ASME Section VIII Div.1 Subsection A, UG-44 Flanges and Pipe Fittings
In this newsletter we will get the insight into ASME Section VIII Div.1
Subsection A, UG-44 Flanges and Pipe Fittings.
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Flange and Pipe Fittings
? Some Basic type of Standard Flanges
? Some Basic type of Fittings
UG-44(a) Standards
UG-44(a) specifically addresses the various available standards for flanges/fittings which are acceptable. The important part is that we do not need to desgin these flanges/fittings as per code, just selection of these based on material, temperature and pressure is sufficient.
MAWP of Flange based on Pressure -Temperature Rating
How to select a standard flange as per ASME B16.5?
Depending on the material, there will be specific tables. For example, for flanges made from A-105, find the corresponding table. In that table, based on different temperatures, you'll see flange ratings. For instance, if your design temperature is 150°C and you choose a 150 rating flange, it can handle a pressure of 15.8 bar pressure.
If your design pressure exceeds the rating, you need to select the next available rating, such as 300. with a maximum pressure capacity of 45.1 bar. There are no intermediate options. Simply refer to the appropriate table based on your flange material and consider the design temperature and pressure to make the right selection.
UG – 44(b) External Load
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Let's consider a scenario where a standard flange is connected to a nozzle, and piping is connected to the flange. Due to this piping, external loads and moments may be applied to the flange. The standard flange selection was based solely on temperature and pressure, without considering external loads. Therefore, it's uncertain whether the selected flange is suitable for these additional forces and moments.
Do you think, the maximum pressure taking capability of flanges is same or it has reduced due to additional forces and moments?
Let's consider the Maximum Allowable Working Pressure (MAWP) previously determined, such as 15.8 bar in the previous example. Now, when external loads are applied, the MAWP of the flange will decrease. This may raise questions or confusion about why the flange's pressure capacity is reduced due to external pressure.
To simplify, let's consider the example of a crane with a lifting capacity of 2 tons. If we attach a lifting hook that is not part of the crane's safe working load rating and weighs 0.1 ton, we need to determine how much weight can be lifted?
The calculation is straightforward: 2 - 0.1 = 1.9 tons. Similarly, the same principle applies to flanges. The flange's capability is not inherently reduced, but rather it has already taken some load in terms of pressure and moments. Thus reduction in maximum pressure capacity, or MAWP, is again same as the crane's reduced lifting capability due to the additional load. This is called derating of standard flange.
It's important to note that the calculation described in UG-44(b), which reduces the MAWP or pressure rating of the flange, only applies to weld neck flanges connected to nozzles. This reduction does not apply to slip-on flanges or other types.
UG – 44(b) External Load (Derating Calculation of flanges
In UG-44(b), an equation is provided to determine if the selected flange is suitable based on external loads. If the external loads satisfy the given condition, it indicates that the flange selection is adequate. Otherwise, it may be necessary to increase the flange rating.
The equation considers different types of external moments, such as ME, which consist of longitudinal (ML) and circumferential (MC) moments. Torsional moments (MT) do not contribute. The resultant moment ME is calculated using ML^2 + MC^2 and taking the square root. FE represents the external force in tension, while G is the gasket reaction diameter. PR is the flange pressure rating from B16.5, and PD is the vessel MAWP at the nozzle location after subtracting the static head. FM is the moment factor obtained from table 44-1. By evaluating these terms, we can determine the external load. If the equation is satisfied, the selected flange is deemed suitable for the pressure and temperature rating. If not, it may be necessary to increase the pressure rating, potentially moving to the next available rating, such as 300. This process is known as "Derating of flange" and accounts for the reduction in the flange's pressure capacity due to external loads.
Torsional moments, longitudinal shear load and circumferential shear load are not considered for flange derating as they do not contribute to leakage or in simple words, they do not try to open the flange joint.
Value of FM (Table UG-44-1)
The moment factor is determined using table 44-1. For flanges of size ≤12 inches and a rating of 150 according to B16.5, the moment factor is 1.2. It is a straightforward process to utilize this information.
Derated MAWP of Flange
Derating refers to the reduction in the pressure-taking capability of a flange. Previously, we checked if the selected flange met certain conditions to determine its suitability. However, if we need to calculate the new pressure-taking capability or the derated value of the flange, we utilize a derived equation. By equating the condition to an equality instead of an inequality and rewriting the equation in terms of PD (vessel MAWP at the nozzle location), we can determine the derated value. PR represents the normal pressure rating of the selected flange. The lower value between the derived equation and PR becomes the pressure rating of the flange. If the value is slightly lower, it indicates the derated value, but if it is still higher than PR, there is no change in the pressure-temperature rating due to the load. This calculation allows us to determine the Derated value of the flange.
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