Corrosive Deadlegs

Deadlegs are the Components of a Piping System that normally have little to no significant flow. A Potential Corrosive Deadleg (PCD) has potential for accelerated corrosion (compared to Design CR for the main Piping System). An unstable Deadleg is a PCD 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.

The table below helps to explain the different types of Deadlegs.

Deadleg management in IMS:

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

See Step 6 and 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 

Step 10.

The Confidence Assessment it not applicable for Corrosive Deadlegs.

IS and MII – Step 11 & Step 12 

See Step 11 and 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:

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 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.