## Why is the thickness of dished head so thick?

A friend asked me, what is the difference between the calculated thickness of dished head and cylinder?
I said that generally, 10% of the dished heads are about 54% thicker than the cylinder thickness.
He asked: Why did I encounter a device with a head twice as thick as the cylinder.
I replied: it’s impossible. Look at the formula, it’s almost 1.54 times the cylinder thickness.
He continued to insist, really, twice as thick.
I thought for a moment and asked: is your design code ASME VIII I, and is the material of high strength?
He replied: Yes, how do you know? It is the duplex steel 2205 dished head with ASME VIII I, material duplex steel 2205, pressure of 1MPa, normal temperature and diameter of 3000.
I replied: no wonder.

And listen to me slowly.

The dished head processed in China is generally 10% of the standard dished head, and its surface is much smoother than that of 2:1 elliptical head.
It is easier to process than elliptical head, with small processing capacity, good forming quality and cheaper cost.
The material used is smaller than the blanking size of elliptical head and saves more materials.
The disadvantage is that the stress is not as good as that of the elliptical head. The thickness of the same diameter, pressure and material is about 1.54 times thicker than that of the elliptical head.
If the internal pressure of the equipment is relatively small, it is very economical to use dished heads.

## Internal pressure instability of dished head

The following figure is the stress nephogram of dished head under internal pressure:

Stress variation diagram of the inner surface along the radial direction, SZI is the circumferential stress:

Stress variation diagram of outer surface along radius direction, SZO is circumferential stress:

From the stress change curve and the deformation of cloud diagram, it can be clearly seen that in the corner transition zone, the circumferential stress changes from tensile stress to compressive stress. When the wall thickness is thin, it may lead to the circumferential instability of internal pressure.

## Normative response

In GB150, in order to avoid internal pressure instability, it is specified that the minimum thickness of standard elliptical head is 0.15% of the inner diameter of head, and the minimum thickness of 10% dished head is not less than 0.3% of the inner diameter of head.

In ASME, dished heads are classified in order to avoid internal pressure instability.

TS is the thickness and l is the inner radius of the equivalent spherical shell.
When the ratio of TS/L is less than 0.002, it shall comply with mandatory annex 1-4 (f).
Appendix 1-4 (f) is not only to calculate the thickness of general elliptical head, dished head and conical head. In addition, the maximum allowable internal pressure of elliptical head and dished head is calculated to avoid internal pressure instability or yield failure in the corner transition zone of elliptical head and dished head.
Firstly, calculate the internal pressure PE that may lead to elastic instability and the internal pressure py that may lead to yield, then calculate the PE/PY value to determine the internal pressure PEK that is expected to lead to the failure of the corner transition zone, and then take the safety factor of 1.5 to calculate the maximum allowable internal pressure of the head.
Therefore, in ASME, detailed calculation is used to ensure that the head will not lose stability under internal pressure.
It may be found through calculation that the elliptical or dished head of ASME is likely to be thinner than GB sometimes. After all, sometimes when the pressure is relatively low, the method of 0.3% minimum thickness will be slightly conservative.
In HG/T 20582-2011, the algorithm in Chapter 5 is also the same as ASME.

## Is the allowable stress value of the head reduced?

As mentioned above, GB should be thicker than ASME. Why is ASME thicker than GB dished head?
That is because for dished heads, in UG-32 (d), when the minimum tensile strength is higher than 485Mpa, the maximum allowable stress of the head can only be 138Mpa.

The dual phase steel for friend’s dished head, SA-240 S32205, has a tensile strength of 665mpa. Therefore, when calculating the dished head, the allowable stress of 186.9mpa can not be used, but the reduced allowable stress value of 138mpa is used.

Back to the initial question:
The dished head is 1.54 times that of the cylinder, and the allowable stress is reduced from 186.9 to 138Mpa, so the thickness of dished head is thicker than that of the cylinder:

• 1.54*186.9/138=2.1

This is why friends will find that the dished head is twice thicker than the cylinder.
Therefore, for high-strength steel, the use of dished heads is often thicker and uneconomical.
If it is changed to elliptical head, there will be no problem.

## Why should this be stipulated?

The total stress near the corner transition zone of dished/elliptical head is very large, and the stress direction is reversed.
On the one hand, avoid brittle failure of high-strength materials. On the other hand, in manufacturing, shape deviation or local thinning defects, the instability load of high-strength materials may be greater than the internal pressure limit load, and the instability precedes the strength failure.
Therefore, ASME stipulates that for all dished heads and elliptical heads with k > 1 (i.e. relatively flat elliptical heads), it is necessary to reduce the allowable stress.

## Summary

The national standard uses the minimum thickness to control the internal pressure instability of convex head, and ASME uses more detailed calculation to control the internal pressure instability. If the large-diameter head needs special thickness due to the influence of the minimum thickness in the national standard, you can refer to the algorithm of ASME or HG/T 20582 for detailed calculation, which can save costs.
If the dished head of high-strength steel such as duplex steel and chromium molybdenum steel is used in the national standard, it is necessary to pay attention to whether there may be internal pressure instability. It is best to reduce the allowable stress for calculation, or replace it with elliptical head.

Source: China Dished Head Manufacturer – Yaang Pipe Industry (www.steeljrv.com)

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)