Flare water seal

In the case of heavier-than-air purge gas, there is no buoyancy mechanism causing air entry into the stack, and there is thus no incentive to include a dry seal. Unlike a water seal, a dry seal cannot prevent a flashback from traveling upstream if a combustible mixture has been formed by the entry of air into the safety valve or flare headers. It only protects against internal burning flashback... 

If HjS is never present in the flare gas, seal water effluent should be routed through an open fuimel (to permit checking of seal water flow) to a manhole in a vented section of the oily water sewer. The water inlet to the manhole must be sealed. 

A major cause of pulsing in flare systems is flow surging in the water seal drum. One of several reasons why it is important to eliminate pulsing is to reduce flare noise. Combustion flare noise has been shown to increase as the steam rate increases. Since the amount of steam required to suppress smoke in a flare is set by the flaring rate, flow surges will require a higher steam rate than for a steady flow. 

Flashback protection is required for H2S flaring systems, either by water seal or continuous gas purge. If a water seal is used, special requirements apply to the disposal of the effluent seal water. In the case of an HjS flaring system handling a flow of HjS which in uninterrupted throughout the period that a plant is in operation, and which stops only when the producing plant is shutdown, then flashback protection is not required. However, steam or inert gas connections are required to permit purging the flare line startup and shutdown. 

The gaseous stream exiting the reactor has a flowrate of 0.6 kg/s and contains 600 [ m cresol. It passes through two water scrubbers before it goes Anally to a flare system for burning. To prevent backward propagadon of Are in the flare, water is utilized in a water valve to seal the flare. Water uAlized in scrubber 1 (W ), scrubber 2 (W7), and flare seal pot (Wg) possess the following constraints ... 

Flare and vent systems should be simple. It is better to avoid water seals than install steam heating systems and low-temperature alarms, which might fail. 

The common theme of many of these items is that blowdown lines and flare and vent stacks should be kept simple because they are part of the pressure relief system. Avoid flame arrestors, molecular seals, water seals, and U-bends. Avoid steam, which brings with it laist and scale and may freeze. 

Flares ideally bum waste gas completely and smokelessly. Two types of flares are normally employed. The first is called the open flare, the second is called the enclosed flare. The major components of a flare consist of the burner, stack, water seal, controls, pilot burner, and ignition system. Flares required to process variable air volumes and concentrations are equipped with automatic pilot ignition systems, temperature sensors, and air and combustion controls. 

Figure 6-8 is a simplified flow diagram for a typical multijet. The flare uses two burners. A small burner handles normal leakage and small blows, while both burners operate at higher flaring rates. This staging is controlled by two water-seal drums set to release at different pressure levels. 

A third emergency release is provided in the center of the stack, bypassing the multijet burners. The water seal to this release will blow at flaring rates higher than the design capacity of the flare. When the over-capacity seal has been blown, the flare is both luminous and smoky. But the unit is usually sized so that an overcapacity blow would be a rare occurrence. The over-capacity line may also discharge to an elevated flare rather than to the center of the multijet stack. 

Fa/T—mounted at the base of the flare and used for forced-draft operation Knockout drum with water seal Flare header... 

I m sure you have seen the flame of a flare surging. That is not typically due to a relief valve opening periodically. It is due to periodically blowing the water seal in the flare seal pot (often located integral with... 

At the request of one VP of refining in India, who found the flare flame surges objectionable, I reduced the depth of the water seal, and the intensity of the flame surges were thus reduced. But, of course, not the amoimt of hydrocarbons consumed. 

Maintaining a water seal of about 10 inches of water in the flare seal drum is the smart way of preventing air intrusion into the relief valve collection system. 

It is not uncommon for my clients to maintain a positive pressure in the flare system by continuously purging the most distant ends of the flare header piping with fuel gas, natural gas, or even propane. This is an expensive alternate to the water seal method 1 described above. It eliminates the need for the water seal and also the annoying sight of the pulsating flare flame. But to me it seems like a wasteful and, hence, dumb alternate. 

But what I find even worse is the use of both options together. For what purpose, I cannot imagine. At least in every refinery I have worked in, there were sufficient relief valve leaks and unreported venting of hydrocarbons to keep the flare system imder a higher than desired back-pressure. But, even without such leaks, the flare system would still be maintained positive just with the water seal. 

In conclusion, please do not use both hydrocarbon purges and the flare stack water seal together. One or the other Using both does not enhance plant safety and makes no engineering sense. I tried to explain all this to the refining VP referred to above, but to no avail. 

With the flare tip and flare seal pressure drop and flare elevation fixed, the flare stack, headers and laterials are sized for the largest release, while not exceeding the maximum allowable operating pressure on the associated blowdown drums and water disengaging drums. These maximum allowable operating pressures are in turn determined by ... 

Flare system designs must also include means of preventing freezing of seal water in the flare seal drum, if entering vapors may be below 0°C. 

It is important to note that even if the blowdown is effective in disengaging liquid and vapor, further condensation could occur downstream especially if the vented vapor exits the drum at a temperature above ambient conditions. A proportion of such condensible materials in the blowdown drum vapor release may condense as a result of cooling in the flare header and contact with seal water, and then disengage in the flare seal drum while condensible vapors which are not condensed out at this stage may condense in the flare stack or its inlet line, thus creating the potential for hazardous fallout of burning liquid from the flare. Condensed hydrocarbon in the seal drum can be entrained out with the... 

Provide settling facilities to separate hydrocarbon liquid from the flare seal water effluent, and appropriate means of disposal, e.g., to slop storage. 

For services where ambient or inlet temperamres may fall below 0°C, flashback protection is provided by a special seal drum or loop seal in the inlet line. This equipment is designed specifically for the particular liquid and vapor materials being flared. In these cases a 150 mm minimum water layer is included in the bottom of the pit to prevent oil seepage into the ground, and the hydrocarbon inlet distributor is mounted 150 mm above the water surface. Details of the inlet distributor are shown in Figure 5. 

The Seal Drum - A typical flare seal drum for an elevated flare stack is illustrated in Figure 7. A baffle maintains the normal water level, and the vapor inlet is submerged 75 mm to 100 mm. Drum dimensions are designed such that a 3 m slug of water is pressured back into the vertical inlet piping in the event of... 

A flare seal drum may also be used as a sour water disengaging drum, if economically advantageous. In such cases, special care should be given to ensure that the drum is adequately sized to simultaneously meet all design features required for both functions. Also a separate source of makeup water must still be provided to ensure continuity of the seal. 

The drum is usually equipped with steam injection if required for winterizing or cold releases. Refer to Figure 7 for some of the details. If winterizing is necessary, then the steam should be temperature-controlled in order to maintain the seal water temperature at 4 to 10 C. It is important to note that the drum should be located at a minimum safe distance from the flare. 

A secondary seal loop is provided for water withdrawal during major blows when turbulence at the downstream overflow connection to the primary seal loop interferes with normal drainage. Extending the base of the flare stack 3 diameters below the sloped inlet line provides vapor disengaging for the secondary seal leg. The bottom of the stack and inlet line up to 1.5 m above the seal water level are gunite lined for corrosion protection. 

If HjS is continuously present in the flare gas or if the flare seal drum also functions as a sour water disengaging drum, then the effluent seal water must be routed to a sour water stripper, desalter, or other safe means of disposal. Withdrawal from the drum is by pump in place of the normal loop seal arrangement. Two pumps are provided one motor driven for normal use, and the other having a steam turbine drive with low pressure cut-in. The seal drum level is controlled by LIC with high and low alarm lights plus an independent high level alarm. 

Disposal of effluent water from multijet ground flare seal drums should comply with paragraphs (1), (2), and (3) above, except that ... 

Flare systems must be protected against any possibility of partial or complete blockage by ice, hydrates, solidification, etc. Seal Drums and Y-seals requiring winterizing should be provided with temperature-controlled steam injection to maintain the seal water temperature at 4 to 10 C. This limits the quantity of water vapor entering the flare stack. 

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