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Handbook of Sulphuric Acid Manufacturing
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Maintenance and Inspection
- Storage Tanks
April
2, 2008
|
Introduction Routine Inspections In-Service Inspections Internal Inspections Review of Data |
Associated
Links Materials of Construction Anodic Protection |
Tank inspection is required to ensure that tank containing concentrated sulphuric acid and oleum are maintained in a safe condition. The degree and frequency of inspection will depend on how the tank has been operated and the maintenance done on the tank.
Documentation and Records
Proper documentation and records for each tank should be maintained. These records should include the following:
A regular visual inspection of a tank ensures that there has been no obvious damage to the tank due to deterioration of accident. This is accomplished by walking around and on top of the tank. This type of inspection is limited to the exterior of the tank. Some of the things to look for during an inspection are:
Storage tanks are often located a distance from the main plant and are not visited frequently during the normal operation of the plant. Tanks should be inspected once a day if possible with a minimum frequency of once a week.
In-service inspection of the tank involves both a detailed visual examination of the exterior of the tank as well as ultrasonic thickness measurement of the shell. As the name implies this inspection is performed while the tank is still in service. The in-service inspection should include all the inspection points of the routine inspection as well as ultrasonic thickness measurements of the tank shell and roof.
Ultrasonic thickness measurements should be performed in the following areas:
a) On the side wall on a vertical line through the radius of any top inlet which is less than 2.44 m (8 ft) from the edge of the tank. Testing should be carried out on a 300 mm (1 ft) grid extending 1.83 m (6 ft) from the centreline on each side from top to bottom.
b) On the side wall on a vertical line through the radius of any top opening at 300 mm (1 ft) intervals on the centreline from top to bottom.
c) On the side wall on a vertical line every 90 degrees around the tank. One or more of these line may correspond to the area tested in a) and b).
d) All side openings and outlets should be tested thoroughly in the whole area extending up to 300 mm (1 ft) from the edge of the nozzle and nozzle reinforcement.
e) On each vertical seam weld on each tank course, consisting of a thickness scan of a 125 mm (5 in) length of weld.
f) On the tank roof on a line every 90 degrees from the centre of the tank to the side wall at 300 mm (1 ft) intervals.
g) On the side wall at an elevation of 25 mm (1 in), 75 mm (3 in), 914 mm (36 in) and 1829 mm (72 in) from the bottom of the tank with a minimum of 4 test points between each vertical weld seam. A test point will be located within 25 mm (1 in) of each vertical weld seam.
h) On the side wall along each horizontal weld seam with a minimum of 4 test points between each vertical weld seam. A test point will be located within 25 mm (1 in) of each vertical weld seam.
The readings obtained should be compared to previous data to determine the extent of corrosion on the tank shell, roof and bottom. Where higher than anticipated metal losses are found, more extensive testing is required to confirm and establish the severity and extent of corrosion.
To obtain good ultrasonic measurements, the following should be noted:
Ensure that the area to be tested consists of a good sound metal surface, free of paint and corrosion inhibitors.
If testing is done on paint an allowance of 3 mils for each mil of paint should be subtracted since the sound velocity through paint is about 1/3 of the velocity through steel.
The instrument should be calibrated frequently.
The inspector should be experienced with performing this type of testing.
Internal inspections are required since there are areas of the tank where corrosion can only be detected by an internal inspection. Internal inspections should be done at least once every five (5) years. Internal inspections should be done in conjunction with the in-service inspection where detailed ultrasonic thickness measurements are performed.
Sandblast welds to remove slag and sulphate. Visually inspect welds for undercutting, excessive porosity, grooving, and other defects. If required, welds should be radiographed. All defects should be repaired by grinding out the defect and repaired by welding. The finished weld should be ground flush with the surface of the surrounding metal.
Ultrasonic thickness measurements shall be performed on the interior surface of the shell and roof when performing an internal inspection. This replaces the ultrasonic thickness measurements taken from the outside of the tank during in-service inspections. In additional to all the measurement points required during and in-service inspection, the bottom of the tank will be tested. Normally, the bottom is only accessible during an internal inspection for ultrasonic thickness measurements.
Ultrasonic thickness measurements should be performed in the following areas on the tank bottom:
a) Every 22.5 degrees 25 mm (1 in), 914 mm (36 in) and 1829 (72 in) in from the shell.
b) On a line every 45 degrees from the centre of the tank to the side wall at 300 mm (1 ft) intervals.
c) If the tank is supported on steel beams, thickness measurement should be taken on the bottom interior directly over the beams at 300 mm (1 ft) intervals.
The readings obtained should be compared to previous data to determine the extent of corrosion on the tank shell, roof and bottom. Where higher than anticipated metal losses are found, more extensive testing is required to confirm and establish the severity and extent of corrosion.
The data collected during an inspection should be reviewed and an assessment done to determine if further testing should be done, the extent of tank repairs or if changes in tank operation are required. The evaluation of the data should establish the following parameters for the tank:
Corrosion allowance remaining
Maximum allowable liquid level
Estimated tank life
Schedule for next full internal inspection of tank and inspection frequency
