312 CHAPTER 6 Asset integrity REFLUX VESSELS Reflux vessels feed cool liquid to columns and vessels. Therefore, during an emergency, the reflux system will generally be kept in operation to ensure that heat continues to be removed. The reflux vessel and its associated pumps and piping should be protected against external fire either with fireproofing or water sprays. SMALL PIPE CONNECTIONS AND ISOLATION The number of small pipe connections on a vessel should be limited. Threaded gauge connections, sample points, and level control nozzles are all subject to mechanical damage, vibration fatigue, and corrosion. The potential failure points are minimized by: - - - Installing only those connections actually needed. Making the pipe attachments as short as possible. Using extra heavy pipe nipples and 3 4 " minimum diameter to the first valve off the / vessel. Using socket-weld fittings, especially between the vessel and the first valve. - PRESSURE TESTS All vessels must be pressure tested before they are put in service. There are two types of test: a tightness test and a strength test. Tightness tests If a vessel has been opened d say for inspection or cleaning d but not modified in any way then a tightness test has to be carried out before it is put back in service. The test ensures that the vessel and its appurtenances are leak-free, but it does not test the integrity of the vessel itself. Pressures used for tightness tests are never above the equipment MAWP or system design pressure or the vessel's relief valve set pressure. Piping that has been unbolted should also be tightness tested before being put back into service. When testing a piping system attached to a pressure vessel and it is not considered practicable to isolate the piping from the vessel, the piping and the vessel may be tested together. The test pressure for pressure vessels is 1.5 times the MAWP for ASME Section VIII, Division 1 vessels; 1.25 times MAWP for Section VIII, Division 2 vessels; and 1.5 times MAWP for Section I, ``Power Boilers''. Once the system is at test pressure, it is held at that pressure for say 15­20 minutes (one hour maximum). If the pressure does not fall then the system is known to be tight. If the pressure does come down then the leak must be found. This can be done using soap solution if gas is being used as the testing medium.