Page 2 of 3 FirstFirst 123 LastLast
Results 14 to 26 of 27
  1. #14
    Join Date
    Dec 2014
    Posts
    2,896
    Post Likes
    Curious...where is this cavitation possibly happening? In the coil after the 2-way? Flashing from the sudden pressure drop?

    How does vena contracta function in a small pipe?

    What is 'recovery factor' on a valve?

    Sent from my SM-G950U using Tapatalk

  2. #15
    Join Date
    Feb 2016
    Location
    Louisburg Kansas
    Posts
    3,523
    Post Likes
    Thread Starter
    The cavitation I'm concerned that might happen is at the outlet of the control valve. The valves that are controlling almost closed will have a lot higher velocity at the outlet than normal. The sudden pressure drop may cause flashing because the velocity will be high. The high velocity may push the vena contracta past the valve outlet before it recovers. If that happens will I get noise from the collapsing of the gas bubbles? I don't know the answer to that.
    Vena contracta functions about the same way in any pipe size because the size of the contraction is proportional to the pipe size.
    The recovery factor is a factor used when calculating the maximum pressure drop of a control valve before damaging cavitation results. Different size valves have different factors. The factor was empirically determined and is dimensionless.
    The formula for determining the damaging cavitation threshold pressure drop is
    (Water Entering Pressure in PSIA x the recovery factor) quantity divided by the vapor pressure. The results is PSIG.

    That is with the valve wide open.
    No man can be both ignorant and free.
    Thomas Jefferson

  3. #16
    Join Date
    Aug 2019
    Location
    Central Florida
    Posts
    87
    Post Likes
    Since you have a VFD I would set the total to each floor by the return water temp . Then you can play with it a bit to find the DP setpoint for the VFD . Being you have infinite control of the pump I wouldn't worry about cavitation ( yet …lol ) You mention the design fan cfm is too high . I always use the following if the design numbers are wacky......Design max primary cfm - Design min primary cfm = fan cfm . Good luck , wish this job was close I would come over and between the two of us we could really create a mess .

  4. #17
    Join Date
    Feb 2016
    Location
    Louisburg Kansas
    Posts
    3,523
    Post Likes
    Thread Starter
    True brad we could discuss it to death if nothing else. To do what you said about return temperature it would require balance of the individual circuit setters. There are so many miss-sized I don't want to do that. I don't disagree with the theory but in this application it isn't feasible.
    I finally decided to not balance any of the system except DP set point. The control valves will take care of heating the building with no problem even though some of them will hunt. The control guy reset the PID to 180 seconds. That should help some. He also said the valves would go full open upon a call for heat then throttle back. That will keep debris from collecting on the valve seat of the oversized valves. I don't know how to explain all this in the balance summary without getting technical and losing everybody. Nobody on this job has a good technical understanding of hydronics so how do I communicate? If you have words that will help I'm interested.
    On the FPB fan airflow the minimum airflow is too high but the outlet temperature is good because the coils are also oversized. The only way to get the airflows lower is to reset the manufacturer's SCR minimum adjustment. I can set the minimums down to 60% of the motor input voltage without damaging the motor but don't want to do that unless there are complaints.
    No man can be both ignorant and free.
    Thomas Jefferson

  5. #18
    Join Date
    May 2014
    Location
    Bay Area California
    Posts
    24,756
    Post Likes
    You need to tell it like it is. As gently as possible, while stating the facts. Start out something like . . . Due to improper engineering and oversized equipment and valves, a conventional balancing and commissioning is/was not possible.

    I've walked up to a few jobs in my day that were messed up. You just look for the easiest way out. But it always starts the same way. You gotta state right up front that things are messed up. In these/those cases, I usually do everything in steps. Meaning, step one is the cheapest and fastest. Steps two, three, and on get more expensive if required.

    You could probably incorporate something along those lines. Just to show that you did your due diligence under the circumstance you were given.


    Quote Originally Posted by WAYNE3298 View Post
    True brad we could discuss it to death if nothing else. To do what you said about return temperature it would require balance of the individual circuit setters. There are so many miss-sized I don't want to do that. I don't disagree with the theory but in this application it isn't feasible.
    I finally decided to not balance any of the system except DP set point. The control valves will take care of heating the building with no problem even though some of them will hunt. The control guy reset the PID to 180 seconds. That should help some. He also said the valves would go full open upon a call for heat then throttle back. That will keep debris from collecting on the valve seat of the oversized valves. I don't know how to explain all this in the balance summary without getting technical and losing everybody. Nobody on this job has a good technical understanding of hydronics so how do I communicate? If you have words that will help I'm interested.
    On the FPB fan airflow the minimum airflow is too high but the outlet temperature is good because the coils are also oversized. The only way to get the airflows lower is to reset the manufacturer's SCR minimum adjustment. I can set the minimums down to 60% of the motor input voltage without damaging the motor but don't want to do that unless there are complaints.
    If you were a real tech, you'd solder a relay on that board and call it good to go.

    I do a triple evac with nitro to remove non condensables.

    I use 56% silver on everything except steel.

    Did you really need the " If you were a real tech " ??

  6. #19
    Join Date
    Feb 2016
    Location
    Louisburg Kansas
    Posts
    3,523
    Post Likes
    Thread Starter
    What has made it difficult BBeerme is the system heats really good and that has everyone convinced everything is OK. I basically have to accurately predict the future in regards to how the valves will stand up to being abused when I'm the only one that knows it is happening. In their minds I can't even prove the system isn't right.
    This is a redesign of a building HVAC system that never made the building comfortable. It does now and everyone is happy. My job is to bad mouth part of the design. It's like telling someone they have a disease when they have no symptoms.
    I'm trying to do this without looking like a smart ass trying to baffle them with BS.
    The hardest part is convincing them there is good reason not to attempt balancing the heating water system. There are balancers a dime a dozen that don't know how to design these systems or even how they work that will jump at the chance to do the balance. They won't back anything with facts or theory just set the circuit setters without concern for the outcome.

    Do you think pointing out the obvious over sized control valves and circuit setters is enough without getting any more technical than explaining how equal percentage works? That seems pretty simple to understand to me.
    No man can be both ignorant and free.
    Thomas Jefferson

  7. #20
    Join Date
    May 2014
    Location
    Bay Area California
    Posts
    24,756
    Post Likes
    Keeping it simple is always important. Much of what you have said in this thread could even be used. Saying what you did about the programmer opening the valves 100% to keep them clear due to them being oversized and running near closed also seems rather simple. Things like that would show that you did the best you could under the circumstances.

    Probably wouldn't work in your case here, but often when I am talking to customers about a piece of air conditioning equipment, I make the relation to an automobile. Because everybody knows what a car is so complicated things seem to make more sense to them.
    If you were a real tech, you'd solder a relay on that board and call it good to go.

    I do a triple evac with nitro to remove non condensables.

    I use 56% silver on everything except steel.

    Did you really need the " If you were a real tech " ??

  8. #21
    Join Date
    Sep 2002
    Posts
    26,504
    Post Likes
    I don't know anything about those valves but my first thought would the difficulty of obtaining anything like stability in the operation. With valves that (relatively) large the tendency to overshoot the control point will be substantial. And an overshoot soon leads to undershoot. <g>. And as all the valves may be doing it at the same time it seems like a big bag of worms to untangle.

    Are the valves two position or modulating? Can you limit their full-opening to a percentage? Or control the modulating rate to dead-slow? <g>

    Oh wait; "circuit setters" I now see you wrote. Are there also individual control valves or is the flow always constant through the coils?

    With the balancing valves oversized will there be any tendency towards noise?

    PHM
    --------



    Quote Originally Posted by WAYNE3298 View Post
    I'm doing TAB on a job that has been interesting from start to finish. I thought you guys might like to hear about some of the fun.
    The building has a history of not heating or cooling properly. For this reason the redesign is over kill of the highest order. Every space has a fan powered box and every space has a design airflow and heating design that considerably exceeds the need.
    The present problem is that most of the FPB water heating coils were downsized at the last minute but they forgot to down size the circuit setters and control valves. A few coils were up sized. The circuit setters are Nexus Orturi and the design orifice DP'S vary from 0.81 inches of water to 129 inches of water to provide design GPM. The control valves were provided the same size as the circuit setters.
    I'll give my take on this after getting some of your comments but thought you guys would like to add your two cents.
    PHM
    --------
    The conventional view serves to protect us from the painful job of thinking.

  9. #22
    Join Date
    Sep 2002
    Posts
    26,504
    Post Likes
    After reading through all of what everybody else had to say; maybe the above was my one cent. <g>
    PHM
    --------
    The conventional view serves to protect us from the painful job of thinking.

  10. #23
    Join Date
    Feb 2016
    Location
    Louisburg Kansas
    Posts
    3,523
    Post Likes
    Thread Starter
    The system won't be stable Poodle but it will heat the building flawlessly. That's why this is difficult to explain. Moet engineers don't really understand what they have designed on water systems like this. The only thing circuit setters in series with equal percentage two way control valves are good for is to prevent excess flow when the control valve is full open. When the valves first open on a call for heat they may go full open but immediately close some to control flow to meet the load. The bottom line is it was a waste of money to install them in this system.
    Yes Poodle if I try to set the oversized circuit setters to design flow there will be noise. Believe it or not it would also be more difficult for the control valve to control flow since the valves are in series.

    BBeerme your advice about telling them what steps have been taken to assure the system functions with minimum hunting sounds like a winner to me. That made me think that explaining what I did and why in laymen terms might be the best approach.
    Laying the foundation now and leaving the door open to a more technical explanation just may be the right approach. Getting more technical can be done later if they choose to challenge the report.
    No man can be both ignorant and free.
    Thomas Jefferson

  11. #24
    Join Date
    Aug 2019
    Location
    Central Florida
    Posts
    87
    Post Likes
    Quote Originally Posted by WAYNE3298 View Post
    True brad we could discuss it to death if nothing else. To do what you said about return temperature it would require balance of the individual circuit setters. There are so many miss-sized I don't want to do that. I don't disagree with the theory but in this application it isn't feasible.
    I finally decided to not balance any of the system except DP set point. The control valves will take care of heating the building with no problem even though some of them will hunt. The control guy reset the PID to 180 seconds. That should help some. He also said the valves would go full open upon a call for heat then throttle back. That will keep debris from collecting on the valve seat of the oversized valves. I don't know how to explain all this in the balance summary without getting technical and losing everybody. Nobody on this job has a good technical understanding of hydronics so how do I communicate? If you have words that will help I'm interested.
    On the FPB fan airflow the minimum airflow is too high but the outlet temperature is good because the coils are also oversized. The only way to get the airflows lower is to reset the manufacturer's SCR minimum adjustment. I can set the minimums down to 60% of the motor input voltage without damaging the motor but don't want to do that unless there are complaints.
    What I meant about setting each floor by return temperature was using the main valves to each floor , one adjustment only to get the return water from each floor the same temperature . ( glad this job is yours and not mine ...lol )

  12. #25
    Join Date
    Feb 2016
    Location
    Louisburg Kansas
    Posts
    3,523
    Post Likes
    Thread Starter
    There are no such valves to set. I agree with your suggested approach but can't do it. It is going to be hard to explain in understandable terms why water balance shouldn't be done. Nobody in the loop even knows what an equal percentage control valve does. Without them knowing that I'm afraid a technical explanation would make them think I'm trying to baffle them with BS.
    How would you approach explaining this Brad?
    It looks like at this point the technical explanation is my only choice. They will probably muter among themselves but act like they understand.
    No man can be both ignorant and free.
    Thomas Jefferson

  13. #26
    Join Date
    Aug 2019
    Location
    Central Florida
    Posts
    87
    Post Likes
    I will pm you with my number later on ….give me a shout .

Page 2 of 3 FirstFirst 123 LastLast

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •