The existing head standards in our country have been formulated separately according to different structural types (elliptical, dish-shaped, conical) and forming methods (stamping, spinning). This not only leads to the unreasonable situation of inconsistent quality requirements among different head standards but also creates certain difficulties in selecting standard heads and revising the standards themselves.

First, previous head standards were all designed solely to complement GB150, “Steel Pressure Vessels,” meaning they addressed only the manufacturing, inspection, and acceptance requirements for heads designed according to conventional rules. Yet as early as 1995, China had already established a dual-standard system for pressure vessels—comprising GB150 and JB4732 (the “Analysis and Design Standard for Steel Pressure Vessels”). The absence of head standards that are fully compatible with the analysis-and-design approach undoubtedly represents a significant shortcoming in China’s standardization efforts for pressure vessels.

Second, GB150 is a mandatory standard, whereas the head standards compiled according to and complementary to GB150 are merely advisory (recommended). This is clearly unreasonable and makes it difficult to ensure the quality of heads—critical pressure-bearing components.

Material Analysis

The inner diameter of the vessel, Di = 4000 mm; the calculated pressure, Pc = 0.4 MPa; the design temperature, t = 50℃; the head is a standard elliptical head; the material is 16MnR (the allowable stress of the material at the design temperature is 170 MPa); the negative deviation of the steel shall not exceed 0.25 mm and shall also not exceed 6% of the nominal thickness; the corrosion allowance, C2 = 1 mm; the weld joint coefficient for the welded joints of the head assembly, φ = 1. Determine the calculated thickness, design thickness, and nominal thickness of the elliptical head.

Calculate the thickness δ = 2[σ]tΦ - 0.5pc =4.73 mm

Calculate the thickness δd = δ + C2 = 4.73 + 1 = 5.73 mm

The effective thickness δe of the standard elliptical head should be no less than 0.15% of the head’s inner diameter Di; thus, the effective thickness δe = 0.15% × Di = 6 mm.

δe > δd, C1 = 0, C2 = 1, nominal thickness δn = δe + C1 + C2 = 6 + 0 + 1 = 7 mm

Considering the standard specification thickness of the steel material, a 1-mm increase was applied to the initial design rounding value Δ1 = 1; therefore, δn is taken as 8 mm.

According to the technical data from the professional head manufacturer, for a head with Di = 4000 and δn = 8, the wall-thickness reduction allowance C3 is 1.5 mm, and the second-rounding value for thickness Δ2 is 0.5.

If the required forming thickness of the head must not be less than the nominal thickness δn minus the steel plate negative tolerance C1, then the material thickness to be used is:

δs = δn + C1 + C3 + △2 = 8 + 0 + 1.5 + 0.5 = 10 mm, whereas the minimum thickness after forming is 8.5 mm. If the formed head thickness is required to be no less than the design thickness δd (where δe should be taken as the value of δe), then the initial material thickness should be: δs = δd (δe) + C3 + △2 = 8 mm, resulting in a minimum formed thickness of 6.5 mm—this thickness is greater than the effective thickness δe, and also greater than both the design thickness δd and the calculated thickness δ.

As shown above, the two different requirements result in a 2-mm difference in the material thickness of this head, while the weight difference amounts to as much as 300 kg.

Thickness definition

The thickness definitions in GB150 and related standards for heads are somewhat unreasonable, particularly with regard to the thickness requirements after the forming of vessels and heads. Specifically, the required forming thickness for convex heads and hot-rolled shells must not be less than the nominal thickness minus the negative tolerance of the steel plate (δn - C1). As a result, both design and manufacturing may end up adding extra thickness on top of the designed thickness just to ensure that the formed thickness meets the requirement. To address this issue, the concept of “minimum forming thickness” was once proposed: “The thickness that must be guaranteed after the hot-rolled cylindrical shell or convex head has been formed shall not be less than the design thickness.” In other words, designers should indicate both the nominal thickness and the minimum forming thickness (i.e., the design thickness δd) on the drawings. This allows manufacturers to decide, based on their fabrication processes and the rounding allowances specified in the original design, whether to further subtract additional material thinning allowances during fabrication. This definition and notation of thickness have been the prevailing practice in the international pressure vessel community as of 2013 and are reasonably justified. However, in China’s current standards, there remain two issues that need to be addressed.