Steam quality and drum sizing

[curve3104]

I intended for this question to be posed in this forum, but listed it in another by accident. I have copied questions and responses. Please feel free to respond with your comments and suggestions to this question. It would be appreciated.

Thanks!



curve3104 (Mechanical) May 27, 2004
A certain area of water surface is required in a boiler from which to release the steam at an acceptable velocity with minimal carryover. I am looking for design standards or calculation methods for determining steam drum diameters and lenghts to ensure that the area of the normal water level would be acceptable for a given steam load.
Anyone know where I could find these answers or does anyone have experience with this topic?
Thanks!




Montemayor (Chemical) May 27, 2004
curve3104:

Proper disengagement, when considering 2-phase separation, is more related to the volume on top of a liquid surface - not to the area of the liquid surface. As an example, consider the difference in disengagement effect when dealing with a vertical cylindrical vessel as opposed to a horizontal cylindrical vessel. How much disengagement height is required (even prior to entering a demister pad) is a matter of the fluids in question, the pressures, and the physical configuration inside the vessel. This height value is normally an empirical one, dependent on the experience of the designer. There are no hard-and-fast rules here. Prior success and results deem what is more appropriate and recommended.

I believe that by basing yourself on a liquid surface area you are making a mistake. The answer is much more involved and complex than that. The Souders-Brown relationship is sometimes used for this application and in others, the settling velocity of droplets is used to size the separation vessel (particularly horizontal vessels). Most text books, Perry's Handbook, and the GPSA deal with this subject.

I hope this helps you out.
Art Montemayor
Spring, TX



athomas236 (Mechanical) May 28, 2004
In the steam drums of water tube boilers, separation almost always takes place in two stages. The first stage is usually cyclone separators that use centrifugal force to separate the steam from the steam and water mixture entering the drum from the evaporator circuits.

This first stage is not 100% efficieny hence the need for a second stage. The second stage is usually demister pads that separate any remaining water from the steam.

With this approach the drum size is then determined to allow this separation equipment to be installed in the drum with access for maintenance. Some times there is a requirement for a minimum water storage quantity in the drum to cover the case when a feed pump is tripped and the standby pump started.

Each major boiler supplier has his own proprietary design of separation equipment which then leads to each supplier have different drum sizes for the same boiler operating conditions. I have kept records of drum sizes offered to us since about 1980 and there are very wide variations in steam drum loadings.

athomas236



curve3104 (Mechanical) May 28, 2004
After giving this some additional thought, I am in agreement with you Montemayor. The height of the water level to the top of the drum or demister pad is the critical variable to determine steam quality. The overall length of the drum can be considered to be fixed based on other design criteria such as the required number of tubes in the boiler and heat release rates.
The question now becomes, is there a relationship between steaming capacity, the quality requirement of the steam, and the height to the water level that can be estimated to calculate the drum diameter?
Thoughts anyone?

 

[davefitz]

For high pressure boilers, the drum length and diameter is set by the mfr in order to ensure he can fit all the required steam water seperators and chevron driers in the drum, for the design steaming capacity at design pressure. The design steam purity is based on the ABMA standards , which are based on consensus between boiler and steam turbine suppliers.

Some other variables might occasoionally affect the drum size, such as :
a) an off design case where the steam output at low pressures is high, such as a case where 100% rated steam flow is required at 60% arted pressure
B) water residence time between NWL and Lo-lo trip or hi-hi trip, to allow time for the operator to respond prior to trip. This ranges from 2-5 min for combined cycle plants , but for large utility boilers it may be less than 1 minute.
c) the allowable rate of change of drum metal temperature varies with the square of teh wall thickness, so a unit that requies a fast startup may choose to use a long drum with small diameter and thin wall as opposed to a large diameter and short length.

The drum disengagement height is only relevant for low pressure drums with no seperators . If seperators are installed, the max drum water level is based on avoiding have the water height exceed teh steam outlet of teh seperator, since that would cause reentrainment of water.

 

[hrsgguru]

I concur with davefitz here entirely.

With the exception of low pressure boilers, steam-water separation is accomplished with static, mechanical separators. Typically there are two stages of separation. The first stage occurs at or near the normal water level and the second at the steam outlet nozzle(s). Relying on the size of the drum to effect the steam separation in a high pressure boiler would require a very, very large steam drum indeed (assuming it would even be possible) due to the increasingly narrow gap in enthalpy.

 

[curve3104]

Thanks to all for the info. I am simply doing some research to determine why and how boiler manufacturers determine the size of their drums. Typically, diameters of steam drums are supplied as 30, 36, 42, 48, and 54". I am trying to determine, why not 34, or 40, or 45?? Since the steel plate can be rolled into any diameter, why are these the industry standard? I am trying to find a relationship of steam capacity, steam disengagement area, and liquid-vapor separation properties to determine, if in fact, that for a boiler of X capacity, one could use a 35" drum instead of a 36" drum. Most of the design information is proprietary of the boiler manufacturer, or it's based on the fact that "it's been done like this for years, so we must follow tradition." Or, is it simply a matter of being able to fit all of the steam separation equipment into the drum, and the actual diameter is irrelevant to the capacity of the boiler??
These are the questions I am trying to answer. I appreciate everyone's input. Maybe I have been unclear on my intended goals here.

 

[davefitz]

As far as I know, the standard drum diameter used by a specific vendor was set according to what standard dies they had in their foundry.

The drum cylinder is hot bent in sections using a press, and the radius of curvature is checked after each impression to confirm correct diameter and curvature error is less than 1%. If the foundry has on hand radii dies in 2" increments, then those are the only options from that foundry.

After about 1990 there was percieved to be global overcapacity in foundries, and most US foundries sold their presses and dies to Mexico , China and Korea. Nearly all drums are fabbed overseas now, and metric drum sizes are available. These 3rd party foundries probably will make any diameter drum , so it is no longer neccesary to assume 2" increments.

 

[curve3104]

Thanks davefitz. Just not quite the answer I was looking for. That's kind of why I'm doing this research...since the drum can be made to any size with today's technology, what are the design limitations that come into play and how are they associated with one another.
Basically, re-inventing the wheel (well, really optimizing)

 

[rmw]

Curve...,

You can book one thing. Boilers are manufactured in a globally competitive marketplace. So factors more than just 'this is the way we have always done it' come to bear.

Metal is purchased by the pound, and is handled, rolled, welded and drilled, etc., by the hour (shop labor).

Your competitor has the same elements available to him/her to accomplish the same task.

Therefore, if certain shop equipment is already set up and cost effective to roll and weld a drum of X dimension, versus X minus 1 inch optimal size, then the costs of manufacutring are going to govern over optimization of diameters, to name just one consideration.

So, within the range of limitations and mitigating factors, such as de-entrainment device sizing, cost and selection, to name one, or dished end (typically an outside purchased item) diameters, to name another already mentioned above, standard sizes are reached, and can be used to fit a range of applications.

No design engineer, no matter how idealistic, is allowed to reinvent all the shop processes just to "optimize" the design.

So, if your research is going to arrive at any meaningful conclusion, I think you are going to have to search in a direction based on the above.

rmw

 

[davefitz]

Based on my current understanding of the boiler business, the only 2 design constraints that are required by longstanding tradition are (a) priced lower than the competition and (b) will last 1 week longer than the time it takes to collect final payment. Allother issues are traditionally ascribed to either operator error or water chemistry.

 

[rmw]

Davefitz,

I think your (b) above needs to be modified to read, "...final payment, or until the end of the warranty period."

rmw