# Flash drum sizing

• Over 20 Years of Design & Fabrication Experience From Wellhead to Pipeline
• What is a Flare KOD | Working, Functions, Sizing, and Design of a Knock Out Drum (PDF)
• Don't Model in a Vacuum
• Don't Model in a Vacuum Check simulator results against calculations using actual data. This point deserves endless repetition because uncritical acceptance of computer output contributes to many process problems.

The proposed system Figure 1 uses a vacuum flash to eliminate residual solvent from the slurry. After decanting, the mixture enters a flash vessel.

The heated flash drives solvent and water vapors to a vacuum system. The purpose of the vacuum flash is to reduce solvent concentration to 50 ppm in the water slurry mixture. Steam in the jacketed flash drum provides heat for solvent vaporization. To check the results, they located literature values for Henry's law coefficients for the system.

Henry's law states that the solubility of a gas in a liquid is directly proportional to the partial pressure of the gas. However, significant deviations occur, especially at low concentration levels. Different Results Table 1. Calculations from simulator and based on literature data disagree sharply in several key areas. In this case, plugging in the Henry's law vapor pressures provided significantly different results than the simulation.

In general, using data from low concentration ranges tends to show separations are more difficult than extrapolations from high concentration ranges indicate. However, this isn't always true. Besides giving a much higher amount of solvent in the feed, the calculations based on literature data exhibited a much stronger temperature dependence than the simulator extrapolations — and the temperature effect was in the opposite direction.

Proposed Configuration Figure 1. Simulation results grossly underestimated duty load for jacket and vacuum system. Table 1 compares the differences in vacuum system load. Absolute pressure doesn't vary much because water partial pressure dominates the system.

However, the required partial pressure of solvent differs significantly. The data-based results give a much higher solvent concentration in the feed — increasing the amount of solvent that must be removed by five times. The lower partial pressure of solvent in the vacuum flash boosts the water load to the vacuum system by more than twenty-fold.

Together, these effects greatly raise the water vapor load on the vacuum system and the corresponding duty on the vacuum flash vessel. The impact on the vacuum system actually exceeds the difference in total vapor rate above due to the shifting composition and much greater relative flow rate of low-molecular-weight water in the vapor stream. Using the correct basis gives 25 times the duty load for the heated jacket and vacuum system pre-condenser. The rest of the vacuum system is merely five times larger.

One other caution needs to be highlighted here. Most data for solvent in water are measured at very low solvent concentrations. This is a completely different operating region that requires checking against suitable data. Don't use data methods in simulations outside the ranges upon which they are based.

Remember that extrapolation to ppm levels often creates problems. And always perform a reality check on any computer results. You can e-mail him at ASloley putman. You can e-mail Scott Henson at Hensonscott aol. Related Content.

It is also known as a flash drum or a knockout pot. The knockout drums are designed and sized according to a specific length-to-diameter ratio between 2 to 4 to maintain the vapour velocity low enough so that the liquids drop out or settle out. We dramatically reduce the speed of the gas so that the liquid droplets settle out. We manually drain or pump the liquids out regularly to ensure the flare KO is always empty of fluid.

Flare Knockout Drum Function A knockout drum is used to remove any oil or water from the relieved gases. Vapour travels upward at a designed velocity which minimizes the entrainment of any liquid droplets as it exits the top of the vessel. Flares are important safety devices used in oil and gas processing.

They safely burn excess hydrocarbon gases which cannot be recovered or recycled. During flaring, excess gases are burnt off in the flare system to produce water vapour and carbon dioxide.

A horizontal knockout drum must have a diameter large enough to keep the vapour velocity low enough to allow entrained liquids to settle or drop out. All flare systems are designed to include a liquid knockout drum.

They are operated at an atmospheric pressure that allows the greatest liquid volume anticipated at the maximum rate of liquid pump out and build up. For offshore activities, a separation-retention time of 1 — 3 minutes with regards to API gravity must be provided, and an emergency dump design without valves is recommended to handle the maximum liquid flow.

The maximum allowable working pressure MAWP of the knockout drum is typically kept at 50 pounds per square inch gauge psig to maintain an explosion. We can provide the best possible solutions when it comes to installing a flare KO drum. You can trust our reliable products to prevent liquids from getting into the flare stack, thus minimizing the risk of fire or an uncontrolled explosion.

To learn more about our products and services or to request a project discussion, get in touch with our consultants by calling or You could also fill out our online form for a project quote. Blog Pod img.

They safely burn excess hydrocarbon gases which cannot be recovered or recycled.

## Over 20 Years of Design & Fabrication Experience From Wellhead to Pipeline

During flaring, excess gases are burnt off in the flare system to produce water vapour and carbon dioxide. A horizontal knockout drum must have a diameter large enough to keep the vapour velocity low enough to allow entrained liquids to settle or drop out. All flare systems are designed to include a liquid knockout drum. They are operated at an atmospheric pressure that allows the greatest liquid volume anticipated at the maximum rate of liquid pump out and build up. For offshore activities, a separation-retention time of 1 — 3 minutes with regards to API gravity must be provided, and an emergency dump design without valves is recommended to handle the maximum liquid flow.

Horizontal drum with the vapor entering in the center and exiting at each end on the horizontal axis. Vertical knockout drums are only used when the liquid load is low and there is a scarcity of plot space. Vertical flash drums are well suited for incorporating into the base of the flare stack. Vertical vessel with a tangential nozzle. Sometimes, a combination of a vertical drum in the base of the flare stack and a horizontal drum upstream to remove the bulk of the liquid entrained in the vapor is also found in process plants.

The sizing process is constrained by a set of fluid dynamic and mechanical relationships and is based on the gravity-settling theory.

The principle of settling theory is that the liquid droplets will settle due to gravitational force. The size of the knock-out vessel is decided by the anticipated liquid and vapor flow.

### What is a Flare KOD | Working, Functions, Sizing, and Design of a Knock Out Drum (PDF)

The KOD diameter is estimated using the maximum allowable vapor velocity. The knockout drums are designed and sized following a specific length-to-diameter ratio in the range of 2 to 4.

This maintains a low vapor velocity such that the liquids settle out. The KO drum design requires the expert application of many thumb rules. Because of multivariable manual trial-and-error procedures, the knockout drum design is usually done by experienced process engineers.

A liquid level indicator is always installed as these knockout vessels shall remain drained and free of excess liquid. The KO drum sizing is done following the bellow-mentioned two steps as per API Step The starting step of Flare knock-out drum sizing is to determine the drum size required for liquid entrainment separation.

Liquid particles separate: If the residence time of the vapor or gas is equal to or greater than the time required to travel the available vertical height at the dropout velocity of the liquid particles, and if the gas velocity is sufficiently low to permit the liquid dropout to fall.

This vertical height is considered from the maximum liquid level. To prevent large slugs of liquid from entering the flare, the vertical velocity of the vapor and gas should be low enough. This liquid may result from: condensate that separates during a vapor release, or liquid streams that accompany a vapor release. Proposed Configuration Figure 1. Simulation results grossly underestimated duty load for jacket and vacuum system.

## Don't Model in a Vacuum

Table 1 compares the differences in vacuum system load. Absolute pressure doesn't vary much because water partial pressure dominates the system. However, the required partial pressure of solvent differs significantly. The data-based results give a much higher solvent concentration in the feed — increasing the amount of solvent that must be removed by five times. The lower partial pressure of solvent in the vacuum flash boosts the water load to the vacuum system by more than twenty-fold.

Together, these effects greatly raise the water vapor load on the vacuum system and the corresponding duty on the vacuum flash vessel. The impact on the vacuum system actually exceeds the difference in total vapor rate above due to the shifting composition and much greater relative flow rate of low-molecular-weight water in the vapor stream.

Using the correct basis gives 25 times the duty load for the heated jacket and vacuum system pre-condenser. The rest of the vacuum system is merely five times larger.

One other caution needs to be highlighted here.

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