Monday, December 10, 2007
There are many head types attached to a vessel, column or drum such as hemispherical, semi-hemispherical, elliptical, flanged, etc. All these have been used in many application and however the selection of the head type is normally not documented in any process nor mechanical document and drawings. The question of application of head type has been raised many time by young engineers and the answer has been repeated many times. There is some incentive behind to list out some common reasons in application of head type.
There are few factors affecting the selection of head type :
- Allowable stress & Material used
- Access internals
- Constructibility / fabrication limitation
- Cost Saving
From stress and material saving perspective, for a vessel exposing to very high pressure hemispherical head is having highest allowable stress and minimum material. This will be followed by elliptical head and last flanged head (least allowable stress). Generally an elliptical head is in used.
In large vessel with the needs of accessing internals, normally an access hole (normally used manhole) will be provided.
However, in small vessel with internals, a flanged head is preferred. Most of the time a swing davit will be installed on the flanged head to held the flange without assistance of any hoist.
Constructibility / fabrication limitation
Flanged head some time is used due to complexity of fabrication. There are many event where a inline condenser is mounted on top of a column.
In some event where the column is small diameter (i.e. 1000mm) and a condenser with 800m ND needs to be mounted on top, there will be difficulties to use a elliptical head. those a flanged head with mating concentric reducer will be used. Apart it serve as manhole to access the internals such as demister.
Sometime process and operation may required a manhole (for any specific reason) on a small vessel. Apart from construction limitation, providing a flanged head will solve the constructibility problem, it also act as manhole. Some cost saving is expected.
Above are some common reasons for selection of head type. I am sure there are others special reason(s) for the selection in Petrochemical & Chemical industry. If you have any example, why not sure with us ? OR if you have any photos for above applications, why not sent to me so that i can upload here ?
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