MULTI-STAGE OVER PRESSURE PROTECTION AND PRODUCT CONTAINMENT ON HIGH PRESSURE POLYMERISATION REACTORS P.W. Thomas* The manufacture of ethylene-vinyl acetate-vinyl chloride polymer emulsions in equipment with design pressures up to 100 bar has led to the development of a multi-stage over pressure protection system designed to reduce the emission to atmosphere of potentially hazardous material whilst adequately protecting the equipment. The combination of operator intervention alarms and automatic systems with bursting disc protected relief valves gives a high level of integrity to the plant whilst allowing control of any over pressurisation situation. Downstream vent tanks are used to largely prevent liquid discharges whilst flammable gases are vented through a high level stack with inert gas dilution. INTRODUCTION The protection of chemical plant from over-pressurisation entails satisfying the contradictory requirements of preventing the emission of hazardous products and ensuring that the plant cannot suffer catastrophic failure. The systems described below have been developed over the past ten years primarily on emulsion polymerisation plant producing vinyl acetate - ethylene - vinyl chloride copolymers in vessels with design pressures up to 100 bar. However, basically the same principles can be applied to many other processes. With hazardous feed stocks such as ethylene or vinyl chloride, emphasis has been on minimising the emission of unreacted materials to atmosphere, the dispersion of any such emissions to prevent flammable cloud formation, and the containment of the liquid phase within the plant. The same principles will apply to many processes within the chemical industry where protection of the environment is paramount. In the examples of systems described in the paper, operating levels of the various stages of protection are expressed as a percentage of the system design pressure, rather than in engineering pressure units. This is intended to illustrate the principles of operation and to assist in their application to equipment with differing pressure ratings. * Vinyl Products Limited, Eastford Road, Warrington, WA4 6HG. 229
DEVELOPMENT OF THE RELIEF SYSTEM In 1974 Vinyl Products Limited installed it's first production scale high pressure emulsion polymerisation vessel at Warrington. The design of the vessel was based on pilot plant information and over-pressure protection was simple but adequate for the process. (A schematic of the plant is given in Fig. l). Major protection for the vessel was provided by a conventional bursting disc arrangement mounted on a stack venting directly to the atmosphere about 12 metres above ground level. To prevent loss of material under noncritical conditions, such as a premature failure of the bursting disc or pinholing, a second disc of the same pressure rating was installed in series with the first. An interstage pressure detector and alarm were fitted to give a warning of a primary disc failure. Two levels of vessel high-pressure alarm were installed, the lower being an operator attention alarm, and the higher being an automatic device, which closed all the feed lines to the vessel and opened a small-diameter vent valve to atmosphere also about 12 metres above ground level. The automatic vent valve reshut as soon as the vessel pressure dropped below set point, thus minimising emission. The two high-pressure alarms were activated by two independent pressure transmission devices. In operation a high-pressure alarm would alert an operator who would manually stop feeds into the vessel, and vent excess pressure to the vent tank via a control valve. The vent tank was a non-pressure vessel three times larger than the reaction vessel; thus a significant volume of gas could be vented to the vent tank, to be nitrogen blanketed and purged off through the flame arrestor. The vent tank also served to separate liquid and gas and prevent the exhaust of liquid into the atmosphere. Experience, however, indicated that the system had several drawbacks and was not totally adequate for the process. Defects in System 1 1) The dual discs in series offered little protection against premature failure, since debris and/or the shock wave from the lower disc damaged the upper disc and caused it also to fail. 2) A non-pressure vent tank meant that the venting of gas through the flame arrestor could not be controlled, owing to the risk of damage to the tank. 3) The advent of vinyl chloride as a feed stock required a much greater control of emissions to the atmosphere, i.e. concentrations of parts per million at ground level, rather than quantities of flammables, which though well below the explosive limit were many times above the hygiene standard. 4) The venting of liquid-gas mixture from high pressures generated large volumes of low density foam that could not be contained in open vessels. System 2 This system incorporates much that has been learnt from the early system and is shown schematically in Fig. 2. The major difference is the adoption of a dual parallel bursting disc system with a relief valve mounted over the primary disc. The vent tank is now a pressure vessel and is equipped with a number of pressure protection devices; 230
the vent gases are now carried to a 40 metre high vent stack with oxygen analysis and automatic nitrogen purging. In operation, the vent tank is kept under vacuum at a pressure of about 0.15 bar absolute. In the event of a high pressure alarm, the operator manually shuts all feeds to the reaction vessel, and via a pressure control valve vents the gas space to the vent tank. The vent tank remains closed until the pressure rises to above atmospheric, when the vent valve opens allowing gas to pass through to the high-level vent stack. A foam alarm on the vent tank top prevents liquids being entrained with the gas to the vent stack by closing the vent valve. When the pressure in the vent tank rises to 75% of its burstingdisc rating, the inlet valve closes, preventing further pressurisation until the foam alarm clears and venting resumes. With vent tank capacity three times that of the reactor and the vent tank normally evacuated during the process, the reaction vessel pressure can be reduced considerably before a combination of vent tank pressure and high foam level causes the inlet valve to close. In the event of the pressure continuing to rise the reaction vessel extra-highpressure alarm system can still vent to atmosphere. If this system fails then the bursting disc under the relief valve will rupture giving a controlled discharge via the relief valve. Ultimate vessel safety is ensured should all the foregoing fail, by a single high pressure disc set to burst at 1.3 times the vessel design pressure. It is now thought that the refined system 2 offers the most effective method of containing the products of overpressurisation while preventing catastrophic plant failure. THE EFFECTS OF SAFETY DEVICES ON WORKING PRESSURES The system described above, with it's four stages of pressure relief, requires discreet pressure steps between each stage. Codes of Practice for vessel protection lay down guide lines for the rating of pressure relief devices in relation to the design pressure of equipment, and the limitations on safety devices also contribute to a general lowering of actual maximum working pressures. In satisfying the conditions of the vessel design code BS 55OO relating to pressure relief devices and the application of BS 2915, the relevant paragraphs are as follows:- BS.5500 J.8.1. BS.550O J.8.5. Relief valves shall be set to operate at a pressure not exceeding the design pressure of the vessel at the operating temperature, except as permitted in J.8.2. In the case of bursting discs fitted in parallel with relief valves to protect a vessel against the consequences of explosion hazard, the bursting disc shall be rated at atmospheric temperature to burst at a pressure not greater than 1.3 times the design pressure. The relief valve shall be set in accordance with J.8.1. 231
BS.2915 5.7. BS.2915 5.8. BS.2915 5.9. Bursting discs may also be used in combination with relief valves: (a) in series to protect the valve against corrosion or to prevent leakage; (b) in series to prevent total loss of contents from the pressure system in the event of the disc bursting; (c) in parallel as an additional safeguard. A bursting disc fitted in series with a relief valve shall meet the following requirements. (a) Shall be specified at the disc operating temperature to burst at a pressure not exceeding the design pressure of the vessel. (b) Shall have a sufficient opening after rupture to prevent interference with the proper functioning of the relief valve. (d) Shall, if fitted upstream of the relief valve, have the system so designed that particles of a disintegrating disc shall not prevent the relief valve from opening. A bursting disc fitted in parallel with a relief valve as an additional safeguard, such as to protect a vessel against the consequences of rapid rises in pressure, shall be specified at the disc operating temperature to burst at a pressure not exceeding 1.3 times the vessel design pressure. It is significant that J.8.5. states that the bursting disc will be rated at atmospheric temperature to burst at a pressure not greater than 1.3 times the design pressure, as opposed to BS.2915, 5.9. which merely states "at the disc operating temperature". Thus in a vessel designed for a working temperature of 100 C, the primary relief system can be specified to operate at 100 C, but the secondary relief system must operate no higher than 1.3 times the design pressure at ambient temperature, to satisfy BS.5500. This can lead to a significant loss of pressure rating if the vessel works at elevated temperatures due to the temperature co-efficient of the disc material. Additionally, both BS.5500 and BS.2915 specify that the maximum burst pressure of the safety disc is not greater than either the design pressure for the primary disc, or 1.3 times the design pressure for the secondary disc. This definition of maximum burst pressure can also seriously affect the working pressure of the vessel, since bursting discs are typically rated ± 5% on burst pressure; thus the rated pressure of the primary disc can be no more than 95.2% of vessel design pressure, while manufacturing tolerance allows disc failure as low as 90.5% with a maximum of 10O%. Similarly, at ambient temperature the secondary disc rating would be 123.8%, with a burst range of 117.6-13O%. Luckily, disc manufacturers can offer some assistance by supplying discs with a smaller tolerance above the rated pressure than below, or by providing ± 2% discs which significantly reduce the possibility of overstressing the secondary disc before failure of the primary disc. The choice of disc construction can have a pronounced effect on maximum working pressure. Conventional (pressure-under-the-dome) bursting discs typically have a lower tolerance to pressurisation close to the burst pressure than reverse-buckling or tear-out discs. Cycling of pressure also affects bursting disc rating adversely, leading to creep on conventional discs with 232
consequent premature failure. Additionally, the bursting disc under the relief valve must not fragment on failure, otherwise damage to the relief valve seat and nozzle will occur, causing failure to contain the vessel contents after operation. Some of the factors described above can best be illustrated by considering their effect on the working pressure of a reaction vessel with a design pressure of 10O units at a design temperature of 100 C. See Table 1. The assumptions made are that a vessel basically as illustrated in Figure 2 is equipped with reverse-buckling bursting discs, with a pressure tolerance of ± 5% on rated pressure and with a 10% temperature co-efficient between 20 C and 100 C. Several interesting points arise from studying the figures and considering the problems that are posed. The first point of note is the effect of temperature on the two bursting discs. The primary disc is designed to operate at the design temperature, and so at ambient temperature will operate at a higher pressure. For stainless steel discs the difference can frequently be higher than 10% over even a small temperature range like 20 C to 100 C. Conversely at elevated temperature the ultimate disc will apparently fail earlier, being specified to burst at ambient temperature to satisfy BS.5500. (Points 1-4) The effect of temperature and pressure tolerances combine to give an apparent overlap where the primary disc can actually fail later than the secondary disc. Clearly the vessel cannot be hot and cold at the same time, but the design of the bursting disc mountings and their relation to the vessel must be dealt with thoroughly to prevent the discs experiencing widely differing temperatures. These limitations are imposed by the Codes of Practice relating to vessel protection and one can only improve the situation by installing closer tolerance discs manufactured from materials with better temperature co-efficients. In a situation of rapidly rising pressure and temperature, such as can be experienced in a polymerisation reactor, the vessel high-pressure relief system must have time to operate. It is, therefore, necessary to reduce the set pressure for this operation and allow 4-5% reduction below the minimum likely burst pressure of the primary disc. (Point 6) Similarly, for the reactor high-pressure alarm, which requires the intervention of an operator to take remedial action, a further 7% offset has been allowed. Although this may be generous, the accuracy of detection instruments and their operating tolerance can again reduce this differential significantly. Once again the operator needs time to intervene to prevent the next stage in the relief system from operating. (Point 7) From these considerations it is apparent that the realistic maximum working pressure of the vessel has effectively been reduced to 75% of the original vessel design pressure. If working pressures are closer to the alarm setting, only minor process variations are needed to trip the pressure alarm and render it less effective, owing to familiarity with its repeated operation "breeding contempt". (Point 8) Application of the system to computer controlled plants In computer controlled plants the principles of operation are modified slightly because constant manning of the control station is no longer 233
required. The operator may now be watching more than one control station, with the computer carrying out all the control and sequencing functions. It is also felt that it could be hazardous to give computer control to a single device such as an automatic vent valve, since computer failure can occasionally have a positive control action causing the valve to open. Hard-wired devices can be more effectively constructed to give safe failure on signal, air or electrical break down. For this reason the computer system has a number of extra steps. The recipe for each product contains information on the normal process pressure attained during the cycle; this is used to trigger a process deviation alarm should the pressure rise significantly above this figure, typically between 5 and 10%. This first alarm will alert the operator to consult the computer, but will not necessarily trigger any further action. If the pressure continues to rise, the normal vessel high-pressure alarm operates, shutting off feeds to the vessel, suspending the sequences of operation and raising a hard-wired alarm as well as a computer alarm. Pressure increase to the high state will cause the computer to vent via a block and control valve to the vent tank, which is equipped with the same safety devices as System 2; hard-wired and computer alarms will also be activated. Should the computer fail to reduce the pressure correctly, manual control of the venting system is possible, with finally a separate key switch vent valve allowing the operator to vent directly to atmosphere, so reducing the vessel pressure and averting bursting disc failure. To ensure the integrity of the system, dual pressure sensing units are employed, and the computer will suspend sequences if the two elements indicate different pressures. 234
SUMMARY The systems described represent an effective and tested method of applying multi-stage over-pressure protection to high-pressure chemical plant, with the emphasis on containment prior to the final stages of pressure relief. The systems can be effectively applied to lower pressure plant, although problems can arise from the smaller pressure changes between each step. The systems require the employment of accurate and repeatable measuring elements for the operation of each stage. Finally, it is our firm belief that bursting discs should never be called upon to burst under process conditions; the safety of the plant should be ensured by the preliminary pressure relief systems, and relief to atmosphere should be a secondary element within those systems. ACKNOWLEDGEMENTS The author wishes to acknowledge that the development of the systems described has been a joint effort by members of the engineering department at Vinyl Products Limited, and would like to thank the following for their assistance in the preparation of this paper. Mr. A.T. Anderson - Senior Projects Manager Mr. I.M. Thorne - Project Engineer Dr. R. Straus - Divisional Engineering Manager (Retired) Miss. C. Tappenden - Engineering Dept. Secretary References BS.550O : 1976 British Standards Institution BS.2915 : 1974 British Standards Institution 235
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Fig. 2 Refined System 2 Design 238