Risk of Transient Conditions That Can Be Missed During Design and RBI

By: Baher Elsheikh

Background

Design of equipment shall account for all operating modes; normal operating conditions, start up, shutdown, upset conditions,? and transient conditions.?

Different types of transient conditions can be occurred; different operating modes, start-up, shutdown, Start Of Run (SOR), End Of Run (EOR), dry run, thermal gradients, impingement areas, and steam out conditions. In addition to the associated with the configuration of the equipment.?

In case transient conditions of the equipment or part under the analysis are not well identified and subsequently not properly considered in the design or the development of the inspection plans, it can lead to immature failure.

Operating transient conditions:

There are different types of transient conditions, below are some:

Minimum Safe Pressurization Temperature for Heavy Wall Cr-Mo Steel Hydro-processing Reactors:

These reactors are susceptible to temper embrittlement or hydrogen embrittlement. The minimum safe pressurization temperature must be carefully determined to avoid these issues.

Start-up and Shutdown of Ferritic and Duplex Stainless Steel:

Operating these materials within the temperature range of 475°C (885°F) embrittlement (370°C to 538°C) requires special attention during start-up and shutdown.

Steam Block in Boiler Coils:

Improper warm-up or inadequate drainage after shutdown can lead to steam blockage in boiler coils.

Dry Out Conditions in Convection Sections of Fired Heaters and Reformers:

Coils in these sections can experience dry-out conditions, affecting their performance and integrity.

Polythionic Acid Stress Corrosion Cracking (PASCC) of Austenitic Stainless Steel:

During start-up or shutdown, air and moisture on sensitized austenitic stainless steel fired heater tubes can lead to PASCC due to sulfur acids forming from sulfide scale.

Accelerated Corrosion During Prolonged Shutdown:

Improper layup and preservation practices can result in accelerated corrosion during extended equipment shutdowns.

Differential Damage Mechanisms on Hot and Cold Sides:

The hot side of equipment may experience different damage mechanisms compared to the colder sides.

Partial Condensation in Hot Services:

Thermal stresses or thermal shocks can occur due to partial condensation in hot services

Steam out conditions can cause vacuum.

Cooling of steam results in condensation and reduction in pressure. In such cases, the equipment / piping will be subjected to vacuum conditions especially if there is no sufficient venting to allow ingress of air and balance the pressure.

Case histories

Below are some case histories where damage or degradation occurred under different transient conditions:

Case-1: Corrosion under insulation in drain lines

Branch connections and dead ends with low or no flow would have a temperature drop along the small bore pipe branch until the critical temperature range of CUI.

Selection of the insulation material and frequency of the CUI inspection did not address? the high potential of? CUI as it the normal operating temperature is well above the CUI range

CUI Temperature effect:

CUI in Carbon steel is more sever from 77 °C to 110 °C and in Aust. stainless steel 60 °C to 175 °C

Corrosion Rate increases with temperature in a manner similar to that open system the graph.

Case-2: Thermal stresses and stress corrosion cracking in reformer tubes

In a top-fired reformer in a plant, the insulation around the top flange and part of a catalyst tube at the top of the reformer loosened and separated from the tube. The temperature at the reformer deck increased and operators were no longer able to work in the area. As a temporary solution, large fans were installed on the reformer deck to decrease the temperature and enhance the working environment.

A few months later, gas leaks were detected due to cracks initiated at the welded joint of the top flange to the catalyst tube. Investigation and metallurgical analysis revealed that it was stress corrosion cracking as the temperature of the process gas dropped below the dew point in this area.

Maintaining the process gas temperature well above the dew point is essential to avoid stress cracking. Similar issues could occur in refractory lined parts like transfer lines or the reformer casing where the temperature at the backside falls below the dew point.

Case-3: Hydrogen Embrittlement

Hydrogen Embrittlement (similar to other embrittlement mechanisms) takes place at temperatures lower then the normal operating temperature.

Hydrogen embrittlement (HE) is loss of ductility of high strength and/or fracture toughness steels due to the diffusion of atomic hydrogen into material matrix and can lead to brittle cracking.

Higher Hydrogen solubility in the steel and weldment at high temperature can cause HE when heavy sections operating at high temperature are rapidly cooled even under low applied stresses.

Special care for possible desponding of cladding layers as hydrogen pressure build up between the substrate and the cladding layer with peak value at the interface between them.

Minimum depressurization temperature is a critical parameter should be considered to avoid embrittlement

Important notes

Transient different operating conditions can result from the configuration of the equipment as well as the different operating modes.

During design and material selection all operating modes and conditions shall be considered

Identification of the damage mechanisms during RBI or preparation of the inspection plans shall consider the configuration and actual design of the equipment.

Identification of the damage mechanisms / corrosion loops based on PID or MSD can overlook the transient conditions resulted from the configuration of the equipment.

Conclusion:

Identifying and properly considering transient conditions are essential for robust equipment design and effective RBI. Engineers and inspectors must collaborate to ensure that all relevant operating scenarios are accounted for, minimizing the risk of premature failures.

Please share your thoughts and enrich the discussions