Feed into twin drum watertube boiler

[Mike Cole]

I am getting to the end of the boiler build, I had water in it for the first time at the week end and after some leek stopping see 100 psi using my test pump.
I am having a engine driven feed pump and hand pump. I have to feed one into the top steamdrum and the other into the lower drum. Both drums the same size. Should I use the upper or lower feed for the engine pump or no difference? The top clack feeds into the drum
at the very bottom same level as bottom of glass.

Mike

[gondolier88]

Hi Mike,

I would definitely go with the engine driven pump into the lower drum- this will aid circulation in the boiler, whereas feeding into the top drum will slow circulation and cool the best areas of steam production in the boiler.

As the hand feed pump is usually used in an emergency or to top up when static at jetties these steam-killing properties of feeding into the top are actually beneficial for pressure control.

Two-drum boilers are very susceptible to temperature fluctuations and to run reliably need a constant feed of water and excellent water circulation in order to produce steam well.

Greg

[johngriffiths]

Hi Mike

Its a good idea to give feedwater plenty of space to get rid of dissolved oxygen before it impinges on a tube or other part you don't want oxidised away. I've noticed on 3 and 4 inch traction engines that tubes opposite feeds can rust out quickly and know of one instance it was in four years. On full size boilers it is sometimes the centre of a tube array that shows most corrosion, much heat, concentration of oxygen because of the close proximity of tubes.

JohnG

[fredrosse]

Feeding into the bottom drum will tend to promote uneven temperatures within the boiler. The colder feed will tend to make a stagnant slug of water in the lower drum, because the colder water is considerably more dense than the hot saturated water. It is my understanding that this condition is why it took so very long to startup cold large scotch marine boilers. You could put your hand on the bottom of the shell plates, while the top surface was steaming. This stressed the boiler with significant differential expansion.

The feed should go into the top drum, the cold feedwater will mix rapidly with the turbulent steam/water mixture, quickly releasing any dissolved O2. An internal feed pipe, directing several feed holes into the drum, should be arranged so that incoming feedwater should not impinge on boiler internal pressure vessel parts. There has already been discussion about this subject on this forum.

[barts]

Are there downcomers between the two drums? I'd be inclined to pump water into the top of the downcomers to promote circulation...

- Bart

[Mike Cole]

Well, thanks a bunch guys!!
When i first got Greg's reply, I thought great, that’s my answer, feed into the bottom. As I read it on my phone, I thought go to work and write a nice thank you when I get home on the laptop.
I have not reckoned on the ability of steamboaters to think and do the opposite of others with two well reasoned replies later on from Fred and Bart. John your reply makes me think I should be piping the feed water all along the drum. Also doing this will equalise the temp more, but too late now.
Given that they are around a dozen one inch downcomers , and that the colder feedwater will fall down them I am going to go with engine pump to the top drum.
It won’t be a big job to change later when Greg turns out to be right.



Mike

[gondolier88]

Feeding into the bottom drum will tend to promote uneven temperatures within the boiler. The colder feed will tend to make a stagnant slug of water in the lower drum, because the colder water is considerably more dense than the hot saturated water. It is my understanding that this condition is why it took so very long to startup cold large scotch marine boilers. You could put your hand on the bottom of the shell plates, while the top surface was steaming. This stressed the boiler with significant differential expansion.

The feed should go into the top drum, the cold feedwater will mix rapidly with the turbulent steam/water mixture, quickly releasing any dissolved O2. An internal feed pipe, directing several feed holes into the drum, should be arranged so that incoming feedwater should not impinge on boiler internal pressure vessel parts. There has already been discussion about this subject on this forum.

A dispersing pipe will make a big difference, but you are still cooling down the boiler in areas it doesn't not want to be cooled down- we are not discussing a large shell boiler here, but a watertube boiler with perhaps <4gal working volume- a watertube boiler relies utterly on a good water circulation, and to feed cold water, even using a dispersal pipe, will inflict a cooling effect on the top drum.

This isn't really a specific subject as different boilers can withstand different conditions, but the two drum WT boiler needs all the help it can get to maintain the top drum as hot as possible, unlike a Yarrow design where the top drum is in the direct path of the flue gasses and produces a significant heating surface in itself, the two-drum design relies solely on being supplied with steaming temperature water by the tubes which are in the direct path of the flue gasses and the radiant heat of the grate, which the tubes also block from the top drum. Remembering that the very small top drum is where the steam take off is, and is effectively at one with the steam dome itself it must be kept as hot as possible.

Use physics to it's advantage; put cold in the bottom drum where it wants to be, it passes through the hottest part of the furnace immediately as it enters the tubes right above the grate and absorbs the heat to rise rapidly generating steam to enter the top drum- if it meets a cooler temperature water here it will be like hitting a brick wall regarding circulation.

Every boiler I have ever steamed/worked on/fitted out has always had the feed as low down as design permits and putting cold water in the hottest part of the boiler seems rather counter-intuitive.

However, if needs be it is easily reversible as Mike says, and the boiler is only a secondary device, as long as the primary device onboard is working (the Windermere Kettle) it's all fine!

[Oilking]

During my time as a US Navy BT, I operated both 600 & 1200 PSI D-type boilers (two drum). On both the feed water was deliver to the steam drum through a perforated pipe that ran the full length of the drum. Here are some things that likely differ from the smaller launch boilers , there were six large down comers, 4"-6", at each end of the drum connected to the water drum, screenwall, sidewall & backwall headers. Also there was a belly pan or baffle plate in the steam drum over the ends of the generating tubes that directed the wet steam to the steam separators from which the steam was directed to the dry pipe and the separated water to the water in the drum.

All that said, the boiler still has to heat the water that comes in to it no mater where it's introduced.

Dave

[steamboatjack]

I would feed into the top drum with the engine pump, I agree with Fred's comments, also I had a nasty incident once in Scotland! My boiler fed into the bottom drum, I had just topped up and got the fire going, the feed bypass was left open. Without my noticing until it was too late, the feed check had stuck open and the entire contents had dumped into the Union canal!
Had the feed been into the steam drum, the result would not have been quite so drastic. Also remember never leave the feed by-pass open.
Regards
Jack

[Mike Cole]

Sorry Greg, but the primary device will not be fitted until later. It is currently on top of my bookcase.