Maximum Recommended Boiler Tube Length

[sullypape]

Are there special considerations and concerns using a single pass water filled boiler tube composed of welded segments forming an 1800 foot long single pass boiler exposed to solar energy. Tube will be supported from rollers supported by steel pipe rack structure and will cycle daily.

 

[JEB66]

Expansion

 

[metengr]

You better make sure you know and understand ASME Section I Power Boilers to determine minimum wall thickness and mean wall temperature to select the correct tube material.

 

[GenB]

if it is a low pressure hot water, need no be certified,
only concern should be expansion (liquid) you should install an expansion tan or approved device
I respect your idea of a million ft of pie for solar, our solar tubes are only 8 ft long.
genb

 

[davefitz]

If it is only 1 boiler tube supplied by a pump then no flow maldistribuion or flow instability is expected. However, if there are more than 1 tube in parallel, 1800 ft long each, then major tube-to-tube flow unbalances and instability would be expected.

If dry-out is expected ( and I certainly expect it to dryout under some scenarios) then the boilerwater must meet once thru unit "holy water" quality requirements to avoid depositon of salts and internal fouling . Layout of deposits generated from upstream equipment ( such as feedwater heaters) can be minimized if you use the "combined oxygenated " fedwater treatement method, holding 200 ppb O2 in the condensate feed tothe feedwater heaters.

To avoid rapid tube failure due to overheat, you would need to avoid DNB departure from nucleate boiling- use of multi lead internal ribbed tubing should help in that regard- see papers from KWU authors Kiefer, Kohler and Hein circa 1980-1992. Tubes available from vallourec-Mannesman or Sumitomo. Use of T23 tubing would also allow temporary overheats to about 1000 F. Would also be recommended to vary the tube diameter, from smaller dia at the inlet to larger dia at the outlet section.

 

[MJCronin]

A single straight pass boiler tube..??? 1800 feet long ?

Did I understand this right ?

Thermal expansion of this single straight tube will be tremendous !!

How will steam, say...generated at the midpoint, exit the end of the tube without degrading heat transfer downstream ?

This is why God gave us steam drums !!!

I am getting the feeling that America's solar energy effort is not being supported by Stanford, MIT and Cal Tech.

 

[sullypape]

To further clarify:

So, here is a similar system

http://www.rechargenews.com/energy/solar/article194840.ece

First, I am a structural engineer, responsible for the foundation system for various sites around the world for this system with various modifications, and am trying to confirm the loads from the boiler receiver/reflector support structure and loads from the boiler tubes and receiver enclosure.

As a structural engineer with 30+ years of experience, I have always seen limits of approximately 400 feet for steel pipe rack structures, ductwork structures, and buildings between expansion joints.

I am willing and able to design foundations for a longer structure without expansion joints but then base rollers or sliding joints for the support interface in the reciever at the top would be required.

There are 2 sets of boiler tubes. Within each set, 3 to 4 small tubes go north and then joinat the north end and return in a single larger dia tube. Steam temp: < 700 degress F design. Steam pressure: < 1220 psig. Feed water pressure:< 1800 psig

There is a single anchor at the center, with appropriately placed rollers spaced out from the center.

So what should the boiler tubes be designed for under the ASME Pressure Vessel Code in addition to pressure and temprature? Especialy since the code is a minimum.

1. There is the extreme length of tube (1800' at present - 2500' maximum future) that is being pushed during expansion and pulled during contraction. This has to influence the secondary stresses to a greater extent that what the ASME PVC intended. Any thoughts or references?
2. Should there be expansion joints/loops?Any thoughts or references?
3. What about upset conditions? Water hammers? Condensate laying in tubes resulting in non uniform tube loads? (i.e. loading of adjacent pipe spans due to deflected shapes; loading of alternate pipe spans due to defelcted shapes?) Lifting of tubes off rollers? (Should these have u-bolt to minimize unwanted movement?
4. Any other comments, thoughts, and/or references would be greatly appreciated.

Thanks,

Sullypape

 

[davefitz]

I think the layout you presented has major design issues, and basically sounds like an outline of why Ausra did not succeed with their solar thermal projects.

There would be expected to be major tube to tube flow unbalances, based on the assumption that the tubes recieve unequal heat absorptions , using Ledeneg static stability theory ( so called multi channel flow stability). Since there are only 4 tubes, it should be feasible to provide active flow control for the 4 tubes, to avoid that issue and avoid the conventional solution of installing high pressure drop at the inlet of the circuit ( using capillary tubes).

The most severe operating upset should be analyzed, and the expansion joints would be located or spaced at locations which would limit the worst temperature gain over that spacing to not more than 200 F- likely case would be dryout in the "worst tube" .

But its your problem- good luck.

 

[racookpe1978]

How are you (your team?) going to handle "off-perfect" solar input (clouds, dust, humidity, rain - sunny - rain - rain - sunny) compared to "perfect days" when it IS clear, low humidity, no clouds?

You're going to burn out (get superheated steam) too early then in at least one tube, or - if the collector is designed for perfect days - then never get enough energy on off-peak days to get steam.

I'd like to be proved wrong - But I can't see this working well. Water problems also - as noted by the previous.

 

[EdStainless]

So what is the actual internal pressure in the tubes to assure that you don't get any boiling?

What I think will happen is that at night when it starts to cool and tries to contract 8' you will inevitably get localized yielding at some location (rollers will bind). With every cycle the tubes will get a bit longer, until some location gets thin enough to fail.


= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

 

[chicopee]

Your link does not show a boiler. It is more in line with a once through forced flow steam generator.

 

[sullypape]

chicopee:

From my perspective as a structural engineer I have been told it is a boiler. If it is a steam generator then what would the differences be? My general comments still apply. Can you provide guidance?

 

[chicopee]

No furnace, no mud drums, no water drums, no steam drums

 

[sullypape]

Nope. Just the tubes. One end has the feedwater in and steam out via risers with instrumentation and valves down low along with drains, etc.; other end has the turnaround weldment where the smaller pipes on each side into the larger diameter return pipes on each side.

 

[MJCronin]

Bravo davefitz !!! Good remarks !!!

The OP must be made to understand that he is "cutting some new ground" here in boiler design and may be in an area where there is little practical design guidance.

This is not another pipe in a pipe rack.....

It is certainly a fitting tribute to America's management expertise that this crucial part of an important plant design is assigned to an experienced structural engineer with little boiler design experience....

There is no sense in bringing in a heat transfer/mechanical consultant

Kudos to all you MBAs and PMPs out there......

 

[racookpe1978]

The mirrors below the tube focus the sun's energy on a row of very straight, very level, very long pipes attempting once-through "water" flow (from the middle apparently) into steam going out at both ends.

Some professor "designed" this thing and sold it to California politicians to get funding from California and national taxpayers .... Our money.

My sympathies. Are you going to put check valves in the center so steam flashing on one side doesn't block flow and force the rest of the water out the other side?

How will you drain it? The photo shows a level pipe -> no conenctions, no bends, no piping.

Outlet steam won't "flow" down like that in the photo under startup - will you use pump flow?

How will you set up circulating or counter flow for convection and unequal conditions down each pipe (real world heat transfer is not "theorectically perfect and exactly even everywhere.)

Even a long shallow "V" going up from the center will help somewhat.

Photo's caption says a conventional boiler will be used for 18 of the 24 hour day. (Solar heat is effective only between 9:00 am (local sun time) and 3:00 local sun time.) I'd plan on that boiler running 24-7, since a six-hour heatup cooldown time can't be done if the conventional boiler is going to last more than a year.

In fact, I'd design the thing as if only the conventional boiler is going to be available.

 

[sullypape]

MJCronin:

Compact Linear Fresnel Reflector (CLFR) technology is the concept for the boiler or solar steam generator.

There is an entire team of boiler and heat transfer "experts" on the project. Along with civil/structural engineers. I am at the end of the food chain responsible for the foundations. As such, I am concerned about the load information and indicated load paths being provided since it goes against what I have been exposed to in 30 plus years of practice.

First, from my perspective, under ideal conditions, an 1800 foot long straight tube anchored in the center and supported by rollers with downcomers at one end for feed water supply and steam exit; and a turnaround at the other end is beyond the intent of any ASME code. Additionally, there will be adjacent and alternate loadings due to condensate settling out when the boiler goes down. Then on start-up I see potential water hammers and othe atypical loadings, as well as, expansion and thinning of the tubes over time. All of this with possible overheating of the tubes and adjacent support structure until the suns position changes.

 

[davefitz]

I think I would investigate the use of low pressure mineral oil as the indirect heat tranfer fluid in the overhead lines, heated from 500-1100 F, transfer its heat to a binary H2O-NH3 kalina type cycle, which is then pased thru the ammonia/ steam turbine.

Or, fresnel-focus the suns rays onto a limited length of tubig that contains compressed air that feeds a sterling engine, each engine producing perhaps 500 KWe.

But 1500 psig steam overhead in a 1800 ft long pipe seems a bit scary. and will lose a lot of heat very quickly when the clouds come out.