Flange Pressure Class Explained

Flange Pressure Class

Before reading further, it is worth noting that the terms ‘flange pressure class’, ‘flange class rating’ and ‘flange class’, mean the same thing and can be used interchangeably. The term ‘flange pressure rating’ does not mean the same as those mentioned previously (the reasoning will be explained in this section). The class rating of a flange is often followed by a ‘#’, ‘Lb’ or ‘lbs’. For example, a Class 150# flange.

Many engineers believe that the pressure class of a flange indicates the maximum allowable working pressure (MAWP) in pounds per square inch (psi) that a flange can endure without failing, this is not true. As mentioned in the previous paragraph, the pressure rating of a flange is not its class rating. A flange’s pressure rating is its MAWP; the two terms refer to the same thing and can be used interchangeably.

The class rating of a flange is a dimensionless number, it does not indicate any unit of measurement (psi, bar etc). Although the class system may seem counter-intuitive, the logic behind the system is quite simple. The MAWP of a flange depends upon the maximum temperature at which it will operate and the material from which it is manufactured; it is therefore not possible to know the MAWP without knowing the other two factors (max temperature and material). To elaborate this point further:

The design MAWP of a flange decreases as its maximum design temperature increases.

The design MAWP of a flange increases as its maximum design temperature decreases.

A higher class indicates that a flange has a higher-pressure rating for a given temperature, and a given material. As the maximum design temperature increases, the class must increase in order to keep the flange’s pressure rating within design limits; this will be explained using examples in the next section.

A flange’s pressure rating is the MAWP of the flange at a given temperature, for a given material. A flange’s pressure class is assigned based on the flange’s maximum pressure rating, maximum temperature rating, and the material of manufacture. The maximum pressure and maximum temperature ratings are collectively referred to as the ‘pressure-temperature ratings’.

Pressure classes are not material specific. It is possible to have a class 150 carbon steel flange and a class 150 stainless steel flange. However, the pressure-temperature ratings associated with a certain class are specific to a given material. For example, a class 150 carbon steel flange has completely different pressure-temperature ratings compared to a class 150 stainless steel flange. This means that the pressure rating at which each flange will fail are completely different. For this reason, it is ill advised to mate flanges that are manufactured from different materials.

Are you enjoying this article so far? Then be sure to check out our Flange Fundamentals Video Course. The course has over five hours of video content, a 52 page colour illustrated handbook (this article is an extract from the handbook), a quiz, and you will receive a certificate of completion when you finish the course. Enjoy!

Flange Pressure Class Examples

Example 1

The below image shows a cross section of various flanges. Each flange is manufactured from the same material and all are designed to operate at the same pressure rating. The only difference between the flanges is that the flange furthest to the left has a much lower temperature rating than the flange furthest to the right. For a flange of a given pressure rating and a given material to operate at higher temperatures, its dimensions must increase, thus its class also increases.

Flange Pressure Class Comparison

In the above example, the pressure rating is fixed whilst the temperature rating has a relationship with the dimensions and class of the flange i.e. as the temperature rating increases, the dimensions and class of the flange increase.

Example 2

Flange material is the same for all flanges.

Flange maximum temperature rating is the same for all flanges.

Because the maximum temperature rating is constant, and the material is constant, the maximum pressure rating of the flange has a relationship to the flange class and flange dimensions. This is useful because the material thickness of a flange is indicative of a flange’s pressure rating i.e. a thicker flange has a higher-pressure rating for the given temperature rating and material of manufacture.

The below image is the same as that shown previously, but the constant temperature rating, and fixed material, mean that the flange shown furthest to the left has a lower pressure rating than the flange shown furthest on the right. 

Flange Pressure Class Comparison

Summary

Example 1 and 2 show that both temperature and pressure have a relationship to the pressure class of a flange. It is incorrect to assume that a flange’s pressure class is solely attributed to its pressure rating, although this mistaken assumption is common.

Determining Pressure Class

The pressure class of a flange depends upon three factors:

• The maximum pressure at which the flange will operate.
• The maximum temperature at which the flange will operate.
• The material from which the flange is constructed.

Using tables from given standards e.g. ASME B16.5, it is possible to determine a flange’s pressure class. The pressure and temperature values are referred to as the pressure-temperature ratings of the flange, and these can be used to determine its class. In the ASME B16.5 standard, there are seven flange classes:

150, 300, 400, 600, 900, 1500, 2500

Each of these pressure classes will often be expressed as:

150#, 300#, 400#, 600#, 900#, 1500#, 2500#

The next section explains how to use class tables to ensure a flange is allocated the correct pressure class.

Pressure Class Tables

To allocate a pressure class, it is necessary to:

1. Access the relevant standard to which the flange must conform e.g. ASME B16.5. 
2. Within the standard, locate the correct material group table based on the flange’s material of manufacture. For example, the ASME B16.5 standard has over 30 individual material groups. 
3. Determine the maximum pressure at which the flange will operate i.e. the maximum pressure of the system to which the flange will be attached.
4. Determine the maximum temperature at which the flange will operate i.e. the maximum temperature of the system to which the flange will be attached.
5. Locate the maximum temperature rating in the left-hand column. Select the row where the maximum temperature rating is equal to, or exceeds, the given value.
6. Scroll along the selected row until reaching a pressure rating that is equal to, or exceeds, the given value.
7. The header of the column where the intersection of the pressure-temperature ratings intersect, is the pressure class.

A pressure class table taken from ASME B16.5 is shown below; the table is for carbon steel flanges. For ease of use, the table has been modified to show imperial and metric units.

Pressure Class Table

Pressure Class Table Examples

The best way to learn how to determine the pressure class of a flange is by using examples.

Imperial Example

Standard: ASME B16.5

Material: ASTM A-105 – Carbon Steel Forgings for Piping Applications

Temperature Rating: 700⁰F  

Pressure Rating: 1,000 psi.

Using the below table, the suitable flange class would be 600.

Pressure Class Table – Imperial Example

Metric Example

Standard: ASME B16.5

Material: ASTM A-105 – Carbon Steel Forgings for Piping Applications

Temperature Rating: 400⁰C  

Pressure Rating: 70 bars

Using the below table, the suitable flange class would be 900.

Pressure Class Table – Metric Example

Flange Pressure Class Effects

A flange’s pressure class is effectively a way of grouping certain pressure-temperature ratings based upon a flange’s material of construction. There are visual indicators that give clues as to the type of service a flange is likely to encounter, these include:

• A relatively thick flange indicates a high-temperature or high-pressure application, or both.
• The outside diameter of a flange increases as its pressure class increases.
• The bolt circle diameter of a flange increases as the flange’s pressure class increases.
• The diameter of a flange’s bolt holes increases as the pressure class increases.

For a flange with a given nominal pipe size e.g. NPS 10, the inner diameter will not change as the pressure class of the flange increases. If the flange has a raised face, its height will not change as the pressure class of the flange increases.

Effects of Flange Pressure Class

It is worth noting that some flange types are not suitable for higher classes due to their design. For example, a flat face flange design will not be used for any pressure class exceeding 250. This is because the flange is poorly suited for high pressure applications, irrespective of its maximum design temperature; the ASME B16.5 standard does not provide information for flat face flanges exceeding class 250 for this reason.

Other types of flange such as raised face, ring-type joint etc. are suitable for all flange classes, and relevant data is provided within standards such as ASME B16.5. If a flange design has data available from multiple flange classes, it shows that the flange design is suitable for a range of pressure and temperature applications (broad pressure-temperature suitability).

Additional Resources

https://blog.projectmaterials.com/flanges/flange-rating-pressure-temperature-chart

https://www.carverpump.com/flange-classes-explained

http://www.wermac.org/flanges/flanges_pressure-temperature-ratings_astm_asme.html