water tube boiler reduced pressure / derating 2

[tlona]

Client currently has a 52kPPh and 26kPPh BW watertube D style, dual fuel (2oil & NG) boilers operating at 125psi. Site steam load being reduced to 20psi with load between 5kPPh and 15kpph peak. Is anyone aware of any concerns to the boilers with operating these boilers at reduced pressure. For example, I herd that reduced pressure results in poor circulation in the tubes causing tube failures. Do the boilers need to be need to be derated?.

 

[davefitz]

Each case needs to be evaluated in detail, but here are some observations on the data you provided:

a) running at 20 psig ( 34.6 psia) vs the design 125 psig ( 139.7 psia) implies roughly 4.5 times higher steam velocities in the steam drum driers and in the heated risers to the drum at a given load. This could cause water carryover issues at loads above about 22% orginal MCR steam flow, and likley cause waterwall circulation problems and erosion/corrosion of the heated risers if they are simple carbon steel construction. So, one could artificially reduce teh velocity by operating the boiler with the outlet valve thrttled , holding a drum pressure high enough to ensure the exit steam velocity does not exceed original rated velocity.

b) low firing rates associated with the described reduced loads could yield flame instability , and also low boiler efficiency due to hi hi excess air. The buners may need to be reviewed and modified for better turndown and a 2 speed fan used to get better combustion system turndown. Combustion air flowrates below 25% MCR air flow may have isssues with correctly measuring the airflow withthe original air flow element.

If the economizer is configured as heated downflow then long operating periods at low loads can lead to flow reversal and flow instability in the economizer tubes.

 

[quark]

If you run at 20psig, you should provide 71 btu of extra energy per lb of steam produced.

The specific volume is 4 times higher, so you have to squeeze 4 times higher volumetric flowrate through existing piping. Check if this is ok with the reduced steam flowrate.

 

[MJCronin]

davefitz, as always, gives superb advice....

I think that it is also important to discuss the original operating pressure and possible new problems regarding the steam distribution system.

You state: "Site steam load being reduced to 20psi"

What was the original deisgn pressure ?

What kind of higher steam velocities will the existing system be subject to ?

I would expect higher noise and possible cavitation at control valves.

There will now be higher erosion in the piping system. Will it be a "huge surprise" to the MBA bosses when the have to replace expensive underground piping, due to erosion, a few years from now ?

What is the reason for the drop in pressure ? (I hope that its not to reduce the rate of steam/condensate leaks in an old system)

My opinion only

 

[rmw]

I think your piping system will take care of the derating of your boiler(s) - if carryover in your steam drum doesn't. Your boiler(s) are designed to operate at a given pressure and the internal water circulation patterns are a function of that pressure (range).

Circulation in a natural circulation boiler is created in a two fold manner. One part is due to the pressure differential between the colder (relative term) water from the drum in the downcomers as opposed to the less dense (hotter) water and steam mixture on the hot side exposed to the flame and flue gasses, and the other is the inducing effect of the bubbles that have been created on the hot side as they make their way to the drum and induce water flow as they rise.

Operating at the lower pressure will mean that the specific volume of the bubbles on the riser side is higher meaning larger bubbles hence higher velocities on that side. It could well mean that the onset of boiling is earlier in the circulation loop further disrupting natural circulation.

Will that be bad? I can't say, but my feeling is that it won't be good. The velocity on the riser side will be too high and it will sweep the hot side tubing clear of water exposing the tubes to high temps.

The higher exit velocity will also cause excessive moisture liberation from the surface of the water level in the drum possibly overwhelming the moisture removal equipment.

Will any of that actually happen. Not sure, but that is how my mind envisions it.

rmw

 

[steamdog]

Since you are lowering the pressure AND reducing the steam flow it seems that the total FT3 of steam leaving the boiler will not increase too much so that the exit velocity should not increase too much and the threat of carryover should not be too great.

I would make sure you look at the safety relief valves setting and capacity and the lower pressure and the feedwater rate with a higher DP.

 

[athomas236]

On an occasion like this I would ask the boiler supplier.

Best regards,

athomas236

 

[boilerone]

I worked for boiler OEM companies for many years. The typical D-style boilers cannot run safely and continuously at 10% design capacities but might be possible at some higher loads. There are many possible problems for low-load operations. Not being able to maintain stable combustion, like some mentioned, is the first but not the only important reason. So, you almost cannot expect at all to run the 52kpph boiler to produce 5kpph steam.

My best recommendation would be,
1) Decommission the bigger 52kpph boiler and only run the smaller 26kpph one at 20%-57% load range (which gives 5kpph-15kpph). Consult a boiler OEM or service company first to make sure your 20% is okay for you 26kpph boiler otherwise some modifications need to be done.
2) To deal with the pressure difference, simply add a pressure-reducing station on main steam line to drop the pressure from 125psi to 20psi. DO NOT simply reduce the boiler operating pressure to 20psi - you'll likely to mess up a lot of things.

By the way, not sure if the 5kpph-15kpph range you gave is for 125psi or 20 psi. If it is for 20psi then the boiler needs to operate at even lower % loads.

The water circulation at lower pressure, nevertheless, is NOT one of the concerns. Lower pressure actually helps natural circulation. Why? The combined density of the steam-water mixture in the furnace wall tubes is much lower at lower pressure compared with what's inside the downcomers (water only) so there is much higher driving force to help natural circulation.



Boilerone

 

[tlona]

Thank-you all for the input. I had gotten pulled away from this project for a short period and now I am back on it. My clients system uses the 125psi mainly for distribution. They had steam absorbers which required 125psi steam but are now gone. There are steam reducing stations in the main building where the 20psig steam is used. Most of the 20psig steam was used for steam to water Hx for the AHU pre&reheat coils. The client is going with small localized gas boilers to heat the re-heat water loop. This will reduce the steam required to the re-heat Hx. My thought was to contact the manufacturer and look at a two staged burner, like you had suggested install and possible install a two staged fan or VFD and also investigate a stack damper. Rather that remove the 52kpph boiler and replace with two smaller, we were trying to see it it was worth while to keep this boiler since it is only 4yrs old.