COMPUTATIONAL FLUID DYNAMICS IN EQUIPMENT DESIGN AND FAILURE ANALYSIS

Presented TXLA NACE Chapter October 15, 2013 Presented by Carl Matherne – BASF TOTAL Petrochemicals

Problem Background • The BASF TOTAL PETROCHEMICAL LLC

Ethylene Steam Cracker installed 5 welded plate heat exchangers in 2006 - 2007.

• Primary process quench water coolers for the quench tower service.

• ‘A’ stream is quench water, a mixed process water and hydrocarbon stream: 90-95% water with trace hydrocarbons.

• ‘B’ stream is cooling water from the cooling water distribution for the ethylene unit, see Figure 1.

Problem Background

Investigation Team

• Knighthawk Engineering

• The BASF TOTAL Petrochemicals LLC – (BTP) Port Arthur olefins facility

Problem Detection

• Leak detected by monitoring of the cooling water systems

• Exchangers sampled to determine the specific unit with the leak

• Operations isolate and sample based on speciation

• Units began to experience failures after 3-4 years

Problem Background

• Original exchangers in service in 2001 – Stacked elastomer gasket plate stainless

plate units – Units conventional plate and frame design

• Failures seen – Stress Corrosion Cracking – Elastomer compatibility process temperature

and chemistry • Units upgraded to welded plate design with no

elastomer

Problem Background – Original Design

Problem Background – New Design

Repair Solutions

• Short term – Damage analysis and return to service – Field repair using weld and chemical patch – Inspected, closed, returned to service, and

tested (16-24 months) • Long term

– Remove to controlled authorized facility for repair

– Testing to find the damage areas – Repair: isolate by blocking off or replace

entire core

Problem Investigation - RCFA

• 3 different failures since initial installation in 2006 (5-yrs) – Occurred on original pair of units installed – Second failure occurred on the unit installed

in 2007. • The general location of the failure was similar

in all failures to-date. – Cracks were developing along a seam

located on the left side in the top right corner.

Problem Investigation - RCFA

RCFA – KHE Metallurgical Work

• Unit sacrificially analyzed for metallurgical and mechanical defects. Analysis indicated: – Fluid induced vibration.

• Dominant fracture mechanism associated with cracking of the heat exchanger plates is cleavage, where cracking occurs across the grains of the material (transgranular). – Consistent with a brittle fracture.

• Cleavage fracture mechanism observed is the result of an increase in hardness in the HAZ of the welds from finer grains and a possible increase in ferrite content in the cracked versus un-cracked region.

RCFA – KHE Metallurgical Work

Root Cause Theories for Defect

1. Flow induced vibration • Damage caused by unsteady flow. Design of the unit allows for

movement in the center of the plates susceptible to fluctuating flows while the edge of the unit is rigid and resists this movement. This behavior would result in local cyclic stresses.

2. High thermal stresses from process environment • Reduced flow in the plates from fouling causing thermal stresses. • Materials of the unit are well suited for relatively high thermal

gradients and resist corrosion from deposition of the fouling materials. Based on the lack of corrosion damage and operational comments the unit fouling on the cooling water side of the unit is not an issue.

3. Chemical attack • Austenitic material was suspected and researched as a third

hypothesis. The analysis ruled out chemical attack damage mechanism. The units in this service were selected for corrosion resistance so this theory was dismissed from discussion after review.

RCFA Solutions / theories for correcting

• Dynamic loading coming from the process fluid was the most viable damage mechanism from field analysis and the design of the unit.

•KHE performed field measurements that captured the dynamic pressure of the fluid entering into the heat exchanger.

Inlet Flow Analysis Using CFD

• The goal of the modeling was to identify the damage mechanism for validation and to change design to mitigate causes.

Computational Flow Model

• CFD model was developed for the inlet piping pair – exchangers fed from the same piping circuit.

• Remaining units identical in design, therefore the dual exchanger system with a single header was the common design for analysis.

• Due to the symmetry only half of the flow path needed to be modeled.

Assumptions and conditions

• The model includes the strainers and butterfly valves • The grid is a hex dominant mesh with some triangular

shaped cells at the ‘T’ junctions. • Appropriate mesh grading is applied on all walls to

satisfy wall function constraints. • The fluid properties assumed in the model and process

(boundary) conditions applied to the model are listed in Table

Model Development

• The model was developed for the velocity contour plot and the contours of fluid velocity across the symmetry plane and outlet plane. – Both indicate that the sharp elbow prior to exiting

results in a low momentum zone that causes a ‘jetting’ behavior into the plate heat exchangers.

• Figure shows the streamlines that a particle would follow in the proximity of the butterfly valve.

• The flow is asymmetric about the butterfly valve. • Note the preferential path to one side of the pipe,

corresponding to the jetting flow. The other side of the pipe displays a recirculation zone.

Model Development

Solution Development and Testing

• The CFD tool was combined with field measurements, metallurgical analysis, and vibration monitoring to develop a failure model and determine nonconformities.

• Results indicated mal-distributed flow entering the heat exchanger inlets cause unsteady forces on the heat exchanger plates. – Non-uniform flow field was predicted using CFD and

its magnitude indirectly measured in the field using dynamic pressure sensors.

• Analysis led to a conceptual proposal for design that will greatly improve the flow field entering into the heat exchanger. A model of this engineered solution is shown …

Solution Development and Testing

Conclusion • CFD as an analytical tool can aid in RCFA.

– Useful analytical and computational modeling for the solution of flow induced phenomena.

– Successful design generated and tested using the model.

• The model and supporting field data provide a defensible design for installation to increase the probability of success. – Better understanding of the damage mechanism and

RCFA for a reliable design.

• Damage mechanism theories and solutions can be proven prior to design changes in the field to increase the likelihood of success.

Thanks!