In what situation is a pneumatic test at one kg/cm2 to be preferred to a hydro test at the design pressure of a vessel?
Answers
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29/07/2010
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Jayaraj Jayam, Chennai Petroleum Corporation Limited, njayaraj@gmail.com
In hydrocarbon industry, pressure test of a vessel to 1.0 kg/cm2 with air is only a remote possibility. However, vapor line like OVHD lines which handle only vapor, flare headers are the likely equipments (pipe lines) are subjected to pneumatic pressure test. This is mainly because of the inability of the lines for handling the weight of water supposed to be used for the test.
Vacuum tower may undergo a pressure test up to 1.0kg/cm2. Here steam or air is being used by different refiners. Before starting the pressure test of the vacuum tower, instruments tappings are to be isolated otherwise the instruments which are designed to work at vacuum condition may fail under pressure condition especially the diaphram types.
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05/06/2009
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keith bowers, B and B Consulting, kebowers47@gmail.com
It depends entirely on the circumstances. Firstly, the 1 ATM leak test is meaningless in proving the vessel integrity unless it has a design pressure equal to or less than that. It can not be substituted for the testing pressure required by the design code under which the vessel was manufactured.
The real issue is risk mitigation and management. A first principle of vessel integrity proving is to do so in a manner that is safe and does not present a risk of damage to other equipment and personnel if failure does occur. Pneumatic pressure testing, particularly of a large vessel, requires a large amount of energy input (in compressing the gas) to develop the pressure. Should a rupture happen, all that stored energy will be discharged through that rupture. The effects can be catastrophic.
Vessel integrity proving with an incompressible liquid requires vastly less energy input to achieve the same stress level in the vessel. Thus, if failure happens, there is much less stored energy to dissipate, and risk of collateral damage tremendously reduced.
If the vessel design pressure is over that you mention, I am unaware of any situation where that pneumatic test would be preferred over or substitute for the real thing.
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