6 replies to this topic

[zezima]

I've attached a photo to help illustrate the below. Vent flow is currently directed towards the path I have highlighted in pink. The drawing is semi-accurate elevation wise. 

 

I work for a chemical manufacturing facility as a project engineer. We employ (2)- 28,000 PPH Keeler boilers. One runs in lead while the other runs in lag. We average 21,000 lbs/hr combined steam production flow between the two over the course of the year, but we sometimes reach as high as 42,000 lbs/hr during middle of winter when higher load demand in the plant is present.

 

We send feedwater from a pressurized deaerator tank through an economizer connected to the flu stack of our boilers. The feedwater from the DA tank to the economizers is ~235F. 

 

The DA tank is a Kewanee brand, no longer in business. We installed back in 1994. We operate the DA at 8-10 psig. The rated capacity of the DA tank is 60,000 lbs/hr.

 

We currently vent the noncondensables (oxygen, CO2, nitrogen, etc.) that our removed from the feedwater along with the motive steam, through what I believe to be a 1 1/4" vent line off the top of the DA tank that runs outside, then spans the entire vertical length of our boiler house building. An engineer back in the day had the "bright" idea to run this 1 1/4" vent line into a 12" stack that spans up the exterior of our brick boiler house about 40'. The original 1 1/4" line and the 12" line run parallel to one another and only extend 2 feet beyond the height of the parapet on the roof. 

 

My guess is that the engineer thought that by running the 1 1/4" line off the DA to a 12" stack it would reduce the velocity of the exhaust significantly. That, plus the increased heat transfer rate from more surface area on a 12" pipe, would be result in an increase condensation of the exhaust. I say this because there is a drain at the bottom of the 12" stack that is then routed back to our condensate recovery tank. This condensate recovery tank feeds our DA tank with condensate returned from all of our processes. 

 

After speaking with a vendor who is knowledgeable on DA tanks, they pointed out how this is counterproductive. We are recondensing the exhaust stream that contains all the molecules we wish to purge from our feedwater, and then sending this oxygen rich condensate back to our system to ultimately be fed right back into the DA tank! The very "stuff" we wish to purge from our system are being sent right back into the system.

 

At some point in time, the operators switched the flow of the exhaust from the 12" pipe stack back to the 1 1/4" pipe stack. The 1 1/4" line has no dedicated drip leg at the bottom, although it could technically flow into the 12" stack drain if you look closely in the picture. We see heavy amounts of condensate flow out the top of this 1 1/4" stack in addition to the vapor exhaust. 

 

A large section of the exterior masonry of the building is in dire need of repair due to the heavy condensate being spewed from the stack against the wall. In the wintertime we get massive ice formation against the wall, then thall freeze thall freeze repeatedly leading to extensive spalling of the brick. My plan is to demo both stacks (1 1/4" and 12") and run one new properly sized stack and extend it another 6-8' above the parapet to reduce the spewing of the condensate against the wall of the building. I may even run the new stack inside the building through the roof, then install a drain for any condensate to be directed back down to ground level. This way the wall deterioration problem we no longer be a factor.

 

In addition to this, I plan to put a drip leg at the bottom of this newly ran stack with a steam trap. The condensate will have a path to our sewage system, instead of routing back into the boiler system. This will also mean less condensate spewing out of the top of the stack.

 

My question is in regard to the sizing of this new stack line. I've read online from Spirax-Sarco and Inveno that DA tank venting is usually accomplished through manual adjustment. This can be done with a manually adjustable ball valve, needle valve, or orifice plate.

 

1) How is the sizing of the exhaust line calculated? 

2) What considerations do I need to make for cold startup of the system? Our boilers run 24/7, 365. We do two annual shutdowns, but keep one boiler running for shutdown 1, and the other running during shutdown 2, therefore cold start up would be extremely infrequent. 

 

 

If I'm missing any pertinent information or am unclear, please let me know.

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Edited by zezima, 01 February 2024 - 04:20 PM.

[breizh]

Hi,

"

After speaking with a vendor who is knowledgeable on DA tanks, they pointed out how this is counterproductive. We are recondensing the exhaust stream that contains all the molecules we wish to purge from our feedwater, and then sending this oxygen rich condensate back to our system to ultimately be fed right back into the DA tank! The very "stuff" we wish to purge from our system are being sent right back into the system. "

 

Agree with the comments above! 

Breizh 

 

[zezima]

breizh,

 

The 3 design/sales engineers of the DA tank manufacturer companies I spoke with all said you should not return the oxygen rich condensate back to your condensate recovery tank. The large pipe contractor who was going to manufacturer the new 12" stack prior to me discussing with DA tank manufacturers said he has fabricated DA tanks in the past and has seen recovery lines back to the condensate tank. Thought being that you would still recover the heat and the steam is already chemically treated. 

 

Furthermore, if the condensate recovery tank is atmospheric, partially open top. I believe the drain line is flex lined into the top of this condensate recovery tank. Would that allow the noncondensables to vent off partially upon entry of this drained stream into the condensate recovery tank?

 

So far, I have 3 DA tank design engineers proclaiming not to return the exhaust stream condensate back to a condensate recovery tank and 1 large pipe fabricator, who happens to make DA tank occasionally, saying it's something that can be done.

 

Perhaps the contractor has bias and wants to sell me the 50' 12" stack lol but I'd like more opinions.

 

Thoughts?

[Pilesar]

My guess would be that the deaerator steam condensate does not have that much oxygen in it compared to the cold feed water. As long as the recovered condensate were sent through the deaerator again, I would not be concerned about the oxygen content. Would this condensate be routed through a mixed bed for treatment before being sent to the boiler? If so, consider how the deaerator steam condensate and its chemicals would affect the resin. I've only seen deaerator steam vented, not conserved. I am surprised saving this condensate would justify much capital. 

[zezima]

Piping for the condensate to the condensate recovery tank from the da exhaust stack drain already exists. No capital needed.

 

As outlined in original post the flow is condensate recovery tank --> DA tank ---> boiler flu gas economizer --> boiler--> steam to process/plant --> condensate back to condensate recovery tank, repeat. I believe treatment chemicals are metered into the DA tank.

 

Regardless of whether we choose to send DA exhaust condensate back to the condensate recovery tank, how can I work out the proper steam/gas exhaust pipe sizing based on the parameters I provided? That is my main concern. Surely I don't need a 12" stack for the DA tank to exhaust the noncondensables sufficently, nor to collect condensate to send back to the condensate recovery tank if we so choose.

 

The exhaust penetration off the DA tank is 1 1/4" according to the manufacturer's drawing. The small stack that parallels the 12" stack is actually 1.3" OD; 1" sch 40 carbon steel according to my caliper. We are exhausting through the 1" stack, not the 12" stack currently. We are spitting condensate out the top of the stack, quite aggresively. 

[breizh]

Hi ,

To support your work and calculation:

https://www.spiraxsa...s?sc_lang=en-GB

https://www5.eere.en...r?random=random

https://invenoinc.co...ractices_23.pdf

Breizh

[Pilesar]

The last link from Breizh (Inveno Engineering document) gives guide for how much steam to vent and also says condensing the vented steam for recovery is standard practice and readily returns the capital cost. Perhaps the deaerators in my experience had vent condensers that I did not recognize. So size the vent pipe for the expected flow.

Here is a nice table for steam line sizing:  https://www.engineer...ipes-d_266.html

Is it really a steam/condensate separator you want to size instead of a vent line?

Edited by Pilesar, 03 February 2024 - 08:19 AM.