Main stages of pipe cleaning
1.1 Cleaning of prefabricated pipe sections
As for the cleaning of pipes before module assembly, all carbon steel pipes should be pickled and phosphated after prefabrication in the workshop to remove impurities, welding slags and rust spots on the pipes. After welding, all stainless steel pipes should be cleaned by pickling and passivation to remove impurities in the pipes. After all pipe sections have been completely cleaned, both ends need to be properly sealed until reinstallation.
 
1.2 Module cleaning
Once the pipe segment modules are assembled, thorough cleaning should be performed using air blowing. During this cleaning stage, considering valve contamination due to air purging, it is recommended not to install valves and flow meters before purging unless it has been proven that the structure of the valve will not be affected by air purging. After the purge is complete, install valves and instruments that were not suitable for the previous purge. The remaining welded joints should use less slag control to control secondary pollution. Once cleaning is complete, on-site inspections should be performed in critical areas, with endoscopes confirming the cleanliness of pipes such as major control and shutoff valves, and low pipelines.
 
1.3 Cleaning on board
When each module is installed on the ship, the internal cleanliness of the pipes is still better under the above conditions. At this stage, only a simple air blow is required after the integrity of each duct system is inspected. The purge at this stage is only a further confirmation of the additional cleaning of the system. Under normal circumstances, each pipeline can only be blown by continuous airflow for 30 minutes. Cleaning may be considered acceptable if there are no obvious impurities at the outlet.
 
2. Preparation
2.1 Cleaning plans
Each pipeline to be cleaned must have a file, which clearly indicates the following information: The pipeline to be cleaned is marked on the P&I D and ISO diagram; the cleaning method, cleaning medium, maximum working pressure and temperature cannot exceed the pipeline limitations of design parameters; use bypasses or short pipes to remove, isolate and separate components, disconnected flanges and blind flanges to be installed or removed; identify the location of manual valves and define the sequence of valve operations; process or utility pipelines should be cleaned starting at the upstream end of the pipeline after the flanges and accessories at the end of the pipeline are removed. Branch pipes are cleaned sequentially in the same manner. When the flushing or purging circuit includes equipment such as heat exchanger side pipes, the purge point should be assigned to the inlet flange of this equipment. To prevent debris from entering the equipment through the equipment flange inlet, the equipment should be protected. After the air purge is complete, downstream cleaning can begin through the equipment. Equipment packages or skids provided by the supplier are not included in the flush or purge procedure. Typically, they are supplied by the factory and include all factory cleaning certificates.
 
2.2 Instruments
All different measuring elements that may be clogged with dirt should be removed from the pipeline, such as orifice plates, flow meters and special parts. The control valve will be removed to allow flushing in both directions through the bypass valve and valves on the pipeline. All PSVs need to be isolated or removed. All disassembled parts should be protected from contact with dirt and atmospheric environment, such as wind and rain. Prepare a temporary storage room. In particular, control valves must have protective measures to protect the valve seat.
 
2.3 Pipelines
All isolation valves along the cleaning pipeline must be fully open, except valves that isolate equipment or valves that isolate the system being cleaned. Compressed non-asbestos gaskets are used for temporary cleaning. Verify any additional tube supports required to support the weight of the tubing and cleaning or testing media. Verify that all pipe supports, retaining brackets, guide pipe clamps and similar components are installed by the detailed details of the construction drawings. Check that the spring hanger is installed correctly and the pre-installed latches are in the correct position.
 
2.4 Equipment
All equipment should be isolated during flushing or purging and should be flushed or purged individually and not cleaned together with the system; containers should not be used as flushing containers and should not be flushed. All connections to heat exchangers and other equipment with internal inaccessibility and no bypass possibility should be disconnected for cleaning or purging without damaging internal parts. The appropriate operations are as follows: remove the inlet blind plate; cover the inlet nozzle with a metal sheet, and flush or purge the suction port of the equipment until cleaning is complete.
 
3. Air purge
Air purging uses the kinetic energy of airflow to remove garbage and production residues from pipelines. Blow with air. Since the specific gravity of air is lower than that of water, a high speed is required to obtain great kinetic energy. Minimum outlet speed must meet requirements for cleaning. Instrument quality air will be used to purge the instrument air, nitrogen and cryogenic lines to ensure moisture removal.
 
3.1 Preparation
Make special arrangements when purging suction and exhaust pipelines to avoid potential damage. The following should be paid attention to:
(1) If there is a detachable joint, the joint should be removed or rotated away from the equipment.
(2) If there is no detachable joint, temporary pipes should be used to lead the outlet to be away from the equipment.
(3) Block the inlet and outlet of the equipment to prevent the entering of debris during purging. When the purge pipeline includes equipment such as heat exchanger end pipes and heater pipes, the purge point should be arranged at the equipment inlet flange, and blind plates should be used to prevent debris from entering the equipment. After the air purging is completed at the upstream end of the equipment, air purging can be carried out from the downstream end. Temporary piping for pressure relief or pipe discharge to the atmosphere needs to be properly secured. The flushing medium is generally oil-free dry air with a dew point of 0°C, from a portable air compressor or public air. When accumulation is used for purging, the air bottle pressure is recommended to be 3 to 4 bar. However, the design pressure of the system must be checked to ensure that this pressure is not exceeded during purging. Purge areas should be taped and warning signs posted; ensure the safety of personnel in the surrounding area by patrolling. It is crucial that all accessories such as cocks, blind flanges and manhole covers. need to be tightened in the purging process.
 
3.2 Execution
Air accumulation: Air is accumulated in the column, barrel, container or tube, and then released to the downstream pipeline to the atmosphere to remove garbage and production residues. Direct purge: directly purge to the atmosphere by using temporary pipes or hoses to connect to the gas source. The purging speed should not be less than 20m/s, and should be carried out in order of the main pipe, branch pipe and discharge pipe. When purging, use a white cloth or white target plate at the discharge port to check whether the pipeline is purged well. If there are no obvious large particles on the target plate within 5 minutes, it is considered qualified. The size of the particles on the display panel should be no larger than the mesh holes of the corresponding system filter. Air purges may need to be repeated to meet cleaning standards. For systems corresponding to air purging, the dew point temperature after purging should be lower than the lowest ambient temperature (0°C).
 
4. Rinsing with water
Water flushing utilizes the kinetic energy of the liquid. When struck by fluid flow, foreign matter such as production debris and grit in the pipeline will be ejected into the open end or collected in a temporary strainer. After flushing the piping system with water, the residual water in the piping should be drained and dried until the dew point temperature is not lower than the minimum ambient temperature (0°C).
 
4.1 Preparation
Use fresh water with a known chlorine content. Stainless steel pipes or austenitic steel must be cleaned using water with a chlorine content of less than 50ppm or in compliance with the project specifications. For stainless steel pipes, the flushing water needs to be drained immediately and the pipes must be dried immediately. The flushing water velocity should be higher than 1.5m/s. The flushing pressure should not be higher than the design pressure, so sufficient flow needs to be provided through calculation. Be careful to avoid water pressure shock when flushing. Make sure dirt and debris not getting into the equipment. Supply water through temporary hoses or temporary pipes. If possible, perform manual cleaning before flushing to minimize clogging. Firewater pipelines should be flushed first as they can serve as a water source for subsequent flushing efforts. Use compressed air to drain water from every low point.
 
4.2 Execution
When using fire water as a flushing water source, the following points should be noted: the flushing pipeline must be an open pipeline, and debris should not pass through the fire pump. The fire pump should be connected to fresh water, and seawater cannot be used as cleaning water. Drain water through pre-storage columns, buckets, or containers.