Knock out drum with Ellipsoidal head vs Flat head

[hemvig]

When should we use a Ellipsoidal head instead of Flat head.

I have a pressure vessel with 42" diameter it has a Ellipsoidal head made out of rolled steel and another pressure vessel with 36" has a Flat head (Flanged)The fabricator has used a 36" pipe for the second pressure vessel.

Please let me know if there is any requirement as such in ASME pr. vessel std, to use a Ellipsoidal head for pressure vessel above 36" diameter.

Thanks in advance for your help.

 

[vesselfab]

no requirement

many times, on small diameters, a flat bolted head is used for access for internals, such as mist extractor when a manway would be too large for the shell diameter.

I can assure you, no fabricator on a fixed price project would use a bolted flat head unless asked to.

look at the cost between a head and small hand hole vs a full body size wn flange, blind bolts & gasket.

 

[unclesyd]

What are your design conditions of said vessel?

what is the material?

 

[racookpe1978]

As the vessel gets larger, and as the internal/external pressure differential gets higher, the thickness of the "flat plate" has to get ever thicker and thicker.

At some point, it becomes cheaper to buy a standard elliptical vessel head (or fabricate it yourself). The rolled vessel head is more expensive, but it canbe much thinner, and so less expesnive - but that depends on the materail needed and the material strength/weldability/heat treatment, etc.

Go to even more pressure differential, and eventually the very expensive hemispherical heads are needed to shave off construction expense so the head can be thinner.

Design the thing, consider a few alternatives that will meet your operating conditions and erection needs (crane availability, size, shape, weight limits, etc.) and get a few bids. Evaluate the costs, but don't spend extravagant amounts of money over-analyzing the problem.

That's why you became an engineer rather than a bureaucrat only able to read a federal law written by a thumbrule writer - to figure out better ways to do the job 8<)

 

[bhushan76]

Apart from Mechanical design perspective correctly pointed out by vesselfab , you need to consider process reasons as well.

You cannot change from ellipsoidal head to flat plate without changing overall height of vessel. Vapour exit nozzle on top head is required to be at certain distance from mist eliminator. Hence if you change from ellipsoidal head to flat plate you are in reality reducing this distance.

 

[r6155]

An alternative is ellipsoidal head with bolting flanges

Regards

r6155