Computing Minimum Design Metal Temperature According to ASME Section VIII, Division 1 Intergraph Intergraph CAS Ray Delaforce
The Minimum Design Metal Temperature is also known by the acronym: MDMT. Carbon steels can become brittle at low temperatures, and can fail catastrophically at these temperatures. Normally vessel components are designed u sing the allowable design desig n stresses provided by t he code. These design stress are determined using safety factors that ensure the vessels can be safely operated within the design parameters. parameters. However, at low temperature, temperatur e, a vessel can fail suddenly at stresses well below the allowable stresses provided by the code. These failures are sudden, and unpredictable. We shall now see how the MDMT is set for a component in a pressure vessel that is made of Carbon steel.
Determination of the MDMT The factors used by the code are as follows: 1. The nominal thickness of the plate 2. The material specification 3. The heat treatment to which the component material is subject Let us look first at the material specification. specificati on. According to the code, a carbon carbon steel falls into any one of four four material groups. These groups are designated designated as: A, B, C or D. We can see this grouping in the ASME ASME code at Figure UCS-66 which we reproduce below. If we look for the material SA 516 70, it is in Group B. However, if it is supplied in the Normalized state, it is a Group D material.
Once we have settled on our material specification, the next thing we have to determine is the nominal thickness. So let us choose a material: a) Thickness 1.25 inches b) Group B (Curve B) material We now look at Figure UCS-66 from which we can determine the MDMT as follows:
From the Graph, it is determined that the MDMT is 42 OF. But, we also have to consider the required of calculated thickness of the component. As an exercise, let us say we have the following data for our component: A cylinder Inside Diameter Shell thickness Design Pressure Joint Efficiency Design Stress
120 inches 1.25 inches 300 psi 1.0 20 000 psi
We can compute the required thickness of this cylinder as follows:
It is apparent, that the required or calculated wall thickness is less than the actual corroded thickness. We can get some credit for lowering the temperature from the 42 F we have deduced from the graph UCS-66 as shown above. We need the ratio of the required thickness to the actual thickness. According to figure UCS66.2 we need to compute this ratio:
Let us perform this calculation as directed:
Now that we have the ratio (0.809), we can now enter graph Figure UCS-66.1 as follows:
We get a further reduction of 19 OF. So our final MDMT is: From Figure UCS-66 From Figure UCS-66.1
42OF -19 OF
Final MDMT
23 OF
If you had done this using PV Elite, the MDMT calculation would look like this:
Minimum Design Metal Temperature Results:
Govrn. thk, tg = 1.250 , tr = 0.910 , c = 0.1250 in , E* = 1.00 Stress Ratio = tr * (E*) / (tg - c) = 0.809 , Temp. Reduction = 19 F Min Metal Temp. w/o impact per UCS-66 Min Metal Temp. at Required thickness (UCS 66.1)
42 F 23 F
Elements Suitable for Internal Pressure.
As a side issue, PV Elite rounds temperatures off to a whole number.
MDMT Depends on the Stress Reduction Ratio The stress reduction ratio depends the computed thickness of the component. This is the symbol tr used in the equation below:
Obviously then, tr depends on the pressure to which the component is subjected. We have to consider two pressures in the case of a pressure vessel as a whole. They are: 1. The design pressure – the pressure used for designing the vessel 2. The MAWP (Maximum Allowable Working Pressure) – the maximum pressure the vessel can sustain, determined by its weakest element. The MAWP is higher than the Design Pressure. This can be illustrated by a simple example: Suppose in calculating the minimum thickness of a component for the design pressure, the thickness turned out to be 0.716 inches. Clearly, one could not purchase a plate that is 0.716 inches thick. However, one could purchase a plate that is 0.75 inches thick. The 0.75 inch plate can sustain a higher pressure than the 0.716 inch plate. Thus, the higher pressure becomes the MAWP for the 0.75 inch plate. So, the question arises: What pressure do we used to compute MDMT? It could be the Design Pressure, or, it could be the MAWP. PV Elite gives you the choice. If you go to the configuration dialog screen here by clicking on Tools Configuration as shown Here:
There is a place on the configuration screen where you can choose to compute the MDMT based upon either the design pressure or the MAWP. It is here:
If the box is checked the, the MDMT will be computed at the MAWP of the vessel. If it is not checked, the MDMT will be computed at the Design Pressure.
How Does PV Elite Compute the MDMT from Figure UCS-66 PV Elite can internally compute the MDMT from Figure UCS-66 is several ways. If we look again at the Configuration Screen, there is a drop down list box that gives us two choices. It is here:
Traditionally, in years gone by, the code only had Figure UCS-66 from which the designer had to squint his eyes to read the graph and hope that the answer he got was reasonable. PV Elite was first written in those times, and internally a series of equations was developed (know as polynomials) to obtain the desired MDMT. Later, ASME issued a table that gave the MDMT
values for various thicknesses. So, two methods are available. That is the reason why either method can be chosen in the Configuration Screen.
Metric values for the MDMT evaluation The code uses two sets of units that are used throughout the code. They are known as , and M etric Uni ts . Unfortunately, the code does not use the exact conversions Customary Uni ts for these units. For example, the equivalent of 1 inch is 25 mm, not 25.4 mm. The same is true when it comes to temperatures. There are two tables in the code for the MDMT evaluation. If they are put side by side like this:
The temperatures do not match completely. For example, consider the MDMT for 0.875 in thick material according to curve A. This is what we have:
Now, 0.875 inches converts to 22.225 mm, and 61 deg F converts to 16.111 deg C. So there is a discrepancy of a fraction of the degree. The next question that arises is this: Do we round up or round down to get the final MDMT. PV Elite does all its calculations (the computing engine) in Imperial (English) units. This means that in deriving the final MDMT for a designer using the metric system, the program could be off by as much as a degree. This is unavoidable since the code uses ‘soft conversions’ from Imperial to Metric Units.
