Working on piping, vessels, and dosing lines that are under pressure, often hot. Released pressure can spray chemical, flash hot water to steam, or throw a fitting.
Never loosen a fitting, sensor, or probe on a line that has not been isolated and depressurized. Hot pressurized water flashes to steam when released and causes immediate, severe scalding.
Over-pressurizing a pressure-containing device can make it rupture or explode, with a risk of serious injury or death. Know the maximum working pressure of every component and keep the system below it.
- Isolate. Close the isolation valves on both sides of the work.
- Relieve. Open a drain or vent and let the pressure go to a safe place, away from people and electrical equipment.
- Verify. Confirm zero pressure at a gauge. A closed valve can leak through, so check again before breaking the connection.
- Let it cool. Boiler and hot-loop water can be well above 100 °C (212 °F) under pressure.
- Break the joint slowly, standing to one side, with a container or rag ready for what remains in the line.
See Isolating Equipment for the full lockout procedure.
Pressure does not go away when a pump stops.
- Closed loops, expansion tanks, and accumulators hold pressure for as long as the system is sealed.
- Dosing and injection lines hold pressure against a check valve or injection quill, and will spray chemical when opened.
- A pump can be restarted by a controller while you work. Isolate it electrically as well as hydraulically.
¶ Relief valves and pressure ratings
- Every part of the system must be rated for the highest pressure and temperature it can see, including pump deadhead pressure. Plastic components lose pressure rating as temperature rises. See Material Compatibility.
- Do not plug, cap, or remove a relief valve, and do not adjust it above the rating of the weakest component it protects.
- Pipe relief discharge to a safe place. A relief valve that lifts discharges hot fluid or chemical at full pressure.
- Do not fit a check valve or other automatic backflow device where it would trap pressure or hide it from a pressure sensor, unless the equipment’s manual calls for one.
- Test new piping before putting it into service, after solvent-welded and threaded joints have had their full cure time.
- Test with water, not air or gas, unless the piping system is specifically rated for pneumatic testing. Compressed gas stores far more energy than water, and plastic pipe that fails under air pressure can shatter.
- Do not test above the rating of the lowest-rated component in the section, including gauges, sensors, and transducers.
- Support the piping. Do not let controllers, manifolds, pumps, or sensor bodies carry pipe loads.
- Fit isolation valves and unions so equipment can be serviced without draining the system.
- Plan for leaks. Place sensors and injection points where a failed seal drains safely, not onto electrical equipment or a walkway.
¶ Boilers and pressure vessels
Work on boilers, pressure vessels, and their fittings is separately regulated: commonly under the ASME Boiler and Pressure Vessel Code and state or provincial boiler rules, with equivalents such as PED 2014/68/EU in Europe. Pressure-retaining components may only be altered by appropriately certified personnel. Do not modify safety valves, and do not defeat a low-water cutoff or any other safety interlock.