Blowdown system

Cyclone Separator with Separate Catch Tank This type of blowdown system, shown in Fig. 26-17 and 26-18, is frequently used in chemical plants where plot pan space is hmited. The cyclone performs the vapor-liquid separation, while the catch tank accumulates the hquid from the cyclone. This arrangement allows location of the cyclone knockout drum close to the reactor so that the length of the relief device discharge hne can be minimized. The cyclone nas internals, vital to its proper operation, which will be discussed in the following sections. 

Accumulator vents Blowdown systems Pumps and compressors... 

Most refinery process units and equipment are manifolded into a collection unit, called the blowdown system. Blowdown systems provide for the safe handling and disposal of liquids and gases that are either automatically vented from the process units through pressure relief valves, or that are manually drawn from units. Recirculated process streams and cooling water streams are often manually purged to prevent the continued buildup of contaminants in the stream. Part or all of the contents of equipment can also be purged to the blowdown system prior to shutdown before normal or emergency shutdowns. 

Blowdown systems utilize a series of flash drums and condensers to separate the blowdown into its vapor and liquid components. The liquid is typically composed of mixtures of water and hydrocarbons containing sulfides, ammonia, and other contaminants, which are sent to the wastewater treatment plant. 

The gaseous component typically contains hydrocarbons, hydrogen sulfide, ammonia, mercaptans, solvents, and other constituents, and is either discharged directly to the atmosphere or is combusted in a flare. The major air emissions from blowdown systems are hydrocarbons in the case of direct discharge to the atmosphere and sulfur oxides when flared. 

In some cases, because of severe corrosion problems or for special process reasons, a unit must have its own separate blowdown system. A sulfuric acid alkylation process is an example. Here the discharge from safety valves which can contain acid emulsion presents two particular problems corrosion and slow disengaging of hydrocarbon from acid. 

The first vessel in the blowdown system is therefore an acid-hydrocarbon separator. This drum is provided with a pump to transfer disengaged acid to the spent acid tank. Disengaged liquid hydrocarbon is preferably pumped back to the process, or to slop storage or a regular non-condensible lowdown drum. The vented vapor stream from the acid-hydrocarbon separator is bubbled through a layer of caustic soda solution in a neutralizing drum and is then routed to the flare header. To avoid corrosion in the special acid blowdown system, no releases which may contain water or alkaline solutions are routed into it. 

Another example of an unsteady state condensible blowdown system is the design for a phenol condensible blowdown tank. A blowdown tank is used in phenol treating plants to handle streams containing phenol and heavy hydrocarbons (lubricating oil stocks). The blowdown tank is illustrated in Figure 4. The design basis is as rollows ... 

While a plant was on line, an operator noticed a slip-plate on a tank vent. The slip-plate had been fitted to isolate the tank from the blowdown system while the tank was under maintenance. When the maintenance was complete, the slip-plate was overlooked. Fortunately, the tank, an old one, was stronger than it needed to be for the duty, or it would have burst. 

If the vent line forms part of a blowdown system, it will have to be blanked to prevent air being sucked in. Make sure the blank is put on the flare side of the disconnection, not on the tank side (Figure 1-4). Note that if the tank is to be entered, the joint nearest the tank should be broken. 

Activation of the blowdown, however, will not depressurize a system fully for a considerable length of time. One of the reasons for considering retaining the possibility of human intervention was that the automated blowdown system was not considered completely reliable at the time because of the limitations of the fire and gas detection hardware. This would have the effect of resulting in increasing the likelihood of spurious blowdown production losses. 

The human factors audit was part of a hazard analysis which was used to recommend the degree of automation required in blowdown situations. The results of the human factors audit were mainly in terms of major errors which could affect blowdown success likelihood, and causal factors such as procedures, training, control room design, team communications, and aspects of hardware equipment. The major emphasis of the study was on improving the human interaction with the blowdown system, whether manual or automatic. Two specific platform scenarios were investigated. One was a significant gas release in the molecular sieve module (MSM) on a relatively new platform, and the other a release in the separator module (SM) on an older generation platform. 

Training Review Offshore personnel were questioned about their training and also asked questions to determine their depth of knowledge of the blowdown system. Training personnel onshore were briefly interviewed. 

The most important hardware items appeared to be the detectors themselves. The gas detection system gave frequent spurious alarms, and on both platforms the ultraviolet (UV) fire detectors were also prone to spurious activation from distant hot work for example, and had a limited ability to detect real fires. The tmreliability of these systems had a general effect on response time and would, overall, lengthen the time to respond. The second aspect which was related to hardware was fimction and performance testing of the emergency blowdown systems. It is critical that the workers believe the systems will work when required, and this can only be achieved by occasional use or at least fimction testing. 

Blowdown from the boiler(s) should always be taken to either a blowdown sump or blowdown vessel before discharging into drains. Both should be adequately sized to give cooling by dilution and be fitted with vent pipes to dissipate pressure safely. The boiler(s) should have independent drain lines for the main manually operated blowdown valve and the drains from a continuous blowdown system. Where more than one boiler is connected to either system the line should be fitted with a check or secondary valve capable of being locked. 

One of several different types of BW blowdown systems that automatically controls the frequency and duration of the BD period. Some systems provide continuous BD. 

Cooling towers, compressors, blowdown systems, pressurised or refrigerated storage tanks 2... 

In some instances it may be beneficial to maintain process inventories of certain process vessels until the incident is actually threatening the container. The inventory of the vessel may be crucial to the restart of a facility or the contents may be highly valuable. Loss of the inventory may be criticized if frequent false trips of the ESD blowdown system occur. In these cases an automatic fusible plug blowdown valve could be installed which would only activate from the heat of a real fire incident. In this way, a false disposal of the inventory can be avoided. 

The need for a liquid blowdown system is eliminated. By retaining liquid in the vessel as a heat sink, any increase in temperature of the wetted shell is minimized. 

Activation of layers of protection such as relief valves, interlocks, rupture disks, blowdown systems, halon systems, vapor release alarms, and fixed water spray systems... 

On the night of the accident, operators heard a screeching noise from the relief valve on one of the tanks. Unfortunately, the closed blowdown system had been taken out of service for maintenance. It was later established that while operators were on their shift change or on a break, someone disconnected a pressure gauge from the cover plate on one of the tanks and attached a water hose. A quantity of water estimated between 450 and 900 kg entered the tank and caused a severe upset and release of MIC vapor. With no means of notifying the public and evacuating the community, thousands were exposed to the vapor cloud, resulting in the deaths and injuries. 

The gauge has proved equally accurate for constant pressure and blowdown systems, and can also be adapted to vented systems. A typical curve for normalized krypton concn vs the amt of proplnt remaining in the tank is shown in Fig 1. Also shown is the analytical relationship between tracer concn and proplnt remaining in the tank. Statistical error analyses showed the typical average gauging error thruout the entire range of proplnt expulsion to be less than . 3% with a one sigma deviation of less than . 4%. This illustrates the consistency and reproducibility of this measurement technique... 

Figure 2. Process Flow Diagram for Typical Blowdown System.

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