Page 100 - Pressure Vessel Design Manual

General Design 81

PROCEDURE 2-17
MINIMUM DESIGN METAL TEMPERATURE (MDMT)

Notation accordance with the provisions of UG-20. Some users
have a standard value that has been chosen as the
R =use the lesser of R1 or R2 lowest mean temperature of the site conditions, such
Ri =ratio of thickness required at MDMT to the cor- as 15°F.
roded thickness 2. Exemption MDMT: The lowest temperature at which
Rz = ratio of the actual stress to the allowable stress the pressure vessel may be operated at full design pres-
tMT =thickness required of the part at MDMT, in. sure without impact testing of the component parts.
tDT =thickness required of the part at design tempera- 3. Test MDMT: The temperature at which the vessel is
ture, in. charpy impact tested.
t,, =thickness of the part, new, in (exclusive of thinning
allowance for heads and undertolerance for pipe) The ASME Code rules for MDMT are built around a set
t, =thickness of the part, corroded, in. of material exemption curves as shown in Figure 2-55. These
C.a. =corrosion allowance, in. curves account for the different toughness characteristics of
E =joint efficiency carbon and low alloy steel and determine at what tempera-
SMT = allowable stress at MDMT, psi ture and corresponding thickness impact testing will become
SI,T = allowable stress at design temperature, psi mandatory.
S, = actual tension stress in part due to pressure and all There is an additional exemption curve (see Figure 2-54),
loadings, psi which allows a decrease in the MDMT of every component,
T, =lowest allowable temperature for a given part and thus the vessel, depending on one of several ratios spec-
based on the appropriate material curve of ified. This curve would permit carbon steel, without impact
Figure 2-55, degrees F testing, to be used at a temperature of -150"F, provided the
T2 =reduction in MDMT without impact testing per combined stresses are less than 40% of the allowable stress
Figure 2-54, degrees F for that material. Granted, the vessel would be inore than
twice as thick as it needed to be for the pressure condition
This MDMT procedure is used to determine the lowest alone, but if the goal was to exempt the vessel from impact
permissible temperature for which charpy impact testing is testing, it could be accomplished.
or is not required. The ASME Code requires this be deter- Since impact testing is a major expense to the manufac-
mined for every pressure vessel and the MDMT be stamped turer of a pressure vessel, the designer should do everything
on the nameplate. While every pressure vessel has its own to avoid it. Impact testing can always be avoided but may not
unique MDMT, this may or may not be the MDMT that is be the most economical alternative. Following these steps
stamped on the nameplate. Not only does every pressure will help eliminate the need for impact testing and, at the
vessel have its own unique MDMT, but every component same time, will provide the lowest MDMT.
of that pressure vessel has an MDMT. The vessel MDMT is
the highest temperature of all the component MDMTs. On
occasion, the MDMT is specified by the end user as an ar- 1. Upgrade the material to a higher group.
bitrary value. The vessel fabricator is then responsible to 2. Increase the thickness of the component to reduce the
verify that the actual MDMT of every component used in stress in the part.
that pressure vessel is lower than the arbitrary value 3. Decrease the pressure at MDMT. This is a process
requested for the nameplate stamping. Considering this, change and may or may not be possible. Sometimes a
there are various definitions for MDMT depending on how vessel does not operate at full design pressure at the
it is used. The definitions follow: low temperature condition but has alternate conditions,
such as shutdown or depressurization. These alternate
1. Arbitrary MDMT: A discretionary, arbitrary tempera- low temperature conditions can also be stamped on the
ture, specified by a user or client, or determined in nameplate.