How Flange Ratings Work

Figure 1: Flanges

A flange rating is a measurement that indicates the maximum allowable pressure and temperature that a flange can withstand. A flange rating is also referred to as a flange class or pressure class. Flange ratings are typically designated by a numerical value, such as Class 150, Class 300, and Class 600; the higher numbers indicate greater pressure and temperature resistance. This article discusses flange ratings and how to select a flange for an application.

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Flange class

ASME designed the flange class, considering the various pressure and temperature ratings. There are seven Classes - 150, 300, 400, 600, 900, 1500, and 2500. The rating indicates the maximum allowable pressure at a given temperature. For example, a Class 300 flange can withstand more pressure and is heavier than a Class 150 flange. A Class 300 flange means that the flange's safe working pressure at the rated temperature for a given material is 300 psi (20.6 bar).

Figure 2: Flange ratings

Flange rating standards

Flange ratings are standardized by organizations like the American National Standards Institute (ANSI) and the American Society of Mechanical Engineers (ASME) to ensure that flanges and their associated piping systems are compatible and can safely handle the required operating conditions. The most common flange rating standards are:

ANSI/ASME B16.5: This standard covers flanges and flanged fittings from NPS ½ (DN 15) to NPS 24 (DN 600) and specifies pressure classes ranging from 150 to 2500. The pressure classes indicate the flange's maximum allowable working pressure at various temperatures.
ANSI/ASME B16.47: This standard covers larger flanges from NPS 26 (DN 650) to NPS 60 (DN 1500) and specifies pressure classes ranging from 75 to 900.

Example

Table 1 shows the flange rating in psig for common carbon steel ASME flanges. The table shows that the allowable pressure in a Class 150 flange is 285 psi (19.6 bar) at 100 °F (37.7 °C) and 65 psi (4.5 bar) at 850 °F (454.5 °C). The flange is rated for 150 psi (10.3 bar) only at around 500 °F (260 °C). Higher-class flanges are rated at their nominal pressure only at around 850 °F (454.4 °C) (see Table 1).

Consider an application with a pressure rating of 800 psi (55.1 bar) at 500𐩑F (260 °C). Refer to Table 1 to select a suitable flange for this application. From Table 1, it's clear that a flange with a Class 400 rating meets the requirement as it can withstand 800 psi (55.1 bar) pressure at 500 °F. Always choose the flange rating that matches or exceeds the expected operating conditions to ensure a safe and reliable operation.

Table 1: Flanges pressure rating in psig for common carbon steel ASME flanges

	Pressure (psi)
Temperature (𐩑F)	150#	300#	400#	600#	900#	1500#	2500#
-20 to 100	285	740	985	1480	2220	3705	6170
200	260	680	905	1360	2035	3395	5655
300	230	655	870	1310	1965	3270	5450
400	200	635	845	1265	1900	3170	5280
500	170	605	805	1205	1810	3015	5025
600	140	570	755	1135	1705	2840	4730
650	125	550	730	1100	1650	2745	4575
700	110	530	710	1060	1590	2655	4425
750	95	505	675	1015	1520	2535	4230
800	80	410	550	825	1235	2055	3430
850	65	320	425	640	955	1595	2655
900	50	230	305	460	690	1150	1915
950	35	135	185	275	410	685	1145
1000	20	85	115	170	255	430	715

Other notes

Combination of flange ratings: When two flanges with different pressure ratings are joined together, the assembly's maximum pressure rating will be limited by the lowest-rated component. Adequate matching of flanges is crucial for maintaining the integrity of the system.
Non-standard flange ratings: Some applications may require custom or non-standard flange ratings beyond what ASME or ANSI standards define. These custom flanges must be designed and tested according to specific project requirements and may need approval from relevant regulatory authorities.

Flanges in industries

Figure 3: Valve with flange connection

Flanges allow for the easy assembly and disassembly of valves within a piping system. They can be bolted together to create a strong and secure connection, which helps in the maintenance and inspection of the system. Flanges create a tight seal, preventing leaks and ensuring proper containment of fluid or other materials being transported through the piping system. They can be used with gaskets or other sealing materials to enhance their sealing capabilities.