Minimum Design Metal Temperature Calc Proc

1. Purpose

This procedure provides information on how to determine the Minimum Design Metal Temperature (MDMT) for equipment as required by the ASME Pressure Vessel Codes.

2. General

2.1 The ASME Code provisions regarding MDMT generally apply to all major equipment, excluding fired heaters. Consult the appropriate Technology or Process Specialist for procedures specific to the process unit, potential problem areas, and equipment requiring special consideration.

2.2 The MDMT is a key factor used in determining if Charpy V-notch impact testing of materials is required by the Code. Impact testing is a means of quantifying a material’s toughness, i.e., it’s ability to resist (brittle) fracture. If testing is required, the MDMT strongly influences the temperature at which minimum impact values must be met, possibly necessitating heat treatment or material upgrades. A low MDMT (e.g., below -50F) may require the use of low nickel or stainless steel material, thus increasing the cost of the equipment. There are also some advantages to an MDMT above -20F.

2.3 The Code suggests that the metal temperature during hydrotest be at least 30F (17C) greater than the MDMT. The same minimum margin is required during pneumatic testing. When the MDMT is elevated, it may be necessary to heat the test medium and/or the equipment in order to meet the targeted margin.

3. Minimum Design Metal Temperature

The Minimum Design Metal Temperature is defined as the lowest average temperature through the metal thickness. Note that the MDMT is strictly defined as the metal temperature, not the fluid temperature. This is very difficult to determine, therefore the MDMT provided on the Inflection Point Engineering Project Specifications is the lowest of the following three values (see Appendix):

3.1 The normal operating temperature of the process fluid (as defined in the Inflection Point Engineering Process Specifications) minus 25F. If multiple cases are to be considered (such as different feedstocks, startup or shutdown conditions, regeneration or purging conditions), use the lowest expected process temperature minus 25F.

3.2 The lowest 24 hour mean ambient temperature as it is defined in the Basic Engineering Design Questionnaire (BEDQ) under “Low Ambient Temperature”.

3.3 The calculated auto-refrigeration temperature as defined in Section 4. The auto-refrigeration case may govern the MDMT for extremely light streams, e.g. methane, ethane, or ethylene rich streams. The responsibility for this calculation resides with the Design Engineer who performs the process work.

4. Auto-Refrigeration Calculation Procedure

4.1 Vessels

The governing case for vessels considers the pressure relief valve (PSV) to open and stick wide open until the vessel has depressurized to 35 percent of its design pressure, at which point the PSV is considered to reseat (see Appendix). Use the following procedure for calculating the resulting temperature:

a. Identify the design pressure (gauge) for the vessel under consideration.

b. Calculate 35 percent of the design pressure (again, gauge).

c. Using the stream information from the Inflection Point Engineering Process Specifications, identify the inlet or feed stream to the vessel under consideration. Examples of this inlet stream are:

• Feed to a fractionator for calculation of the column’s MDMT (see Appendix)
• Column overhead material from the condenser for calculation of the receiver’s MDMT
• Cooled material to separators for calculation of the separator MDMT

d. Perform an adiabatic flash calculation (QF) on this feed stream from the Heat & Weight Balance conditions down to the pressure level calculated in “b” above.

e. Record the resultant temperature from the QF as the auto-refrigeration temperature. In most cases, this temperature may be rounded down to the next lower 5F increment for the MDMT (if the auto-refrigeration case governs). If the resultant flash temperature is between -45 and -50F, use the calculated temperature without rounding down. This increases the possibility that killed carbon steel will be adequate for the MDMT.

4.2 Heat Exchangers

Determine heat exchanger MDMT’s in the same manner as used for vessels. The stream to be flashed shall be the cooler of the inlet or outlet streams. For shell and tube heat exchangers, calculate and report MDMT's separately for both the shell side and the tube side.

4.3 Pumps

In the vast majority of cases, employ the procedure described in Section 4.1 for calculating pump MDMT's. Although a pump may be isolated and vented to flare, thereby dropping below 35 percent of the design pressure, there is generally insufficient liquid in the pump and connected piping to exceed the MDMT as calculated in Section 4.1, particularly if some heat ingress is allowed. However, in certain specific cases where the normal operating temperature is less than 10F and the pumped fluid is an LPG type mixture (methane, ethane, ethylene), use the rigorous calculation method described below:

a. Obtain the approximate pump casing weight from the equipment specialist.

b. Obtain the specific heat (Cp) of the pump casing material (assume 0.107 Btu/lb·F for killed carbon steel).

c. Obtain the pumped fluid specific heat (Btu/lb·F).

d. Obtain the approximate volume of fluid in the pump casing and piping from the equipment specialist and calculate the weight of the fluid.

e. Calculate the theoretical auto-refrigeration temperature by flashing the pumped liquid stream from the normal operating temperature and pressure (at suction conditions) to atmospheric pressure. This will provide the theoretical temperature differential (T).

f. Calculate the total theoretical heat to be transferred into the pump casing as:

Fluid Cp * Pump Fluid Weight * T = Heat Transferred

(Btu/lb·F) * Lbs * F = Btu's

g. Calculate the pump casing temperature change assuming all the heat is transferred from the fluid to the pump casing:

Heat Transferred (“f” above)/casing material Cp/Pump Casing Weight

= Casing temperature change

Btu * (lb·F/Btu) * ( l/lbs) = F

h. Subtract the temperature calculated in “g” above from the normal operating temperature to determine the theoretical minimum pump casing temperature.

4.4 Pump MDMT Example:

Pump casing weight = 500 lbs

Pump Casing Cp = 0.107 Btu/ lb·F

Fluid Cp = 0.797 Btu/ lb·F

Fluid volume = 5 gallons, approximately 17 lbs of fluid

Normal operating stream temperature = + 8F

Adiabatic flash temperature (to 0 psig) = -142F

Theoretical T = 8 - (-142) = 150F

0.797 Btu/ lb·F * 17 lbs * 150F = 2032 Btu

2032 Btu * ( lb·F /0.107 Btu ) * ( l / 500 lbs) = 38F

Pump MDMT = + 8F - 38F = - 30F

5. Piping

MDMT requirements do not directly apply to piping. However, in order to provide a consistent level of toughness to all components of a circuit, Inflection Point Engineering applies the lowest MDMT of the connected equipment to the interconnecting piping. For piping, this temperature is called the design minimum temperature.

Appendix

Comments and Explanatory Notes

1. Column MDMT’s are based upon the feed because:

• The properties of the inlet stream are well known.

• In the event of a pressure loss the properties and composition of the inlet stream will not change, whereas the composition of the internal and exiting fluids will change continuously and unpredictably as the pressure declines.

• Flash calculations are performed down to 35 percent of design pressure because:

a. PSV’s have been known to stick open, but a brief literature search (performed around 1990) did not reveal any cases where the valve opened and did not reseat as the equipment depressurized and the net closing force correspondingly increased. Thus flashing to a very low, or no, pressure level is unrealistic.

b. Other means of uncontrolled depressurizing require a component failure. Since failure has already occurred, potential brittle behavior becomes a moot point.

c. Auto-refrigeration is, in part, time dependent. As the vessel depressurizes the driving force decreases hence the rate of depressurizing declines. As the rate of depressuring is reduced the temperature reduction is lessened. This is not considered in the calculation.

d. Before all but the smallest equipment can depressure greatly, action taken by the operators or by safety control systems will control or halt the depressuring.

e. As the pressure declines, the stress in the metal also declines. The Code allows a reduction in the MDMT without impact testing as the stress level drops below the allowable value {see figure UCS-66.1 and Section UCS-66(b)}. This credit is, conservatively, not considered.

• The MDMT is the metal, not the contents, temperature. For ease of calculation they are assumed to be the same, but the metal temperature will actually change less and much more slowly than the contents. That’s because of the barriers to heat transfer between the fluid and metal, the large heat sink of the metal mass, and the difference in specific heats. Flashing the vapor to zero pressure and considering the metal to be at the same temperature is extremely conservative.

• ”Flashing” to a very low pressure could lead to a very low temperature, possibly requiring the use of a high alloy (and expensive) material. For the reasons noted above this temperature will not occur along with the design pressure because, by definition, flashing occurs due to a loss of pressure. The design, however, conservatively considers them to be coincident. Flashing to zero would be ultraconservative.

• When the actual stress level is less than 35 percent of the allowable stress, and the MDMT is less than -55ºF, no impact testing is required {see Section VIII, Division 1, Paragraph UCS-66(b)(3)}. The Code use of 35 percent is the basis for this procedure’s selection of 35 percent. For the above reasons, any value less than 100 and more than 0 percent is reasonable.

• One area that could get cold is the relief nozzle on the equipment. As the contents vent, the velocity in the nozzle could be high. The high velocity head results in a loss of pressure head and significant “flash” cooling.

• We consider the material to be fully stressed at the MDMT, a conservative, but not unrealistic or uncommon approach. This considers that the material may be stressed by loads other than internal pressure (which may not be present when the MDMT occurs, e.g., an MDMT due to ambient temperature cannot occur while the unit is operating at an elevated temperature, even if the ambient temperature condition occurs). For example, stress may occur due to wind, earthquake, residual stress, stress concentrations, piping or eccentric loads, weight, impact, etc. Repressuring is a concern if it occurs before the metal (not just the fluid) is warm. Operational error is a factor too. That’s why, as noted in item 2 above, the Code “credit” for materials stressed to less than the allowable stress, is normally not considered.