TECHNOLOGY New program sizes pressure-relief drums

Alejandro Anaya Durand,
Raul Abrajan Osorio,
Rogelio Hernandez Suarez
Instituto Mexicano del Petroleo
Mexico City

• Equations [44781 bytes ]

In accordance with API Recommended Practice 521, a new procedure has been developed for the design of relief drums. The calculation method determines by convergence the most economical length-to-diameter ratio for gas-liquid separation vessels.

Drum sizing is based on the separation of a two-phase stream, taking into account the special condition of intermittent flow. Design parameters such as settling velocity and residence time also must be calculated to determine an optimum design.

A new program based on a programmable algorithm can be converted from basic language to any other computer language to facilitate vessel-design computations. The program quickly and efficiently computes design values for relief systems used in refineries and petrochemical plants.

Relief systems

Pressure-relief systems in refineries or petrochemical plants comprise:

• Relief valves

• Relief headers (piping system that move fluids to a burner)

• Relief drums (liquid separators, such as flare knockout drums)

• Seal drums

• Gas burners.

Fig. 1 [74546 bytes ] shows a typical arrangement for headers and relief valves.

The relief drum is an important part of the security concept (Fig. 2 [70740 bytes ]). Within acceptable design parameters, it is necessary to review and update common design procedures to derive a mechanism capable of quickly and efficiently describing gas-liquid separation and liquid accumulation in the drum.

The relief drum is a continuously operating "utility." In case of emergency-such as fire, or loss of power, cooling water, or instrument air-the system pressure will increase to the maximum allowable working pressure of the equipment.1 2 Relief valves will have to release gas and liquid streams to reduce the system pressure until conditions are safe and the emergency is handled.

Fig. 3 [82123 bytes ] shows typical horizontal and vertical relief drum arrangements.

Because streams containing hydrocarbons frequently cannot be released to the atmosphere, it is necessary to have equipment that both stores liquid streams and separates the liquid from the gas phase.

L/D ratio

To develop the most economical relationship of length to diameter for a relief drum, the size and materials requirements must first be determined. A preliminary design also must take into account the drum head shape.

Equation 1 can be used to calculate the shell thickness, based on internal pressure (see Equations, Nomenclature).^{3 4} Equation 2 calculates head thickness for torispherical heads, while Equation 3 is used for elliptical heads.

Pressure and stress apply equally at every point in a vessel except the shell-head joint, where the geometrical configuration creates higher loads. For this reason, head thicknesses must be greatest for flat heads.

Reduced head thicknesses can be used at the following pressures:

• For torispherical heads, 50-150 psig

• For elliptical heads, 150 up to 600 psig

• For hemispherical heads, 600 psig.

For vessels operating at high pressures, an L/D ratio of 2 is recommended. For moderate pressures, the L/D ratio should be between 3 and 5. A ratio of 3 is commonly recommended for low pressures.5 Knockout drums operate at pressures close to atmospheric.

Head manufacturers indicate that for low-pressure operation, no torispherical heads are commercially available with a radius greater than 60 in. and a thickness of 3/16 in. or 1/4 in. As a result, it is generally recommended that elliptical heads 5 ft or larger be used. These heads are available in sizes up to 16 ft diameter, in 0.5-ft increments.

Separation

Stokes law describes the process of separating liquid particles from the gas phase inside a relief drum (Equation 4).6 Equations 5 and 6 relate drag factor to system characteristics in, respectively, English and metric units.

Drag factor can be defined as shown in Equation 7. When a particle falls under the influence of gravity, it will accelerate until the frictional drag in the fluid balances gravitational forces. At this point, it will continue to fall at constant velocity. This velocity is called terminal velocity or free-settling velocity (Equation 8).

Separation is a mechanical process. The design program will find a vessel's length and the cross-sectional area of the gas-liquid trajectory, taking into account the fact that, at the end of the trajectory, only gas will be present (Fig. 4 [82617 bytes ]).

When the settling time of the liquid particles is the same as the residence time of the gas inside the drum, the gas velocity will be low. The flow pattern therefore will be in the transition zone.

Particle size

The minimum particle size for a gravity separation process is 100. For design purposes, it is a good practice to use a range of 150-600.⁷

For a knockout drum inside the battery limits of a plant, 600m should be considered because the gas will flow to a second knockout drum near the burner (Fig. 5 [58818 bytes ]).

A burner is able to handle without risk gas streams containing liquid particles as large as 400. Nevertheless, in practice, it is suitable to use a 300 particle-size limit.

The API method

API RP 521's code shows how to calculate drum sizes using a traditional trial-and-error procedure:7

• Calculate the cross-sectional area (Equation 9).

• Determine the area required for the additional liquid load (Equation 10).

• Calculate the required area for the condensate from the inlet stream (Equation 11).

• Calculate the cross-sectional area available for gas flow (Equation 12).

• Determine the liquid level height, then compute the vessel diameter (Equation 13).

• Check whether the vapor space is adequate for the gas-liquid separation, then compute the settling time for the available vertical space (Equation 14).

• Calculate the vapor velocity (Equation 15).

• Compute the minimum vessel length for the gas-liquid separation (Equation 16).

• Compare L_min with L; if L_min < L, the procedure is finished. If L_min L, a new L must be supposed, and the procedure is repeated.

Because many values of L and D can satisfy the condition L_min < L, this trial-and-error design work is extensive. The design must be calculated as many times as necessary before making a decision.

New method

The model for the new calculation procedure is an algorithm based on a convergence toward the most economical ratio of length to diameter. The calculation steps are outlined in the flow chart in Fig. 6 [187644 bytes].

Four considerations are basic to this model:

• The available space for liquid storage in the drum is 50% of the diameter.

• The L/D ratio is between 2.7 and 3.0.¹

• The drum diameter is fitted by a calculated L/D ratio, rather than by assumed values for L and D.

• The available and required volumes indicate whether the drum diameter must be modified using a safety factor, to avoid designing square drums.

Step-by-step procedure

The optimum L/D ratio is determined according to the calculation procedure illustrated in Fig. 6 [187644 bytes].

1. Input data:

- a. Thermophysical fluid properties in the gas-liquid stream

- b. Gas flow

- c. Additional liquid load, including drains, process liquid from blocked outlets, etc.

2. Input minimum liquid height (common practice is h = 0.5 ft).8

3. Compute drag factor using Equation 17.

4. Compute settling velocity of liquid particles.

5. Compute volumetric gas flow.

6. Assume a vessel diameter of 5 ft as a starting point and calculate:

- a. Cross-section drum area

- b. Cross-sectional area available for gas flow

- c. Gas velocity

- d. Settling time of liquid particles

7. Compute the length required for gas-liquid separation (Equation 18).

8. Fix the diameter equivalent to DM and the length to LM. (These are the minimum diameter and length required to accomplish gas-liquid separation.)

9. Compute R = L/D (the limits for the convergence procedure are 2.7 < R < 3.0).

10. Fit the diameter using 0.5-ft increments (i.e., D = D + 0.05 and D = D - 0.05); repeat Step 7, changing the diameter by 0.05-ft increments, until the conditions for R are satisfied.

11. Start the second convergence procedure (Fig. 6) using Dimensions L and D determined in Step 10; check that the drum's available volume, V_f, is adequate to contain the entire liquid load (from Step 1):

- a. Compute the X factor using Equation 19. (This represents the available space from the minimum height (0.5 ft) to the maximum liquid level (0.5D.)

- b. Compute Y (Equation 20).

- c. Calculate the minimum liquid-level area (Equation 21).

- d. Compute the available drum volume (Equation 22).

- e. Compare the available volume, V_f, with the required volume, V_L.

If V_f V_L, the drum's capacity to handle the liquid is correct, and the minimum length and diameter (LM and DM) will be equal to the final, or current, length and diameter. This means that the drum's dimensions are a function only of the gas-liquid separation process.

If V_f < V_L, then LM and DM are not adequate to hold the maximum liquid level at 50% of the drum's capacity. This means that the drum's dimensions are a function of the liquid storage requirements and not of the gas-liquid separation.

To satisfy the liquid storage requirements, it is necessary to find the final vessel length and diameter:

12. Increase the diameter in 0.05-ft increments.

13. Compute R = L/D; if R 2.6, calculate:

- a. Total drum area

- b. Available gas flow area

- c. Gas velocity

- d. Settling time of liquid particles.

14. Calculate the X factor, Y function, minimum liquid-level area, and available drum volume.

15. Compare V_f with V_L to the convergence of V_f < V_L (the procedure then prints the drum dimensions satisfying the design conditions).

16. If R < 2.6, change to R = 2.7 to estimate L (in this case, L = 2.7D).

17. Compute the volumetric gas flow and the parameters in Step 14.

The length and diameter selected using this procedure are not the final commercial dimensions. The diameter must be fitted according to the head shape.9 The length computed when the nozzle diameter is $ 12 in. is shown in Equation 23.

A computer program of the procedure shown in Fig. 6 is available in basic language. If installed on a personal computer, the program can be used to design a knockout drum or evaluate one in service.

Editor's note: To obtain the basic relief-drum program, Journal subscribers can send a blank 3.5-in. diskette formatted to MS DOS and a self-addressed, postage-paid diskette mailer to: Refining/Petrochemical Editor, Oil & Gas Journal, 3050 Post Oak Blvd., Suite 200, Houston, Tex 77056, USA.

Subscribers outside the U.S. should send the diskette and return mailer without postage to the same address. This offer will expire Aug. 15, 1996.

References

1. API Recommended Practice 520, 6th ed., March 1993.

2. API Recommended Practice 521, 3rd ed., November 1990.

3. Scheiman, Adolph D., "Horizontal Vapor-Liquid Separators Quicker," Hydrocarbon Processing, May 1964, pp. 155-60.

4. Gerunda, A., "How to Size Liquid-Vapor Separators," Chem. Eng., May 4, 1981, pp. 81-84.

5. Abrajan, Ral, and Becerra, Diseo, thesis Recipientes de Proceso, Mxico, 1985.

6. Perry, R.H., and Green, D.W., Manual del Ingeniero Quimico, 6th ed., McGraw-Hill, 1973, pp. 5-7.

7. "Lukens Heads" catalog, Lukens Steel Co., July 31, 1978.

8. ASME Boiler and Pressure Vessel Code, Section VIII, Division I, 1977 ed.

9. API Recommended Practice 521, 1st ed., September 1969.

10. Ludwig, E., Applied process design for chemical and petrochemical plants, Vol. 1, 2nd ed., Gulf Publishing, 1977, Houston.

The Authors