Hydrocarbon Processing - August 2021 - 65

Maintenance
and Reliability
F. D'ANTONIO, Raffineria di Milazzo,
Milazzo, Italy
Continuous monitoring of creep in equipment
In this article, the development of a customized program-
designed by the author and used at Raffineria di Milazzo-for
the continuous monitoring of creep in vessels and piping is explained.
The program extracts temperature data for each piece
of equipment from the distributed control system (DCS) and
calculates the equivalent creep hours according to equations
derived from creep parameters introduced by researchers, such
as the Larson-Miller parameter (LMP), the Manson-Haferd parameter
(MHP) and the Orr-Sherby-Dorn parameter (OSDP).
What is creep? All refineries face material creep, a phenomenon
that is seen when the operating temperature exceeds limits-which
is typical for steel and alloy. As shown in FIG. 1, each
metal/alloy has a temperature at which creep starts (Tcreep).
When the temperature is above Tcreep, stress causes deformation.
The speed of the deformation usually decreases for the
first period, remains constant for the second period, and then
increases in the last period.
Deformation is dangerous because it can lead to ruptures
(FIG. 1). These failures have been experienced in several different
plants all over the world, causing prolonged shutdowns and
jeopardizing the safety of plant personnel.
The primary aim of this program is to provide an instrument
that helps in the calculation of the equivalent creep hours per
each piece of equipment in the facility. Typically, this can easily
be accomplished for all vessels and piping, except for steam
reformer tubes, which are not controlled by skin temperature.
Creep control. The number of hours in which equipment
units are in creep is usually controlled, since this number of
hours is equal to the number of plant running hours. Potential
creep damages are typically controlled by using non-destructive
techniques (NDTs) during turnarounds.
This kind of control, even though it is commonly used, is
not ideal. During operations, the temperature can be lower than
Tcreep. Conversely, temperatures above the design temperature
(Tdes), which can be very dangerous, are not considered.
Moreover, turnarounds are usually conducted every two years
or more, so controlling creep via NDTs during a turnaround
would not happen very often. Therefore, a more accurate system
to control creep can be of great advantage.
In the design of sections subjected to creep, a Tdes at which
the material is expected to fail is provided. If the Tdes value is unknown,
then it is possible to calculate it by following these steps:
1. Calculate the stress at the design conditions
of the vessel/tube.
2. Using the master creep curve that can be found
for the given alloy, obtain the LMP (depending on
the material, MHP or OSDP can be used).
3. From the LMP and the design number of hours,
calculate the Tdes in Kelvin degrees.
Once Tdes is known (e.g., 100,000 hr), then the lifetime (t)
at the temperature (T) is obtained by using an equation derived
from the LMP formula (Eq. 1):
t = 10^((Tdes / T)(log(Tdes) + C) - C) (1)
where tdes is 100,000 hr in this example, temperatures are in
Kelvin degrees and C is a constant, depending on the material.
Equivalent equations can be derived for the MHP or OSDP.
To calculate the equivalent creep time for each vessel/tube, the
Raffineria di Milazzo information technology (IT) department
created a custom application that extracts the online temperature
values from the DCS for each piece of equipment, converts it to
Kelvin degrees and introduces the X parameter (TABLE 1). If the
temperature is below Tcreep, then there is no creep for the equipment
and the lifetime is not affected-i.e., X is zero. The assignTABLE
1. Calculating the X parameter
Temperature (T)
0 < T < Tcreep
Tcreep < T < Tdes
T > Tdes
X parameter
1
Td/10^((Tdes/T)(log(Tdes) + C) - C)
FIG. 1. Each metal/alloy has a temperature at which creep starts
(Tcreep). Creep leads to deformation, which can lead to rupture.
Hydrocarbon Processing | AUGUST 2021 65
Hydrocarbon Processing - August 2021

Table of Contents for the Digital Edition of Hydrocarbon Processing - August 2021
