What is Mean By Pressure:
Pressure is force exerted on surface per unit area.
Unit of Pressure is Pascal or N/m2.
Pressure Terms Used in Pressure Vessels and Piping:
- Operating Pressure
- Design Pressure
- Maximum Allowable Working Pressure
- Hydro test Pressure
Pressure which is required for Process,served by vessel, at which vessel is normally operated. It can also be called as normal working Pressure.
Pressure Used in Design of Pressure Vessel or Piping System. This pressure is called as Design Pressure.It is recommended to design vessel and its parts for a higher pressure than operating pressure. it is recommended that always use design pressure should be 30 psi or 10% higher than the operating pressure.
Maximum Allowable Working Pressure (MAWP):
Internal Pressure at which the weakest element of the vessel is loaded to the ultimate permissible point, when the vessel is assumed to be in corroded condition, under the defect of designated temperature, in normal operation position at the top and under the effect of other loading such as wind load, external pressure, hydro-static pressure etc. which are additional to internal pressure.
When MAWP Calculation are not done, then Design pressure may be used as Maximum allowable working pressure.
As common practice followed by many users and manufacturers of pressure vessel is to limits the Maximum allowable working pressure by the head or shell, and not by small element such as flanges.
New and Cold Maximum allowable working pressure term very often used in pressure vessel manufacturing It means that the pressure at which weakest element of the vessel is loaded to ultimate permissible point when the vessel is not corroded (new ) and the temperature does not affects its strength at room temperature ( Cold ) and the other condition need not to be taken in to consideration.
Hydro test is the technique in which pipes, pressure vessels, gas cylinders, tanks are tested at required design pressure of the system or pipes or vessel to check the leak and strength.
Hydro-static Pressure Calculation for Pressure Vessel:
Hydro-static Pressure = 1.3 x Maximum Allowable Working Pressure or MAWP.
When Maximum Allowable Working Pressure or MAWP calculation is not done, then we used Design pressure in place of MAWP.
Hydro-static Pressure = 1.3 x Design Pressure.
if the stress value of the vessel material at design temprature is less than the stress value at test temprature then hydrostatic pressure should increase proportionaly.
Hydrostatic Pressure = 1.3 x MAWP or Design Pressure x ( stress value at Test Temperature / stress value at design temprature ).
Hydrostatic Pressure Calculation for Piping:
Hydrostatic Pressure = 1.5 x Design Pressure.
if the stress value of the Pipe material at design temprature is less than the stress value at test temprature then hydrostatic pressure should increase proportionaly.
Hydrostatic Pressure = 1.5 x Design Pressure x ( stress value at Test Temperature / stress value at design temprature ).
Process of Hydro-static Testing:
Stages of Hydro-static Testing:
First Stage: Raise the pressure to 40% of the final pressure, stop pressurizing, keep it for 5 minutes, and then make a fast visual inspection of the external surface.
Second Stage: Restart pressurizing up to 70% of final pressure, stop the operation, keep for 5 minutes and make a fast visual inspection on external surface.
Third Stage: Restart pressurizing up to 100% of the final pressure, stop the operation, and keep for 45 minutes.
Activities Before Hydro testing:
- Checking all welding already finished and fully accepted by the NDT examination per the project Inspection and test plan.
- Making sure the inner part of the vessel is clean and free of remaining slag or other elements and also Making sure the external surface is dry for the correct execution of the visual inspection during the vessel pressure testing.
- Checking the pressure gauges’ calibration tag and certificate and the range of the lower limit and upper limit of the gauges. It needs to be between 1.5 and 4 of the pressure test value.
- Controlling testing equipment such as the test pump and housing for soundness and tightness.
- Making sure the test temperature will not violate the following values:
- Min. Test temperature= MDMT + 30°F
- Max. Test temperature = 120°F
- MDMT is the pressure vessel minimum design metal temperature, and it is stated in the pressure vessel design document.This reduces the risk of a brittle fracture during the test.
- Making sure which reinforcement pads are already soap tested.
- Controlling the testing water quality and using corrosion inhibitor if it is necessary or when the vessel metal is sensitive material
- Checking of vents. It is required to be placed at high points of the vessel in a position where it is possible to purge air pockets while the vessel is filling.
Activities During Hydro-testing:
- Making sure the filling and pressurizing are done from the lowest point and venting from the highest point.
- Witnessing water overflow through the venting in order to assure that no air bubbles remain in the vessel.
- Controlling and witnessing pressurizing stages.
- When 45 minutes elapse, making sure the de-pressurizing is started and the pressure is dropped to the “Inspection Pressure.”
- Making sure the pressure vessel hydro-static testing pressure calculated correctly.
- Making sure the vessel is immediately and carefully drained after the test and dried by air.