What are the hydrogen cracking and stress corrosion cracking (scc)?
There are two types of SCC normally found on pipelines:
(1) High pH (pH 9 to 13) (2) Near-neutral pH SCC (pH 5 to 7)
SCC defined
SCC is unexpected and sudden failure of ductile metal is also under a tensile stress in a corrosive environment. This condition is further aggravated if the metal is at elevated temperature. For pipeline this would be a temperature above 40oC (104oF).
Hydrogen cracking may also be called cold cracking or delayed cracking. The principal distinguishing feature of this type of crack is that it occurs in ferritic steels, most often immediately on welding or a short time after welding.
What are HAZOP analysis and risk management? List risk assessment building blocks.
HAZOP. A hazard and operability study is a team technique that examines all possible failure events and operability issues through the use of keywords prompting the team for input in a very structured format. Scenarios and potential consequences are identified, but likelihood is usually not quantified in a HAZOP. Strict discipline ensures that all possibilities are covered by the team. HAZOP and failure modes and effects analysis (FMEA) studies are especially useful tools when the risk assessments include complex facilities such as tank farms and pump/compressor stations.
Risk assessment building blocks:
· Checklists
· Safety review
· Relative ranking
· Preliminary hazard analysis
· "What-if" analysis
· HAZOP study
· FMEA analysis
· Fault-tree analysis
· Event-tree analysis
· Cause-and-consequence analysis
· Human-error analysis
·
Calculations
1 In formula P = (2 St/D) *F*E*T, please give F, E, T values? A gas pipeline is constructed of API 5LX-70 steel, NPS 16, 0.250 in. wall thickness. Calculate the MAOP of this pipeline for class 1 through 4 locations. Use a temperature duration factor of 1.00.
P = (2 St/D) *F*E*T = (2*70000*0.25/16)*0.72*1*1=1575 psig
2 An impressed current cathodic protection (CP) system is to be designed for a 24 mi pipeline NPS20 with FBE coating. The maximum protective range for the CP installation is 30 mi. The ground bed is proposed to be installed at the milepost 12. The soil resistivity at the anode burial depth is 5000 ohm.cm. The current required after 25 years of service is estimated to be 6.5 x 10-6 A/ft2 . (1) Draw impressed current system; (2) Determine the total current (current density x pipeline surface area) requirement for the 24 mi pipeline; (3) If silicone-chromium cast anodes are used assuming 45lb weight each, estimate the number of anodes required for the 25-year life span [ N= L x I x C/ (0.7 x W) standard rectifier size I = 8A ]. Assume a consumption rate C of 1.2 lb/A.yr and 70% consumption. 1 mile =5280 feet. 1 feet =12 inch.
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3 During a 4-h hydrostatic test, ΔP =15.5psi, ΔT =2.6 ̊F. At 1.5h after test started, the pressure was dropping , so 5 gal water was injected into pipeline. Calculate the volume change per hour and determine the test passed (±2.45) or not if the steel pipeline tested with water ΔP/ΔT = 19; the measured field value ΔV/ΔP = 0.15.
±ΔVp, volume accounted for by pressure change
ΔVp = ΔP × ΔV/ΔP = −15.5 psi × 0.15 = −2.325 gal
±ΔVt, volume accounted for by temperature change
ΔVt = ΔP/ΔT × ΔT × ΔV/ΔP = 19 × (−2.6) × 0.15 = −7.41 gal
Volume lost plus injected
ΔVp = −7.41
ΔVt = −2.325
ΔVi = 5.0 injected
Total: −4.735 gal or −1.18375 gal/h
California Government Code allows the calculated leakage for a pipeline of this volume to be 1.45 gal/h plus a 1 gal/h tolerance or a total of 2.45 gal/h.
Results: So, with a calculated 1.18375 gal/h leakage, this pipeline has successfully passed its hydrostatic test, PASSED.
Excellent questions
4 Discuss briefly pipeline repair methods and safety procedures.
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