If work is to be performed on a section of the plant while the remainder is in operation, great care should be taken to ensure critical safety interlocks are not compromised, such as via isolation of over-pressure protection devices or safety interlock instrumentation. It might also be necessary to insert blanks into certain flanges. The safest time to do this is when the system is down since blank installation and removal requires process-entry permitting and depends upon effective performance of block valves. Consideration should also be given to provision for thermal expansion or contraction of the liquid in the system.
If spark-producing work is to be performed, the affected piping might require cleaning by steam or solvent, and/or purging with inert gas. Check with the fluid manufacturer for a suggested cleaning procedure. Solvents must be compatible with the HTF and system components, and could be hazardous.
Before beginning any hot work, consider whether there might be residual vapors in the system. It might be preferable to remove sections of piping at flange connections to allow more controlled cleaning and ventilation.
Review safety provisions: Hydrocarbon vapors above their flash point can ignite if in concentrations within their combustible range in air while an ignition source is present. Depending on the amount of combustible vapors in the area, ignition might result in a short-lived flash fire or a continuous burn. Cooling the system will reduce ignition potential, as well as vapor concentrations to which workers might be exposed. Ignition sources can include electrically powered equipment, extension cords, cutting torches/flames and static discharges, as well as sparks from the impact of steel tools.
If work is required near or above the fluid’s flash point, non-powered tools such as hammers, screwdrivers and wrenches should be used where possible. Tools should be coated or made of brass to prevent sparking. Electrically powered equipment should be used only after obtaining an acceptable reading from an explosion meter, and when properly grounded. Static discharges can be prevented by attaching grounding cables.
An explosion meter can be used to help prevent fluid ignition. Most companies have policies that do not allow work to proceed if any flammable vapors are detected. Detection of such vapors might require further decontamination of the piping and/or equipment. Repeat testing during the execution of the job is a good practice, particularly before resuming work after extended breaks in activities.
Getting to work: Using a band saw, as opposed to a cutting torch, for the first cut into piping can reduce sparking and temperature. After the piping is severed, the opposing pipe ends at the cut should be separated. If flanges are to be welded at the cut, balloon plugs can be inserted as far into the piping as possible for isolation from residual organic liquid and vapors. The inner walls can then be wiped down with cleaning solvents to remove residues before grinding and welding commences using standard work procedures. Of course, the balloon plugs should be removed when finished.
If flanges are not to be installed, clean the piping section by first steaming thoroughly, and then inerting with a dry gas prior to any cutting, grinding and welding. If a portion of the piping can be removed or separated, this might facilitate effective cleaning of residual HTF from inside piping surfaces before proceeding. Normal work procedures can then be used to prepare and weld the piping. Take care to remove all debris from the piping. The sidebar gives several references for piping and equipment repairs.
At the conclusion of all work, the unit should be prepared for normal operation, which includes removal of blanks, locks and tags, and returning valves into their normal operating positions; an appropriate inspection should follow. It is the responsibility of facility management to ensure the piping is safe for service based upon proper inspection and pressure testing. ASME B31.3 provides guidance on inspection and examination of chemical plant piping. Insulation should be left off any affected flanged connections until after start-up to allow inspection for leakage.