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  1. #1
    Join Date
    Mar 2005
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    168

    Steam Boiler Control

    What is the most efficient way to control steam boilers?

    I have 3 steam boilers with a heat exchanger with

    1/3 , 2/3, and a third modulating valve that was an old bypass

    but was converted because customer stated they didn't have enough heat in the coldeset days.

    So, any ideas?

  2. #2
    Join Date
    Nov 2009
    Location
    Saskatchewan
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    492
    Teckmar Controls

    Save 30 to 50 % on gas at the same time.

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  3. #3
    Join Date
    Mar 2008
    Location
    NY NY
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    181
    I like to jump in here, but have some questions.

    Are we talking about a steam to hot water HX?

    What is the design steam pressure?

    Assuming steam to hot water HX, what is the design hot water temp?
    Local 30 New York, New York Operating Engineer

  4. #4
    Join Date
    Mar 2005
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    168
    Quote Originally Posted by kdocsr05 View Post
    I like to jump in here, but have some questions.

    Are we talking about a steam to hot water HX?

    What is the design steam pressure?

    Assuming steam to hot water HX, what is the design hot water temp?

    We have 3 steam boilers that supply steam heat to certain places in the bldg.
    Each boiler has pressure controls to switch from low to high fire.
    The current steam header pressure setpoint on the heat timer panel is 11 psi but I don't know if that is right or not. We will be taking out the heat timer and putting in DDC controls. The heat timer was enabling each boiler based on header pressure.
    There is also a steam to hot water heat exchanger that supplys hot water heat to other parts of the bldg. Currently there is a DDC controller that modulates the 1/3 and 2/3 valves based on OA reset. In VERY cold days the valves are open but the system cannot keep up. We had to add a belimo to the manual bypass to get enough heat. So now we have 3 valves that we have to modulate.
    I believe the OA reset that was programmed had 180 deg for Hi HWR temp.

    I don't need to know what equipment but I was wondering
    if someone can tell me the most efficient sequence of operation.

  5. #5
    Join Date
    Aug 2006
    Location
    New England
    Posts
    520
    There's nothing wrong with the sequence of operation, but there is something wrong with your valves if adding steam through the bypass helped the situation. The limitation on heat output of the heat exchanger is its heat transfer capability. The valves should have been sized to supply more than sufficient steam to utilize all the heat transfer capability of the exchanger so either the valves were undersized when installed, the current steam pressure is lower than design or the valves are not supplying their full capacity for other reasons (like the steam trap is leaking by?). One thing to remember about steam is most of the heat doesn't come out until it's condensed. The trap should discharge water, but not steam.

  6. #6
    Join Date
    Mar 2008
    Location
    NY NY
    Posts
    181
    Sorry for the delay in response, the holiday had me occupied the past two days.

    I agree with the post regarding performance issues with the heat exchanger affected by reduction in steam flow. The reduction can be on the distribution or the return side of the loop. The fact that you have increased flow through the bypass, leads me to believe the source is in the supply. I would however look at the temperature on the outlet of the steam traps to rule out a trap that may be passing. I think a trap passing steam would also present additional symptoms typical of by phase flow in the return, water hammer, damaged return pumps, etc.

    Have the control valves been changed in the past? I have seen this before when contractors changed out steam valves ignoring the design cv which reduced the high load capacity on the waterside.

    How about design flow rate on the waterside? This could also be the source of the conditions mentioned.

    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
    Local 30 New York, New York Operating Engineer

  7. #7
    Join Date
    Mar 2005
    Posts
    168
    Thanks for the replies...

    I believe ctrlguy is correct when he says
    the valves were undersized when installed, the current steam pressure is lower than design

    The problem is on VERY cold days, otherwise the bldg is fine.
    I am leaning towards the current steam pressure is lower than design. The Heat Timer panel had only 3 relays and they were either enabling the 3 Boilers and /or switching low-high fire when the header pressure dropped below 11 psi. We arrived the boilers were in hand and we have no documentation. We added local pressure controls on each boiler to control low-high fire and set each one at 15 psi.

    kdocsr05:
    We can double check flow rates if we had documentation but we don't.
    I appreciate and will implement your sequence of operation for the valves.
    Now to enable each boiler, would it be better to enable each boiler based on header pressure or based on HWR demand?
    Hopefully, the current steam pressure setting is the issue and the problem will be resolved by using local pressure controls and increasing the pressure setting.
    After all is complete, I will update both of you guys.

    Again, thanks for the replies!!!!

  8. #8
    Join Date
    Mar 2008
    Location
    NY NY
    Posts
    181
    CMA,

    My next question would be, low fire/high fire do we modulate between a range, or two stage firing low fire and high fire?

    We should have a common pressure control which puts the call on the boilers proportional to the steam load. The local pressure control captures the firing rate at the boiler.
    Local 30 New York, New York Operating Engineer

  9. #9
    Join Date
    Mar 2003
    Posts
    7,326
    how can you pick an arbitrary number of fifteen psig for the steam setting? look around the boiler room and try to find something that indicates required settings. for example, relief valve set points, if at fifteen or thirty, you have other issues, and steam pressure is too high. control valves and traps are more practically the issue. just because you have a 1/3, 2/3 setup, doesnt mean the valves are sized correctly.many people only know to install a valve by line size and do not look at cv. you may be able to get the original specs on the exchanger. at any rate, I would look at my receiver to see if my condensate is flashing, indicating the same is probably occurring in the return lines. this could cause trap issues. try lowering steam pressure to around eight psi for starters, and possibly lower.

  10. #10
    Join Date
    Mar 2005
    Posts
    168
    Quote Originally Posted by kdocsr05 View Post
    CMA,

    My next question would be, low fire/high fire do we modulate between a range, or two stage firing low fire and high fire?

    We should have a common pressure control which puts the call on the boilers proportional to the steam load. The local pressure control captures the firing rate at the boiler.
    It is two stage low/hi fire control.

  11. #11
    Join Date
    Mar 2005
    Posts
    168
    Quote Originally Posted by flange View Post
    how can you pick an arbitrary number of fifteen psig for the steam setting? look around the boiler room and try to find something that indicates required settings. for example, relief valve set points, if at fifteen or thirty, you have other issues, and steam pressure is too high. control valves and traps are more practically the issue. just because you have a 1/3, 2/3 setup, doesnt mean the valves are sized correctly.many people only know to install a valve by line size and do not look at cv. you may be able to get the original specs on the exchanger. at any rate, I would look at my receiver to see if my condensate is flashing, indicating the same is probably occurring in the return lines. this could cause trap issues. try lowering steam pressure to around eight psi for starters, and possibly lower.
    I will take a second check around and see if I can find anything.

    Thanks for the replies...I will update you later!!!

  12. #12
    Join Date
    Oct 2003
    Location
    Minnesota
    Posts
    1,377
    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.
    Interesting ....

    Trying to wrap my head around this sequence you give.

    Granting that everything is sized properly, what's the advantage of your method?

    Just curious.

    I've done a number of 1/3-2/3 valve installations. Haven't had any particular problems as a result of implementing a fairly straight forward control.

    i.e. If using two analog outputs, a separate one for each valve, I typically adjust the two PID loops so that the 1/3 valve modulates first, then the 2/3 starts cracking open when the 1/3 valve is at about 75%. 2/3 valve control loop is watching the 1/3 valve control loop output, using the 1/3 valve control loop output as its MV.

    Or, often we simply use Belimo MFT's and a single analog control loop. 0-10 VDC output range from controller. With 1/3 valve programmed for .5 - 4.5 VDC full range of travel and 2/3 valve programmed for 5.5 to 10 VDC full range of travel. The DB between the signal ranges keeps 2/3 valve from repeatedly cracking and shutting very much.

    These schemes seem to have worked well for us but I'm always on the lookout for a better idea or one that might resolve an issue I haven't yet seen. Thus my interest in what advantages you see to your suggested sequence.
    A site where I stash some stuff that might be interesting to some folks.
    http://cid-0554c074ec47c396.office.l...e.aspx/.Public

  13. #13
    Join Date
    Jan 2003
    Location
    USA
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    1,685
    I have done something similar to kdocsr sequence. You want to minimize the amount of time either valve sits between 0-10% open. When the valves spend a lot of time just cracked open, the steam eats away the seats of the valve.

    We typically use the 1/3 valve first 0-100%. If the 1/3 valve remains 100% for a certain period, then we close it and use the 2/3 valve. If the 2/3 valve remains 100% for a period of time, then the 2/3 stays open and the 1/3 begins to open again.

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