Ammonia Vapor Cloud from Vent Stack of Safety Relief Valve Collector System

The collector system for pressure relief valves of the ammonia separators and refrigeration system was flooded with liquid ammonia because a small PSV was stuck open by a dirt particle. While draining the collector pipe a cloud of ammonia spray was ejected from the vent stack. This paper explains the mechanism of flash eruption while draining a vertical pipe filled with ammonia, the identification of other potential causes and preventive measures thereof.

Hans H. Wagner INEOS, Cologne

work spans 60 manufacturing facilities in 13 Introduction countries throughout the world. INEOS is a young company which has grown through a se- he INEOS ammonia production plant is a ries of related acquisitions to become a leading Kellogg design built in 1966 with 300,000 chemical company with sales today around $ MT/YR capacity. From its original naph- T 28.4 billion. tha feedstock design it was converted to natural

gas feed. Several revamps improved energy effi- This paper encourages you to answer questions ciency and capacity to increase competitiveness as: amongst European ammonia producers. • Have you considered the effects of relief The plant is integrated into the INEOS site at valves not reseating in hazard studies and Cologne, Germany. The site employs around Safety Health Environmental Assurance re- 1,800 people in the manufacture of petrochemi- views? cal products such as ethylene, ethylene oxide, • If you have low boiling point liquids on your ethylene glycol, polyethylene, propylene, pro- asset, do your teams fully understand the be- pylene oxide, propylene glycols, acrylonitrile, havior and effects of draining a low boiling butadiene, C4 oligomers, isoamylene, benzene, point liquid? toluene, ammonia and nitric acid. • Do you have suitable detection systems to know that material is flowing into vent sys- INEOS is a global manufacturer of petrochemi- tems? cals, specialty chemicals and oil products. It • Do you have liquids being relieved into vent comprises 15 businesses each with a major systems designed for gases / vapors? chemical company heritage. Its production net-

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Incident description Sequence of events The plant was in restart after the 2009 revision Reduction of the new catalyst was ongoing and shutdown with exchange of the ammonia synthe- some water containing ammonia collected in the sis catalyst. On 25 Nov at 3 am ca. 700 kg of primary separator D-123 and subsequently in boiling liquid ammonia sprayed out of the vent flash drum D-140. This start up product con- stack at 42 m height. The following description tained water from the synthesis catalyst reduc- was reconstructed from recorded plant data. tion. It was planned to send it to the dedicated off spec tank directly instead of contaminating the refrigeration system with too much water. Setup of the product collection and distribution system On 24 Nov at 11:53 pm pump P-107A was started for the first time for about 4 min.. The In normal operation ammonia is condensed from pump was started with V1240-1 closed, and then the synthesis gas loop in primary (post reactor) ammonia flow to the tank was adjusted by open- and secondary (post recycle compressor) separa- ing the valve. When the level in D124 was suffi- tor drums D-123 and D-122. From here the ciently reduced the valve was closed and the product is routed via the flash drum D-140 to the pump stopped. suction side of centrifugal pump P-107 and also

to the refrigeration system. In the refrigeration One hour later at 00:58 am the pump was again system the product is evaporated and recom- started for a few minutes. Another two hours lat- pressed. Recompressed and condensed ammonia er ice on the collector system of the safety relief gathers in drum D-124 and joins the stream com- valves was noticed, indicating that it was filled ing directly from D-140. The cold side of the re- with ammonia. The operators assumed that the frigeration system accumulates water. Water safety relief valve between P-107 and V1240-1 concentration is controlled by purging some cold had opened and blocked in the pump to isolate ammonia with pump P-109. When the refrigera- the piping on the pressure side. tion system is in recycle mode (e.g. at plant start

up) then P-109 is routed to D-124. Level control The collector system has a manual drain valve in D-124 is done by adjusting the flow with level leading to the DeNOx feed in the convection control valve V1240-1. A schematic drawing of zone of the primary reformer. At 03.08 am the the installation is shown in Figure 1. operators started to drain the system.

Five minutes later a cloud of ammonia suddenly erupted from the top of the vent stack of the collector system. It was described that it fell down like a wall just some meters north of the stack (there was mild wind from the south) and then flowed on ground level through the adjacent acrylonitrile plant (Figure 2). Ca. 100 m away two field operators were caught by the cloud. They suffered irritation of the breathing appara- tus due to ammonia inhalation. The cloud then dissolved very quickly. The rescue team and mobile environmental analysis lab could not no- Figure 1: Ammonia product flow diagram sche- tice any traces of ammonia when they arrived matic only a few minutes later

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shut down system is only 250 ms but the maximum recorded pressure was 22.3 bar. When the piping is blocked in it is protected against over pressure from thermal expansion by a small safety relief valve (DN15/DN25) with set point 25 bar. The correct opening pressure of the valve had been documented in the testing protocol. Al- though the maximum recorded pressure was sig- nificantly lower the safety relief valve had opened during the second time of running the pump. This is explained with a transient pressure shock which under certain condition can also occur when starting centrifugal pumps against blocked outlet.

Other than at the first run the safety valve did not reseat the second time. Product from P-109 kept on flowing through P-107 and expanded through the stuck open safety valve into the collector system (Figure 5). After the incident the safety valve was disassembled and a metal particle was found in the annular gap of the valve seating (Figure 3, Figure 4).

With the maximum free cross section area of the Figure 2: estimated spread of ammonia aerosol safety valve of 44.2 mm2 and the differential cloud pressure from the upstream system of 6.6 bar the flow of liquid ammonia is estimated to have been 3 Direct cause 6.3 m within the two hours until the pump P- 107 was blocked in (Figure 5). The temperature reading on the pump discharge piping T2010-1 showed ambient temperature before starting the pump. It then fell during the time of pumping and rose again afterwards because of heat intake through the non-insulated piping system. From the arrangement of piping on the suction side it was later assumed that mainly a mixture of warm ammonia from D-124 and cold ammonia from P-109 was sent to the tank. After the pump was started and stopped for the second time the temperature did not rise to ambient but it fell to - 18 °C.

The pump was started with V1240-1 closed which resulted in a short hydraulic shock. The piping on the pressure side is protected against Figure 3: Metal particle stuck in the safety valve high pressure by a 2003 pressure high shut down seating of the pump at 24.5 bar. Cycle time of the safety

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was filled with ammonia at 7.6 bar vapor temperature. With the sudden release of pressure this volume had flash evaporation. It condensed again when mixed with the under cooled product from P-107A and the equilibrium temperature adjusted to ca. 0 °C. In this step further ammonia at -18 °C was pushed through the leaking valve.

When the draining of the stack was started the hydraulic pressure on the system fell together with the level. The ammonia in the pipe system was locally superheated in different conditions. The pressure fell continuously about 1.6 - 1.8 bar over several minutes while flashing ammonia pushed on the stack and ejected the liquid ammonia there. This pressure decrease relates to a decrease in vent stack level of ca. 23 - 26 m or 1200 l volume (Figure 7). Because some of the ammonia was drained to the reformer it is estimated that ca. 700 kg were released to atmos- phere (Figure 7).

The water content of this start up product was not quantified but it contributed to the severity of the incident. From the behavior of the vapor cloud quickly falling to the ground an aerosol phase must have been present. Simulations of this release scenario with pure ammonia and the present weather conditions indicated that the ejected ammonia would have been completely evaporated. In this case the cloud would have distributed at the level of the vent stack opening and would have been diluted when reaching the

Figure 4: Drawing of a VSE0 safety valve ground much further away.

The horizontal collector pipe and the vent stack both are DN 250 pipes. The total holdup of the Immediate corrective actions system is estimated to 6.5 m3. With the ammonia Direct cause for the incident was a pressure peak from the open safety valve the stack was almost with opening of the safety valve and the unno- completely filled to the top. This is in line with ticed filling of the collector system over two the operators reporting, that the stack was cov- hours. Impurities in the system leading to the ered with ice to 1-2 m from the top. When the blockage of the safety valve cannot be ruled out pump was blocked and the flow from upstream in future service and must not result in unaccept- stopped the pressure quickly fell to 2.7 able consequences. The following immediate barg which relates to the hydraulic head of the measures were taken to prevent this from hap- ammonia in the vent stack (Figure 6). The piping pening again. in the discharge side of the spare pump P-107B

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vent stack

ammonia from P-109 T = -22°C PSH 2oo3 P- 107A at 24,5 barg 25 barg drain to reformer D- 140 I- 31 I-32 I-30 P-107 B D- 124 V1240-1

Figure 5: Ccold ammonia below boiling temperature pumped to storage, leak of 3.7 m3/h through safety valve after closing V1240-1

vent stack

ammonia from P-109 T = -22°C PSH 2oo3 P- 107A at 24,5 barg 25 barg drain to reformer D- 140 I-27 I- 28 I-29 P-107 B D- 124 V1240-1

Figure 6: decrease of pressure after pipe was blocked, expansion of warm ammonia from spare pumps P-107B

vent stack

ammonia from P -109 T = -22°C PSH 2oo3 P -107A 25 barg at 24,5 barg drain to reformer D-140 I-24 I- 25 I-26

P -107 B D-124 V1240 -1

Figure 7: draining of vent stack with related decrease of hydraulic pressure on horizontal collector pipe, flashing of superheated ammonia ejecting the inventory of the vent stack

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1. 6.6 bar g upstream system pressure, -18 C from mixed streams P-109 and D-124 2. blocking the pressure side of P-107A with pressure decrease to hydraulic head of filled stack 3. evaporation and diffusion of superheated ammonia from P-107 discharge piping 4. decrease of hydraulic head after draining of the vent stack

Figure 8: ammonia vapour diagram with pressure and temperature during the sequence of events

P-107A/B may only be started with the discharge side V1240-1 partially opened to avoid pressure Systematic safety analysis of the peaks. After every startup of the pump the safety collector system valve and downstream collector system is visual- ly inspected for indication of internal of leakage. Our learning from this incident was that the present ammonia safety relief valve collector A pressure gauge was installed in the low point system is not suitable for taking relevant of the horizontal collector pipe. This measure- amounts of liquid. A range of different scenarios ment is equipped with a high alarm indicating were identified which could still lead to this. The buildup of level in the system. strategy of the resulting project was to eliminate such scenarios rather than designing a new col- Several other scenarios were identified where lector system with safe separation and disposal liquid ammonia could enter the system. If this of liquids. liquid is not carried over to the vertical vent stack the horizontal pipe can be safely drained. A systematic safety analysis equivalent to This needs to be done very slowly to allow the HAZOP was carried out to identify scenarios ammonia to cool to equilibrium temperature. leading to a release of liquid ammonia to the vent system. Some of those scenarios and pre-

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ventive measures are summarized hereafter. One In two cases where rerouting to the process is not example is described in more detail. applicable the thermal relief valves are connected to the knock out drum of the flare system. This knock out drum has low pressure steam Identification of scenarios leading to heating to evaporate liquid ammonia. Failure of a release of liquid ammonia to the vent valve is detected by a temperature low and level system high alarm. With the latest shut down the two safety valves were equipped with an orifice in Small amounts of ammonia from drain points the discharge pipe. By doing this the released Several drain points from level gauge parallel liquid ammonia is limited in flow. vessels are connected to the collector system with double block valves. In case of leakage of Large amounts of ammonia resulting from those valves the downstream system would be high pressure as a consequence of high level filled unnoticed. All those drain points were se- Several scenarios with separator drums were parated by blanking plates. identified where an excess of the allowed level would lead to high pressure and opening of a The drain lines of the ammonia refrigeration safety valve. Those safety valves are typically compressor were even connected directly to the designed for gas phase and will allow high flow vertical vent stack. When in use it is quite likely rates. to fill the low point siphon between the vent stack and the horizontal collector pipe separating Some of those separator drums are the surge it from atmospheric pressure. These drain lines drums of the four stage refrigeration compressor will be rerouted to the collector pipe and blocked and the synthesis gas recycle compressor (prima- with blanking plates. ry product separator). As high level would lead to liquid carry over and compressor damage level Liquid ammonia from drain points can safely be high trips in reliable and safe installation were disposed in several sections of the ammonia already in place. However, with the new learn- plant. These are the DeNOx section of the prima- ing’s from this incident those protection circuits ry reformer, the purge gas scrubber for aqueous are now classified as SIL 2 as they not only pre- ammonia production (slowly with temperature vent compressor damage but also release of liq- and pressure control), the flare system with uid ammonia. steam traced blow down drum or mobile disposal systems. The safety valves of the refrigeration compressor surge drums have a temperature low alarm in the Small amounts of ammonia from thermal re- discharge pipe which would also indicate gas relief valves lease or leakage. Some more thermal relief valves for pipe protection are connected to the system. Those relief The product flash drum D - 140 collects three valve discharge lines can be rerouted from the streams of product from primary and secondary vent system to other places in the process, where separator and the high pressure purge gas the design pressure is higher than on the pro- separator. Flash gas is sent to a water scrubber. tected pipe and a gas phase is present. Failure of the level control in D-140 would lead to liquid carry over to the scrubber. This would The safety valve causing this incident will be re- increase differential pressure in the gas outlet placed with an orifice going back to the suction beyond capacity of the pressure control valve. A side of the pump and a block valve in locked two phase flow through the safety relief valve open position. would result.

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The overfill protection for ammonia flash drum D-140 is SIL 2 classified. This demanded for installation of an additional redundant level gauge and double block valves in the ammonia feed line. Blocking the ammonia flow to the flash drum will lead to fast level increase in the upstream separators. Therefore the new shutdown circuit will not only block the feed but also trip the synthesis gas compressor and thus ammonia production.

Overfill protection of secondary separator D- 122 Figure 10: systematic safety analysis of separator drum D-122 part 2 In this example a systematic identification of a safety relevant condition is shown. The analysis This protection circuit was classified in safety in- starts with the scenario of a single failure. In this tegrity levels SIL according to the severity of the case it is the failure of the level control circuit in incident to be protected, the probability of occur- the secondary separator D-122. Under certain rence and the possibility to prevent the incident conditions and when unnoticed by the plant op- with independent measures. The INEOS Cologne erator (this is not regarded as second failure) the site standard for such systematic classification is event will put the plant in a safety relevant con- the application of a risk graph which led to SIL dition. 2. In order to meet the requirements of a certain SIL the installation has to meet certain criteria In this case it was decided to prevent the safety for reliability. In the case of overfill protection relevant condition by a level high trip of the for D-122 two new independent level gauges ammonia synthesis gas compressor. This will were needed to upgrade the sensors. On the in- stop the ammonia production before the pressure itiator side the shutdown system of the steam can increase to open the safety relief valve. turbine on the synthesis gas compressor was improved with new valves for the release of control oil pressure on the steam shut down valves.

Conclusion From the failure of a very small relief valve against over pressure from thermal expansion an incident resulted in which two people almost were killed. The analysis showed that the system that was designed to prevent hazardous situations actually was a threat in itself. The relatively small incident led to the identification of much Figure 9: systematic safety analysis of separator drum D-122 part 1 more severe scenarios.

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