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Produced jointly by BP and IChemE, the BP Process Safety Series is a collection of books, animations and slide rules which guide the user in the application of. download BP Process Safety Series - 16 Title Bookset - IChemE (BP Process Safety Series) - Includes Two Slide Rules For Foam Application Rates on medical-site.info . BP Process Safety Series. IChemE: Rugby, Cite this:Org. Process Res. Dev. 9, 4, View: PDF | PDF w/ Links | Full Text HTML. Article Options.
However their use has also lead to many fatal accidents through the creation of atmospheres which are deficient in oxygen. It is important not to enter any vessel which has been purged with an inert gas until it has been isolated and shown to be safe even placing the head inside a manway may lead to suffocation.
Care must also be taken not to create spaces in which the concentration of inert gas may build up as illustrated by the incident below. The column had been cleaned, several manholes were open, and a nitrogen purge was on the column. Two experienced workers were examining the flange surface of a remote manhole for stress cracks. They sprayed dye on the flanges and used a black light to identify the suspect areas.
A tarpaulin was draped over the flange but it is unclear whether this was to block the wind while they were using dye penetrant or to facilitate using the black light, or both. The confined space created by the tarpaulin was soon filled with nitrogen which asphyxiated both men. One man died as a result of the exposure and the other survived because he collapsed face down on the expanded metal grating, which allowed sufficient oxygen to sustain his life.
The immediate cause of the accident was the inadvertent creation of a confined space environment around an open manhole that was being purged with nitrogen. The basic causes were the failure to recognise a confined space and the risk of asphyxiation from nitrogen coming out of the manhole, and inadequate control of work on a column that was being nitrogen purged.
In certain catalyst or catalyst-handling sections of units, the catalyst must first be regenerated to eliminate hydrocarbons and then purged before air can be admitted. What has been said about the hazard of air and hydrocarbons mixing in an operating unit cannot be emphasized too strongly when applied to a flare system. Here, if a combustible mixture exists, ignition is guaranteed by the continuous flame at the end of the line.
When a flare system is to be shut down with the unit it serves, it should be gas freed; and the ignition source at the flare should be shut off before admitting air. Lines should be purged with steam or inert gas before admitting air. Steam, however, must be used with caution in freezing weather because of the danger that condensing water may freeze and plug the system with ice. The majority of accidents involving flare systems have one thing in common, namely, the entry of materials that the system was not intended to accommodate.
Some of these are air, steam, heavy oil which may congeal and plug the system , corrosive materials, low-boiling liquid hydrocarbons which due to rapid evaporation may cause freezing of water or congealing of heavy oil or an excessive amount of liquid which may blow out at the top of the flare Figure 7. Care must be taken to follow procedures which will prevent the introduction of any material to the flare system except under conditions that will not cause trouble.
Figure 7 Burning liquid can spill from flares if the knockout drums are overfilled see also accident description in section 3. It displaces both gas and liquids, which otherwise might be trapped, and it cools the unit.
If economically practical, it is desirable to use water at a minimum temperature of F 38C. The use of warm water aids the removal of hydrocarbons and other chemicals if they are present.
Water flooding can be used only if the equipment and its foundation have been designed to support the weight and pressure of the water Figure 8 and only if water will do no damage to the process. Figure 8 Purging by water filling can only be used if equipment and foundations can support the weight and pressure. Before the water is introduced, all rundown lines from the unit, the feedline and other lines to the unit should be blinded at the battery limits to prevent water from entering these lines.
For work on some units and equipment, water flooding alone may be sufficient. However, unless a vessel is very clean and has no pockets, water flooding will not gas-free it for hot work or for inspection. If such work is to be done, especially where there are scale deposits, the vessel should be steamed before water flooding. After the water is dropped, the vessel should be tested to determine whether the hydrocarbon level is safe. It may be necessary to use ventilating devices to ensure a noncombustible condition.
Steaming should not be done at this time since an explosion could be set off by the generation of static electricity. The water filling may be done either by flowing the water from one vessel to another until the entire system is overflowing or by filling several parts simultaneously until the system is full. As the water enters the unit, the materials displaced should be vented from a high point of each part of the unit to prevent trapping any oil, gas or other material.
Towers should be flooded over into the reflux drums which are vented at the top while water is entering the tower. Oil flushed into the reflux drums may be pumped out or dropped to the closed drain as desired.
After flooding, the water should be drained to the sewer. As the water level falls to the level of the vents on the respective towers, vessels, etc. It is extremely important that sufficient air enters the unit to prevent pulling enough vacuum on any part of the unit to cause it to collapse. Enough air should enter the unit to permit subsequent entry by maintenance personnel.
To make certain that water is completely removed, water must flow from each drain. Plugged drains must be opened. A log of the drains should be made, showing the drain location, the time and date drained and the initials of the operator who witnessed the draining. Such gases as hydrogen sulphide, arsine and hydrogen fluoride are especially toxic and must be purged from any vessels to be entered. The material and procedure for purging depend upon the type of residual material to be removed.
Residual gases can be removed from a unit with a purge of steam or nitrogen, followed by water filling to overflowing, and then by displacement of the water with air. The reader should refer to the operating manual for the correct purge materials for that particular unit. It is possible that toxic gases may dissolve in the water used to fill the vessel. Care must be taken when disposing of this water to ensure that the gas is not subsequently released to the atmosphere.
Sludges which are the product of acid or caustic treatment of a hydrocarbon may be flushed from the system when the acid or caustic is removed.
Some of this sludge can remain and be removed by the water wash. Disposal of this water must be done carefully to avoid release of the sludge in an unwanted location or the release of a toxic gas if the sludge is inadvertently neutralized. The shutdown procedure will spell out the disposal methods. Much costly damage can occur to piping, exchangers, vessels, pumps, compressor jackets and other equipment from the freezing of water. Freezing must be prevented or the water removed.
In some localities, freezing weather must be anticipated. Equipment to be steamed or water washed, or equipment handling steam or water in normal operations, must be designed to minimize all pockets where water can collect.
Drains must be located at all low points on each piece of equipment. Fractionating tower trays and exchanger baffles must be provided with weep holes so that this equipment can be drained completely when shut down. Hot-air blowers may be installed in towers and vessels to prevent freezing.
Water often can be trapped in large control valves, orifice runs and pumps, particularly multi-stage centrifugal pumps. As far as possible, these types of equipment should have bottom drains which must be used to prevent damage from freezing Figure 9. Exchanger tubes and furnace tubes are frequently bowed due to their service.
In this condition, the tubes can trap water and are vulnerable to freezing and breakage. The shutdown procedure will point out the protective steps necessary such as blowing individual tubes with air or keeping the tubes warmed above 32F 0C. Almost every centrifugal pump will trap some liquid within its internals when drained through the drain connections normally supplied. Such traps can range in size from small to very large. The design of many multistage pumps including barrel-type pumps is such that there is no practical way to drain them completely.
For these, a flushing procedure should be developed to clear the pumps of hydrocarbons and to avoid or remove water accumulations safely. The draining and flushing procedures must include all spare equipment as well as the equipment regularly used. Provisions should be made to prevent water lines from freezing Figure This can be done in several ways. A sufficient flow of water can be maintained to prevent freezing. Main lines and their shutoff valves may be located underground below the frost line.
The firewater system is often used during downtime to provide water for flushing and testing of equipment. The system itself should be flushed and tested during this time. Freezing precautions should be observed on this system as required. Compressor cooling-water jackets and sewer traps, in which the flow of water must be cut off, should be filled with an antifreeze mix such as alcohol or glycol and water. Steam traps and steam lines which are shut off in freezing weather are particularly susceptible to freeze damage.
Proper drains should be provided. If steam tracer lines are shut off in freezing weather, they should be blown out thoroughly with air.
The purpose of freeze prevention is to prevent equipment damage and loss. This point must never be overlooked. All connections to any tower or vessel which technicians are to enter during the shutdown should be blinded to prevent accidental entry of foreign material which might injure the workers. Maintenance workers opening various parts of the unit should wear appropriate protective equipment in addition to the normal safety equipment.
Figure 11 Blinds are required to keep unwanted material from entering. Opening lines to install blinds should be done only after the operator in charge has given his approval and then only with extreme care. Flanges should be cracked open slowly so that any material incompletely purged from the unit can be handled safely. Small amounts of material may be vented or drained. If large amounts of materials are present, the flanges should be closed quickly for further purging of the equipment.
Valves should not be removed until it is absolutely certain that the equipment to which the valves are attached is empty. A list should be made of all blinds to be installed during the shutdown. Each blind location should be assigned a number. The date and time at which each blind is installed and the initials of the operator who witnessed its installation should be noted on the list. Certain lines, vents, drains, etc.
These running blinds should be removed at prescribed times during the shutdown to permit purging and draining of material from the unit. A list of these blinds should be made, and the date and time of removal of each blind and the initials of the operator who witnessed its removal should be noted on the list.
When all the blinds on the blind list are installed or removed, the unit should be opened for cleaning and repairs. It may resist oil, steam and water purges and washes; and when exposed to air, it will ignite spontaneously even at low temperatures.
When wet, it will not ignite Figure 12 but will do so as soon as it dries. The drying time may range from a few minutes to several days depending upon conditions. The simple burning of this material alone may do considerable damage. If hydrocarbon vapours and air are also present, an explosion and fire may follow the ignition of the iron sulphide.
Within one oil company, there is typically one pyrophoric scale fire in distillation units every two to three years. There are many more reported incidents within the process industry. When this occurs inside equipment like columns, vessels, and tanks and exchangers containing residual hydrocarbons and air, the results can be devastating. The fire happened during the replacement of the carbon steel structured packing, which involved cutting out the old column internals and removing each bed of packing.
Despite persistent attempts to extinguish the fire, the incident progressed to the point where the splitter fell over early the next morning. Why do pyrophoric fires occur? This becomes a source of ignition for any nearby combustibles.
The reaction is accelerated by high temperature and if the FeS is dry. Where do pyrophoric fires occur? Pyrophoric scale can occur within process vessels, exchangers and distillation columns. In general, the higher the sulphur in the crude processed, the higher the tendency for FeS production. In refineries, the equipment most prone to pyrophoric combustion induced fires are distillation columns in crude and vacuum distillation units, as deposits of iron sulphide are formed from corrosion products that most readily accumulate at the trays, pump around zones, and structured packing.
Also, pyrophoric fires are a common occurrence in heavy fuel oil and asphalt bitumen tanks. Within vessels and heat exchangers, there is a large thermal mass and the pyrophoric scale will often burn out locally without causing any significant damage. However, with packed distillation columns, there are additional problems: Structured packings have high surface areas with extremely thin sheet metal usually only 0. They have a low thermal mass, and so will heat up very quickly in the event of a pyrophoric fire.
The packing structure often traps the FeS and enhances the contact between the FeS and the oxygen. Severe damage of the packing and vessel wall may occur due to pyrophoric scale combustion. There does not necessarily need to be any hydrocarbon source to cause a serious fire. The high surface area of structured packings and the interface between each of the layers may well result in a greater retention of coke and hydrocarbons than other types of random packing and certainly trays.
This clearly provides the fuel for a potentially destructive fire. Some types of packing metallurgy are very reactive e.
Stainless steel packings generally used in refining are not as reactive and should not result in a metal fire, but as stated in the U. Department of Energy Handbook Primer on Spontaneous Heating and Pyrophoricity [DOE-HDBK] some metals, such as aluminium, iron, and steel, that are not normally thought of as combustible, may ignite and burn when in finely divided form.
A fire started in the evaporator drum, and subsequently ruptured the bottom of the vessel see picture. The drum was randomly packed with thin titanium rings to ensure the vaporization of droplets before they reached downstream vessels where they may cause corrosion. It is believed that the warm air introduced during start-up and the presence of iron oxides and hydrocarbons helped start the fire on the highly reactive metallurgy of the rings.
When the process equipment internals are exposed to air such as during turnaround pyrophoric fires can occur at any time. The risk of pyrophoric autoignition continues throughout the turnaround, and there have been a number of incidents where a fire has occurred many days after opening a vessel. The fire was a structured packed bed fire internal of the main distillation column, located at the bottom of the wash oil packed zone near to the bottom of the column.
Some packed sections retrieved were heavily coked. One day, flames and smoke were observed coming from the tank. Upon investigation, it was found that pyrophoric scale had ignited leading to combustion of residual naphtha in the tank. After the oil was pumped out and tank cleaning commenced, a fire occurred in the rim seal area, probably as a result of pyrophoric iron sulphide being present.
It proved quite difficult to extinguish. How can pyrophoric fires be prevented? The most important aspect of preventing a pyrophoric fire is to ensure that the column internals are thoroughly water wetted prior to allowing air into the vessel. This is essential for packed beds. The internals should be repeatedly water wetted throughout the shutdown to ensure that a fire does not occur at a later stage.
It is possible that these could be blocked or inadequate for the desired wash rates, so it is difficult to ensure that the column packed beds are adequately water washed.
Some refineries have used chemical cleaning methods to help neutralize pyrophoric scale. The following best practice guidelines should be incorporated in the column shutdown procedures, especially for structured packed columns which present a greater pyrophoric risk than random packed beds.
Before opening vessel manways: Normal procedures should be followed for hydrocarbon freeing and steaming of the vessel. Each packed bed should then be individually water washed for at least two hours.
This wash rate is quite high and it may be necessary to use the existing process pumps for circulation of water. Check whether the existing distributors can handle these rates.
It may be necessary to modify the wash rates to suit the distributors. Vacuum distillation unit wash beds will be difficult to water wash at this high rate due to hydraulic limits of the distributor.
In this case, there is also a serious risk of coke retention within the bed. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, without the prior permission of the publisher. Shutdown procedure. Cooling and depressuring. Pumping out. Removal of residual hydrocarbons. Removal of corrosive or poisonous materials. Disposal of water and freeze prevention.
Blinding and opening. Hazards of pyrophoric iron sulphide. Inspection for entering. Layaway of unit. Preparation of auxiliary equipment and services. Elimination of air. Tightness testing. Backing in fuel gas. Elimination of water.
Bringing the unit onstream. Unit operations require many skills and a wide range of knowledge. Skill and knowledge are generally acquired through experience and training.
Experience is sometimes a slow and painful teacher, so training is of paramount importance in operating safety. This and the other booklets in the BP Process Safety series are tools for the training of supervisors, process operators and maintenance men to supplement experience, thereby shortening the learning period. Failure to recognize and eliminate the hazards associated with shutdowns and start-ups of refinery units has resulted in serious injury, death and costly property damage Figure 1a.
Figure 1a Refinery damage after a fire. Kelly and M. Clancy, CEP The petroleum industry has devoted much time and money in designing units to avoid these booby traps. When a hazard cannot economically be eliminated in the design, however, it must be recognized and procedures set up to avoid the hazard. These procedures must be diligently followed. The hazards encountered most frequently in shutdowns and start-ups of units are accidental mixing of air and hydrocarbons, contacting of water with hot oil, and freezing of residual water in piping and equipment.
Other hazards commonly experienced during turnarounds are corrosive and poisonous liquids and gases and pyrophoric iron sulphide. Further hazards associated primarily with start-ups involve pressure, vacuum, and thermal and mechanical shock. These can result in fires, explosions, destructive pressure surges and other damage to the unit, as well as injury to personnel.
Fires occur when oxygen and fuel vapour or mists are mixed in flammable proportions and come into contact with a source of ignition. They may burn out of control or touch off a devastating explosion.
Pressure surges resulting from unplanned mixing of water and hot oil may cause only minor damage or they may wreck equipment resulting in extensive costly downtime on process units. Fire usually follows if the explosion bursts lines or vessels.
Failure to drain water from equipment or failure to maintain a sufficient flow of water or steam through a system may permit freezing which can cause extensive damage. Proper drainage or flow will prevent freeze damage Figure 2. Because history shows that most of the serious refinery fires and explosions have occurred on units during start-ups and shutdowns, procedures must be developed which recognize and avoid hazards.
This booklet presents many of the tried and proven operating practices typical of the industry today, some of which are summarized in Chapter 5. This booklet begins with shutdown, through which the narrative progresses to start-up. It actually covers the steps required for a complete turnaround, but the applicable steps would also apply to a turnaround of part of a unit. The shutdown procedure should be in writing, and it should be followed strictly.
Checklists, with blanks for time and initials, should be used to show the sequence of events to assure safety and efficiency of operations and continuity of work between shifts.
The critical path arrow diagram method Figure 3 has been used effectively in recent years for planning. In actual practice, separate diagrams are usually made for shutdown, repair work and start-up. In addition to serving as a checklist, the critical path the chain of interconnected work requiring the longest overall time for completion can be determined so that the work can be planned most efficiently.
The original diagram should be made in one colour. As each job is completed, the corresponding arrow should be marked over with another colour. The diagram so marked will indicate at a glance the work which has been completed up to that time and the work which remains to be done. It thus promotes good communications and job continuity. These in turn will contribute significantly to safe and efficient shutdown of equipment. Figure 3 Portion of a shutdown arrow diagram. Whenever practical, emergency equipment should be tested regularly while the unit is in operation.
In addition, all emergency equipment including emergency generator, driver trips, spare pumps and emergency instrument-air system should be tested while the unit is being shut down so that any malfunction can be corrected during the downtime. Where possible, utilize the normal temperature controls. Simultaneously, reduce the charge gradually by about 20 percent of normal per hour, until it is about 30 percent of normal.
During this reduction, the products from distillation towers should be held on test by reducing the side-stream drawoff and reflux rates in accordance with the reduction in feed rate and using tower temperatures as a guide. Stripping steam rates should also be reduced accordingly. It is a good idea to throttle the cooling water to overhead condensers as charge is reduced, but not enough to cause slug flow and possible pipe damage due to incomplete condensation.
When the feed rate has been reduced to about 30 percent of normal, all fuel to the main and pilot burners should be shut off. Oil burners should be blown out with steam. A small amount of steam should be left blowing through the tips to keep them cool, or the tips should be withdrawn from the firebox. The secondary-air dampers and the flue-gas dampers to the stack should be opened wide to permit the maximum flow of outside air through the furnace firebox for cooling. Next, the outside or fresh charge oil to the unit should be shut off and internal circulation of the oil in the system started.
Oil should continue to flow from the charge pump through the main components of the unitexchangers, furnace tubes, towers, etc. Circulation should continue until the oil reaches a temperature in the range of F to F to C. This will probably require several hours. When this point is reached, a small flow of dry steam should be started through the furnace coils. The steam should flow with the recycled charge oil through the coils and into the tower. It will pass upward in the tower and through the water-cooled overhead condenser where it will be condensed to water.
The water should be drawn off from the reflux drum. The charge pump to the unit should then be shut down and its discharge valve closed to prevent steam from backing through the pump. The steam to the furnace coils and into the tower may then be increased. The fuel-gas line s to the furnace should be blinded as soon as the furnace is shut down to avoid accidental introduction of gas into the furnace. When the main fuel-gas line is no longer needed for other purposes during the shutdown, a blind should be installed at the battery limits.
On some units, the blind may be installed downstream of the dry-drum block valves. This is satisfactory if the drum is at or outside the battery limits. Any other connections within the battery limits to the main fuel-gas line, such as wet gas or lines to towers, should also be blinded. As an added precaution, gas burners of some units should be disconnected. The fuel lines should be freed of hydrocarbons with steam or other inert gas if required by specific operating instructions.
Steam condensate should be removed. All of these things are done to make it safe to work inside furnaces, or on burners or gas lines. At the same time, the fuel-oil line s should be removed from the furnace and the fuel-oil lines drained to prevent accidental introduction of oil into the furnace.
The fuel-gas burner valves should be checked for tightness, and lubricated plug cocks, if any, should be greased during the shutdown. For units normally operating above atmospheric pressure, the pressure should approach zero gauge as the temperature drops. Excess pressure should be relieved by releasing hydrocarbon gases to a gas collecting system. If the cooling tends to produce a vacuum in the unit, steam or other inert gas should be introduced to maintain the pressure slightly above atmospheric pressure so that the vessels will not be damaged and so that air cannot enter the unit.
Where this is a possibility, water must be drawn frequently, and the depressuring and cooling must be done slowly enough to prevent the water from freezing and possibly closing off important drain points or causing other serious blockage or obstruction. Later these obstructing ice plugs may melt, releasing hydrocarbons which can flow, sometimes unnoticed, to a source of ignition Figure 4.
It will thaw later and release light ends. When shutting down towers, care should be exercised to adjust cooling so that water is not condensed in the upper part of the tower and then dropped into hot oil in the bottom of the tower. The tower-top temperature should be kept safely above the water condensing temperature at the pressure involved.
Where a catalyst is involved, special cooling procedures may be required to avoid catalyst deactivation or physical damage to the catalyst. The procedure will depend on the specific process and the catalyst.
Each material, depending upon its composition, should be routed to a prescribed place. Centrifugal pumps should be watched carefully to see that they do not lose suction. Reciprocating pumps are best for pumpout service because they have superior suction characteristics and are less susceptible to damage. As the oil is pumped from the unit, it should be cooled further in heat-exchange equipment to a safe storage temperature.
Some equipment drains used during the shutdown operation may not have permanent connections to a pumpout or closed-drain system. If the material released from these drains can burn and then injure persons and damage equipment, temporary facilities should be installed to drain the material to a closed system or another safe place.
Inert gas or steam should be admitted to the unit during the pumpout operation for two reasonsfirstly, to prevent entrance of air into the unit; and secondly, to prevent collapse of any equipment not designed to withstand a vacuum. On shutdown of vacuum towers, special techniques are needed to avoid O2 entry to the tower, to control liquid levels, to avoid formation of liquid water as the tower temperatures decrease, and to avoid excess pressure drop across the trays which could upset them.
These techniques are spelled out in the shutdown procedure and must be followed carefully. The electric and steam-driven pumps and compressors which are to be worked on during the turnaround should have the main disconnect switches padlocked in the OFF open position, and the main steam valves to the drivers padlocked in the closed position. Exhaust steam lines attached to steam-turbine cases should not be shut off unless the turbine itself is to be worked upon.
This is a safety measure to prevent rupture of the turbine case which could occur if high-pressure steam was admitted to the turbine with the exhaust valve closed. A Repair Hold Card tag should be attached to each padlock to prevent its removal before proper authorization to do so is given. Some refineries may use different methods such as the multiple padlock procedure, wherein each person working on the equipment in question puts his own padlock on the main disconnecting device.
In any event, local safety regulations covering such situations should always be strictly followed. Any gland oil or seal-gas system employed on the unit should be shut down along with the equipment it serves. The type of unit determines the appropriate purging material. Displacement with inert gas Steam is the preferred inert gas for oil processing units, unless it would damage equipment or material in the unit, such as a catalyst.
Distillation and coking units are typical units in which steam should be used. In certain catalytic process units in which steam would damage the catalyst, nitrogen, carbon dioxide or gas from an inert-gas generator should be used instead. The choice would depend upon the catalyst involved and the cost of the respective purge gases. When a gas is used for purging, it is commonly admitted at or near the inlet to the unit and flows from one vessel to another in succession, including finally the blowdown or flare system.
This reduces the chance of bypassing any area and gives an orderly procedure. If parallel paths of flow exist in a unit, care must be taken to ensure that each path is purged. If dead-end spaces exist in the system such as behind metal shrouds in reactors , it may be desirable to pressure-depressure the system with inert gas to dilute the hydrocarbon to the acceptable concentration. The purge gas should continue to flow into and through every part of the unit until tests show that the gas flowing from each drain and vent on the unit has a hydrocarbon content of less than 1 percent.
The limit specified by operating instructions should be followed when combustibles other than hydrocarbons are present. Except where steam is being used, each vent and drain on the unit should be closed as soon as the test shows that the hydrocarbon content of the gas at that point is less than 1 percent. This will conserve purge gas and reduce purging time. If steam is used, all drains and the vents at the highest points of the unit should be left open until tests show that only steam or water, or both, is issuing from each drain and vent on the unit, and again, that the hydrocarbon content at each drain and vent is less than 1 percent.
This step ensures continuous drainage of all condensate as it forms and prevents any air from flowing into the unit during purging. When the purge is complete, the purge gas should be replaced by air. Various devices available for measuring the hydrocarbon content of purge gases are shown in Figures To read percent hydrocarbon directly, they must be calibrated for the gases involved, and the proper scale must be used.
Care must be taken to select an instrument which has been calibrated for the combination of purge gas and hydrocarbon being tested. The hydrocarbon content of issuing purge gas cannot be measured directly with the normal combustible gas tester, as this type of instrument requires air to burn the combustibles in the instrument. However, at least one type of these testers can be equipped with a dilution tube which permits a known ratio of air to mix with the purge-gas sample. Thus, this specially equipped instrument can be used when nitrogen is the purge gas, but not steam.
Be sure to return this type of tester to its normal testing mode after using it in an inert purge sample so that the dilution mechanism does not interfere with the normal sample. Infrared detectors are now able to detect flammables in inert gases. Another instrument available for measuring the hydrocarbon content of nitrogen or other non-condensible purge gases uses the thermal-conductivity principle to measure the amount of hydrocarbon present Figure 5.
This instrument must be calibrated for the particular purge gas and hydrocarbon combination to be tested. Other manufacturers provide suitable equipment, and the foregoing discussion is not intended to exclude that equipment.
When steam is used for purging, neither of the instruments previously discussed can be used for testing the purge stream. A representative portion of the steam flows through the sample chamber, heating the sample chamber to steam temperature.
Placing the sample chamber in the insulated container speeds up this step.
A sample of the steam is then trapped in the chamber by closing in quick succession the inlet valve and outlet valve. The chamber is then lifted from the insulated container and cooled to F 38C. Condensation of the steam produces a vacuum in the chamber. The amount of pressure reduction is a function of the relative proportions of steam and non-condensibles air in the original mixture, and the air content can be calculated.
This instrument cannot be used to determine concentration of air in inert gas. After purging with inert gas, absence of oxygen must be confirmed using one of several types of portable oxygen analysers suitable for this purpose.
This instrument was available commercially from Mine Safety Appliances Company MSA for a number of years, although it is not presently in production. A number of these analysers are currently in use in the Amoco heritage refineries and the drawings are available from these sites.
Steam purge streams containing C7 and heavier hydrocarbons can be tested using the American Gas Association AGA technique shown in Figure 6b on page This device is quite simple, being a direct measurement of the hydrocarbon present in the condensed sample. The method is shown as a laboratory setup, but a portable testing device could be developed.
When the AGA technique and equipment are used, sufficient cooling water should be available at the temperature of the condensate from the cooling coil to approximately 70F 21C. After flooding. It may be necessary to use ventilating devices to ensure a noncombustible condition. Steaming should not be done at this time since an explosion could be set off by the generation of static electricity.
Figure 8 Purging by water filling can only be used if equipment and foundations can support the weight and pressure. As the water level falls to the level of the vents on the respective towers.
If economically practical. Before the water is introduced. As the water enters the unit.. Oil flushed into the reflux drums may be pumped out or dropped to the closed drain as desired. Towers should be flooded over into the reflux drums which are vented at the top while water is entering the tower. The water filling may be done either by flowing the water from one vessel to another until the entire system is overflowing or by filling several parts simultaneously until the system is full.
It is extremely important that sufficient air enters the unit to prevent pulling enough vacuum on any part of the unit to cause it to collapse. Equipment to be steamed or water washed. Much costly damage can occur to piping. Some of this sludge can remain and be removed by the water wash. The shutdown procedure will spell out the disposal methods. Enough air should enter the unit to permit subsequent entry by maintenance personnel.
Fractionating tower trays and exchanger baffles must be provided with weep holes so that this equipment can be drained completely when shut down. Disposal of this water must be done carefully to avoid release of the sludge in an unwanted location or the release of a toxic gas if the sludge is inadvertently neutralized. It is possible that toxic gases may dissolve in the water used to fill the vessel. Such gases as hydrogen sulphide. Drains must be located at all low points on each piece of equipment.
Care must be taken when disposing of this water to ensure that the gas is not subsequently released to the atmosphere. To make certain that water is completely removed. Sludges which are the product of acid or caustic treatment of a hydrocarbon may be flushed from the system when the acid or caustic is removed. Residual gases can be removed from a unit with a purge of steam or nitrogen. In some localities.
A log of the drains should be made. The material and procedure for purging depend upon the type of residual material to be removed. Plugged drains must be opened. Freezing must be prevented or the water removed. The reader should refer to the operating manual for the correct purge materials for that particular unit. This can be done in several ways.
The draining and flushing procedures must include all spare equipment as well as the equipment regularly used. Exchanger tubes and furnace tubes are frequently bowed due to their service.
As far as possible. Provisions should be made to prevent water lines from freezing Figure The design of many multistage pumps including barrel-type pumps is such that there is no practical way to drain them completely.
In many cases. A sufficient flow of water can be maintained to prevent freezing. Water often can be trapped in large control valves. For these. A section can be heat traced and insulated.
Almost every centrifugal pump will trap some liquid within its internals when drained through the drain connections normally supplied. In this condition. Such traps can range in size from small to very large. Hot-air blowers may be installed in towers and vessels to prevent freezing. Freezing precautions should be observed on this system as required.
Steam traps and steam lines which are shut off in freezing weather are particularly susceptible to freeze damage. Proper drains should be provided. Main lines and their shutoff valves may be located underground below the frost line.
Maintenance workers opening various parts of the unit should wear appropriate protective equipment in addition to the normal safety equipment. The firewater system is often used during downtime to provide water for flushing and testing of equipment.
The system itself should be flushed and tested during this time. If steam tracer lines are shut off in freezing weather. This point must never be overlooked. The purpose of freeze prevention is to prevent equipment damage and loss. All connections to any tower or vessel which technicians are to enter during the shutdown should be blinded to prevent accidental entry of foreign material which might injure the workers. Compressor cooling-water jackets and sewer traps. The drying time may range from a few minutes to several days depending upon conditions.
Valves should not be removed until it is absolutely certain that the equipment to which the valves are attached is empty. The date and time at which each blind is installed and the initials of the operator who witnessed its installation should be noted on the list. If large amounts of materials are present. A list should be made of all blinds to be installed during the shutdown. Opening lines to install blinds should be done only after the operator in charge has given his approval and then only with extreme care.
If hydrocarbon vapours and air are also present. When wet. Flanges should be cracked open slowly so that any material incompletely purged from the unit can be handled safely.
The simple burning of this material alone may do considerable damage. Small amounts of material may be vented or drained. It may resist oil. Each blind location should be assigned a number. These running blinds should be removed at prescribed times during the shutdown to permit purging and draining of material from the unit.
A list of these blinds should be made. Figure 11 Blinds are required to keep unwanted material from entering. When all the blinds on the blind list are installed or removed.
Certain lines. Within one oil company. The fire happened during the replacement of the carbon steel structured packing. There are many more reported incidents within the process industry. When this occurs inside equipment like columns. Despite persistent attempts to extinguish the fire.
Why do pyrophoric fires occur? Pyrophoric scale FeS is generated during crude oil processing. Stainless steel packings generally used in refining are not as reactive and should not result in a metal fire. Within vessels and heat exchangers. Severe damage of the packing and vessel wall may occur due to pyrophoric scale combustion.
The reaction is accelerated by high temperature and if the FeS is dry. There does not necessarily need to be any hydrocarbon source to cause a serious fire. A fire started in the evaporator drum. Some types of packing metallurgy are very reactive e. The drum was randomly packed with thin titanium rings to ensure the vaporization of droplets before they reached downstream vessels where they may cause corrosion.
This becomes a source of ignition for any nearby combustibles. In general. They have a low thermal mass. The high surface area of structured packings and the interface between each of the layers may well result in a greater retention of coke and hydrocarbons than other types of random packing and certainly trays.
This clearly provides the fuel for a potentially destructive fire. The packing structure often traps the FeS and enhances the contact between the FeS and the oxygen. It is believed that the warm air introduced during start-up and the presence of iron oxides and hydrocarbons helped start the fire on the highly reactive metallurgy of the rings. In refineries. Where do pyrophoric fires occur? Pyrophoric scale can occur within process vessels.
Some packed sections retrieved were heavily coked. The risk of pyrophoric autoignition continues throughout the turnaround. The fire was a structured packed bed fire internal of the main distillation column. When the process equipment internals are exposed to air such as during turnaround pyrophoric fires can occur at any time. This is essential for packed beds. It proved quite difficult to extinguish. The internals should be repeatedly water wetted throughout the shutdown to ensure that a fire does not occur at a later stage.
One day. Upon investigation. After the oil was pumped out and tank cleaning commenced. How can pyrophoric fires be prevented? The most important aspect of preventing a pyrophoric fire is to ensure that the column internals are thoroughly water wetted prior to allowing air into the vessel.
Consideration should be given to back-filling the VDU with water to above the level of the wash bed. It may be necessary to modify the wash rates to suit the distributors. In this case. With other columns. This can be achieved by using cold wash water. Check whether the existing distributors can handle these rates. Some refineries have used chemical cleaning methods to help neutralize pyrophoric scale. The following best practice guidelines should be incorporated in the column shutdown procedures.
Each packed bed should then be individually water washed for at least two hours. The use of chemical cleaning solutions containing neutralizing agents. This wash rate is quite high and it may be necessary to use the existing process pumps for circulation of water.
Vacuum distillation unit wash beds will be difficult to water wash at this high rate due to hydraulic limits of the distributor. It is possible that these could be blocked or inadequate for the desired wash rates. In view of the potentially destructive nature of a column fire fuelled by coke or residual hydrocarbons. If there is an indication of a localized fire. During the period when air is introduced into the column. Before opening vessel manways: Note that it may be necessary to check that the foundations can take the additional weight.
The column should then be flooded with water. FCC main fractionators. This could include packed debutanizers. Mineral seal oil should never be used in towers or vessels because it is flammable. Monitor the air at the top of the column for increasing levels of SO2. In certain places where water might be undesirable or freezing would be a problem.
This can be carried out using hand held water hoses. Do not perform hot work above or below packed beds.
All attempts should be made to remove the packing prior to performing hot work. During the shutdown: The wet iron sulphide can then be mechanically removed and taken to an area where its subsequent ignition and burning will cause no damage Figure Failing this. To allow anyone to enter a vessel. If hot work is required.
Progressively open the manways from the top downwards. The water wash frequency will depend on the ambient conditions. Do not cut. CO and CO2. Whether the material has been prewet or not. Figure 15 Burn pyrophoric iron sulphide at a safe distance from units.
Each packed section should be periodically water washed to prevent dry out. At the time of the accident the tank was not in service. The critical factor. A permit must be issued with authorization by a responsible person.
The shutdown procedure must provide specific instructions for entry. The fire was extinguished after 40 minutes. Because of the absence of any other credible ignition source. Valve isolation only. It was isolated for planned repair work on a local level indicator but it still contained some hydrocarbons.
Oxygen entered the tank via the pressure and vacuum safety valve as a result of the breathing process triggered by temperature fluctuations in the days prior to the incident. Detailed procedures for laying up a unit are beyond the scope of this booklet but are available in operating manuals. Enough manways at other locations should be opened to provide adequate ventilation before personnel are allowed to enter.
Laboratory analysis indicated that FeS was present in the tank. Towers and vessels must have the top manway or vent nozzle open. The roof of the tank was blown off against the stairs of an adjacent tank.
If a foul atmosphere persists in the vessels. A detailed procedure should be set up similar to the shutdown procedure. The importance of communications between shifts and between individuals must be emphasized. There should be a requirement to follow a Management of Change process for start-ups after an emergency shutdown. The procedure should include the following consecutive phases: Some overlap of supervision between shifts can improve communications and continuity of work.
On start-ups it will be necessary to use checklists in combination with the arrow diagram. Each shift must clearly understand what has been done on prior shifts and what is expected of it. As mentioned in Chapter 2. Preparation for the start-up of a unit should begin with a complete review of the start-up procedure by the supervisors and operators. In some cases management and engineering staff may be involved in assisting the normal shift team in the start-up.
It is recommended that the operators record and initial the completion of each step. The activities of those on the unit should be coordinated with the activities of the pump houses. At the final inspection. All vessels. Failure to remove shutdown blinds and failure to install running blinds can cause serious upsets and damage.
A blind list should be included as a part of the start-up procedure. Pumps and piping should be inspected during and following these tests. Tests should be made in accordance with a procedure prepared prior to the start-up. The final inspection should include a tightness check of these closings. Furnace fireboxes should be inspected and all debris removed from them.
If the towers and other vessels and supporting structures were designed for it. Scaffolding materials. The start-up procedure. Instruments and controls should be checked. It is extremely important that critical instruments and alarms and automatic shutdown devices are checked out prior to start-up to be certain that they are reliable.
The electrical system supplying power should then be placed in service. If blinds are reinstalled after having been removed and signed out. These checks should confirm that any isolation valves that were closed during maintenance have been returned to the operational state. The safety practices specified for removing and installing blinds in section 2.
Critical valves should be broken loose to verify the valve position and operability. Shutdown systems. No blind may be installed or removed without permission from the operators. If a blind is overlooked. Figure 17 Records are important to make sure blinds are installed and removed.
The flare stack failed due to low temperature embrittlement. This will allow the system to warm up slowly. Figure 18 Steam systems must be warmed up slowly. The plant was therefore started up with two vital alarms inoperative. It had been fully isolated in order to permit entry. After the incident both instruments were found to be isolated at their top tappings.
To avoid these hazards. Two days before the incident. The knock out drum was fitted with two independent level instruments. All drains and vents on the lines inside the unit and the bypasses on all steam traps should be opened.
Had these instruments been available. Slow opening of the valves will prevent temperature shock and water hammer which could cause serious damage to lines and equipment Figure In carrying out this task. Steam systems When steam is turned into a unit. The knock out drum had been inspected internally during the shutdown. Then the steam block valves to the unit should be opened slowly.
The instrument air system can then be commissioned according to the procedure which has been established for the system. Complete draining will prevent water hammer and reduce the hazard of water in the process system during later stages of the start-up. Water systems The hazard to avoid when filling the water system is unintentionally allowing water to get into places where it can freeze Figure To prevent the accidental entry of water.
The connections may then be closed and the system pressured up to plant air pressure. Then the steam block valves to the unit should be slowly opened wide. Steaming of lines must be continued until water is out and dry steam issues from each vent and drain. Air systems Before putting the plant air system in service.
All utility station air-hose connections should be opened and. When there is no danger of freezing. Stay clear of open connections at this time. During freezing weather.
If the system has been drained. If the first method was used. Activation of the main fuel-gas line should begin with the removal of the battery-limit blind. Fuel-gas system The first demand for fuel gas will probably be for backing into equipment after purging see Section 3.
The main fuel-gas line will have been blinded at the battery-limits shutoff valve. Air must be purged from the header with As with other hydrocarbons. The purpose of these methods is to prevent freezing and damage to equipment. For this reason. The firewater system. If parts of the system have been steam heated. High points should be vented. If the temperature has been below freezing.
If steam is used in freezing weather. At this point. Care must be taken to minimize the amount of fuel gas blown to the atmosphere. The main fuel-gas line should then be cracked open and fuel gas admitted to replace the purge material.
When gas emerges. If steam was used. The burner valves should be open during the purging operation. For more information on lighting a furnace. The fuel line s to the furnace should then be purged and activated. The flow should continue until all purge material. When purging. The burner valves should not be reconnected until just before the furnace is to be fired. Fully closed systems are safer—avoid flammable releases to deck.
When the replacement of purge material is complete. Figure 20 Use fuel gas to force purge material out of the system drains. No gas should be admitted to a burner until it is ready to be lighted and a torch is in front of it. It must be shut off at the glands until each pump is put in operation. Eliminating air oxygen from any unit before hydrocarbons are introduced is most important to ensure safe startups. An example of what can happen if this is not done is shown in Figure If a gland-oil system is used.
The system should be filled in the same manner as the fuel-gas system in order to prevent mixing of hydrocarbon vapour and air. The pump-out. Miscellaneous auxiliary equipment At this time. It is desirable. If air and fuel are permitted to mix in flammable proportions. The line through which water overflows should be connected to the topmost point of a vessel Figure If the equipment and its foundations are designed for hydrostatic testing with water.
When the air has been displaced by purge gas. Steam is commonly used to purge air from units. The gas used should not have an oxygen content of more than 0. The choice of purge material depends upon the materials of which the unit is constructed. After water overflows the vessel. Figure 22 Vents or overflows should be at the high points of equipment and drains at the low points.
It is necessary to replace air to avoid the serious fire and explosion hazard likely to exist whenever air and hydrocarbons are brought together in a closed system. In some units. Drains and vents designated in the written start-up procedure must be opened before the steaming is started.
Water sides of coolers and condensers must be drained and vented. It is important that a note is made of all tappings that have been isolated in order that they are re-opened again at later stages of start-up.
Specific details require attention before and during the purging of some units. Additional steam may be injected at the base of each tower and stripper and perhaps at other points. For example. A sufficient number of vents must be left open to avoid overpressuring any part of the unit. If neoprene or buna-N is used in certain pump seals.
Drains must be kept open until only dry steam is vented and can then be closed but must be opened intermittently to remove additional condensate. All vents except those on the tower tops may be closed after there has been a free flow of steam for 20 to 30 minutes.
When steaming through heat exchangers. Figure 23 Instruments which would be damaged by steam must be shut off during steam purge. These sketches drawings of line systems can be used to show where steam is to be introduced and where drains are to be opened for water removal and vents opened for venting gas.
Bypasses around any check valves that might prevent flow of steam must be opened. The oxygen content of the purge gas entering the system must be kept as low as possible. Specific instruction on oxygen content will be shown in the start-up procedure. If tests show that oxygen is still present in a concentration greater than 1 percent by volume equivalent to 5 percent air.
The pressure should be held as low as possible to reduce the gas quantity required to complete the purge. The steam purge of a unit accomplishes three things: