API RP 581 - 3rd Ed.2016 - Add.2-2020 - Risk-Based Inspection Methodology

5-28 API RECOMMENDED PRACTICE 581
5 Heat Exchanger Tube Bundles
Overview
This section describes a methodology to assess the reliability and remaining life of heat exchanger bundles.
It also provides a methodology for performing cost benefit analysis to assist in making risk-based inspection
and replacement decisions and to determine the optimal replacement frequency of heat exchanger bundles.
The purpose of the module is to manage heat exchanger bundle inspection and replacement cycles and to
reduce annual operating and maintenance costs of heat exchanger bundles. The costs considered include
bundle fabrication and installation costs, environmental impact costs, and lost opportunity costs due to unit
and plant unplanned shutdowns or unit rate reductions as a result of an unplanned bundle failure.
Background
Analyzing each heat exchanger bundle service history generally does not consider the financial
consequences associated with a bundle failure. Many exchangers experience few or no bundle failures while
some failures may not have occurred at the current operating conditions or practices. In addition, statistically
significant data may not exist in order to make an accurate prediction of future performance or POF for the
heat exchanger bundle.
Basis of Model
The application of risk principles to exchanger bundle inspection allows decisions to be made based on the
consequences of bundle failure, including costs associated with lost production and environmental impact
costs associated with leakage into utility systems and the replacement and maintenance costs associated
with bundle replacement.
The combined experience of heat exchanger bundles of similar design and service is combined and
statistically analyzed to provide a prediction of future performance. The exchanger bundle under evaluation
is matched to similar bundles and statistically analyzed using a Weibayes or similar analysis to estimate the
POF of the bundle. The results from the analysis are used to determine if the exchanger bundle will operate
safely and reliably until the next scheduled maintenance opportunity. See Annex 5.A for an explanation on
how to determine Weibull parameters.
Required Data
The data listed in Table 5.1 shows the minimum data requirements for each heat exchanger bundle.
Methodology Overview
Calculations for the risk and inspection for heat exchanger bundles are performed following the flow chart
shown in Figure 5.1.
An overview of the steps for calculating Risk are shown in the following steps:
a) STEP 1.1⎯Gather and input the bundle basic input data required as defined in Table 5.1.
b) STEP 1.2⎯Gather and input inspection historical data, if available.
tube
tube
c) STEP 1.3⎯Determine the maximum acceptable POF, P based on the calculated
tgt
C and the risk
f
target, Risk .
tgt
d) STEP 1.4⎯Provide MTTF or Weibull parameters for the bundle failure rate curve based on historical
bundle failures in the same or similar service using Weibull analysis or some other statistical approach.
e) STEP 1.5⎯Calculate the POF at the current date, RBI Date, Plan Date, Turnaround Date 1 and
Turnaround Date 2 using the Weibull data.
tube
f) STEP 1.6⎯Determine consequences of bundle failure, C .
f
g) STEP 1.7⎯Calculate the risk at the plan date with and without inspection.
h) STEP 1.8⎯Define a recommended inspection plan.
i) STEP 1.9 – Calculate a bundle replacement frequency.