Vent collection system

As vent collection systems normally contain vapor/air mixtures, they are inherently unsafe. They normally operate outside the flammable range, and precautions are taken to prevent them from entering it, but it is difficult to think of everything that might go wrong. For example, an explosion occurred in a system that collected flarmnable vapor and air from the vents on a number of tanks and fed the mixture into a furnace. The system was designed to run at 10% of the lower explosion limit, but when the system was isolated in error, the vapor concentration rose. When the flow was restored, a plug of rich gas was fed into the furnace, where it mixed with air and exploded [17]. Reference 34 describes ten other incidents. 

A fire in a bulk storage facility at Coode Island, Melbourne, Australia, in August 1991 caused extensive damage and many complaints about the pollution caused by the smoke plume, but no injuries. The tank vents were connected together and piped to a carbon bed vapor recovery system. There were no flame arrestors in the pipework. Whatever the cause of the initial fire or explosion, the vent collection system provided a means of spreading the fire from one tank to another. 

The explosion would not have occurred if the recommendations made after the first explosion had been followed—if there had been a flow of inert gas into the vent collection system and if the atmosphere inside had been tested regularly for oxygen. 

Detonation arresters are typically used in conjunction with other measures to decrease the risk of flame propagation. For example, in vapor control systems the vapor is often enriched, diluted, or inerted, with appropriate instrumentation and control [5]. In cases where ignition sources are present or predictable (such as most vapor destruct systems), the detonation arrester is used as a last-resort method anticipating possible failure of vapor composition control. Where vent collection systems have several vapor/oxidant sources, stream compositions can be highly variable and this can be additionally complicated when upset conditions are considered. It is often cost-effective to perform hazard analyses such as HAZOP or fault... 

The outside surfaces of refrigerated storage vessels are thermally insulated to assist in maintaining the desired storage temperature. Thermal excursions increase product vaporization, raise vessel pressure, and increase the amount of gas relieved to atmosphere orto a relief vent collection system. Fire exposure of a refrigerated vessel can increase product temperature and vessel pressure, possibly exceeding the capacity of relief valves or relief vent collection system, and could result in vessel rupture with major fire or explosion consequences. 

However, the vent collection system that served the low-pressure feed tank also served a nearby distillation column that operated at nearly 15 psig and contained a toxic component. Physically the low-pressure tank was on the vent piping between the distillation column and the flare stack. During a distillation column upset, there was an increased flow to the flare. The vent header pressure increased. The vent piping diameter between the distillation column and the flare proved to be too small. Toxic fumes from the higher pressure system column were released from the low-pressure tank emergency tiffing lid. 

Make specific allowances for distribution systems within the process area for the various utilities cooling water, high/low pressure steam/condensate, service water/air, nitrogen, instrument air, vent collection systems, etc. Lengths and sizes must be determined based on arrangement and flows. Allow at least 30 ft. of pipe from distribution system to each individual user. 

When water started to react with MIC, no control of the temperature/pressure started, because it was late at night and the operational staff was reduced to a minimum, and the MIC tank s alarms had not worked for 4 years. The gas leakage followed, for the first 30 min, approximately the inverse route of water entrance (Figure 1.6), except that which reached the atmosphere through the vent collection system (VCS). However, the flare tower and the vent gas scrubber (Figure 1.7) had been out of service for 5 months before the disaster. After the first 30 min, the rupture disk bursts and this increased the rate of release of MIC. 

There must be a piping system to carry the gas relieved by opening a device to its destination. Chlorine should not under ordinary circumstances be released to the atmosphere. Two approaches are in common use. One involves direct relief to an absorption system the other places an expansion tank between the source of gas and the absorber (GEST 87/133). The discussion of emergency vent scrubbing systems in Section 9.1.10.3C includes some of the considerations that apply to vent collection systems. 

There is a need in many chemical processes for protection against propagation of nnwanted combnstion phenomena snch as deflagrations and detonations (inclnding decomposition flames) in process eqnipment, piping, and especially vent manifold systems (vapor collection systems). 

It was not nndl the 1950s that detonation flame arresters made of crimped metal ribbon elements were developed and began to be used more freqnendy (Binks 1999). The major impetus for die use of crimped metal ribbon detonation flame arresters in the US was the enactment of clean air legislation (Clean Air Act of 1990) which inadvertently created a safety problem by requiring reductions in volatile organic compound (VOC) emissions. To do this, manifolded vent systems (vapor collection systems) were increasingly installed in many chemical process industry plants which captured VOC vapors and transported them to suitable recovery, recycle, or destruction systems. This emission control requirement has led to the introdnction of ignition risks, for example, from a flare or via spontaneous combustion of an activated carbon adsorber bed. Multiple... 

An in-line detonation flame arrester must be used whenever there is a possibility of a detonation occurring. This is always a strong possibility in vent manifold (vapor collection) systems, where long pipe runs provide sufficient run-up distances for a deflagration-to-detonation transition to occur. Figure 3-3 shows the installation of in-line arresters of the detonation type in a vent manifold system. 

Fignre 9-1 is a schematic drawing of the major eqnipment and protective measnres that comprise the vent manifold system. In the system shown, the vent vapors (offgas) from the condensers of two vacnnm col-nmns are collected in a manifold which goes to the vacnnm pnmp system. From the vacnnm pnmp system, the vapors go to a seal dmm (hydranlic flame arrester), and then to the firebox of a process heater, where they are incinerated. 

Figure 3-3 (page 21) shows where detonation flame arresters should be located in a vent manifold (vapor collection) system and upstream of a... 

Pressure relief/vent collection Release devices (flares, scrubbers, etc.) Plant equipment isolation Critical alarms/interlocks Fire detection/protection Gas detection Flame arrestors Emergency system ser ices Appropriate grounding and bonding... 

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