Chemistry and Technology of Fuels and Oils, Vol. 49, No. 6, January, 2014 (Russian Original No. 6, November – December,2013)
EQUIPMENT
DETERMINATION OF THE MINIMUM SAFE PURGE GAS FLOW RATE IN FLARE SYSTEMS WITH A VELOCITY SEAL
Yong-Zhong Bai 1,2, Peng Wang,2 and Jun-cheng Jiang1
Purge gas in flare systems is used to avoid burnback in the flare stack. We have used computational fluid dynamics (CFD) methods to calculate the purge gas flow rate in a flare stack with a velocity seal. We constructed a simulation model of the purging process for different purge rates. As the results show, the baffle size has a significant effect on the oxygen distribution in the flare. When the first baffle/seal diameter ratio is d 1/D = 0.92 and hydrocarbons are discharged in the flare, the minimum safe purge gas flow rate is 0.05 m/s. However, when the hydrogen content in the discharge gas is high, the minimum safe purge rate is higher: 0.1 m/s. But if the purge rate is maintained at the level recommended by API 521-2007 (0.012 m/s when discharging hydrocarbon gases), the ratio d 1/D should not be greater than 0.75. In this case, when discharging gas with high hydrogen content, the minimum safe purge rate is 0.02 m/s. Key words: velocity seal, purge rate, flare system, burnback. In order to prevent major problems arising during operation of flare systems (burnback and explosions in the stack), flares are provided with a system for purging with oxygen-free gas. The gas prevents air from coming into the flare stack and forming a flammable mixture in the stack. Velocity seals are widely used to reduce consumption of purge gas. The most important operating parameter of a velocity seal is the purge gas flow rate. A purge rate that is too high leads to unwarranted increase in operating costs; a purge rate that is too low may cause burnback and explosions in the stack. In April 2008, an explosion occurred in a flare system at a refinery in China after emergency relief involving a hydrotreating unit. The reason was the low purge rate [1]. ____________________________________________________________________________________________________ 1 School of Urban Construction and Safety Engineering, Nanjing University of Technology, Nanjing 210009, China. 2 Qingdao Safety Engineering Institute, SINOPEC, Qingdao 266071, China. Translated from Khimiya i Tekhnologiya Topliv i Masel, No. 6, pp. 12 – 15, November – December, 2013. 474
0009-3092/14/4906–0474 2014 Springer Science+Business Media New York
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1) baffle; 2) air; 3) purge gas.
Fig. 1 Schematic drawing of velocity seal:
Atmosphere
Fig. 2 Velocity seal simulation model: 1) flare stack; 2) baffles; 3) wind.
Atmosphere
A significant contribution to optimization of the purge rate in flare stacks was made by Husa [2], who proposed a formula for determining the purge rate needed to maintain the required oxygen content in the stack at a certain distance from the open end of the stack. Based on these results, Tan [3], Panchenko [4], and Shore [5] proposed their own formulas for calculating the purge rate. Bryce and Fryer-Taylor [6] determined the length of the flammable zone within a flare stack, where hazardous flammable concentrations are present. API 521-2007 recommends a purge rate of 0.006-0.012 m/s for flares with a velocity seal, but no source is indicated for these data. We have not encountered any other research papers devoted to determination of the safe purge rate. The traditional method for determining the safe purge rate calls for conducting experiments, generally requiring significant material costs and time. An alternative is the numerical method. The aim of our work was to use computational fluid dynamics (CFD) to determine the minimum safe purge rate by analysis of the oxygen distribution in the velocity seal for different purge rates. The velocity seal is a cylinder with one or more cone-shaped baffles; when air flow entering from above encounters the baffles, the flow is deflected, encounters the purge gas flow, and is pushed out of the flare stack (Fig. 1). We simulated the most widely used type of velocity seal in the Ansys Fluent 13.0 environment (Fig. 2). Computational fluid dynamics is based on solving conservation equations or transport equations for mass, momentum, energy, and the participating species:
v S m t v v v P g F t E v E P k eff T h j h j J j eff v F S h t j j
where is the density; . v is the velocity; t is time; is the vector differential operator; S m is the mass added to the continuous phase due to input of mass from the dispersed phase, for example during vaporization of liquid droplets (in our case, S m = 0); P is the static pressure; . is the stress tensor; g and F are the gravitational body force and external body forces; k eff is the effective conductivity; T is the gas temperature; h j is the sensible enthalpy of species j; . J j is the diffusion flux of species j; S h represents the other heat sources.
Flow of purge gas in a velocity seal is turbulent flow, and so we need to additionally solve the model equations for turbulent flow. In this paper, we use the k-e model for turbulent flows. It is believed that safe operation of a flare system is possible for not more than 6 vol.% oxygen in the gas below the seal (~50% of the concentration at which a flammable mixture is formed). Therefore the purge rate ensuring 6 vol.% oxygen in the flare stack is the minimum safe purge rate. Safe purge rate in existing velocity seal designs First we calculated the oxygen concentration in the velocity seal for purge rate 0.012 m/s and baffle/seal diameter ratio d 1 /D = 0.92. The oxygen concentration profile in the velocity seal is shown in Fig. 3. As we see, below the baffles the oxygen concentration is significantly lower than after the baffles. But 476
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Fig. 3 Oxygen concentration profile (a) and velocity profile (b) in velocity seal.
a
b
close to the lower boundary of the seal, the oxygen concentration exceeds 6 vol.%., which suggests risk of burnback and explosion in the flare stack. Therefore a purge rate equal to 0.012 m/s cannot be considered safe. In order to identify the reason why a velocity seal cannot ensure safe operation of a flare for purge rate 0.012 m/s, we compared the velocity profiles in the seal (Fig. 3b). When the wind blows over the tip of the flare stack, some of the air enters into the velocity seal and flows along the stack wall. When the air flow encounters the baffles, some of it is forced away from the wall, turns back, and flows out of the stack. However, some of the air still passes through the velocity seal, enters the downstream part of the stack, and forms a flammable mixture with the hydrocarbons. This is because the distance from the side of the seal to the edge of the baffle is only 50 mm, while the diameter of the velocity seal is 1300 mm, i.e., the baffles are too small to force all the air flow to turn back and make it flow out from the seal. In order to determine the minimum safe purge rate, we simulated the velocity seal purging process for different purge rates. The averaged oxygen concentrations at the bottom entrance of the seal are shown in Fig. 4. We see that 6 vol.% oxygen at the bottom entrance of the seal is achieved for purge rate 0.05 m/s, which can be considered as the minimum safe purge rate. Because of the wide explosive range of hydrogen/oxygen mixtures (hydrogen concentration from 4 to 75 vol.%), the probability of formation of flammable and explosive mixtures is higher when hydrogen-containing streams are discharged into the flare system than when hydrogen-free streams are discharged. Furthermore, hydrogen/air mixtures require very low energy for ignition (~5% of the energy needed for ignition of a methane/air mixture). When hydrogen-containing gases are discharged, the risk of burnback in the flare stack increases. The oxygen concentration corresponding to half the upper explosive limit for hydrogen/oxygen mixtures is 2.6 vol.%. From Fig. 4 we see that an oxygen concentration at the bottom entrance of the seal equal to 2 vol.% is reached for purge rate 0.1 m/s. This rate can be considered the minimum safe purge rate in flare systems discharging hydrogen-containing streams. Optimization of the velocity seal structure Baffle size in existing velocity seals is insufficient for forcing out all the air entering the flare stack for
Oxygen content, vol.%
purge rate 0.012 m/s. Consequently, in order to prevent infiltration of air into the flare stack for low purge rate, we
Purge rate, m/s Fig. 4 Oxygen content at bottom entrance of seal vs. purge rate.
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Fig. 5 Oxygen concentration profiles in velocity seals a-e
Oxygen content, vol.%
Purge rate, m/s Fig. 6 Oxygen content at bottom entrance of seal vs. purge rate (d 1 /D = 0.75). need to optimize the structure of the velocity seal. Since the key components of a velocity seal are the baffles, especially the top baffle, the optimization process must be focused specifically on the baffles. We considered several velocity seals with different first baffle/seal diameter ratios d 1/D: a ................................................................................ 0,92 b ................................................................................ 0,85 ñ ................................................................................ 0,80 d ................................................................................ 0,75 e ................................................................................ 0,70 The oxygen concentration profiles in seals a-e for purge rate 0.012 m/s are shown in Fig. 5. As we see from Fig. 5, the baffle diameter has the determining effect on the oxygen concentration profile in the velocity seal. When the ratio d 1 /D is equal to 0.92, the average oxygen concentration at the bottom entrance of the seal is 12 vol.%. It is approximately at this oxygen concentration that a flammable mixture is formed with hydrocarbons, i.e., burnback is possible in the flare stack. As the baffle size increases and consequently so does the ratio d 1 /D, the oxygen concentration below the top baffle gradually decreases. For d 1/D = 0.75, the average oxygen concentration at the bottom entrance of the seal is 5.6 vol.%, which is 0.4% lower than the safe oxygen concentration. When d 1 /D = 0.7, the average oxygen concentration at the bottom entrance of the seal is almost the same as for d 1/D = 0.75. For the model d 1/D = 0.75, we determined the oxygen concentration at the bottom entrance of the seal for different purge rates (Fig. 6). As we see, for purge rate 0.02 m/s, the average oxygen concentration at the bottom entrance of the seal is 2 vol.%. For d 1/D = 0.75, the minimum safe purge rate ensuring safe discharge of hydrogen-containing streams is 0.02 m/s. Thus we have found that the baffle size has a decisive impact on the oxygen distribution in a velocity seal. The most widely used baffles do not guarantee prevention of burnback and explosions in the flare stack for the purge rate recommended by API 521-2007. For a velocity seal with d 1/D = 0.92, the minimum safe purge rate is 0.05 m/s when discharging hydrocarbons and 0.1 m/s when discharging hydrogen-containing streams. If the purge rate of 0.012 m/s recommended by API 521-2007 is maintained, the first baffle/seal diameter ratio should be
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not more than 0.75 when hydrocarbons are burned in the flare. For d 1/D d” 0.75 and discharge of streams with high hydrogen content, the minimum safe purge rate is 0.02 m/s. REFERENCES 1.
W. Y. Dang and A. F. Yu, “Study on the design pressure for water-sealed drum in refinery flare system,” China Safety Science Journal, 20, No. 9, 75-80 (2010). [in Chinese]
2.
H. W. Husa, “How to compute safe purge rates,” Hydrocarbon Processing and Petroleum Refiner, 43, This work was supported by the National Basic Research Program of China (program 973), grant No. 2010CB226902.
3. 4.
S. H. Tan, “Flare system design simplified,” Hydrocarbon Processing, 46, 172–176 (1967). V. I. Panchenko, “Determining the safe purge rates for flare tubes,” Chemical and Petroleum Engineering, 29, No. 5,
5.
205-209 (1993). D. Shore, “Making the flare safe,” Journal of Loss Prevention in the Process Industries, 9, No. 6, 363-381 (1996).
6.
S. G. Bryce and R. E. J. Fryer-Taylor, “Reducing the purge in hydrocarbon vent stacks,” Journal of Loss Prevention in the Process Industries, 7, No. 3, 249-255 (1994).
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