Determination of thickness of large diameter blind flange

Determination of thickness of large diameter blind flange

Blind flange set in the requirements of the pipeline system or equipment need to cut off the part, such as equipment before and after the cut-off valve or between the two flanges, the main role is to completely isolate the process media to prevent the shut-off valve due to poorly closed to affect production and cause accidents. From the outside, generally divided into spectacle blind flange, blind flange and pad ring (blind flange and pad ring for each other blind pass).

Usually DN600 and below the thickness of the figure-8 spectacle blind, blind flange and pad ring can be obtained by consulting foreign engineering companies, such as Shell and TEC standard drawings or “pipeline steel insert, pad ring, 8 word blind” HG21547-93. DN600 or more large diameter blind and pad ring thickness in Shell and TEC standard drawings and HG21547 is not given, but in the actual engineering design and will inevitably encounter, so we must try to determine the thickness of large-diameter blind flange.

Thickness determination

In a Shell coal gasification project, a DN1000 large-diameter blind flange and gasket ring are set for the crude syngas inlet of the high-temperature and high-pressure filter, and the thickness of this blind flange is not given in the attached drawing of the DEP standard provided by Shell.
This DN1000/PN10.0 blind is used as an example to illustrate the method of determining the thickness of the large-diameter blind.
The operating temperature of this pipe is 340°C, the design temperature is 380°C, the operating pressure is 3.96MPa(G), the design pressure is 4.40MPa(G), the pipe material is 0Cr18NiTi, and the flange is DN1000/600lb.
(1) “Industrial Metal Piping Design Code” GB50316-2008 section 6.7.2 gives the formula for calculating the thickness of the blind flange clamped between the two flanges, the blind flange clamped between the two flanges is shown in Figure 1.
Its calculated thickness can be determined in accordance with the formula (1); for permanent blind flange should be increased by the additional amount of thickness in accordance with the formula (2).

t_m=0.433d_G[P/([σ]^tE_j)]^0.5(1)
t_pd=t_m+2C₂+C₁(2)

In the formula.

• t_m for the blind flange calculated thickness, mm;
• d_G is the inner diameter of the concave or flat flange gasket or the average diameter of the ring groove gasket, mm;
• P is the design pressure, MPa;
• [σ]^t is the permissible stress of the material at the design temperature, MPa;
• E_j is the welded joint coefficient;
• t_pd is the design thickness of the blind, mm;
• C₂ is the additional amount of abrasion or corrosion, mm;
• C₁ for the thickness of the additional amount of thinning, mm.

Figure.1 Blind flange clamped between flanges
The internal diameter of the gasket d_G at the flange is 956mm; the [σ]^t of 0Cr18NiTi steel plate at 380℃ is 80.8MPa; the welded joint coefficient E_j is 1.0; the additional amount of abrasion or corrosion C₂ is 0mm; the additional amount of thickness reduction C₁ is 0.15 times the design thickness of the blind flange.

t_m=0.433d_G[P/([σ]^tE_j)]0.5
=0.433×956×[4.4/(80.8×1.0)]^0.5
=96.60mm
t_pd=t_m+0.15t_pd
t_pd=113.64mm

The DN1000/PN10.0 blind thickness can be taken as 114mm.
(2) According to the definition of shear stress to calculate:

τ = Q/A_Q

A_Q = πd_G t_pd

Q = PA = πPd²_G/4

In the formula.

• τ for the shear surface of the work shear stress;
• Q for the shear surface of the internal force, that is, shear force;
• AQ for the shear surface area.

Working shear stress on the shear surface shall not exceed the permissible shear stress of the material [τ] that:

τ = Q/A_Q ≤ [τ]

Therefore, the blind thickness t_pd ≥ Pd_G / (4 [τ]), if the shear force [τ] of the steel plate, according to the process parameters can be found in the blind required minimum thickness.
(3) The defining formula of shear stress is derived, and the force analysis is shown in Figure 2.

τ=Q/A_Q=Pd_G/(4t_pd)

Figure.2 Force analysis
According to the shear strength conditions, the working shear stress on the shear surface shall not exceed the allowable shear stress of the material [τ], that is, when the blind thickness is taken as a minimum value, τ = [τ] to obtain:

d_G/t_pd=4[τ]/P

For different materials used in different occasions of the pipeline, the design pressure P and allowable shear [τ] are different, assuming that d_G/t_pd is a constant, verify its validity by Table 1, Table 2 and Table 3.
Table.1 Standard attached parameters of DEP provided by Shell (PN10.0/D2222)

DN	NPS ( in)	tpd(mm)	dG ( mm)	dG/tpd
150	6	24	174	7. 3
200	8	31	225	7.3
250	10	38	274	7.2
300	12	44	329	7.5
350	14	48	359	7.5
400	16	55	412	7.5
450	18	61	473	7. 8
500	20	68	520	7.6
600	24	81	628	7.8

Table.2 Blind thickness chart parameters provided by TEC (PN10.0/D2222)

DN	NPS ( in)	tpd(mm)	dG ( mm)	dG/tpd
150	6	22	174	7.9
200	8	28	225	8
250	10	34	274	8.1
300	12	41	329	8
350	14	45	359	8
400	16	51	412	8.1
450	18	59	473	8
500	20	65	520	8
600	24	78	628	8.1

Table.3 Blind thickness table parameters provided by HG21547 (PN10.0/D2222)

DN	NPS ( in)	tpd(mm)	dG ( mm)	dG/tpd
150	6	26	174	6.7
200	8	32	225	7
250	10	38	274	7.2
300	12	44	329	7.5
350	14	48	359	7.5
400	16	54	412	.7.6
450	18	61	473	7.8
500	20	67	520	7.8
600	24	79	628	7.9

Through the above table, the blind flange made of stainless steel with PN10.0 is studied, and the d_G/t_pd of Shell, TEC and HG21547 is calculated and found to be basically constant, and there is a trend of slow increase. Therefore, the d_G/t_pd value of DN600/PN10.0 in the attached chart of DEP standard can be used to calculate the thickness of DN1000/PN10.0 blind flange, i.e.:

(d_G/t_pd)_DN1000=(d_G/t_pd)_DN600=7.8t_pdDN1000=123mm

Analysis and discussion

The following analysis discusses the safety, economy and feasibility of the blind thickness calculated by these three methods.

• (1) The first method of calculating the blind thickness is not only safe and reliable, but also very economical, but because the designer is limited by design experience and design information, the determination of E_j, C₁ and C₂ parameters in the calculation formula will vary from person to person or can not be found, so the calculation results are not completely consistent. In addition, special materials of the blind can not be calculated using this method.
• (2) The second method is calculated from the force principle of the blind flange for force analysis, and the safety and economy of the calculation results are not in doubt, but the method is less feasible because of the difficulty of finding the allowable shear stress [τ] of the material.
• (3) The third method is the derivation of the second calculation method, which avoids finding the material allowable shear stress [τ], and can be used for special steel blind flanges for which data such as allowable stress [σ]^t at the design temperature of the steel plate cannot be found and cannot be calculated by the first method. The thickness of the blind flange calculated by this method is also safe and reliable because the thickness of the blind flange provided by Shell is common for carbon steel and stainless steel, and the thickness of the blind flange calculated by this method is also safe and reliable because the thickness of the blind flange provided by Shell is common for carbon steel and stainless steel. This method is not as economical as the first calculation method, especially for large diameter blind flange is more obvious.

Conclusion

Through analysis and comparison, the calculation of large-diameter blind thickness is recommended to use the first method when the data is complete, and the third method can be used when the data is incomplete.

Author: Junjie Yin

Source: China Blind Flange Manufacturer – Yaang Pipe Industry Co., Limited (www.pipelinedubai.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.)

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Reference.

1. GB50316-2000, Industrial Metal Piping Design Code [S].
2. HG/T20570.23-1995, the setting of the blind flange [S].
3. Tang Shanfu, Zhu Siming. Chemical equipment mechanical fundamentals [M]. Shanghai:East China University of Science and Technology Press, 1991.