIDs, so maybe I just haven’t found the best TR/PB yet. I’m already at 15deg TR in one room and still overshooting during warm-up.
Limiting the output of a PID?, No. Retuning? Absolutely.
The lower Cv rating gives appropriate valve authority. That means the control valve is st least half the pressure drop for the entire coil system (coil, ISOs, balancing valve, control valve). If the Cv is higher, you may hunt. You LIKELY will hunt.
Keep the Cv the same unless you have someone who understands the design make that change. Your best bet is to go back to a Lower Cv valve. You will try to not have to and will VERY likely fail.
Hmmmm….smells like numbatwo to me.
Good tips. In some cases I have been limited in what valves I could select because Siemens (as do everyone) have valves of limited Cv ranges. I’m replacing old Powers pneumatic Powermites for which parts have been difficult to obtain. I have to go see what I selected.
I actually have not been too bad with hunting, though that has been changing as I’m trying to eliminate the overshoot at warm-up. It’s also hard to get things right when it hasn’t been too cold outside here yet, and the steam plant shuts down when it warms up outside.
Honeywell has historically had very small increment steps in their vav reheat valves, making a good fit easier. If you can’t get the same Cv, go higher, unless it is a big jump. For example, an original valve is 2.4, a new one is either 2.0 or 3.2. Go 2.0, the 50% increase will likely hunt.
These are all ballpark figures. Your best bet is to either find prints that give Ideas on design, or reverse engineer a single box. That should give a good glimpse into the overall design.
What you want is a pressure drop that your pumps can still deal with (if you go lower Cv). Or that are not outliers with a high Cv. Google is your friend, it shows how to calculate pressure drop from Cv rating. A good first step.
Hmmmm….smells like numbatwo to me.
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So, the valves have already been purchased, and as I mentioned earlier, these are replacing Powers valves; the dimensions are very similar so as to be easy swaps, considering this building is all black pipe. In one of the offices where the valve has been changed, the new one is a 2.5. The submittals I have from the original controls didn’t have the cut sheet for those Powermite valves. I chose the 2.5 instead of the 4.0 for the reasons you articulated. What about the ramp time for the PID to go to 100%? I have it set to zero. Would it be wise to slow it down so the valve can’t jump to full open under low load warm-ups?
2.5 is a Lower Cv than 4. So it will allow less flow with a higher pressure drop than the 4 Cv valve.
This means it should provide better control, or less flow at a given valve position… but it might not provide enough at a full open position.
So you shouldn’t need to limit the full open position here, and the pid that worked on a 4 Cv valve will likely still work, but be a little slower.
If you put a 4 Cv valve in place of a 2.5, the PID that worked on a 2.5 valve will be too fast on a 4 Cv valve and may hunt.
Hmmmm….smells like numbatwo to me.
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You may have better results by re-scaling the analog output rather than the PID. This will allow you to limit the valve travel and if you find out later you need a little more then change the voltage range of the AO.
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I should add that I done this before and it has been a while…. I am a good figure-out-er and a bad remember-er…. so the specifics fade. It may be that your Cvs are not different enough to warrant a big change. Depends on several factors…
Everything posted is correct, but it is good to go into the weeds to have a good handle on exactly how much change is needed.
Hmmmm….smells like numbatwo to me.
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Originally Posted by theoldman
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You may have better results by re-scaling the analog output rather than the PID. This will allow you to limit the valve travel and if you find out later you need a little more then change the voltage range of the AO.
That’s an interesting idea. How would this be different than changing the max limit of the PID? I imagine it is to prevent possible integral windup.
Originally Posted by numbawunfela
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2.5 is a Lower Cv than 4. So it will allow less flow with a higher pressure drop than the 4 Cv valve.
This means it should provide better control, or less flow at a given valve position… but it might not provide enough at a full open position.
So you shouldn’t need to limit the full open position here, and the pid that worked on a 4 Cv valve will likely still work, but be a little slower.
If you put a 4 Cv valve in place of a 2.5, the PID that worked on a 2.5 valve will be too fast on a 4 Cv valve and may hunt.The good news here is that, even though it hasn’t gotten too cold yet, these new valves at 2.5 Cv have no problem heating up these offices from 63 in the morning to 70 for occupied!
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put a ramp pon the output of the pid. you can slow the valve down. Also put a high limit on the output of the valve,
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In passing you mentioned steam…
Any chance that the steam pressure is higher than originally designed for?
What other loads are on the same lines as your steam fin tube?
Is there any chance of lowering the steam pressure? This is obviously an older building and along the way it wouldn’t surprise me if someone decided that is some steam pressure is good, more is better. Lower steam pressures would call for valves with larger Cv.
Could you share exactly what valves you’re installing?
Originally Posted by CraftyNRingwise
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In passing you mentioned steam…
Any chance that the steam pressure is higher than originally designed for?
What other loads are on the same lines as your steam fin tube?
Is there any chance of lowering the steam pressure? This is obviously an older building and along the way it wouldn’t surprise me if someone decided that is some steam pressure is good, more is better. Lower steam pressures would call for valves with larger Cv.
Could you share exactly what valves you’re installing?The steam pressure is set to 2 psi. The submittals from Powers for the pneumatic system specified 5# pressure drop. Even with the old Powermite valves, the 2# I’ve been running for the last 9 years had no problem heating any space in the building. As far as other loads, I assume you mean other branches off the steam main. That would be fin tube and a few unit heaters (building is a former public school now used as a church and private school). The valves I’m installing are Siemens 599 Powermites (stainless trim) with the SSC61.5U actuators.