Attachment 1- Corrosion Rate Determination (As per API 510)

A-1       For a new vessel or for a vessel for which service conditions are being changed, one of the following methods shall be used to determine the vessel's probable Corrosion Rate:

a.     Corrosion Rate shall be calculated from data collected on vessels in the same or similarservice.

b.     If data for vessels in the same or similar service is not available, Corrosion Rate may beestimated from the company's experience or from published data on vessels in comparable
service.

c.     If the probable Corrosion Rate shall not be determined by either item a or item b above,on-stream thickness measurements shall be made after approximately 1000 hours ofservice. Subsequent measurements shall be made after appropriate intervals until theCorrosion Rate is established.

If it is determined that an inaccurate Corrosion Rate has been assumed, the rate to be used for the next period shall be increased or may be decreased to agree with the actual rate.

The long-term (LT) Corrosion Rate shall be calculated from the following formula:

                                                t _initial - t _actual

Corrosion Rate (LT) = ___________________________________

                                    time (years) between t _initial and t _actual

 The short-term (ST) Corrosion Rate shall be calculated from the following formula:

                                             t _previous - t _actual

Corrosion rate (ST) = ____________________________________

                                    time (years) between t _previous and  t _actual

where:

t _actual    = the actual thickness, in millimeters, measured at the time of inspection for a given location or component.

t _initial    = the thickness, in millimeters, at the same location as t _actual measured at initial installation or at the beginning of a new corrosion rate environment.

t _previous = the thickness, in millimeters, at the same location as t _actual measured during a previous inspection.

Long-term and short-term corrosion rates shall be compared as part of the data assessment. The Authorized Pressure Vessel Inspector, in consultation with a corrosion specialist, shall select the Corrosion Rate that best reflects the current process.

A representative number of thickness measurements must be conducted on each vessel to satsify the requirements for an internal or on-stream inspection. For example, the thickness of all major components (shells, heads, cone sections) and a representative sample of vessel nozzles shall be measured and recorded. The Remaining corrosion Life and next inspection interval shall be calculated for the limiting component.

A decision on the number and location of the thickness measurements shall consider results from previous inspections, if available, and the vessel component criticality. Measurements at a number of Thickness Measurement Locations (TML's) are intended to establish general and localized corrosion rates in different sections of the vessel. A minimal number of TML's are acceptable when the established corrosion rate is low and not localized.

For pressure vessels subject to localized corrosion, it is vital that those knowledgable in localized corrosion be consulted about appropriate placement and number of TML's. Additionally, for localized corrosion, it is important that inspections are conducted using scanning methods such as profile radiography, scanning ultrasonics, and / or other suitable NDE methods that will reveal the scope and extent of localized corrosion.

NOTE: New thickness measurement locations shall not be used to increase the remaining half life determined from the previous measurement locations.

A-2   Remaining Corrosion Life Determination

The Remaining Life of a vessel shall be calculated from the following formula:

                                             t _actual  –  t _required

     Remaining Life (years) = __________________

                                           Corrosion Rate

where:

t _actual  = the actual thickness, in millimeters, measured at the time of inspection for a given location or component.

t _required   = the required thickness, in millimeters, at the same location or component as the t _actual measurement, computed by the design formulas (e.g., pressure and structural) before corrosion allowance and manufacturer's tolerance is added.

1.              The primary method of estimating the remaining corrosion life is by analysis of UT thicknessreadings, referred to as TML readings. ONE-M GROUP will utilize a computer based data management program Ultra-PIPE, Capstone, or similar. The programs other primary use is to determine the TML gauging frequency.

2.              The remaining corrosion life of the vessel equals the remaining life of the limiting TML
point. The TML corrosion rate is the worst case corrosion rate determined by the long termcorrosion rate (first to last reading) or short term rate (last two readings).

3.              No single TML will be allowed to reach the retirement thickness. Exceptions will require athorough analysis to ensure the thinned area is fit for service and is localized, approval by theMaterials Reliability Supervisor.

A-3       Frequency of Thickness Measurement Inspection

•         The collection of TML data shall be prioritized and scheduled in accordance with theequipment RBI ranking.

•         TML thickness data shall be taken at intervals equal to the lesser of half the RBI internal
inspection frequency or one half the remaining corrosion life.

•         A "TML due for Inspection" report shall be generated as a minimum on a monthly basis. All
vessels on this list shall undergo an on stream UT inspection.

•         Any TML inspection due date will be cause to re-inspect the entire vessel / circuit.  

                                                                                                                                    *End*