Boilers: Design of "keep warm" systems 1

[MJCronin]

To all..

We have a proposed client in Texas that has been asking us about the best type of retrofit for an existing boiler.

Our client want to use a "steam sparging" system to keep the boiler warm. Spargers would be supplied with 150 lb saturated steam from an auxiliary boiler.

My only experience with " keep warm" systems has been with steam-fed tube coils in lower mud drums or with small auxiliary guns in the burners.

Of all of these systems, which is the best and what are the advantages and disadvantages ?

Does anyone offer a packaged "steam sparger" sytem with controls etc..?? Or are all such systems unique ?

Thanks much for your time....

Regards

-MJC

 

[davefitz]

For a drum type boiler, natural circ design, I am only familiar with the same sort of warming system you described. However, in europe, there is system used on once -thru boilers that might be revised to be used on drum boilers, as follows:

On european once thru units,the boiler recirc pumps are left on during the outage. The furnace outlet fluid is manifolded into a common transfer pipe prior to feeding the integral steam/water separator bottles. 150 "# sat steam is sparged into the comon transfer pipe at the top of the furnace; this allows the entire boiler to be kept at 350 F, well into the ductile regime of the advanced ferritic alloys used- this permits a very fast boiler restart with minimal thermal stress and reduced startup oil consumption, and also prevents low temp corrosion during the outage. CO2 footprint is reduced by using a biomass fired 150# aux boiler.

Duplicating this effect on a drum boiler would require some piping chages at the economizer outlet pipe that feeds the steam drum. Remember: per code, the drum feed nozzle already has a thermal sleeve ( if the drum is rated above 150 psig) so this connection is not expected to have high stresses, compared to the older case of sparging downcomers or waterwall panels. A small recirc pump would source suction from several downcomer drains, and pump the water thru the economizer ( new connection to economizer inlet header required, maybe include a thermal sleeve). The economizer outlet transfer pipe to the drum would have installed a sparger connection, designed to prevent pipe hammer due to collapsing steam bubbles.This sparger to be located at the highest practical location, to maximize the sat liquid temperature feasible with this configuration.

Other code and design requirements not detailed in above description.

 

[MJCronin]

davefitz.....

Thank you very much for the completeness, time and effort that you put into your response...

I have a couple of follow-up questions:

(The Texas boiler installation has a MAWP of 2875 psig)

- What did you mean by "compared to the older case of sparging downcomers or waterwall panels" ?

- Have you heard of any specific problems with spargers in the downcomers ? Should thermal sleeves be used

- Can you offer any specific details about or any design guideline for the steam spargers ? Is there any national code or standard that addresses steam spargers ? Is there a vendor that you can refer me to ?

Thanks again Dave, and best Regards !!

MJCronin

 

[davefitz]

MJCronin:

in response to the above questions...
- The attachments to the drum for risers and downcomers typically do not have thermal sleeves, so if one tries to warm-up a cold unit using spragers in the water inlet headers or downcomers, one would expect thermal stresses to be developed at these weld lines. I do not know of any failures associated with this means or warmup, though.
-In the case of a large radiant boiler with membrane waterwalls, one should not allow the occurrence of high membrane panel to membrane panel temperature differences in excess of ( 100 F). The older means fo pre-warming from the bottom does not ensure that all panels will initially get the same amount of warming water or steam, so it may occur that some panels heat up more than others. In general, the desing of all the furnace components ( downcomres, panels, risers , feeders) assumes they are all at the same temperature- so a sparger desing that causes some parts to run hotter than others will be generating stresses not originally considered by the boiler oem.
- the design of the sparger was detailed by the lead boiler engineer for LC Steinmueller ( later deusche babcock) in an english text dated 1990- I'l have to dig around for the reference and post later.

 

[davefitz]

OK, I thought the sparger design was published by J. Merz, but it was instead by R. Leithner. Refer to the text edited by S. Kakac "Boilers , Evaporators and Condensers" chap 7 pp 327-331.

The above referenced chapter in turn references a prior work by J. Merz ( 1988) . See the other text by S. Kakac "Two Phase Flow Heat Exchangers " chapters 2, 6 + 7(1988). So Merz is the primary contributor to this technology.