Dished heads and caps
Different names are used to qualify dished heads: vessel head, dished ends, tank heads, ends, caps …These names can vary according to their application.
We distinguish several dished heads families: dished heads, flat heads, conical heads, spherical caps and hemispherical heads.
Dished heads are defined by the following elements:
- External or internal diameter
- Crown radius (CR)
- Knuckle radius (KR)
- Nominal thickness of the plate or minimal thickness of the head
- The right side (RS): which can be completed by different types of bevelling.
Dished heads can be manufactured according to different standards. In this article we present the French standard NFE81 -100:
– GRC dished heads according NFE 81-102: «large knuckle radius»= « grand rayon de carre». It is the most commonly used dished head profile to manufacture pressurized vessel heads.
– PRC dished head according NFE 81-101: « small knuckle radius »= « petit rayon de carre ». It is a type of head non recommended for pressurized vessels. This model is mainly used for non pressurized storage tanks and for its lower costs.
– ELLIPTICAL dished head according NFE 81-103: much deeper than the GRC, this type of head is mainly chosen for pressurized vessels.
– MRC dished head according NFE 81-104: « medium knuckle radius »= « moyen rayon de carre », is the kind of dished head less used.
GRC DISHED HEADS NFE 81-102: large knuckle radius
The main features of GRC dished heads according to NFE 81-102 are:
- The Crown radius (CR) is the same as the head diameter.
– The knuckle radius (KR) is 10% of the head diameter.
PRC DISHES HEADS NFE 81-101 : small knuckle radius
The main features of PRC dished heads according to NFE 81-101 are:
– The Crown radius (CR) is higher to the head diameter value.
– The knuckle radius (KR), which depends on the head diameter, can only be:
* kr = 30 for a diameter < 800mm
* kr = 40 for a diameter between 800mm and 1000mm
* kr = 50 for a 1000mm < diameter
We can conclude that the bigger is the crown radius (CR), less shallow is the head. That is why the PRC head is less deep than the GRC dished head.
ELLIPTICAL HEAD or SEMI ELLIPSOIDAL HEAD NFE 81-103
The main features of ELLIPTICAL dished heads according to NFE 81-103 are:
– The Crown radius (CR) = 0.8 x head diameter
– The Knuckle radius = 0.154 x head diameter
with all this values, the elliptical head is deeper than the GRC head.
CONVEX DISCS OR SPHERICAL CAPS
The spherical caps are shaped discs according to a given radius upon their whole surface. A spherical cap is a portion of a sphere and it is featured by its diameter and its crown radius (CR). A convex disc or spherical cap has not a right side.
The smaller is the Crown radius, the deeper is the spherical cap and more difficult to manufacture.
Spherical caps are not standardised items.
A flat head is a disc with a flanged side according a radius. Flat heads are used in the agri-food industry for turning or distribution plates.
The inner height of the head is given by the addition of the knuckle radius and the right side of the head.
As spherical caps, flat heads do not follow any standard.
A conical head is featured by a cone and a flanged side upon its big diameter.
The principal elements of a conical head are:
– the big diameter
– the small diameter
– the knuckle radius
– the right side
– the total height or the value of the angle at the base or the half angle at the top.