Steam drum gen bank tube penetration 1

[1464649]

While working on a 1960's vintage B&W boiler, we noticed that the generating bank tubes only penetrate the steam drum about one inch, or one half its thickness. The steam drum is 36 inches ID and is 2.25 inches thick. We're used to seeing the tubes penetrate fully plus 3/16 plus or minus and flared inside the drum.

If someone else has seen this and understands why it would be done this way I'm curious to know.

Thanks,
Brian

 

[rmw]

If I understand your post you are stating that the tubes are only inserted one inch into the 2.25 inch thickness of the drum????

NOT GOOD!!!!!!

Is there any indication that something mechanically has pulled these tubes from their proper position (which would be as you describe; inserted somewhat into the drum with a flare rolled into the tube end) down into the tube hole?

Something very strange here.

rmw

 

[saplanti]

The connection that you described is in accordance with old B&W standards. The type of the connections were changing with the wall thickness and the type of the roller to be used. There was not any groove used. Depending on the tensile force and/or bending on the tube at the connection they may/may not be adequate. Today they are still used in the applications with light forces at connections.

Today, these type connections were replaced by the grooved wall tube connections that have more force and bending capacities at the connection and safer against the leak.

Some heat exchanger standards are calling for the pull test for the tube/tubesheet connections to make sure the connection is adequate.

Regards,

Ibrahim Demir

 

[1464649]

All of the tubes appear to be rolled into the steam drum the same way. They do appear to be intentionally installed this way. Thanks for your feedback saplanti but I'd still like to know WHY the tubes do not take full advantage of the full steam drum thickness. The tubes in the mud drum penetrate full +/- 3/16 inches and are flared. The mud drum tube holes are also grooved, I don't know about the steam drum tube holes as we have not removed any of them.

Thanks,
Brian

 

[rmw]

Are you sure the tubes aren't seal welded to the T/S down inside the hole? I was around in the early '70's and in those days was the kid engineer who was young enough to be the one that had to go inside the drums so I have been in my share, B&W and the rest of them. (I was a lot smaller then too.) The boilers that would have been in service in that period would have been the ones from the '50's and '60's. I have NEVER seen what you described. I have looked through my older books (my first B&W Steam was 1963 edition and they clearly show tube rolling using rollers with a flare roll. All the other texts of the period that I have always show a flare roll. (Nice trip down memory lane reading some of those older books.)

I'm still scratching my head over what you describe. It would seem that any tube rolled as such with less contact area would have to have grooves, not be without them.

If anyone has any documentation please scan it and upload it.

rmw

 

[GenB]

Hi,
check the outside, they may be welded tubes and you can not see it from the inside. the technique is used in heavy wall/high pressure boilers.
genb

 

[saplanti]

I could not reach the source of the information of the B&W drum-tube connections. However, if you think about the cost of rolling and the roller dimensions, rolling the full length is very costly and rolling the tube in a long lengths is very difficult. 2.25" of length with grooves is almost the limit length for roller application, and requires long time to roll the tube. Plasticizing the tube in that length it requires very big power in case there is no groove. Effectiveness of sealing without grooves in that length is always questioned even though flaring is used. This length is only used for 3" and larger wall thicknesses in the old B&W standards.

Harvey's "Theory and Design of Pressure Vessels" Fig 5.25 gives some kind of comparison of plain, one groove and two grooves expanded joint holding forces depending on the seat lengths. I guess this can give you a good idea about most commonly used lengths for expansion.

A year ago I designed drums for a boiler application and used 3 and 4 grooves with 12 mm distances between grooves with another 12 mm distance to the inner face. Even with those grooves due to the lack of proper expansion a couple of tubes left their location during the hydraulic test. Luckily it did cost only taking some line of the tubes away to be able to reach them to repair. Unfortunately the company I was working for did reject my earlier suggestion for the test at the connections and hardness limitation on the tube by relying on their previous drum applications with low pressure.

I recommend that the connection design should consider pressure trust (in case the tube change direction), and the other structural loads (the lower drum might be supported by the upper drum) in addition to sealing. Some tubes may not be contributing the load transfer and some might be overloaded due to the arrangement and distribution on the drums.

Ibrahim Demir