Pressure Vessel Loads
The forces applied to a vessel and its structural attachments are called loads, and the first requirement in vessel design is to determine the loads and the conditions to which the vessel will be subjected in operation.
The major loads acting on a pressure vessel are caused by:
1. Internal pressure
2. External pressure
3. Weight of vessel and contents (including internal components that transmit
loads to the pressure vessel)
4. Wind and seismic forces
5. Connecting piping and the weight of external appurtenances (platforms, etc.)
6. Differential thermal expansion (or temperature gradients)
7. Cyclic forces
These forces must be considered during design in order to prevent failure from any of the failure modes mentioned earlier.
The loads are usually static, or the amplitude and frequency of their fluctuations are such that they can be considered to be so. However, cyclic loads of sufficient magnitude can result in a fatigue failure, and it may be necessary to consider them in the design of a pressure vessel. For example, pressure fluctuations that exceed 20% of the design pressure and cyclic temperature gradients greater than 50°F between adjacent locations can cause fatigue.
Vertical Three-phase Separators Liquid Phase Separation
Flow of Liquids
Oil and water flow downward to the left of the horizontal baffle in Figure 300-7. Bulk oil passes through the coalescer pad; water droplets are coalesced and settle from the oil in the oil drawoff compartment to the right of the coalescer pad. Bulk water flows downward to the left of the coalescer pad and collects in the bottom head below the oil-water interface.
Interface Levels
The high interface level (HIL on Figure 300-7) is determined by the maximum velocity allowed for oil flow through the coalescer pad (see the next paragraph). The normal interface level, NIL, is Dimension “J” (usually 1 foot) below the HIL and Dimension “I” (usually 6 inches) above the lower edge of the pad. The low interface level, LIL, is 6 inches below the bottom tangent line, below the pad. The interface might have to be held at the LIL if the coalescer became plugged.
Coalescer Pad Area
The pad is not necessarily located on the vessel centerline. Pad width (not thickness) will be equal to or less than the vessel diameter. If the interface were at the HIL, bulk oil would flow only through that portion of the pad between the HIL and the horizontal baffle at the top of the pad. The HIL is placed to make the oil velocity through that cross section equal to 5 ft/min per Section 335. The total pad height is the height just determined plus the sum of Dimensions “I” and “J.”
Elevation of Horizontal Baffle
Dimension “C,” height of the baffle above the bottom tangent line, is the total pad height plus any additional height left for flow of oil if the coalescer should plug (usually zero but up to 1 foot). The minimum recommended value of Dimension “C” is 3 feet 0 inches.
Separation of Water Droplets from Oil
Placement of the pad in the horizontal direction is shown by Dimension “G” in Figure 300-7. The oil holdup volume for separation of water droplets is bounded by the horizontal baffle supporting the pad, the NIL, the vessel wall, and the left (upstream) face of the pad. Oil residence time is the holdup volume divided by oil flowrate. Settling distance is from baffle to NIL. As in Section 353, use Equation 300-5 or 300-7 to estimate the water droplet size which will move through the settling distance during the oil residence time. Apply the quality of separation criteria in Section 334.
Vertical Three-phase Separators Liquid Surge
Surge volume for hydrocarbon is the space between the high liquid level (HLL) at the “donut” baffle ring and the low liquid level (LLL) at the horizontal baffle which supports the coalescer. Surge volume for water is the space between the high interface level (see “Interface Levels” in Section 364 below) and the bottom of the coalescer pad.
Vertical Three-phase Separators Vapor Space
Allowable mass velocity, based on the full cross section of the vessel (not the circular space inside the shroud) is Gc from Equation 300-1, multiplied by a derating factor. That factor is 0.5 for hot separators in residuum desulfurizer and vacuum residuum desulfurizer plants, 0.7 for other high pressure separators in hydroprocessing plants, and 0.9 for other separators and knockout drums at pressures above 800 psig. Vessel diameter, Dimension “D” in Figure 300-7, is at least that which corresponds to the allowable mass velocity just determined. The diameter may be larger if liquid surge requirements would require an L/D ratio greater than 3.
The head is hemispherical. The vapor outlet nozzle is in the head. The demister, if used, is mounted on a baffle ring at the tangent line. The width of the ring (crosssectional area left for vapor flow through the demister) is determined by Equation 300-3. The baffle ring is at least 2 feet 0 inches above the shroud. If there is no demister, then an outlet baffle is placed on the centerline as in Figure 300-8.
Vertical Three-phase Separators Feed Inlet
Feed may enter tangentially as shown in Figure 300-7 (expensive in a thick-wall vessel) or straight in with a right-angle turn (Figure 300-8). Feed pipe entry through the head, not shown, is another alternative; feed is released into the vessel at the same point as in the other designs. A shroud ring with a sloped ramp baffle is used to direct liquid to the wall. Radial width of the space within the shroud is 0.1 times vessel diameter. (Note that the cross-sectional area for vapor flow upward is reduced to 64% of its value elsewhere in the vapor space; this is taken into account by use of the derating factors in Section 362.) The inlet nozzle diameter is sized for velocity of 30 ft/sec. Height of the shroud ring is twice the inlet nozzle diameter.
Details to Improve Separation
A horizontal separator is not often used ahead of a compressor because of the possibility of entrained liquid in the vapor. The situation can be improved by use of a demister pad over the vapor outlet. The pad cross-sectional area is found by Equation 300-3. The pad is supported in a housing of either round or rectangular cross section as shown in Figure 300-4.
Difficult hydrocarbon-water separation may be helped by a wire mesh coalescer pad. The pad is located as close as possible to the inlet end of the separator. See Figure 300-4. The pad extends up to the maximum liquid level. Vane-type pads may be used in production separators.
The water rate may be so large, or the upward settling rate of oil droplets so slow, that a water boot of impractical diameter would be required. In this case, water holdup time and hydrocarbon-water interface area are provided by holding water volume within the main compartment of the separator. See Figure 300-4. The hydrocarbon outlet is raised to a point 2 to 6 inches above the highest hydrocarbon-water interface level.
If the primary concern is to provide interface area and water holdup time for separation of oil droplets, then a transverse weir may be used to assure that water volume within the main compartment is maintained. Drain holes are provided in the weir to empty the water at shutdown. The water-hydrocarbon interface is controlled in the boot as before. Since the water behind the weir is not controllable, it does not serve as surge volume.
If increased water surge volume must be provided, then one controller maintains the hydrocarbon-water interface within the vessel and another controller handles the hydrocarbon level. A water boot is not required.
Water Drawoff Leg
The water surge volume in the drawoff leg is found from criteria in Section 333. This is the volume between maximum and minimum levels shown in Figure 300-6. The rate at which 100-micron oil droplets will move upward is found from Equation 300-8 (or 300-7). Size the diameter of the drawoff leg so that the water velocity downward is less than the oil droplet velocity upward.
Water Separation from Hydrocarbon
If water separation is not a requirement, any of the vessel sizes meeting the surge volume and L/D requirements in Section 352 may be chosen. Otherwise, water separation must be checked. Using Equation 300-5 or 300-7 depending on the Reynolds number, compute the settling velocity for a 100-micron water droplet in hydrocarbon. Hydrocarbon residence time (seconds) is surge volume divided by the hydrocarbon flow rate. The settling distance is from maximum liquid level to minimum liquid level. If a 100-micron water droplet will fall through the settling distance within the hydrocarbon residence time, separation will be “good.”
Check the various trial sets of drum dimensions found in Section 352. For constant surge volume, settling distance decreases and separation improves as L/D increases. Increase L/D until water separation becomes satisfactory or until the maximum L/D of 6.0 is reached. In the latter case, increase vessel size at constant L/D. Hydrocarbon liquid surge will be in excess of the requirement.
Water will collect along the bottom of the vessel before running into the boot. The hydrocarbon liquid outlet is therefore raised above the bottom in order to avoid drawing water. The clearance is 2 to 6 inches depending on the volume of water expected.
Shell Length and Diameter
Determine shell dimensions by trying various combinations of diameter, length, and liquid depth. For any trial diameter, the maximum liquid level is known from the vapor cross-section calculation above. The minimum liquid level is at a height of one-eighth diameter from the bottom. (See Figure 300-6.)
The volume in between is hydrocarbon liquid surge. For each trial diameter, compute the required drum length. Include portions of the heads. Several combinations of length and diameter will be found which satisfy the volume requirement. Use a length-to-diameter ratio (L/D) of at least 2.0. The distance from inlet to vapor outlet should be at least 4 feet. It has been found that the cost of a horizontal vessel of given volume does not vary greatly as length-to-diameter ratio (L/D) varies from 2 to 10. However, wave action is a concern in long vessels. Limiting the L/D ratio to a maximum of 6.0 is recommended for horizontal vessels. For large vessels, a tangent-tangent length of 20 feet is a convenient length.
Vapor Cross Section
The cross-sectional area for longitudinal flow of vapor will be the largest of the following:
1. An area equal to 10% of the vessel cross section.
2. The area which results when the vertical height above the maximum liquid level (Dimension “D” minus Dimension “H” in Figure 300-6) is equal to 12 inches.
3. The area determined by applying Equation 300-1. (Derate or use a higher value of velocity for less critical service as discussed in Section 331.)
Note that in this case the vapor moves horizontally and the liquid droplets settle out at right angles to the vapor flow. This would seem to be an advantage. However, it is partially offset by turbulence and entrance/exit effects. If the vapor flow area is to be determined by Equation 300-1, some designers use 125% of the allowable Gc; others use Gc directly. The latter practice is recommended.