Steam Boiler Control - Page 2
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  1. #14
    Join Date
    Mar 2008
    Location
    NY NY
    Posts
    181
    Osyio,

    The key to this control sequence relies on proper selection of the valves.

    A typical good design for parallel transfer has the 1/3 valve at 25% and the 2/3 valve at 75% total pressure drop. Assuming this is application correct the control sequence as stated by Orion is intended to limit modulation of the 2/3 valve close to the transition point or close off point. This will minimize the potential for unwanted erosion of the valve trim.

    In my area when using parallel transfer at steam pressures above 15 psi, a method to limit modulation of the 2/3 below 10% is required in the steam code.

    The code sequence is easy, the program may be a little more involve then what you are currently doing. Since I am not a controls programmer I will not be of help there. The key to good tuning is the loop transition point and the timing need to be adjustable.
    Local 30 New York, New York Operating Engineer

  2. #15
    Quote Originally Posted by kdocsr05 View Post

    I have included a control sequence for your application, which works well with properly sized equal percentage valves.

    1. upon a call to run, the 1/3 - 2/3 steam valves will be modulated by a PID control loop to maintain the required hot water temperature set point. The valves will be controlled in the following manner:

    a. The 1/3 valve will be modulated first.

    b. If the output of the PID control loop exceeds 25% (adjustable) for more than 2 minutes (adjustable), the 2/3 valve will become the first valve modulated making the 1/3 valve control range switch to the 66% to 100% output range of the PID control loop.

    c. If the output of the PID control loop exceeds 60% (adjustable) for more than 2 minutes (adjustable), the 1/3 valve will become the first valve modulated (the 1/3 valve will be 100% open at this time) making the 2/3 valve control range switch to the 33% to 100% output range of the PID control loop (the 2/3 valve will be modulating at this time).

    d. The reverse sequence shall occur at the 40% (adjustable) and the 25% (adjustable) points as steam demand decreases.

    e. The sequence shall also reset anytime the output of the PID control loop equals zero.


    Hope this helps.


    Regards
    Thats really good.

    Are you a ME?

  3. #16
    Join Date
    Mar 2005
    Posts
    168
    Just an update...

    Found no documentation but the relief valves are rated for 15 psi.

    We installed local pressure controls for each boiler to modulate
    low and hi fire at 12 psi.
    I put the header pressure DDC setpoint at 12 psi to enable each boiler
    We will monitor the boiler reaction for now then we will adjust the
    setpoint accordingly to enable each boiler.

    Then I implementated the HX valves sequence as
    Originally Posted by kdocsr05.

    Thanks for the replies...once we let the system operate for awhile
    and everything is operating well, I will give you guys an update.

  4. #17
    Join Date
    Oct 2003
    Location
    Minnesota
    Posts
    1,369
    Quote Originally Posted by kdocsr05 View Post
    The key to this control sequence relies on proper selection of the valves.
    Indeed, the key to the proper operation of a lot of things is proper valve selection.

    In any event, Thanks to both you and Orion for the info. Very interesting.

    I have been very familiar with the problem of steam cuts in the past. Ex-Navy, used to dealing with 600 to 1200 PSI steam systems. But have been away from dealing directly, hands-on, with steam system operation, maintenance, and repair for many years. Other than installing and programming DDC controls on low pressure systems. So hadn't really given much thought to the possible problems of steam cutting on barely cracked valves in low pressure systems.

    Now you have my curiosity perked. And after the holidays I'm gonna have to quiz the fitters in our service department as to how much of an issue this has been on the 1/3-2/3 systems we've installed, that've been in service for sufficient time for them to say if it appears to be a significant issue.

    All the systems we've dealt with in recent history are <= 15 psi. Usually less than. A far cry from the type of issues I had to deal with back in the old engineroom/fireroom days aboard a Navy ship.

    Quote Originally Posted by kdocsr05 View Post
    The code sequence is easy, the program may be a little more involve then what you are currently doing. Since I am not a controls programmer I will not be of help there. The key to good tuning is the loop transition point and the timing need to be adjustable.
    No problem as concerns the programming. Piece of cake.

    Again, thanks for giving me something to think about that I'd not even considered.
    A site where I stash some stuff that might be interesting to some folks.
    http://cid-0554c074ec47c396.office.l...e.aspx/.Public

  5. #18
    Join Date
    Mar 2008
    Location
    NW Ohio
    Posts
    949
    Quote Originally Posted by kdocsr05 View Post
    Osyio,

    The key to this control sequence relies on proper selection of the valves.
    Really?

    The code sequence is easy, the program may be a little more involve then what you are currently doing. Since I am not a controls programmer I will not be of help there. The key to good tuning is the loop transition point and the timing need to be adjustable.
    Why don't you want to post the example that goes with the SOO you posted?

  6. #19
    You mean this?
    Attached Files Attached Files

  7. #20
    Join Date
    Dec 2007
    Location
    chicago
    Posts
    249
    Really good info guys, a good read.

  8. #21
    Join Date
    Dec 2009
    Location
    Canada
    Posts
    39
    If you have trouble tuning with a single PID loop on both valves, try a pair of PID loops - one for each valve. Simply lock them at 0 or 100% according to your sequence. That way you get separate tuning parameters for each valve, may help it be more stable if the single PID is trouble.

    We started doing this when we got good self tuning PID's rather than our default 90% tuning parameters that were "good enough".

  9. #22
    Quote Originally Posted by DreamCar View Post
    If you have trouble tuning with a single PID loop on both valves, try a pair of PID loops - one for each valve. Simply lock them at 0 or 100% according to your sequence. That way you get separate tuning parameters for each valve, may help it be more stable if the single PID is trouble.

    We started doing this when we got good self tuning PID's rather than our default 90% tuning parameters that were "good enough".
    You get too gnats ass on the low end ...you will go through seats faster than Tiger Woods goes through mistresses'

  10. #23
    Join Date
    Dec 2009
    Location
    Canada
    Posts
    39
    Quote Originally Posted by freddy-b View Post
    You get too gnats ass on the low end ...you will go through seats faster than Tiger Woods goes through mistresses'
    One PID or 2 doesn't change the rest of your sequencing, just how it tunes. You still need to do all the rest - ie make sure it doesn't spend too much time below say 10%, and open very slowly on cold start to prevent hammer.

    The gnats on Tigers mistresses should be safe!

  11. #24
    Join Date
    Jul 2002
    Posts
    862
    I vote for Orion. Is there any building feedback? If there has been no call for heat for one hour I hold the boiler water at 180 for three hours. If no call in three hours it shuts down. (you would need boiler water sensors though)
    Second place is the first loser.

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