Primary/Secondary system problem

[remp]

I am working on upgrading an old boiler installation in a large site. The utility building boilers ( 3 off x 5mW each) serve 6 different buildings through a tunnel under the site. There are primary and secondary pumps but it was origionally 3 port valves on the secondary side. Most have been changed out to 2 port over the years. There is 2 differential pressure sensors going into 2 off the building and the sec. pumps ramp up/down to to maintian the worst case. The set point is 2 bar.

The problem is even in the middle of last winter the boilers are only running a half duty with 3 of them ON and the delta T is only 10 Deg C, the design Delta T is 25 deg C. The sec. pumps have not really ramped down a whole lot since winter and now i am in summer. OK the site is a manufacturing and hot water is used in some processes but the delta T now is only 5 deg C... but maintenance want ot keep all boilers on.
With the low Delta T is looks like one boiler can do the job but if i turn off the other 2 boilers it is too much water from the sec loop to get thro the running boiler...mind you I dont knwo how much water is in the sec loop for sure... I got it check with ultra sonic but i dont believe it 100%... I was very high...
Am i missing something...maybe I have to check the site for more by pass valves and more 3 port valves that need changing...
Maybe the differential pressure set point is too high although 2 bar DP between F&R going into a building does not seem to high either but i might drop it and see if there is any complaints...this will slow down the sec. pumps. Iwas going to close off 1 bulding at a time and see which building give the biggest chaneg in system pressure..
Any tips...??

 

[tys90]

If you can't turn off 2 boilers because there is too much water flow in the primary loop then there is a problem.

 

[ChrisConley]

The whole advantage to primary/secondary piping is that you can run different flowrates on either side.

If your secondary flow is much higher than your primary flow (one boiler, one boiler pump?) then you can continue to expect to see low system delta T.

If you have some budget, I'd consider looking at an intergrated pump from Armstrong (or others, I just know Armstrong). Could be some pretty major savings on pump power, plus you'd be dropping your secondary flow rate enough that you'd get rid of your delta T problem.

http://www.armstrongpumps.com/ivs/index.html

Once I wrapped my head around what they're doing, and saw some installation along with some energy savings, I became quite interested in the concept.

 

[remp]

Chris

I dont really know what the flow rates are yet. Im in the process of putting in a decoupling line with a flow meter for future staging. the sec. system pressure needs 2 bar. The existing decoupling line is in the wrong spot although it will take the excess or deficate flow but i dont like the location. there is a process on site that requires 2 bar in the lines or else there is trouble.

Today I ran 2 boilers and switched off 2 boiler ( actually there is 4 boilers on site) the two boilers are designed for 50l/s and 115/135 deg C. The 2 boilers were running at 100% modulating ( i must check if that means 100% capacity) and kept the supply temp at 135 but the return was only 125deg C. Half the design. The water flow may have been double the desing for all I know at this stage as there is no balancing valve in the boilers

I closed the existing by pass so the primary and sec pumps were sort of running in series and the sec. pumps slowed down..

Just kind of messing about with the system at the moment. Will know more when the decoupling line is in and balancing valves on the boilers.

Still a bit worried as i have suggested removing one of the boilers since 3 in winter were only running at 50% capacity..

 

[ChrisConley]

Possible to upload a schematic of your system (with any known flows)? I'm not seeing what you're describing correctly.

 

[marcoh]

Is your 125 return temp from the secondary pumping circuit or the across the boilers?

If it is from the field then you have a coil/valve/control issue not related to the boilers. You mentioned there are still some 3 way valve which will always bypass water and lower return water so you should be aiming to replace these as soon as possible.

 

[remp]

Turns out that the old air handling unit coils are sized for a 12 DegC Delta T . 2 off the old boiler sized for 12 degC delta T and 2 off newer the boilers are sized for 25Deg C delta T - Hecne the high water flow rate in the sec. side.
I found out the consultant on the job 5 years ago deciede to make this fundamental change when the 2 newer boilers were going in. Explain why they ever only fire up to 50% in the middle of winter and the old one next to it is doing 100%. I will increase the flow rate in the 2 new boilers to decrease the Delta T so it will match the older boilers.

Thanks to all