No.143 WHICH DO I BELIEVE

Transcription

1 Page 1 No.143 WHICH DO I BELIEVE Also in this issue 143/2 A valve plug, made from the wrong material, disappeared in a few hours 143/3 Do not disarm more trips than you need to 143/4 Another failure of an air supply while a man is inside a vessel 143/5 Why a steam main exploded 143/6 An unusual prosecution 143/7 Electrical accidents to children in the home An Engineer s Casebook Back pressure on relief valves A problem on area classification IMPERIAL CHEMICAL INDUSTRIES LIMITED PETROCHEMICALS DIVISION

2 Page 2 143/1 ONLY FOOLS BREAK SAFETY RULES RULES ARE FOR THE GUIDANCE OF WISE MEN AND FOR STRICT OBSERVANCE BY FOOLS Newsletter 128/1 described some of the absurdities that can result when we interpret rules or codes too literally and apply them to situations not foreseen by those who wrote them. The item caused some comment and I would like tb discuss some of the points raised. Proverbs and other sayings are supposed to enshrine the accumulated wisdom of the ages. Unfortunately, they contradict each other Look before you leap and He who hesitates is lost. Similarly in safety: Only fools break safety rules. Rules are for the guidance of wise men and for strict observance by fools. Which is correct or rather when should we take the advice of the first and when should we follow the second? Let us look at an example entry to vessels and other confined spaces. The rules to be followed are laid down by the Law (Factories Act, Section 30) and in Works Rules, while Engineering Specifications describe how plants should be designed so that the rules can be followed. A feature of the Law s requirements is adequate isolation and in Petrochemicals Division we take this to mean slip-plating or physical disconnection as close to the vessel as possible. Valve isolation, even doubleblock and bleed, is not sufficient and we also insist that each vessel is isolated individually we do not allow groups of vessels to be isolated as a unit because liquid may be trapped in interconnecting pipework or in valves. Any person who enters a vessel, or allows someone else to enter a vessel, without adequate isolation is a fool and numerous incidents in earlier Newsletters (for example, 137 and 125) will convince anyone who doubts this. However, cases occasionally arise where the rules as written are difficult to observe and in these cases we should ask if an equal standard of safety can be obtained in another way. For example, if a distillation column has to be entered a very big slip-plate is needed for the overhead vapour line. If we put it next to the column there are access problems so we sometimes put it just above the condenser. Access is easier but the slip-plate is still big, expensive and difficult to handle. Sometimes therefore, we put the slip-plate below the condenser. Only a small one is now required. It is true we have isolated two vessels the still and the condenser as one, but there are no valves or depressions in the overhead line in which liquid can collect. The design of the condenser should be checked to make sure liquid cannot collect there. Slip-plates are normally designed to withstand the same pressure as the pipeline in which they are to be put, and they are normally made of the same grade of steel. However, a big slip-plate in an overhead line will only be used when the plant is shut down and swept out. It will not have to withstand full pressure or corrosive chemicals and could therefore be thinner than standard or made of mild steel. So a wise man will not follow the rules on entry regardless of circumstances but will depart from them if there are special circumstances and if doing so does not lower the standard of safety. He does not change the objective of the rules but asks if that objective can be achieved in some other way.

3 Page 3

4 Page 4 Since we cannot have everyone changing the rules all the time, departures have to be authorised by approved people. In the case of entry to vessels, in Petrochemicals Division this is normally the Assistant Works Manager. Note that exceptions cannot be authorised by members of the maintenance organisation, who may be under pressure to get the job done. However, everyone, whatever their level of job, can suggest changes. So only fools deliberately flout the intentions of safety rules, while wise men look for better ways of achieving the same objectives. Sometimes people say that if we allow any changes to a rule, people may think the rule is not important. I do not think this is so; most people can recognise the difference between ignoring a rule and achieving its objective in another way. So far as the Law is concerned, we have to work to the letter, whether it is sensible to do so or not. This is why most of the regulations made under the Health and Safety at Work Act are inductive, they say what is required but not how it is to be done. If you follow all the rules literally, you may feel that you may do too much but at least you will be safe. This is not so; the rules may be out-of-date. See Newsletter 88/1. Finally, one way of solving the problem of slip-plating the overheads line from a still is to use an internal condenser or dephlegmator. There is then no overhead vapour line, only a liquid take-off, and the problem disappears! 143/2 A VALVE PLUG, MADE FROM THE WRONG MATERIAL, DISAPPEARED IN A FEW HOURS Earlier Newsletters (136/6, 132/4, 125/3, 117/4, 113/3&4, 104/2, 98/8, 78/4, 71/6, 61/7-4 and 34/4) have described incidents which occurred because the wrong material of construction was used. Now another incident has occurred in the Division. A plug valve was supplied with a pure nickel plug instead of one made from 304L stainless steel. The valve body was made from the correct material. The valve was installed in a nitric acid line and after 4½ hours the plug had disappeared. Acid passed the stem seal and escaped, causing corrosion of external mild steel parts. The manufacturers had provided a material test certificate stating that the valve was made from 304L steel. However, the valve had not been Metascop tested (see Newsletter 71/6). We cannot rely solely on manufacturers identifications and on high hazard pipelines the material should be checked as described in Newsletter 71/6 (which also defines high hazard ). 143/3 DO NOT DISARM MORE TRIPS THAN YOU NEED TO Sometimes trips have to be disarmed during the start-up of a plant, for example, a low pressure trip may have to be disarmed until the pressure in the plant has built up to the trip setting. On one plant in the Division a pressure measurement device operated both a low pressure trip and a high pressure trip so it was disarmed during start-ups. One day the pressure rose rapidly as the result of a fault elsewhere in the plant. The high pressure trip was disarmed and could not operate, a bursting disc blew and the plant had to be shut-down. The low and high pressure trips have now been separated so that only the low pressure one need be disarmed during start-ups. Always keep disarming to the minimum. Indicate clearly which trips are disarmed.

5 Page 5 143/4 ANOTHER FAILURE OF AN AIR SUPPLY WHILE A MAN IS INSIDE A VESSEL Previous incidents were described in Newsletters 34/1, 49/1, and 1 37/2. A senior engineer recollects an incident that occurred 20 years ago. He was inspecting a 60 inch diameter gas main, wearing breathing apparatus supplied from the compressed air lines, and had moved 200 feet from the end when his mask started to fill with water. He pulled it off, held his breath and walked quickly out of the main. He discovered that the air line had been connected to the bottom of the compressed air main instead of the top. As a young engineer with very little works experience he had assumed that the Safety People and the works instructions would do all that was required and that he could enter the main with confidence. It is the responsibility of the Process Team, before issuing an entry certificate or clearance certificate, to make sure that the job has been properly prepared. Nevertheless it is a good idea for the man doing the job to carry out checks when he can. The man who asks to see the clearance certificate, checks that valves are locked, slip-plates fitted, breathing apparatus in order and so on, is not being awkward he is making a positive contribution to safety. Reminder: Another incident in which a compressed air line was connected to the bottom of a main was described in Newsletter 49/1. 143/5 A LOOK BACK AT NEWSLETTER 43 (August 1972) A steam main exploded A 10 inch diameter steam main operating at 600 psi suddenly ruptured, injuring several men. The failure occurred because the main was grossly overstressed by water hammer. The incident occurred soon after a shut-down. The main had been up to operating pressure for several days but there was no flow along it. The failure occurred while the operators were starting the flow of steam. The steam trap was known to be faulty and had been isolated for maintenance. It is believed that the amount of live steam entering the condensate recovery system had made it necessary to gag the remaining valves to the point of closure. The arrangement is shown below.

6 Page 6 Condensate had thus accumulated in the steam main. When a flow of steam was started this caused the condensate to move with very great force, fracturing the main. A full report of the incident, No ,760/A, may be obtained from Division Reports Centres. A number of recommendations are made about steam trapping on steam mains and the report highlights the hazards of steam mains with no flow and of operating with pressurised condensate recovery systems. 143/6 WHAT THE LAW SAYS No 22 A wheel came off a car, narrowly missing several pedestrians. The wheels had not been put back properly after balancing. The police decided not to prosecute the company which did the work but the Safety Officer to the local council prosecuted them under the Health and Safety at Work Act, Section 3(1), for failing to take reasonable care for the safety of people not in their employ. The company was fined 300 on this charge and 600 on a number of other charges which resulted from the Safety Officer s visit. From Protection, May 1980, p /7 ACCIDENTS IN THE HOME An American study of electrical burns to children showed that wall sockets were not the danger point that everyone thought. Far more injuries arose from children sucking extension cord sockets or biting the cord itself. Care in the Home, Oct 1980, p 5. So if you have young children or grandchildren, don t leave electrical equipment plugged in when not in use. Pull the plug out. 143/8 UNUSUAL ACCIDENTS No 103 A man choked to death during a contest to see how far a ping-pong ball could be blown. He breathed in by mistake. Mond Division Safety Report, December /9 RECENT PUBLICATION Process alarm systems as a monitoring tool for the operator, by D Kortlandt and H Kragt, one of the papers presented at the Third International Symposium on Loss Prevention and Safety Promotion in the Process Industries, analyses the alarms on two plants, a fertilizer plant and a polyethylene plant. On both plants only 7% of the alarm signals were followed by action. However, 45% of the alarm signals occurred after an action by the operator and told him the results of his actions. 48% of the alarms resulted neither in actions nor from them. Not all these alarms may be unnecessary sometimes operators need to be aware of process conditions even though they are corrected automatically. However, perhaps some of them could be removed. For more information on any item in this Newsletter please phone P.2845 or write to us at Wilton. If you do not see this newsletter regularly and would like your own copy, please ask us to add your name to the circulation list. January 1981

7 Page 7 An Engineers Casebook No 43 BACKPRESSURE ON RELIEF VALVES When a relief valve which discharges to atmosphere lifts, the initial backpressure acting on the disc and in the discharge pipe is nil. As flow builds up the exhaust pressure rises to expel the discharge from the tail pipe to unrestricted atmospheric pressure. This has the effect of reducing the pressure differential across the valve and results in a rise in the pressure in the vessel to sustain the dynamic flow conditions determined by the nozzle area of the valve and the fluid properties. When equilibrium is reached the amount by which the set pressure is exceeded, referred to as the accumulation, is limited by pressure vessel Codes to 110% of the design pressure at the design temperature. Most valve discharge capacities are sized on the basis of 3% accumulation. The designer must ensure that the relief valve tail pipe is large enough and short enough so as not to restrict the discharge capacity of the valve. Some relief valves discharge into an enclosed gathering system, such as a flare header, or into a closed vessel at some lower pressure which may itself by fitted with a relief valve. In these cases, if for example the flare system is already receiving discharge(s) from other valves or the closed vessel pressure is greater than atmospheric, an additional pressure will act on the relief valve seat. Unless steps are taken to correct this situation the relief valve will lift heavy, i.e., at a pressure greater than the set pressure, the extra pressure being equal to the backpressure in the system. To prevent this happening balanced valves must be used. Balancing is achieved either by using a flexible bellows between the top of the valve disc and the valve body with the inside of the bellows vented to atmosphere via the valve bonnet or, through the use of a differential area piston. Either solution ensures that the valve will always lift at the correct set pressure irrespective of the pressure in the exhaust piping. Balanced valves may, of course, be used for valves which discharge directly to atmosphere. Conventional unbalanced valves must not be used in closed systems in which there may be a backpressure. The integrity of any bellows, or impulse piping in the case of valves with differential area pistons, must be checked when balanced valves are overhauled. E H Frank This is the last contribution by Harland Frank who retired at the end of November On behalf of all our readers, we wish him a long and happy retirement. 7

8 Page 8 A PROBLEM ON AREA CLASSIFICATION A fuel gas pipeline ran in a pipe-trench under a road bridge inside a factory. There was a flanged joint, fitted with a compressed asbestos fibre (caf) gasket near the edge of the bridge and 2.5 m from the edge of the roadway. For a caf gasket in a fuel gas line there is normally a Zone 2 area for a radius of 3 m (See ICI Engineering Codes and Regulations, Group B, Vol 1.5. Electrical Installations in Flammable Atmospheres, Table 1, Item 3(b).) Road vehicles, unless specially modified, are a source of ignition and are not therefore allowed unrestricted access to Zone 2 areas. Access to this road bridge had been restricted but the factory wanted to open the bridge to unrestricted traffic. Two courses of action were suggested: 1. Replace the caf gasket by a spiral wound one. This would reduce the radius of the Zone 2 area to 1m. However this would involve an inconvenient and expensive shut-down of the line and the joint, which had not leaked in 10 years, might leak once it had been disturbed. 2. Leave well alone. Forget about the area classification code for once; it is for guidance; it is not law. On the other hand this might give the impression that we disregard our codes as soon as it becomes inconvenient to follow them. What would you do? Answer next month.