Dynamic forces excerted by flue gas in ducting

[pch1]

Can anyone help me locate a standard or similar that will guide me in determining the dynamic forces excerted by a gas flowing through a circular duct. (I did have access to an American piping book which showed an example of fluid flowing through a pipe and the force excerted by the fluid when the direction was changed by use of an elbow, and I thought there may be similarities between the piping example and the ducting. Unfortunately I no longer have access to that book.)

I do have access to most British and European standards, but only a few American standards if anyone is able to point me to a particular standard.

The conditions are:
Duct Size: DN1000
Duct Material: Carbon Steel to BS EN 10025 S275
Duct wall thickness: 8 mm
Flue gas pressure: -0.5 barg
Flue gas flow rate including water vapour: 32318 kg/h
Solids in flue gas: 1250 kg/h

The duct starts at a waste heat boiler (WHB) and feeds to an electro static precipitator (ESP). Between the WHB and ESP, the duct undergoes two directional changes of 45 degrees each in the Z plane, separated by a straight section of duct approximately 2 metres. There are expansion joints (Bellows) located at both WHB and ESP.

Thanks in advance
Paul Hearn

 

[JohnBreen]

Pch1

I designed such a system late last year (48 inch OD schedule 40 ASTM A-240, type 304H). My system was supported by a structural steel truss type "bridge" between the WHB and the
ESP (about 80 feet above a plant railroad right-of-way). I put X, Y and Z restraints at the mid-point on the "bridge (did not restrain any moments). The supports on the "bridge" upstream toward the WHB and downstream toward the EXP were guided sliding saddles of the same material as the pipe (we used polished SS plates under the saddles to minimize friction). The mid-point restraint system was designed to accommodate the loads including the flow forces (but as I remember the flow forces were insignificant, however, my angled offsets were only about 18 degrees) and the "bridge" designer took care of them from there. I specified the bellows type EJ's to accommodate the expansion/contraction movements and the EJ manufacturer did not seem to mind - the installed length of the EJ's was set by removable travel stops to the "ambient temperature" length. Yes, we remembered to remove the bars before start-up

The duct had to have abrasive resistant lining (attached internally with welded SS hex mesh) at the directional changes. but other than that it was nothing exceptional.

Regards, John

 

[pch1]

The civils group needs to know the exact forces excerted by the duct so they can design their structure accordingly.
I was hoping to use a formula similar to the force excerted on a fixed vane to determine the resultant forces on the duct.
Would this be the correct or can anyone suggest a better solution?

Regards
Paul

 

[BigInch]

As John said, the fluid forces should be relatively nothing.

Do you have any thermal loads going to anchor points? I'm sure they must be talking about those. The rest would only be sliding friction, wind, snow, ice, earthquake, etc. and the structural boys should be able to take care of those all by themselves.

http://virtualpipeline.spaces.msn.com

"What gets us into trouble is not what we don't know, its what we know for sure" - Mark Twain

 

[pch1]

I did a quick calc on the forces exerted by the flue gas at a single mitred bend and have come up with 118 N in the horizontal direction and 49 N in the vertical direction. (This supports your views that they would be relatively nothing).
Please note that these calculations may not be spot on as I have assumed a gas density of 0.657 kg/m^3 and a velocity of 18 m/s.
(I took a bit of help from this website:

http://www.nawabi.de/chemical/hrsg/page6_7.htm

)

Thanks John and BigInch

 

[BigInch]

Needs some good duct tape.

http://virtualpipeline.spaces.msn.com

"What gets us into trouble is not what we don't know, its what we know for sure" - Mark Twain

 

[pch1]

I've re-read my post...the mitred bend is made of 5 sections, I was referring to one of the two bends in the duct - hope I didn't confuse everyone.

 

[JohnBreen]

pch1

I think the only thing you should concern yourself about (beyond what we all have addressed above) is the abrasive catalyst impinging upon your mitered bends. I hope that you intend to install an appropriate anti-abrasive lining.

John

 

[pch1]

The original ducting had no lining and lasted approximately 9 years, it was made from 8mm thick Carbon Steel BS EN 10025 S275.
Our client has asked for a design life of 25 years, and as I don't design ducting on a regular basis I have written a specification for our Sub-Contractor, stating the ducting material to be suitable for the flue gas and also having a design life of 25 years.

 

[DSB123]

pch1,
You may well have written a spec for the sub-contractor specifying a design life of 25 years but when he offers you a specific "design" and material you will have to assess the "offer" technically and ensure that the "offer" meets the design intent. Also where does the liability lie for the ducting? With your company? or the sub-Contractor? If you are the main contractor and the subby;s design/materials are not up to the job your company may also get bad feedback from your Client.
You must be in a position to know that the subby's offer will meet the design intent.

 

[BigInch]

By simple ratio that indicates at least a 1" wall thickness "duct" is needed for 25 year life. Is that an acceptable solution, or shouldn't some alternate means be investigated before you get something too heavy for the existing pipe supports, high axial forces, friction loads, or worse?

http://virtualpipeline.spaces.msn.com

"What gets us into trouble is not what we don't know, its what we know for sure" - Mark Twain