Cooling Tower Bypass Valve Actuator

[apprentice3]

I have an existing job that used to have a 6” Delta control valve for the cooling tower bypass. The valve in question modulates to maintain a condenser water supply temperature. On cold starts the bypass valve bypasses the cooling tower to allow the chillers to warm up their condenser water. Recently they have been having issues with the replacement Belimo actuators failing every 2 years or so. I noticed that the replacement modulating valves had a 20 second stroke time. As soon as the control loop responds, the valve is already moving into position rather than having a slower moving actuator to stabilize a little better. The original Delta actuator (DMS24-3500) doesn’t have its stroke time listed. I cannot find a specification sheet anywhere for this actuator. We even called the manufacturer who bought them out, and they don’t know the stroke time either. Would anyone happen to have access to the specification sheet for this actuator? What is the recommended stroke time for a modulating cooling tower bypass valve? I would think at least 60-90 seconds.

Any feedback is appreciated! Thank you!


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[ehsx]

PID is more important than speed. Condenser water temperature rate of change should be limited to < 1*f per minute. Screw chillers are the most critical for oil management.

Check the valve for ease of movement. Belimo may be stacked for higher torque apps.

[numbawunfela]

X2
Instead of shackling the PID to a slow valve, just slow the PID and the fast valve will move slower.
You may be better off ensuring the belomos have the same torque rating as the old Delta actuator. Assuming the valve can open and close freely.

[orion242]

I would think at least 60-90 seconds.

You can bet the farm on that 9/10 times, especially with valves sizes in that application.

Early deaths like your talking about and given the application almost always one of two things. Weatherproofing failure, ratio of drive time vs idle time. Besides install stupidity, using an HVAC actuator as an RC drive motor is almost always the case when I find repeated early deaths.

If that PID is driving the actuator constantly, its going to be a short life. 2yrs is still within Belimo's warranty. Don't expect to cash that in more than once or twice. All but the bottom models calc the motion vs powered ratio. App like this really has no reason to reposition more than 4x a minute. Even then it shouldn't be 0%, 100%, 0%, 100%. You have a large thermal mass and if the valve is sized correctly most of its life should be gradual controlled changes. Building might occupy and require some large changes, but in general it should be small tweaks. Aka low reposition vs powered ratio. If that's high, your PID tuning / control is crap and any actuator will die early every time.

Ask me how I know...

[numbawunfela]

Ask me how I know...

I bet it is the same way I know 480v hurts like the dickens.... (I got juiced today)

[orion242]

I bet it is the same way I know 480v hurts like the dickens.... (I got juiced today)

Lol. Got lit up this week as well which is very rare. Damn single strand 180 deg outside a wire nut through me to the ceiling grid. Somehow OSHA / building inspectors will let HVAC equipment be installed in the most retarded locations even though it violates code. Yet I have to prove me touching it isn't my problem. Pays to know where things get sketchy and conductivity at least. Wife wonders why I don't put metal on my body...

[Right Control]

I bet it is the same way I know 480v hurts like the dickens.... (I got juiced today)

Dang man be careful one of your nine lives is now gone. Took a really bad one from 277vac about 10 years ago. That one was a trip to the hospital to get the ticker checked. They found it was still beating and I was glad.

[apprentice3]

Thanks for all of the replies! My initial thought was to adjust the PID loop parameters to slow things down. However, after I got into the control system (1995 DOS based control system), I found that there was some limited functionality. The line code program that was controlling this valve was strictly proportional. After I realized I was dealing with this old of a system, I started to ask some questions. They have only recently started to see the actuator failures every 2 years. Basically they had the original Delta actuator on the valve for nearly 18 years. Then from there every Belimo actuator only lasted about 2 years. I have confirmed that the torque rating on all of the actuators match. However, I saw that the stroke times were different across a few of the Belimo actuators. So it made me question what the stroke time was on the original Delta actuator. I figured that a poorly tuned PID loop OR a proportional only loop would really cause some havoc with a fast stroke time. The Belimo actuator in question has a 75% duty cycle, I assume that it is probably exceeding that at times. I just felt that a slower moving actuator would react more slowly to smaller changes on the control loop. The control loop basically hunts between about 0% to 20% on start up. I felt one factor might of been the actuator stroke time since it never seemed to do this originally.

Ultimately, they need to upgrade their control system to something more functional that will allow modern day PIDs to take control over this valve. This old system doesn't have loop tuning parameters listed very clearly in the code. It will just say something like TSIPid(3,CTBYPASS,CWS,65.0,SPA,0.0,100.0,0,20.0,0. 0,0.0,0.0,0.0);. All of these values are hardcoded, and have to be redownloaded every single time (which continually starts the whole process over again). I am sure that one of the zeros is probably integral. Not being able to make loop tuning parameters changes live, can make this process difficult.

Thanks again for the responses.

[ehsx]

" The control loop basically hunts between about 0% to 20% on start up. I felt one factor might of been the actuator stroke time since it never seemed to do this originally."

Valve may need to start with a minimum position. Not clear if this is a 2-way or 3-way valve. I've had to put manual stops on 2-way valves to limit closure and stabilize operations on startup. Butterfly valves are a pain to regulate since not proportional.

[apprentice3]

" The control loop basically hunts between about 0% to 20% on start up. I felt one factor might of been the actuator stroke time since it never seemed to do this originally."

Valve may need to start with a minimum position. Not clear if this is a 2-way or 3-way valve. I've had to put manual stops on 2-way valves to limit closure and stabilize operations on startup. Butterfly valves are a pain to regulate since not proportional.

It is a 2 way butterfly valve.


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[ehsx]

It is a 2 way butterfly valve.


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They suck for low flow control. I used to cut a bushing to limit the valve to ~10% to stabilize on startup.

[dieseldude]

Damn!! be carful. If you were well grounded, you wouldn't be typing here

[numbawunfela]

Hehe - when I did mech work I was a little more exposed. I have not been juiced at all in 4 or 6 years. But I been juiced maybe a dozen times, 3 or 4 times by 480, in the last 15 years. Usually just a brush up against something.
I always wear the fat insulated steel toe boots I wore as a Mech guy... it buys a little street cred with the fitters (more than loafers do) and is handy in case something like this happens. So no, not grounded.
Hope for the best and plan for the worst.

[Right Control]

Hehe - when I did mech work I was a little more exposed. I have not been juiced at all in 4 or 6 years. But I been juiced maybe a dozen times, 3 or 4 times by 480, in the last 15 years. Usually just a brush up against something.
I always wear the fat insulated steel toe boots I wore as a Mech guy... it buys a little street cred with the fitters (more than loafers do) and is handy in case something like this happens. So no, not grounded.
Hope for the best and plan for the worst.

In 26 years I've never been shock by 480, have worked on it hundreds if not thousands of times. Got the 277vac from a fan powered box in a ceiling over a desk like I said in my previous post. Anybody that has done this is going to get bit that is a fact, but 480vac I where gloves and safety glasses every time. Was standing next to a guy when his meter exploded while checking 480vac that started that habit. I also never hold the meter in my hand if I can help it, using the magnet on my meter instead. Maybe I'm over cautions, but my fingers come in handy and I do like having them.

[orion242]

They suck for low flow control.

Ditto.

Might want to put a deadband on your set point when the valve is <25% open to try and keep it from hunting all the time.

[orion242]

Was standing next to a guy when his meter exploded while checking 480vac

The ol wire around the fuse trick or harbor freight meter?

[CraftyNRingwise]

1. is the bypass outside or inside?
2. What kind of climate are you in?
3. Is this a year round application or seasonal?
4. Is this two butterflys on a tee or some other kind of valve? Butterflys aren't the greatest control element but they do make a lot of sense for tower bypass.
I haven't seen mention of the other half of the equation. How are the tower(s) controlled? Perhaps a description of the tower configuration would help (multiple cells, multiple fans, etc.) Am I correct in assuming that the bypass valve works in sequence with some kind of tower capacity control? My expectation would be a VFD arrangement. IF so I would think that after initial startup the bypass valve would wind up in the full flow to the tower position and actuator repositioning wouldn't be much of a problem.

[Right Control]

The ol wire around the fuse trick or harbor freight meter?

Well neither. He was on a roof all day in the rain the day before. He left his meter in a bucket full of rain then takes the meter to a electric boiler with a 1200 amp service. I know nothing about his prior stupidity and was just standing there. It scared the s**t out of me!! That boiler was decommisioned this year for a new gas plant, so glad not to be digging around in that thing anymore.

[orion242]

Well neither. He was on a roof all day in the rain the day before. He left his meter in a bucket full of rain then takes the meter to a electric boiler with a 1200 amp service.

That certainly could fill ones shorts.

Fluke 289, blew the ma fuse at some point. Needed to check a 4-20ma circuit and figured out the stupid $15 fuse was still blown. Wrapped it to get'er'done. All fine for that, stupidest thing ever but got it done on some far flung remote site. Few weeks later, have a side job doing resi home electrical. Utility meter in, at the 200A panel ready to light things up but the main is off yet. Resi mains at its finest. Before I turn on the main breaker for the first time, I want to verify the hots are what are and the ground neutral are good. First check, phase to phase, don't notice my meter leads are still in ma current when I hit those leads on the lugs.

When the second lead made contact I knew something was very wrong. Will say that Fluke contained the result impressively well. In hand, next to face...likely no harm. Do that on 2K 480V service, could be very different. That meter still worked in every mode but current after that which blew my mind after looking at the internal carnage. Will never buy a cheap meter and carry plenty of expensive fuses ever since.

[MaxBurn]

This scary stuff is why I got out of working on UPS systems, everything in here will kill you. Those are buss bars, all hot. Some of it is in the 500v DC range which will hold you too. Also in training they told us to not use other peoples tools, meters/leads especially. I used to go through a set of leads every year.



Stay safe out there.

Back on topic; yeah they've gotten a decent long life out of that contols system it seems. I might try finding an exact replacement for the old actuator.

More likely something changed after the 18 years and killed that actuator and every other one. That might be hard to track down.