Inspection Manual/Section‐5/Specific Requirements‐2 (Drums & Vessels) Page 2‐1
Section ‐5
Vessels & Drums
Specific requirements 2. Specific Inspection Requirements for VesselsThis section covers the specific inspection requirements of the pressure vessels without the special purpose internal structures like treys, down comers, or catalyst beds. The text in this section can be used for the internal and external inspection of the types of vessels including but not limited to Drums, Flash tanks, LPG Bullets, Two phase and Three phase Separators, sludge catchers, deaerators, and steam drums etc.
The coverage of the inspection in this procedure is limited to the first flange, either blinded or connected with the attached piping. Past the first flanged connection the inspection of piping should be performed according to the requirements of Section 5 Para 9 of this manual. The inspector should consult the design details, operation parameters, history of operational upsets, and the previous inspection reports before commencing the inspection.
It is the responsibility of the inspector to make sure all of the minimum safety requirements as per section‐4 of this manual and any additional requirements specific for the vessel being inspected are in place. Additional safety requirements as deemed necessary must be adopted.
2.1. External Inspection
2.1.1. Support Structure:
The support structure should be inspected as per the General requirements Para 1.4.
2.1.2. Ladders and Platforms:
If applicable the leaders and platforms should be inspected as per General Requirements Para 1.6.
2.1.3. Insulation Inspection:
If vessel is insulated, the inspection of the insulation should be performed as per General requirements Para 1.2.1. In case CUI is suspected than remove the insulation from the affected area and inspect the shell as per General Requirements 1.2.2.
2.1.4. Shell and Heads:
If vessel is not insulated than inspection of heads and shell should be performed as per General Requirements 1.2.2.
2.1.5. Welds:
The structural, nozzle and attachment welds should be performed as per General Requirements Para 1.3.
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2.1.6. Nozzles:
All of the nozzles should be inspected as per General Requirements Para 1.5.
2.1.7. Attachments:
Inspection of all of the attachments should be performed as per General Requirements Para 1.8
2.1.8. Gauges:
Inspect condition of all of the gauges as per General Requirements Para 1.7
2.1.9. Grounding Connection:
Integrity of the grounding connection should be checked as per General Requirements Para 1.9.
2.1.10. Gaskets:
Check for the leaks and gasket integrity as per General Requirements Para 1.10
2.1.11. Attached Piping:
a. Inspect the general condition of the attached piping. To determine if the attached piping is not exerting undue stress on the vessel, attention should be paid to the pipe supports on the piping directly attached to the vessel.
The unsupported piping exerts the stress on the localized areas of the vessel which could be above the designed limits of the vessel. The pipe support inspection should be extended to at least to the 1st support away from the vessel.
b. Look for the vibrating sections of piping directly attached to the vessel.
Vibration trend of the piping can be determined by shaking the piping with light force. The vibrating section of attached piping can induce the
mechanical fatigue at the nozzle weld areas.
2.2. Internal Inspection:
Vessel should be prepared for internal inspection as mandated in section 4.
2.2.1. Shell & Heads:
a. The shell, heads, drain & gas boots (if applicable) should be inspected as per General Requirements Para 1.2.2.
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b. The area of the shell or head below and above the normal product level is typically susceptible to pitting and general corrosion. Sometimes the
generalized wall loss takes place which is not easy to detect visually. The Zero degree Ultrasonic thickness measurement should be done in this case.
c. The area of the shell adjacent to the baffle plate (on the downstream side for separators) is susceptible to erosion, pitting and general corrosion. This area should also be examined with ultrasonic to determine the actual wall thickness of the shell.
d. Depending upon the contents of the vessel, the bottom of the vessels especially areas around the drain nozzles and the drain boot are susceptible to under deposit and various kind of the corrosion mechanisms including sour (wet H2S damage) and biological corrosion pitting.
e. If the wall loss falls below the design tmin, the fitness for service analysis should be conducted using the guide lines of API‐510 Para, 7.4. If the analysis determines that the corroded area is not fit for continuous service at the design parameters, the damaged area should be restored using approved welding procedures.
f. If restoration of excessively corroded areas is not possible at time being the vessel should be derated and operated at reduced pressure limits until the repair is done. New working pressure should be calculated as follows.
• New working pressure due to corrosion wall loss on shell:
0.6
• New working pressure due to corrosion wall loss on dished head:
0.1
Whereas t = Measured minimum thickness in corroded areas (in inches) P = De‐rated working pressure (measured in psi)
E = Joint efficiency ( E=1 if the corroded areas is away from weld)
R = Radius in vessel ( in inches)
S = The allowable stress, (1/4 times tensile strength for vessels Manufactured before 1999, and 1/3.5 times tensile strength for vessels manufactured after 1999).
g. The PSV setting on the derated vessel should be adjusted accordingly.
h. Since the corrosion process is active in the derated vessels, further deterioration is eminent during the derated service, (unless some
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operational procedures are not altered relative to the active demerge mechanisms). Hence an ultimate safe thickness value (ultimate required tmin) of the corroded shell should be fixed. The vessel must come out of service if the minimum thickness of the subject corroded area falls below this ultimate required tmin. Due to this reason the subject corroded areas must be periodically monitored with ultrasonic scanning (from outside). The time period of monitoring should be determined based on the corrosion rate, and the severity of the service.
i. Once the ultimate required tmin is determined the remaining life of the vessel (in derated service) should be determined as given below. The remaining life will determine the time interval allowed for the continued service.
.
tprev = The min thickness at time of previous inspection. (if no thickness is available the design thickness should be used.
tlatest = The measured minimum thickness at present.
tmin req = The ultimate minimum thickness decided.
Supplementary note:
i. This approach should also be used to determine the remaining life for vessels which are not in the derated service.
ii. If enough OSI data for the corroded area is available so that the long term and short term corrosion rates can be calculated (as given is API‐510 Para 7.1.1.1). The highest of two should be used to determine the remaining life. However it must be noted that the general corrosion rate of the vessel might not be applicable at the localized area which shows excessive corrosion. Therefore it is safe to develop separate corrosion rate of these areas with stringent approach.
2.2.2. Coated Vessels:
If vessel is internally coated the inspection should be done according to General requirements Para 1.2.3.
2.2.3. Lined Vessels:
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The vessels in erosive services are internally lined with the metallic liners or weld
overlaid. Following are the minimum requirements for the proper inspection of the weld lining inspection.
a. No corrosion is occurring either on the surface of the lining or below.
b. The lining is properly installed, and no imperfections like disbanded or cracking exist. All weld overlaying should be inspected with Liquid Penetrant inspection.
c. Bulges or buckling of internal liners usually indicates that the process fluid or gas has been trapped beneath the liner. This may present a potentially dangerous situation if the pressure has not been relieved prior to inspection.
Such situation should be carefully reviewed prior to initiating any inspection work inside of the pressure vessel. The trapped fluid could be an explosive mixture of hydrocarbons. All means should be adopted to relieve the trapped pressure behind the bulges.
d. In case of the bonded lining (done by the explosion lining process), where ever suspected the integrity of the lining should be checked with ultrasonic.
e. Check shell and nozzle thicknesses just above and below cladding.
Sometimes the shell besides the lining shows excessive generalized corrosion which could not be judged by visual inspection.
2.2.4. Weld Inspection:
Inspect all internal structural, nozzle and attachment welds as per General
Requirements Para 1.3. The attachment welds are of significant importance. Most of the vessel failures are associated with the defects originating from the attachment welds. If the service related defects are suspected at some welds, the Magnetic particle inspection (if not already part of scope) should be added to the scope of inspection.
2.2.5. Nozzles Inspection:
a. All nozzles (including manways) should be checked thoroughly, for corrosion or any other service related defects.
b. Check nozzles for plugging with deposits, especially small draw‐off and instrument nozzles.
c. Look at the gasket face for corrosion and erosion. The gasket faces are prone to have crevice corrosion if the debris is trapped in between.
d. Report any condition that could affect the sealing and cause the leaks. Over torque can sometimes damage the mating faces of the flanges.
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e. Look for the appropriate depth of machined serrations, both on the nozzle flanges nozzle and the cover (or mating flange) gasket faces. Best visual judgment is adequate to find out if the serration depth is appropriate or not, however the micrometer measurements can also be taken if required.
2.2.6. Internal Attachments:
a. Check the structural integrity of the internal attachments like vortex breaks, Baffle plates, inlet deflectors, impingement plates, stiffener rings, demister pad holding assemblies etc. The damage to these attachments significantly affects the operation parameters of the vessel and alters the damage mechanism. Repairs should be recommended if the attachments are found excessively corroded, dismantled or damaged to the extent that intended function is not achievable.
b. If applicable the demister pads and/or filters should be removed and cleaned for inspection. Replace these components if found corroded or damaged to the extent that the mesh or assembly would not survive the next service cycle.
c. Check inlet distributors and surrounding area for abnormal wear or corrosion
2.2.7. Magnetic particle inspection should be added to the scope of inspection, if the cracking is suspected. The scope and extent of MT is decided based upon the vessel history, potential damage mechanism and the severity of the service conditions.
However Wet Fluorescent MPI or LPI (whichever applicable) must be done at the welds in following vessels.
a. Vessels in sour service.
b. Vessels in refrigerated service.
c. Vessels in hydrogen service.
d. Vessels subjected to thermal and mechanical fatigue.
e. Where ever stress corrosion cracking of any kind is suspected.
2.2.8. Internal instruments like floaters, thermo wells, Corrosion coupons, internal cathodic protection anodes etc (if not disassembled) should be inspected for the integrity and the intended functions. These instruments could be subjected to the resonant vibration because of the flow dynamics inside the vessel. Check for the fatigue signs on these instruments.
2.3. Additional Inspection with the specialized None destructive methods like AUT, acoustic emission, remote visual, magnetic flux leakage and Eddy currents etc. should be carried out as
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the special requirements. The detailed procedure and precautions for using these techniques is beyond the scope of this manual.