Pressure testing is a vital step in equipment maintenance and modification. When everything goes well, we have confidence in equipment integrity. However, a failed pressure test creates safety concerns. We must design our equipment, develop testing procedures and implement safety steps that assume failure can occur. After all, if we knew equipment would pass every time — there wasn’t any possibility of failure — we wouldn’t have to pressure test at all.
The most important decision in pressure testing is what to select as the test fluid. Most vessels are tested with either air or water. Sometimes other gases (e.g., nitrogen) or liquids are used but these cases are rare. Air and water have overwhelming benefits in being cheap, readily available and safe to handle.
The major safety issue in selecting between air and water is what happens if the vessel fails. Pneumatic energy release is much greater than hydrostatic energy release because air expands much more than water when pressure containment fails.
Even so, some plants use air for pressure testing. Water contaminates specific processes. If you put water in a sensitive process and can’t get it out again, water may not be a choice.
Water testing is more complex than air testing. You must configure the equipment and piping to allow for correctly filling, holding and draining the water. In spite of the added complexity, I prefer the safer approach of using water whenever possible.
Here are some of my guidelines and design criteria for using water for pressure testing. Many of these points are especially important in dealing with tall vessels, such as the 130-ft. tall distillation column depicted in Figure 1. There, the water height adds 56 psi of static pressure on the bottom.
- Confirm that water delivery pressure is sufficient to get to the top of the vessel or equipment loop.
- Have the capability to vent from all enclosed spaces and loops in the process when filling.
- Make sure that the system is designed for full vacuum or has means of avoiding pulling too low a pressure when draining the water.
- Allow for hydrostatic head along the height of the vessel in setting design maximum allowable working pressure (MAWP) and test pressures.
- Check that the foundations can bear the load when testing. Depending upon location, this may include verification that the water-filled load is acceptable even during an earthquake. You usually don’t need to consider water-filled load at maximum wind speed. You have plenty of warning of coming hurricanes and wouldn’t be pressure testing during them.
- Fill and drain the system at a controlled rate. Be aware that internal restrictions inside the equipment may set maximum fill or drain rates. For instance, bubble-cap or valve trays inside towers may limit draining rates.
- Assess water quality against the vessel’s material of construction. The prime example of what to watch for is chloride content of the water if you have stainless steel equipment or piping.
- Ensure that connected piping systems that aren’t isolated during pressure testing can deal with the test pressure.
- Thoroughly go over testing procedures in suitable management of change, process safety management, and hazard and operability reviews.
- Keep personnel away from areas where pressure testing is taking place.
The tower in Figure 1 poses five potential trouble spots for a hydrostatic pressure test: two sections of valve trays, two sections with internal partitions creating potential dead spots, as well as the extra 56 psi of static pressure on the bottom.
Restricting fill and drain rates to appropriate values will prevent damage to the valve trays. The dead spots require either internal venting or external venting through nozzles to prevent the relatively weak internals from bearing excessive load (when filling) or having a partial vacuum pulled (when draining). It’s crucial to account for the static head from the vessel height in setting MAWP and test pressures. The vessel was successfully pressure tested with all internals in place by taking all factors into account and allowing for them.
Water is a safe, reliable medium for pressure testing. However, like all other plant work, such testing takes planning and thought to correctly design the equipment and piping.