Take Note
There are some specific cases where the S-RBI assessment approach in IMS PEI changes, depending on the Equipment/Component type.
This section gives guidance for Corrosive Deadlegs.
Deadlegs are the Components of a Piping System that normally have little to no significant flow.
Deadleg Classification
Corrosive vs Normal Deadlegs
A Potential Corrosive Deadleg (PCD), also referred to as Corrosive Deadleg, has the potential for accelerated corrosion (compared to Design CR for the main Piping System).
Unstable vs Stable Deadlegs
An unstable Deadleg is a Corrosive Deadleg where a slight change in process conditions can result in this accelerated (or unstable) CR. For stable Deadlegs a slight change will not cause this. An example of an unstable Deadleg is one with a potential for wet ammonium chloride salt since CR changes more that an order of magnitude when the dry salt becomes wet.
Deadleg management in IMS
The table below indicated the RBI approach applicable to the type of Deadleg.
Type of DL | Mode | How is it managed | RBI Components in IMS | Comments |
|---|---|---|---|---|
Potential Corrosive | Unstable | Class A, B, C (see IS table below) | Corrosive Deadleg | Time limit (MII) – own Circuit |
SRBI | Deadleg | Per SRBI – own Circuit | ||
Stable | SRBI | Deadleg | Per SRBI – own Circuit | |
Non-Corrosive (normal) | Stable | SRBI | Deadleg | Inspect with main pipe |
Define Asset - Step 1
See Step 1.
In IMS a Corrosive Deadleg can be modelled as a separate Component. On this level the RBI Analysis can then take place.
Corrosive Deadlegs with CMLs should be tracked in a Circuit separate from the mainline piping.
Assign DMs to Component – Step 3
See Step 3.
For this Component type, you can assign the DL Analysis, rather than an actual Degradation Mechanism (DM).
RL – Step 6 & Step 11
RL is not applicable, but the RL tab can be used to enter a Legal/Local Interval. This may influence the NID.
StF – Step 7
See Step 7.
The difference between the Potential Corrosive Rates of the Deadleg (PCRD) and the Design Corrosion Rate (DCR) of the main flowing pipe determines the Unstable Deadleg’s StF. Susceptibility to Fail is the vulnerability to actual degradation as compared to the degradation expected per design.
IMS uses the following deltas to determine the StF for Deadlegs:
H: > 20 mpy (> 0.5 mm/y)
M: 10-20 mpy (0.25 - 0.5 mm/y)
L: 5-10 mpy (0.13 – 0.25 mm/y)
N: < 5mpy (<0.13 mm/y)
Note: For a low Criticality rating, IMS advices to analyze the Equipment per normal RBI. In this case the Component Type can be changed, and an actual DM assigned. A different StF questionnaire, applicable to the selected DM, must then be filled in again.
Confidence – Step 10
The Confidence Assessment it not applicable for Corrosive Deadlegs.
IS and MII – Step 11 & Step 12
For Corrosive Deadlegs, IMS will use the Criticality rating to determine the IS class (A, B or C) and MII from a lookup table (see the table below). No IF is determined in this case.
Inspection Strategy (class A, B, or C) and Maximum Inspection Interval lookup table for unstable Corrosive Deadlegs:
IS and MII
| Consequence | ||||||
- | N | L | M | H | E | ||
Susceptibility to Failure (StF) | H | Per S-RBI | Per S-RBI | C (3 yr) | B (≤ 3 yr) | A (≤ 1 yr) | A (≤ 1 yr) |
M | Per S-RBI | Per S-RBI | Per S-RBI | C (3 yr) | B (≤ 3 yr) | A (≤ 1 yr) | |
L | Per S-RBI | Per S-RBI | Per S-RBI | Per S-RBI | C (3 yr) | B (≤ 3 yr) | |
N | Per S-RBI | Per S-RBI | Per S-RBI | Per S-RBI | Per S-RBI | Per S-RBI | |
However, for a low Criticality rating IMS does not specify an IS class and MII, but rather advices the user to analyse the Equipment per normal RBI. In this case the user can change the Component Type to Deadleg and assign an actual DM, rather than the DL Analysis.
For low Criticality, you will be asked to change from Corrosive Deadleg to Deadleg.
You can then assign a Degradation Mechanism to the Deadleg.
A new StF questionnaire, applicable to the selected DM, must then be filled in. The Consequence remains applicable.
Assign RBI results to Corrosion Schedule – Step 12
See Step 12.
The Circuit detail and Inspection Schedules should be tagged according to the IS Class (A,B,C – see table above) by assigning a Special Emphasis to it.
Deadlegs within an IP/MP primary/secondary zone should be inspected as part of the IP/MP inspection strategy.
When inspected, small diameter branch connections, e.g., vents, drains, and bleeders, should be put in one or more separate Deadleg Circuits. CMLs should be representative of these types of connections, considering the potential and extent of Deadleg corrosion. Only in a few cases, should all the piping Deadlegs in a Circuit be monitored, e.g., a high consequence Piping System where experience has shown that corrosive sediment, salts, and/or contaminated water collect in the Deadlegs.