Treatment of pipelines for light oils (such as gasoline)

Heat tracing pipes are added to the pipelines. Where hot work is required, the pipelines must first be purged, either by flushing with water or by using steam or inert gases. It is best to remove those parts that can be taken apart before carrying out the hot work. Before carrying out hot work, a hot work permit must be obtained in accordance with the procedures, and there must be a dedicated person to supervise. Generally, such companies need to have relevant systems in place.

When work involving open flames is required on our company’s gasoline pipelines, we usually first use water to flush out the oil completely and then blow it away with steam.

By the north I mean those long-distance light oil pipelines; if water is used to push out the oil, it’s impossible to completely empty the water from within the pipelines (in our area, this has led to pipelines breaking due to ice forming from water remaining in the lowest points of the pipelines). How should the issue of freezing be addressed?

After water washing, steam cleaning is also required; make sure that every part of the surface is exposed to steam, with no dead corners

For long-distance pipelines, a few pairs of flanges can be added at appropriate locations to divide them into several sections; when working on a particular section, it is sufficient to remove the oil from both ends and install blind flanges there. The method we commonly use is to use water to push the oil in the pipes into the oil tank, and then install blind flanges at the valves or flanges to isolate the area where welding is required from the rest of the system ； Then use a saw to cut the pipe, and insert wet mud bags inside both sides of the pipe. Flanges, valves, or fittings can be added at the cut site of the pipe. This method does not require prolonged purging of the pipelines, results in less oil loss, and the pipelines can be put into use once they have been properly prepared.

In the north, after water is drained from the top, steam is used for purging, and then the condensate at the lowest point must be drained; otherwise, freezing can easily occur in those blind spots.

I hope this can be helpful to you! I. Purpose of thorough purging (1) Through purging, remove slag, sediment that have entered the equipment and pipes during construction, as well as oil and rust in the pipes. (2) Conduct preliminary leak testing and pressure testing on static sealing points such as flanges in equipment and pipelines. (3) Streamline the process, become familiar with basic operations, and identify issues related to pipelines and equipment. (4) Familiarize with the process to improve the technical skills and practical operation capabilities of operators. II. Methods and requirements for purging (1) Before purging, it is necessary to know the purging procedure for each pipeline, the purging medium used, as well as the precautions to be taken. It is also important to identify the point where the purging medium is introduced, or the temporary point for its introduction, as well as the discharge point and temporary discharge point for each pipeline. The emission points must be properly covered to prevent dirt from being blown into the equipment and the pipelines behind it. (2) When introducing steam for purging, care must be taken to prevent water hammer and avoid burns. (3) Before purging, the control valve, orifice plate, flow meter, steam trap, flow-limiting orifice plate, filter element, and check valve disc (or check valve) should be removed to facilitate the subsequent purging of the pipeline. (4) Before purging, valves that are prone to clogging, such as the sampling valve, pressure gauge hand valve, and level gauge, should be closed. Blow out the pipelines and equipment only after they have been thoroughly purged. (5) When purging the pipelines, start with the main lines before the branch lines, and from the higher points to the lower ones. Longer pipelines should be purged in sections; all drain points or vent points along the pipeline must also be opened one by one to remove debris. All branch lines, cross connections, samplers, and glass level gauges should also be opened for drainage until the entire pipeline system is thoroughly cleaned. (6) During pipeline purging, use a hammer (0.8–1.0 Kg) to strike and vibrate the pipeline, focusing on the welds and elbows to improve the purging effect. (7) Blowing air into the interior of the equipment is strictly prohibited to prevent dirt from being blown in. If a blowing medium must pass through the equipment, the flange before it enters the equipment must be removed first, the inlet port must be covered, and the inlet pipelines of the equipment must be cleaned thoroughly before the blowing medium can pass through the equipment and be blown in the reverse direction following the established procedure. (8) Pipelines associated with the tank farm should be purged together with the tank farm; if the conditions in the tank farm do not permit this, the boundary flanges should be removed for discharge. (9) During the purging process, the leakage at each sealing point should be checked; if any leakage is detected, it should be marked and recorded for treatment after the pressure is reduced. (10) When purging the equipment pipelines equipped with safety valves, the safety valves should be removed; once the main pipeline has been thoroughly purged, the pipelines upstream and downstream of it should be purged separately. (11) When purging the pipeline passing through the pump, purge from behind by going around the pump first, then remove the cover of the pump’s inlet filter to purge the inlet pipeline; the pump body itself is not involved in the purging process. (12) Special valves within the unit, such as four-way valves and the feed isolation valve at the bottom of the coke tower, must be installed in place and involved in the purging process only after the upstream pipelines have been thoroughly purged. (13) During the purging of the main pipeline, close the instrument root valve; once the main pipeline is cleaned, contact the instrumentation team to purge the instrument pressure lead lines. (14) The purging area should not be made too large, to avoid insufficient steam supply and low pressure, which could affect the effectiveness and quality of the purging process. (15) To improve the purging effect, pressure retention can be used (the retained pressure must not exceed the design pressure of the equipment). It is possible to first purge for a period of time, then rapidly reduce the amount of purging medium, and subsequently increase that amount again; this approach helps to remove debris that has accumulated in dead corners or within the vertical sections of the pipe. (16) Accessories such as flanges, orifice plates, control valves, etc., that are removed before purging must be carefully recorded and managed by a designated person; after purging is completed, they must be restored to their original state, with records kept for reference. (17) Isolation ball valve at the outlet branch of the heating furnace, TV6118 with short coupling. (18) When working on the large gas lines at the top of the distillation tower and the carbonization tower, TIG welding is required for the root pass, and cleaning should be carried out as work progresses. (19) Keep thorough records of the purging process, and pay attention to preventing freezing and condensation depending on temperature conditions. III. Precautions for purging the pipeline (1) Adhere to the principle of draining condensate first and then introducing steam; introduce steam slowly. Once steam is visible at the outlet, gradually increase the amount of steam supplied, being careful to avoid water hammer. If water hammer occurs, it is necessary to immediately reduce the amount of steam supplied, or even stop it altogether, until the pipeline stops vibrating. After the water hammer stops, dehumidification and steam introduction should be carried out following the principle of first introducing a small amount of steam to establish flow, draining condensate, and warming the lines, followed by a small amount of purging. (2) During purging, a dedicated person must be stationed at the discharge outlet; it is strictly prohibited for the purging steam to pass through the pump body, and overpressure in the equipment and pipelines being purged is strictly forbidden. (3) Since the pipelines are heated during steam purging, it is strictly prohibited to increase the steam flow rate rapidly after introducing steam. This is to avoid sudden thermal expansion of the pipelines, which could cause them to push forward; without enough time for compensation, this expansion can lead to buckling. It can also result in water hammer, with consequences such as the detachment of pipe supports, deformation of the pipelines, fragmentation and removal of the insulation layer, cracking of welds, and separation of gaskets. (4) Steam must be introduced into the equipment slowly, with a preheating process, to prevent uneven expansion of the heated equipment due to sudden temperature rises, which could lead to leaks. (5) Cold exchange equipment is generally not involved in purging; if purging is necessary, its inlet pipeline must be purged first. When the purge medium flows in one direction, the other side must be opened to vent, to prevent leakage in the heat exchanger. (6) After the steam purging is complete, the low-point and high-point vents of the system should be opened to prevent steam from condensing, which could create a negative pressure in the system and damage the equipment. IV. Quality requirements for purging and cleaning pipelines: For pipelines that are to be purged with steam, polished wooden boards can be placed at the exhaust ports to check; there should be no rust or dirt on these boards. Generally, the flow rate for steam purging should be at least 30 meters per second, 20 meters per second for air, and 1.5 meters per second for water flushing. Line cleaning: As the name implies, it involves using a medium to clean pipelines. To fully understand what line cleaning is, one must know its purpose as well as the medium used for this process. Purpose of line cleaning: 1. For natural gas pipelines in operation: to remove water accumulation, light oils, methane hydrates, iron oxide, carbide dust, carbon disulfide, hydrogen sulfide, and other corrosive substances from within the pipelines, thereby reducing the corrosive damage caused by these substances to the inner walls of the pipes. 2. Operating crude oil pipelines: Removing sludge, wax, and scale inside the pipelines aims to reduce backpressure during oil transportation, lower friction losses, and decrease the oil transport temperature. 3. Chemical materials and edible oil pipelines: Clean pipelines containing polymeric materials, and isolate different transported media to enable the transport of multiple products through a single pipeline. Medium used for line cleaning: The type of cleaning medium is primarily determined by the properties of the medium to be purged, while also taking into account the structural design of the receiving tank. When the purging medium comes into contact with the medium to be purged, no intense vaporization, chemical reactions, or the formation of hazardous substances should occur ； Floating roof or internal floating roof tanks should not have oil pumped into them using steam or other gases. The cleaning medium is generally selected according to the following requirements: 1. For crude oil pipelines, hot water is usually used for cleaning; if welding is required on the pipeline, the water in the vent pipe must first be drained, after which steam and compressed air are used for cleaning. 2. Pipelines for residue oil, heavy oil, lubricating oil feedstocks, and heavy deasphalted oil are generally cleaned using steam and compressed air. 3. Finished lubricants, additives, and light diesel are purged using dehydrated compressed air or nitrogen in the pipelines. 4. Pipelines for gasoline, solvent oil, kerosene, jet fuel, aromatics, and light contaminated oil should be purged with nitrogen; water flushing can also be used. If hot work is required on the pipeline, it should be purged with steam. 5. Fuel gas, low-pressure vent gas, liquefied petroleum gas, hydrogen, etc. should be vented and then purged with nitrogen or steam. 6. Acid and alkali pipelines should be purged with dehydrated compressed air or nitrogen. 7. For pipelines transporting hot asphalt, hot residue oil, etc., at temperatures above 120°C, light diesel should be used to flush the pipeline first, followed by purging it with steam and compressed air. Steam and compressed air should not enter the hot oil tank.

Practical experience in work: Precautions: It is not possible to use water to flush gasoline pipelines in winter, followed by a light cleaning with steam. Firstly, the temperature inside the tanks during winter is between minus 10 and minus 20 degrees; water entering such tanks tends to freeze at the bottom of them. Using steam to clean the tanks then can lead to increased evaporation losses of the oil, and it is also not safe. Procedure: First, purge the tank using nitrogen, and control the base valve of the tank to prevent high pressure from flipping the floating disk (it is best not to purge when there is oil beneath the floating disk). After hearing the sound of gas flowing, open and close the valve repeatedly (adjusting the valve opening) to clean the pipeline. First, close the base valve, then close the gas valve, and release all pressure from the pipeline. Install blind flanges on the valves on both sides of the area where welding will take place. If valves need to be added to the pipeline, or for safety reasons, the pipeline must be cut using a steel saw (to avoid injuries caused by direct welding or blasting), and both ends should be sealed. After welding the flanges, the seals can be removed and valves can be installed. If connecting pipelines, after welding the flanges and removing the seals, a tee can be added; in this case, the flanges should be welded in advance as well. This approach makes flange connections relatively safer. Many valves and pipelines have been installed using this method without any issues arising. :)