Appropriate entry point for Boiler Feedwater

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JonRiley56
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Appropriate entry point for Boiler Feedwater

Post by JonRiley56 » Wed Nov 16, 2011 7:07 pm

All,

What is the optimum level on a Steam drum for the boiler feedwater entry point ? (I am talking about a B&W type design drowned tube water tube boiler.)

The UK "Steamboating Guide" includes the following statement in a paragraph about valving commenting on where the water should enter:

"which is normally round about the design level of the internal water". They are not making reference to any specific design however.

What is the "design Level" in a B&W type boiler, 33%, 50% ? I know that with the formation of steam bubbles in the system that the apparent water level will rise, and that priming needs to be avoided, but running at 33% which I have read in a few places seems to allow a lot of headspace

jon
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Re: Appropriate entry point for Boiler Feedwater

Post by barts » Wed Nov 16, 2011 9:45 pm

Many people introduce the feed water into the mud drum, or bottom of the boiler. As far as I know it doesn't matter very much aside from making sure that on dual drum boilers the distribution of water is symmetric to avoid one drum being colder than the other.

As far as where the water level should be, I carry mine as high as possible such that it still doesn't prime... hot water in the boiler is like money in the bank.

- Bart
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Re: Appropriate entry point for Boiler Feedwater

Post by preaton » Thu Nov 17, 2011 12:55 am

There are valid arguments for putting in feedwater high or low.

1. Feedwater should be heated if possible. Then lessens the problem of cold water into a hot boiler
2. Putting it above water level allows the steam to heat the water. On big boilers a spray nozzle was used to help this along
3. In a water tube boiler like the B&W steam generation is very dependent on the circulation. As the water is heated hot water rises by convection and so the water begins to circulate up the heating tubes to the main drum (if there is one) then down the downcomers to the mud drum and back up. Introducing the water so that it doesn't disturb the pattern is important. Introducing straight into the drum could do this. So it should be introduced near the downcomer or as Bart says even into the mud drum. In this way it encourages rather than discourages the circulation.
4. On my boiler which made up of grids of inclined tubes each of which has its own "downcomer" the feed goes into a perforated tube within each downcomer again allowing the water to heat up some before it is introduced to the boiler and also in such a way as to encourage the circulation in the grid.

Cheers

Paul
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Re: Appropriate entry point for Boiler Feedwater

Post by fredrosse » Thu Nov 17, 2011 4:19 am

ASME Code, Power Boilers, General Requirements, PG-59-2, Requirements for Feedwater Connections:

"Feedwater shall be introduced into a boiler in such a manner that the water will not be discharged directly against surfaces exposed to gasses of high temperature or to direct radiation from the fire"

"Feedwater......... shall not be introduced through the blowoff"

PFT-48-2, Feed Piping:

In vertical firetube boilers the feedwater shall be introduced at a point not less than 12 inches (300 mm) above the crown sheet ...... feedwater shall not be introduced through the openings or connections used for the water column or gage glass.
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Re: Appropriate entry point for Boiler Feedwater

Post by JonRiley56 » Thu Nov 17, 2011 8:14 pm

Thanks for the replies,

If I bring the feedwater in through the mud drums I am assuming I should feed both of them so I dont get an imbalance in the temperature of the two sides. I intend to run pre-heated feed water.

jon
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Re: Appropriate entry point for Boiler Feedwater

Post by artemis » Fri Nov 18, 2011 10:16 pm

JonRiley56 wrote:Thanks for the replies,

If I bring the feedwater in through the mud drums I am assuming I should feed both of them so I dont get an imbalance in the temperature of the two sides. I intend to run pre-heated feed water.

jon
Absolutely correct for our little boats. If you use a smaller (1/2"), perforated, end capped pipe running inside each mud drum (may have to make the mud drums a bit larger diameter) that should evenly distribute the water through the drum and avoid any temperature differentials
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Re: Appropriate entry point for Boiler Feedwater

Post by steamboatjack » Sat Nov 19, 2011 8:31 am

I feed into the bottom drum on Grayling using a "piccolo"pipe as mentioned above, the only problem is that should a feed check fail open (and yes it has happened to me) you may lose the entire contents before you know it, where as if the feed line came in the steam drum it would not perhaps be quite so dramatic, Large boilers always feed into the steam drum and loco boilers use “top feed” where the water is cascaded onto a tray to mix and heat it with the steam, obviously not near to the steam draw off point.
regards Jack
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