Types of Tests, Certificates and Records
Storage and pressure vessels are to be tested and checked for the assessment of their reliability. Testing of pressure vessels and their parts is most essential. Mainly two types of tests are in practice, pressure test and non-destructive tests (NDT). Pressure tests include hydraulic, pneumatic and combined. NDT includes radiography, dye penetrant, magnetic particle and ultrasonic, NDT methods can be used before or after pressure test methods. Mechanical tests are also employed for fusion welded seams and test plates. Main tests methods are explained below.
Test: Tanks should be erected after considering its static load (as if fully filled), wind load, factor of safety and other allowances and testing the soil for its bearing capacity and making the foundation accordingly.
The underground tanks should be erected in underground RCC sump so that their leakage cannot pollute the soil or underground water.
The tanks which are not pressure vessels should be tested for its water load (static) test by filling it fully with water and observing any subsidence or damage.
The tanks which are pressure vessels should be tested as per statutory provisions, its design Code and Standards or recommended by the manufacturer.
Periodicity of test should be decided after considering material to be filled, its properties, storing parameters (pressure, temperature, flow, vibration etc.), heating/cooling, method of use, rate of corrosion and type and thickness of metal. Its condition monitoring is most essential. See IS:2825 also.
When tanks are tested, its fittings should also be tested and replaced if necessary.
Major causes of material or equipment failure are as under which need to be periodically tested:
- Suitability (compatibility) of the material of construction (Metal, plastic, lining etc.) with chemical. It can be checked by a meta-scope.
- Mechanical failure due to excessive stress.
- Mechanical fatigue and shock.
- Thermal fatigue and shock.
- Brittle failure due to cold liquid.
- Creep due to plastic deformation and rupture.
- Hydrogen attack due to atomic or molecular hydrogen generated by corrosion. Alloyed steels (with Cr, Mo etc.) are useful to mitigate hydrogen attack.
- Corrosion by water or corrosive chemicals.
Before testing or allowing welding, cutting or repairing, the tanks should be thoroughly cleaned and purged by inert gas if it was containing any flammable material. It should be made gas free if it was containing any toxic or flammable gas. Testing equipment should be safely utilised and a safety work permit should be followed. Oxygen content should also be measured. Top, bottom and side nozzles should be opened and free ventilation should be allowed. First it should be externally examined for manual checks and then it may be hydraulically or pressure tested. Test record should be maintained.
They are of three types as under-
(1) Hydraulic (Proof Hydrostatic) Test:
Water is used to pressurise the vessel for hydraulic test. By a hand pump, the pressure is gradually increased, stopped, observed any leakage if any and again increased. Test pressure is 1.3 to 1.5 times the design pressure. A standard pressure gauge should be used to watch the pressure. Slight higher test pressure is required for an old or repaired vessel than for a new vessel.
Testing by air, steam or gas pressure is more hazardous as air, steam or gas are more compressible than water and contains more energy at equal pressure. Therefore air-pressurised vessel can burst with great explosive force and can make great damage. The water pressurised vessel, if bursts at the same pressure, will release less energy, no shock wave will be generated and make less damage. Therefore hydraulic test is a safer test. It provides easy detection of leak also.
After the test pressure is attained, it is held up for some time (15 to 20 minutes.) If no leakage or no deformation, the positive test certificate can be issued.
Examinations & Tests u/r 61, Gujarat Factories Rules:
They are prescribed as under-
- New vessel to be used for the first time- Hydraulic test at 1.3 × Design pressure.
- Old vessel, unused vessel for more than 2 months or where repair/alteration is made-Hydraulic Test at 1.5 × MPWP (Maximum permissible working pressure)
- Where water is not suitable for test-Pneumatic (air) test at – Design Pressure for new vessel, MPWP for old vessel.
- Glass lined vessel – Hydraulic or Pneumatic test at – Design Pressure for new vessel, MPWP for old vessel.
- Test certificate from the manufacturer or competent person should be obtained before using any vessel for the first time. Inspector may demand it.
- No vessel can be operated at a pressure higher than that mentioned in the certificate.
In-service test/examination as under-
- External test at 6 months.
- Internal test at 1 year.
- Hydraulic test at 2 years.
- Internal or Hydro test at 4 years for vessel in continuous process.
- If external, internal or hydro test is not possible as above, them – NDT for metal thickness and other defects.
Thin – walled vessels of non-ferrous metal-
- Maximum life 20 years.
- 5% reduction in MPWP every year.
- Where pressure reduction on repair is suggested, the competent person shall send a copy of such report to the Inspector within 7 days of the completion of the test.
To determine the ultimate bursting pressure, the vessel is subjected to five or more times the design pressure. Burst pressure tests are used only to test a sample of a large number produced to verify the adequacy of calculations and manufacture.
Precautions necessary while hydro-testing are as under-
- Use of standard and correct pressure gauge.
- Removal of air from the vessel before testing.
- Visual inspection for clearly visible defects if any.
- Use of a rupture disc to test at a high pressure.
- Protected area away from public. A pit or steel-box is preferable.
- Limiting the liquid volume in the test vessel to reduce the energy stored at the time of testing. To achieve this bigger size solid pieces should be inserted in the vessel to take up as much of its volume as possible.
After hydraulic testing if more than 0.2% increase (elongation or deformation) in volume is noticed, the vessel should not be recommended to use as a pressure vessel.
(2) Pneumatic Test:
This test is useful where even traces of water cannot be tolerated in its service or where a vessel is designed and/or supported that it cannot be safely filled with the water or testing fluid.
Such test shall be carried out under close supervision by the inspecting authority. Adequate precautions like blast wall or pit and means for remote observation are essential.
The pneumatic test pressure should be more than the design pressure but less than the hydraulic test pressure. In the beginning the pressure should be gradually increased up to 50% of the test pressure. Thereafter it should be increased in steps of 10% of the test pressure till the required test pressure is achieved. Then the pressure will be reduced up to design pressure and held at that pressure for a sufficient time to permit inspection of the vessel.
(3) Combined Hydraulic and Pressure Test:
Here a test vessel is partially filled with water and then air pressure is applied to the space above the liquid level. The test pressure (not exceeding hydraulic test pressure, but more than the design pressure) shall be deducted by the pressure due to the static head of the water in the vessel.