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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)

Originally Posted by freddy-b

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!

Originally Posted by DreamCar

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'

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".

Really good info guys, a good read.

You mean this?

Originally Posted by kdocsr05

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?

Originally Posted by kdocsr05

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.

Originally Posted by kdocsr05

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.

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.

Originally Posted by kdocsr05

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?

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.

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.

Originally Posted by kdocsr05

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.

Originally Posted by flange

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!!!

Originally Posted by kdocsr05

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.

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.

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.

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!!!!

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

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.