IPE-TM-300 Vessels
Vessel Nozzles, Internals, & Ancillaries
IPE-TM-300-03
1. Table of Contents
1. Table of Contents 1
2. Purpose 2
3. Nozzles - General 2
3.1 Minimum Size 2
3.2 Oversized Nozzles 2
3.3 Maximum Size 3
3.4 Nozzles on Vessel Heads 3
3.5 Nozzle Location Limitations 3
3.6 Manways 4
3.7 Inspection Openings 6
3.8 Ventilation 7
3.9 Steamout 7
3.10 Vents and Drains 8
3.11 Unloading 9
3.12 Instrument/Gauge Glass 9
3.13 Pressure Relief Valves 9
3.14 Refractory Lined Vessels 9
3.15 Drop Leg 9
3.16 Hot Vapor Bypass 10
4. Horizontal and Vertical Vessel Internals 10
4.1 Coalescing Blankets 10
4.2 Demisting Blankets 10
Figure 1 One Piece Blanket Support 12
Figure 2 Sectional Blanket Support 12
4.3 Distributors 12
4.4 Phase Separation Baffles 15
4.5 Vortex Breakers 15
5. Fractionator Nozzles and Internals 16
5.1 General 16
5.2 Liquid Inlets 17
5.3 Vapor and Mixed Phase Inlets 18
5.4 Reboiler Return 18
5.5 Vapor Side Draws 20
5.6 Liquid Side Draws 22
5.7 Preferential Once-Through Reboiler 22
5.8 Absolute Once-Through Reboiler 23
5.9 Vertical Thermosiphon Reboiler 23
5.10 Column Bottom Surge Capacity 23
6. Ancillaries 24
6.1 Vessel Supports 24
6.2 Stiffener and Insulation Rings 25
6.3 Fireproofing and Insulation 26
6.4 Davits 26
6.5 Platforms 26
Attachment 1 Minimum Openings Required for Passage of Pipe Fittings 28
Attachment 2 Reboiler Drawoffs 29
2. Purpose
This procedure describes the Inflection Point Engineering practice for vessel nozzles and certain vessel internals and ancillaries. Design guidelines and criteria are provided to assist with the sizing, location and orientation of nozzles and aspects of internals.
3. Nozzles - General
Connections on vessels shall be flanged. Coupling connections are not permitted, even on atmospheric or non-code vessels.
3.1 Minimum Size
Inflection Point Engineering practice requires a minimum nozzle size of 1 inch on vessels which ensures mechanical strength and reduces the chance of plugging of the nozzle. Sometimes Owners request a larger minimum nozzle size on vessels, e.g., 1-1/2 or 2 inches. This requirement is indicated in the Vessels section of the Basic Engineering Design Questionnaire (BEDQ). There are other minimum size requirements for certain services such as vents, drains, steamout, level, temperature, ventilation, and manways as discussed in this procedure.
Minimum size nozzles used on vessels may be connected to smaller line sizes connecting miscellaneous equipment. Examples are 1/2 and 3/4 inch pressure instrument connections or when gauge glasses, which normally utilize 3/4 inch piping, are connected to the vessel. When this situation occurs, since the minimum nozzle sizes come from a Inflection Point Engineering or Owner engineering practice, these nozzles are not considered to be oversized. For these cases the Piping & Instrument Diagram (P&ID) will not show the nozzle size. The exception to this is to show the nozzle size on the P&ID for temperature or displacer float instruments mounted on a minimum size vessel nozzle. This applies whether the minimum size is dictated by Inflection Point Engineering or by Owner practice. This is done to ensure that instrument specifications for these instruments are correctly prepared.
3.2 Oversized Nozzles
Oversized nozzles on vessels are at times required for the following reasons:
- For stab-in distributors, collectors, and drain screens. Refer to the appropriate Standard Drawing.
- For internal insulation or lining of a nozzle
- Use a 2 inch nozzle size when instruments are connected to a lined vessel. Special level instrument connection sizes can be as large as 3 inches.
- To reduce the velocity of a mixed phase to the inlet of a horizontal vessel
Without exception, show the oversized nozzle size on the P&ID to confirm that it is intentionally larger than the connecting line size.
3.3 Maximum Size
Nozzles larger than half the vessel diameter require additional investigation and possibly additional reinforcement. Nozzles of this size are not normally recommended by Inflection Point Engineering.
3.4 Nozzles on Vessel Heads
According to the American Society of Mechanical Engineers (ASME) Section VIII Pressure Vessel Code, locate all nozzles on vessel heads inside of a circle of diameter 0.8 times the inside diameter of the vessel. On very small diameter fractionators, it may be necessary to install the vapor outlet on the side rather than on the head to satisfy this requirement. Data for Class 150 and 300 flanges is shown on the following page to enable determinations to be made. Computer Vessel Program 254.6 checks to make sure that both the manway and the outlet nozzle will fit on the top head.
| Nominal Pipe Size ins. | Pipe Outside Diameter ins. | Flange Outside Diameter ins. A or B | Flange Outside Diameter ins. A or B |
|---|---|---|---|
| C or D | Cl 150 | Cl 300 | |
| 1 | 1.315 | 4¼ | 4⅞ |
| 1½ | 1.90 | 5 | 6⅛ |
| 2 | 2.375 | 6 | 6½ |
| 3 | 3.50 | 7½ | 8¼ |
| 4 | 4.50 | 9 | 10 |
| 6 | 6.625 | 11 | 12½ |
| 8 | 8.625 | 13½ | 15 |
| 10 | 10.75 | 16 | 17½ |
| 12 | 12.75 | 19 | 20½ |
| 14 | 14.00 | 21 | 23 |
| 16 | 16.00 | 23½ | 25½ |
| 18 | 18.00 | 25 | 28 |
| 20 | 20.00 | 27½ | 30½ |
| 24 | 24.00 | 32 | 36 |
½ (A+B+C+D) + Clearance < 0.8 x Vessel I.D.
3.5 Nozzle Location Limitations
a. Vessel Weld Line
Inflection Point Engineering assumes that a reinforcing pad will be applied to all nozzles attached to a vessel (the vessel vendor could use an integrally reinforced nozzle, which doesn’t require a reinforcing pad; however, not all vendors use these). The reinforcing pad is approximately 2x the nozzle diameter. Locate 2 inch nozzles and smaller a minimum of 6 inches (150 mm) from the tangent line to the centerline of the nozzle to avoid interference with the vessel weld line,. Locate nozzles greater than 2 inches a minimum distance of the nozzle diameter plus 5 inches (125 mm) from the tangent line to allow for the reinforcing pad. Distance from tangent line = DN+5”
b. Tangential Nozzles
Generally, for tangential nozzles on horizontal vessels, the maximum permissible distance from the centerline of the vessel to the nozzle is 0.45 times the vessel diameter.
3.6 Manways
Inflection Point Engineering practice is to specify manway sizes as ID and to provide a minimum manway size of 24" ID, including lining, if applicable. If required specify larger size manways to accommodate internals. Provide nominal sized manways only upon Owner request. If the vessel diameter is less than 48” (1200), see IPE-TM-300-16, Small Diameter Vessels for guidance regarding manway sizing.
a. Horizontal Vessels
For unlined horizontal vessels, normally locate the manway on the side of the vessel at or below the horizontal center line. If the bottom half of the horizontal vessel is lined, locate the manway on the upper side or the top of the vessel. Horizontal vessels with small diameters may have a full flanged head. See IPE-TM-300-16 for guidance regarding manway sizing.
b. Vertical Driers, Treaters, etc.
These vessels usually combine the top manway with the process piping and also use it for loading solids or packing. Use a minimum 24" ID size, which may be larger to permit installation of internals. Provide a manway or handhole where applicable on the side near the bottom of the vessel to unload solids directly into a truck. Also, see Section 3.11. If the vessel diameter is less than 48” (1200), see IPE-TM-300-16 for guidance regarding manway sizing.
c. Trayed Columns
In trayed columns, provide manways as follows:
- On the top head or on the side near the top head
- Below the bottom tray
- At the feed trays
- At any other tray where access may be required frequently.
- At intermediate points such that the maximum spacing of manways in the trayed section does not exceed 60 feet (18 meters).
- If the vessel diameter is less than 48” (1200), see IPE-TM-300-16 for guidance regarding manway sizing.
d. Removable Internals
Size the manway at the distributor of a trayed column to accommodate the largest pipe fitting, without flanges, of the distributor. Refer to Attachment 1, “Minimum Openings Required for Passage of Pipe Fittings”. For example, a 14 inch straight tee must fit through the manway of a column. Attachment 1 shows the minimum opening required is 21 inches. Select a manway size of 24 inches since 22 inch manways are not used. Reactors also require large manways to accommodate centerpipes, etc.
e. Nominal Sized Manways
The ID of a nominal sized manway depends on nozzle construction, corrosion allowance, and code requirements. Generally, manways with Class 150 or 300 flanges in the nominal size range of 18 to 24 inches have an ID about 1 inch less than the nominal size. Higher flange classes and larger sizes require that the ID be specified.
3.7 Inspection Openings
These guidelines apply to vertical vessels in pressure service with diameters less than 48” (1200). The minimum allowable inspection openings are defined in the ASME Code Section VIII.
a. Vessels with an ID of 12 Inches or Less (300 mm or less)
No separate inspection openings are required if there are at least two removable pipe connections of a size not less than 1-1/2 inches located at each end of the vessel on the head or on the shell near the head. If separate inspection openings are required, provide 1-1/2 inch flanged connections as described in the previous sentence.
b. Vessels with an ID 12 Through 16 Inches (300 to 400 mm)
Vessels with removable internals shall have a flanged top head and one 1-1/2 inch inspection opening on or near the bottom head. Vessels without removable internals shall have the same inspection openings as specified in Section 3.7.a.
c. Vessels with an ID of 18 Through 24 Inches (450 to 600 mm)
These vessels shall have a flanged top head or two handhole connections of a size not less than 2 inches located at each end of the vessel on the head or on the shell near the head. See IPE-TM-300-16, Section 4.4.
d. Vessels with an ID of 2'-6" (750 mm or 800 mm)
Vessels with removable internals may have a flanged top head or an elliptical head with a manway on or near the head. Vessels without removable internals shall have a manway located on the shell near the bottom head. See IPE-TM-300-16, Section 4.3.
e. Vessels with an ID of 3'-0" (900 mm) and 3'-6" (1100 mm)
Specify these vessels with a manway. See IPE-TM-300-16, Section 4.2.
3.8 Ventilation
Provide all horizontal vessels 10 feet (3000 mm) and greater in tangent length with a blanked off ventilation nozzle on the top of the vessel near the end opposite the manway.
Provide all vertical vessels and fractionators 10 feet (3000 mm) and greater in tangent length with a blanked off ventilation nozzle on the top head.
These nozzles are a standard safety item necessary for the preparation of the vessel for safe entry. Do not omit upon Owner/Contractor request. Size the ventilation nozzle as follows:
| Vessel Tangent Length | Nozzle Size |
|---|---|
| 10'-0" (3000 mm) through 15'-0" (4500 mm) | 4" |
| Over 15'-0" (4500 mm) through 25'-0" (7500 mm) | 6" |
| Over 25'-0" (7500 mm) | 8" |
The ventilation nozzle is sized by WIN254. This nozzle may be omitted only upon instruction from the appropriate Technology Specialist or the Vessel Specialist.
3.9 Steamout
Provide a steamout connection on each vessel, which is steamed out as part of the normal start-up or shutdown operation. Examples of vessels which will not be provided with a steamout connection are reactors in hydrogen-rich gas circuits, vessels containing caustic, and vessels containing materials such as molecular sieves which could be damaged by water.
The steamout nozzle size shall be the same as the vent with a minimum size of 2 inches. Fit a 2 inch steamout connection with a gate valve and blankoff. When the steamout connection size is 3 or 4 inches, hard pipe the steamout connection with a gate valve, spectacle blind, 3/4" plugged bleed valve, check valve, and gate valve. Locate the steamout connection as follows:
- For horizontal vessels, on the head furthest from the ventilation nozzle
- For vertical vessels, near the bottom tangent line
- Do not locate in the lined portion of the vessel when the lower half of the vessel is lined.
3.10 Vents and Drains
The Inflection Point Engineering practice for minimum sizes of vents and drains is shown below. Check to determine whether any process reasons or Owner instructions (from the BEDQ) exist requiring vents or drains to be sized larger than these minimum sizes.
| Vessel Diameter | Vent and Drain Size |
|---|---|
| 15'-0" (4500 mm) and less | 2" |
| Over 15'-0" (4500 mm) through 20'-0" (6000 mm) | 3" |
| Over 20'-0" (6000 mm) | 4" |
- Inflection Point Engineering practice is to make the vent and drain nozzles the same size. Vent nozzles are not to be less than one size smaller than drain nozzles to prevent a vacuum condition during draining operations.
- In general, with the exception of some reactors in hydrogen service, all vessels shall have a vent and a drain. These reactors contain catalysts, which would be damaged by the presence of water or oxygen. Operating procedures do not allow water to be introduced into these reactors; oxygen is removed with a purge and evacuation procedure. Minimize the number of vessel connections to reduce the potential for hydrogen leakage during normal operation and air ingress during evacuation procedures.
- For vertical vessels, locate the atmospheric vent nozzle and valve on the vessel, or on piping near the vessel, at the highest point and use for venting to atmosphere, for steamout, purging, and hydrostatic testing.
- For fractionators, locate the atmospheric vent nozzle and valve on the top head of the column; most commonly, on the manway or on the neck of the vapor outlet nozzle. Commonly locate the depressuring vent on the overhead receiver or around the pressure relief valve assembly.
- For horizontal vessels, locate the atmospheric vent valve on the blankoff for the ventilation nozzle.
- An atmospheric vent nozzle and valve must be provided when a steamout connection is specified.
- A depressuring connection to the pressure relief header is always required.
- Locate the drain either on the vessel or on the piping near the vessel at the lowest point for draining hydrostatic test water to the sewer or hydrocarbon to a closed drain or pumpout system.
- When a stacked vessel design is applied (common head between vessels), the lower vessel must be provided with its own vent. This vent should be located as close to the top of the lower vessel as possible. This ensures that the upper portion of the lower vessel does not accumulate toxic/flammable vapors. The distance below the weld line from the intermediate head is DN+5” or 6”, whichever is greater.
3.11 Unloading
Install withdrawal nozzles at an angle on the bottom head of radial flow reactors and treaters and at an angle on the side and/or bottom head of most downflow reactors and treaters.
3.12 Instrument/Gauge Glass
For information on nozzles for level instruments and gauge glasses see Procedures , and ”.
Thermowell nozzles must be flanged and specified as 1" ID min.
When multiple pipe columns are required for gauge glasses on vessels, the upper connection for the differential pressure type level transmitter will be attached to the top pipe column and the lower connection to the bottom pipe column. When the customer request block valves between the vessel and pipe column(s), separate 1 inch (minimum) connections to the vessel must be used for the differential pressure type level transmitter and any pressure transmitter that would be attached to the pipe column.
3.13 Pressure Relief Valves
When PSVs are connected to a vessel, Inflection Point Engineering requires the nozzle size to have an ID the same size as the PSV inlet flange unless the inlet piping to the PSV is oversized to avoid excessive pressure drop. For more information refer to Procedure ”.
3.14 Refractory Lined Vessels
Design nozzles, where the vapor flow will impact a refractory lined surface, for a maximum velocity of 35 ft/sec, especially in services with high hydrogen content.
3.15 Drop Leg
Inflection Point Engineering used to provide flanged boots when the vessel was lined and the boot diameter was less than 30 inches. Vessel vendors were unable to line boots less than 30 inches diameter. In that case, a flanged, unlined boot having additional corrosion allowance (6mm instead of 3mm) was provided. The boot was treated as a consumable item, which was replaced by bolting on a new boot when the corrosion allowance of the old boot had been consumed.
Vessel vendors are now able to line the smallest boot diameter (14”) that Inflection Point Engineering provides in its design. It is no longer necessary to provide a flanged boot for heavy, corrosive liquids. The boot lining can be fabricated of the same material as the vessel lining. Therefore Inflection Point Engineering’s standard practice will be to provide all boots as welded, regardless of size or metallurgy.
For sizing criteria for drop legs refer to Procedure Section 4.2. For location of level instrument nozzles on drop legs refer to Procedure IPE-TM-300-02; Sections 7.3 and 8.
3.16 Hot Vapor Bypass
Locate the nozzle for a hot vapor bypass circuit on top of a vessel approximately equidistant between the inlet and outlet ends of a vessel. The nozzle shall have a baffle in accordance with Standard Drawing 3-185.
4. Horizontal and Vertical Vessel Internals
4.1 Coalescing Blankets
Use coalescing blankets to coalesce fine droplets of the dispersed liquid phase from the continuous liquid phase in a shorter period of time than could be accomplished by gravity settling in a vessel. Normally, use stainless steel or Monel, although anthracite coal and fiberglass have been used. If an organic liquid is dispersed in water and coalescing is needed, the material of the blankets shall be Teflon or other organic material which is preferentially wetted by the dispersed phase.
The blanket normally has a thickness of 12 inches (300 mm). Locate the vessel manway upstream of the blanket. Access the downstream area by removing a section of blanket. For sizing and location criteria refer to Procedure IPE-TM-300-04 Section 4.1.
4.2 Demisting Blankets
Use demisting blankets to remove entrained liquid in vapor streams where such liquid may cause damage to downstream equipment such as compressors or result in loss of product to the fuel system, etc. If excessive entrainment of liquid in the vapor outlet stream could be economically significant or detrimental to equipment, safety, or operation, size the vessel to allow gravity settling of liquid from the vapor phase in addition to providing a partial mesh blanket. For less critical situations use a full vessel diameter demisting blanket which meets the vapor velocity criteria. Inflection Point Engineering designs demisting blankets for 100 percent of the optimum velocity required for vapor disengagement. Koch-Otto York Inc. states that adequate results can be achieved in the 30 percent to 110 percent range of the calculated optimum.
Use a single zone mesh blanket for all normal applications of demisting vapor streams with the blanket in the horizontal position. The blanket shall be 6 inches thick. In general, blankets for horizontal installation are fabricated in two styles.
a. One Piece Blanket
Mount the coil-wound, one piece blanket, which is 24 inches maximum diameter for passage through large vessel openings, on a simple x-bar grid which does not affect the blanket size. Refer to Figure 1.
b. Sectional Blanket
The sectional blanket, which is fabricated in 12-1/4 inch maximum width sections to pass through any size vessel manway is mounted on grids which are normally supported by a ring welded to the vessel shell. Refer to Figure 2. In addition to the support ring, use support beams as required to limit the blanket span to a maximum of 6 feet. Add the top surface area of the support ring and flanges of the support beams to the blanket area. The widths of these supports are normally as follows:
| Blanket Diameter, feet | Blanket Diameter, feet | Support Ring Width, inches | Support Ring Width, inches |
|---|---|---|---|
| Less than 8 | Less than 8 | 2 | 2 |
| 8 to 15 | 8 to 15 | 2.5 | 2.5 |
| Greater than 15 | Greater than 15 | 3 | 3 |
Support beam flanges are 3 inches wide. For demisting blanket sizing and location criteria refer to Procedure Sections 4.1 and 5.3. Blanket densities and wire diameters are included in Standard Specification 3-35, Mesh Blankets.
1" x 1/8" Cross Bar
Figure 1 One Piece Blanket Support
Figure 2 Sectional Blanket Support
4.3 Distributors
The table below summarizes the distributors used by Inflection Point Engineering and described further in this section.
| Section No. | Service | Type | Inflection Point Engineering Standard Drawing No. |
|---|---|---|---|
| 4.3.a | Sub-cooled liquids to horizontal drum | Top inlet - open end Bottom inlet - baffle | 3-183, Type B 3-185 |
| 4.3.b | Bubble point liquids to horizontal drum | Top inlet - slotted pipe facing nearest head | 3-183, Type C |
| 4.3.c | Immiscible liquids to horizontal drum | Side inlet - slotted pipe facing nearest head | 3-184 |
| 4.3.d | Mixed phases to horizontal drum | Slot facing nearest head for parallel flow phases. Slot facing sides for opposite flow phases | 3-183, Type C 3-183, Type A |
| 4.3.e | Mixed phases to vertical drum | Tee | 3-190 |
| 4.3.f | Feed to reactor | Depends on service | ---- |
a. Total Liquid Inlets to Horizontal Drums – Sub-cooled Liquids
Examples of sub-cooled total liquid inlets include cold feed surge drums or overhead systems with the final condenser at grade. The liquid inlet may be through the top of the vessel with an internal open end, non-slotted pipe (Standard Drawing 3-183, Type B) or through the bottom of the vessel with a baffle (Standard Drawing 3-185). In either case, the inlet nozzle and the internal distributor shall be the same size as the line size to the vessel. The distributor shall remain sealed under the minimum liquid level.
b. Total Liquid Inlets to Horizontal Drums - Bubble Point Liquids
Total liquid inlets have the possibility of small quantities of vapor existing, examples include bubble point system, low pressure system, or overhead condensing systems with an equalizing line to the condenser rundown line. The inlet shall be through the top of the vessel with an internal closed end, and slotted pipe distributor (Standard Drawing 3-183, Type C). The inlet nozzle and internal distributor pipe shall be the same size as the line size to the vessel. Terminate the closed end 6 inches above the bottom of the vessel. The slotted area shall be approximately twice the cross sectional flow area of the distributor pipe and shall be provided by one slot, oriented on the vessel centerline, facing the nearest head of the vessel.
c. Total Liquid Inlets to Horizontal Drums - Two Immiscible Liquids
Total liquid systems may have two immiscible liquids present in sufficient quantities to establish an interface level at or near the centerline of the vessel, not in a drop leg. Begin the inlet on the side of the vessel with a horizontal pipe distributor perpendicular to the major axis of the vessel (Standard Drawing 3-183). The inlet nozzle and the internal distributor shall be the same size as the line size to the vessel. Locate the horizontal pipe at the normal elevation of the interface between the two liquids with a closed end opposite the inlet and slotted on the side opposite the vessel outlet. The slot area shall be approximately twice the cross sectional flow area of the distributor inlet pipe.
d. Mixed Phase Inlets to Horizontal Drums
The inlets to mixed phase vessels, such as 2-phase receivers, have 2-phase inlet lines are sized for higher velocities than total liquid lines. If these higher velocities were carried into the vessel, the resulting turbulence would interfere with the disengaging function of the vessel. Therefore, expand both the inlet nozzle and the inlet distributor one pipe size greater than the line size to the vessel in order to reduce the inlet velocity. Indicate the expanded inlet nozzle size on the P&ID to confirm that it is intentionally larger than the line size.
Locate the inlet at the top of the vessel with a closed end and slotted internal distributor pipe terminating 6 inches above the bottom of the vessel (Standard Drawing 3-183, Type C). The slotted area shall be between two and three times the cross sectional flow area of the distributor pipe. Start the slotted section of the distributor approximately 6 inches from the top of the vessel and extend it the length of the distributor to the closed end. The slotted area for all vessels except those with a vapor phase and baffles for separate ends for light and heavy liquid phases shall be provided with one slot facing the nearest head of the vessel. For vessels with a vapor phase and baffles for separate ends for light and heavy liquid phases, the slotted area shall be provided by two slots arranged perpendicular to the major axis of the vessel (Standard Drawing 3-183, Type A).
If the elevation of the inlet nozzle must be minimized, the inlet can be on the side of the vessel with an horizontal pipe distributor perpendicular to the major axis of the vessel (Standard Drawing 3-183). Expand the inlet nozzle and the internal distributor one pipe size greater than the line size to the vessel in order to reduce the inlet velocity. Indicate the expanded inlet nozzle size on the P&ID. Locate the horizontal distributor pipe above the maximum liquid level in the vessel and slot it on the side opposite the vessel outlet. The length of the horizontal pipe shall be large enough to permit a slot area approximately twice the cross sectional area of the distributor pipe. Do not use this distributor for vessels with a vapor phase and baffles for separate ends for light and heavy liquid phases.
e. Vertical Drums
For vertical receivers or separators use a tee type distributor located close to the vessel wall. Refer to Standard Drawing 3-190. For vessels with flanged top heads, omit the tee type distributor.
f. Reactors
Reactors use three or four types of distributors. The feed distributor is in most cases a velocity breaker to prevent impingement on the catalyst or a liquid-vapor distributor above the top of the catalyst bed. In some cases a liquid-vapor mixture is distributed with a mixing device at the inlet. Another device often used at the top and intermediate distribution points in reactors with mixed phase streams is a plate with raised vapor pipes. Liquid holes are provided in the side of the pipe and are calculated to give at least 1-1/2 to 2 inches of liquid head at the lowest liquid rate.
Use quench distributors with or without re-distributor trays and locate in the catalyst bed. Normally inject liquid quench into a vapor space between catalyst beds with a spray nozzle. Use suitable means to give uniform flow and temperature of the quench stream. Where necessary, use a double concentric pipe, which satisfies both of the requirements just mentioned as well as preventing catalyst attrition from high velocity exit from the distributor pipe.
Distribution of liquid and vapor is one of the most difficult and important problems faced by the designer. If good distribution is not accomplished, the best catalyst may not perform and may even coke rapidly due to stagnant area. In some processes dead pockets may cause demethylation reactions which have been known to damage reactor walls from the large exothermic heat of reaction.
4.4 Phase Separation Baffles
For criteria for locating of phase separation baffles refer to Procedure Sections 4.1, 5.2 and 5.3.
4.5 Vortex Breakers
a. Bottom
Use Vortex breakers (VB) on any vessel outlet nozzle from which a pump is taking suction. Also use if gas entrainment would present a problem in downstream equipment to which an outlet is feeding. The type of VB depends on the vessel lining if any, on nozzle size and if there is a separate water phase present.
Always provide a raised VB if there is a drop leg on the vessel. The top of the extended pipe vortex breaker shall be 6 inches above the vessel shell unless there is a specific process requirement to the contrary. With no drop leg on a vessel, provide a raised VB to help in water removal if there are certain process considerations. Examples of these considerations include downstream catalyst protection, or a requirement that the product from the vessel meet a dryness specification, both of which would necessitate removal of free water from the equipment at startup. See Standard Drawings 3-122 and 3-123.
b. Side
If the pump suction is taken off the side of a vessel, or on very large nozzles, install a grating to act as a VB when the outlet is close to a liquid-vapor interface. See Standard Drawing 3-322. A VB is not required at the drawoff nozzle for a tray.
c. Small Vessels
For small vertical vessels, such as knockout drums, the vortex breaker may be welded to the bottom head before the head is welded to the shell.
5. Fractionator Nozzles and Internals
5.1 General
The Design Engineer is accountable for the inlet nozzle sizes and the types and sizes of inlet distributors on fractionators. The feed distributor piping will normally be the same diameter as the vessel nozzle. Although Computer Vessel Program 254.6 for designing trayed and packed vessels specifies all distributors suitable for vapor/liquid service, consider expected vaporization at the feed nozzle to ensure velocity criteria are satisfied.
During both the P&ID check and project document review stages, the Design Engineer shall check the effect on column performance of internals in the transition zones between trayed, packed or trayed/ packed sections. Also check drawoffs for once-through reboilers, preferential and absolute, and vertical thermosiphon reboilers.
The default location for top fractionator manways is on the side of the column. Most customers prefer this location as it generally facilitates personnel entry as well as the entry of long structural members. If the customer prefers, the top manway may be located on the top head provided that MD trays are not being used in the column. The use of MD trays requires the top manway to be on the side of the column to facilitate the entry of the downcomers.
The default location for reflux nozzles is also on the side of the column.
5.2 Liquid Inlets
Reflux is sometimes introduced behind a weir provided by the tray vendor.
a. Multipass Trays
Distribute liquid approximately proportionately to the bubbling areas of multipass trays.
b. Velocity Criteria
To prevent hammering due to back flow of vapor, the exit velocity of liquid shall be greater than 2 ft/sec. However, the velocity shall not be excessive, preferably less than 4 ft/sec (The 4 ft/sec velocity constraint may be relaxed for large diameter columns. Consult the vessels specialist to determine if this requirement can be relaxed). If the distributor ends in a tee, the exit velocity from each branch of the tee shall be between 2 and 4 ft/sec.
Occasionally the distributor may have to be reduced from the inlet nozzle size to achieve a minimum velocity of 2 ft/sec. The vessel inlet nozzle should be the same size as the inlet line size running to it. Any reduction in size for the distributor should occur inside the column. This size reduction for the distributor requires a special note on the Project Specification 301.
Once in a while the distributor may have to be increased from the line size to satisfy the maximum velocity of 4 ft/sec. If there is no distributor , this is accomplished by installing a reducer outside the column. The reducer should be located 10 pipe diameters upstream of the inlet nozzle. The distributor should be the same size as the inlet nozzle it is connected to. Check the velocity at each outlet of the distributor to ensure the velocity is within to 2 – 4 ft/sec range. Reduce the distributor outlet cross sectional area as required to maintain the target velocity range of 2 - 4 ft/sec.
If a distributor is present, do not show a line size change on the PID. Instead, indicate on the PID the required hydraulic line size. The larger nozzle size required for the distributor is indicated on the PID with the size and an arrow pointing to the subject nozzle. The contractor will determine how to connect the smaller line size to the larger nozzle.
5.3 Vapor and Mixed Phase Inlets
The recommended maximum allowable velocity for vapor or two phase flow in inlet nozzles (e.g. most reboiler returns, feed and reflux nozzles) is:
V = 5.0 {(L - V)/ V}1/2 (without distributor)
V = 7.5 {(L - V)/ V}1/2 (with distributor)
Where V = velocity, ft/sec
L = liquid density, lb/ft3
V = vapor density, lb/ft3
Velocity “V” (for vapor & mixed phase inlets) is calculated at the vessel shell penetration. Do not calculate “V” at the distributor outlet(s). Note that the minimum velocity is not design parameter for vapor or mixed phase inlets.
Where there is vapor only, the liquid density for the above formula shall be the density of the dew point liquid. Do not submerge vapor feeds and mixed phase feeds below the froth. Computer Vessel Program 254.6 places the bottom of the distributor in the middle of the froth. The distributor is located above the tray floor at a distance equal to 60 percent of the tray spacing.
a. Multipass Trays
For multipass trays, do not isolate vapor feeds from tray panels above. Do not use vapor tunnels in downcomers for this communication.
b. Velocity Criteria
Vapor inlets have an upper velocity limit for good column operation. Vapor and mixed phase maximum velocities are calculated for the nozzle at the shell penetration only, not the distributor outlet(s). Apply the same maximum velocity limit to mixed phase inlets, including reboiler returns. Minimum velocity limits are not applied to vapor & mixed phase inlets.
c. Packing
Advise the packing vendor of the quantity of vapor likely to be in the feed so that he may properly provide the distributor. The specification form for packing provides space for this information.
5.4 Reboiler Return
The orientation of reboiler return nozzles with respect to tray downcomers is important for good vapor distribution to the bottom tray, if no collector tray is used. This is built into Computer Vessel Program P254.6. For information on a particular configuration, refer to Configurations for Trayed and Packed Vertical Vessels. Always locate the reboiler return(s) parallel to the downcomers when using conventional trays (1, 2, 3 or 4 pass trays).
a. Velocity Criteria
Refer to Section 5.3.
b. Single Return Nozzle
When a single reboiler return nozzle is used with one pass, or two pass trays (side downcomers), locate it on the vessel centerline and parallel to the downcomer(s). For three pass trays, locate it midway between and parallel to the side and intermediate downcomers.
c. Two Return Nozzles
Conventional Trays
When there are two or more reboiler return nozzles, orient them parallel to the downcomer(s). If there are two nozzles, locate both on the same side of the vessel. Arrange them, as much as possible, to proportionately distribute vapor to the bubbling areas of the bottom tray. If the bottom trays are two or four pass, they shall have side downcomers. When four pass trays are used, specify two or four reboiler nozzles.
MD Trays
Typically one reboiler return nozzle is sufficient for MD Trays. The return nozzle shall be oriented perpendicular to the downcomers. The distance below the bottom tray to the nozzle centerline shall be 2 reboiler return nozzle diameters or as calculated by P254, whichever is longer.
d. Clearance
The clearance between top of the reboiler return nozzle and the fractionating tray or packed bed above shall be the same for trays or packing to avoid confusion and provide design flexibility. The clearance for packing shall allow for a vapor distributor if necessary. Therefore, it is recommended that the clearance be 0.05 times vessel diameter plus 3 feet.
e. Nozzle Size
If no inlet distributor exists and the nozzle size must be increased to meet inlet velocity criteria, then increase the reboiler return piping for a length of 10 pipe diameters minimum upstream of the inlet nozzle. It is expected that this increase in return piping line size shall occur more frequently for fired heater reboilers than for shell and tube reboilers.
5.5 Vapor Side Draws
In general, a vapor draw taken from one side of a multipass tray tends to cause both high weep rates from the tray above and high froth heights from the tray below the vapor draw, particularly if the side-cut vapor is a large percentage of the vapors rising from the tray below.
In the past, Inflection Point Engineering has occasionally used tunnels in downcomers to balance this vapor traffic. However, field experience has not been good. Furthermore, after a review of the hydraulics of multipass trays, Inflection Point Engineering practice is that tunnels shall not be used. Tunnels severely restrict the capacity of the downcomer. If a special case arises where tunnels are deemed necessary, discuss with Fractionation Specialist before proceeding.
a. Entrainment Critical
If entrainment of liquid into the vapor draw is critical, place a seal welded collector tray above the draw. A bubble-cap tray can be used in place of the collector tray if the risers extend above the liquid head on the tray (weir height plus head over the weir). Use a vapor scoop and locate the centerline of the vapor draw (or draws) as shown below:
See Standard Drawing 3-115, Type III for vapor scoop details. Be aware that entrainment can still be significant at heights exceeding normal tray spacing in lower pressure columns.
b. Special Cases
For some special cases, where some entrainment of liquid into the vapor is acceptable, eliminate the collector tray (or bubble-cap tray). However, at turndown, considerable liquid weeping from the tray above can be entrained into the vapor draw.
For small vapor draws that do not exceed 5 percent of the rising vapor, use a single draw nozzle without providing tunnels in downcomers. Locate the nozzle as described in Sections 5.5.c through 5.5.e.
c. Two-Pass Trays
For two-pass trays, always locate the draw nozzle below a side downcomer tray and midway between the side downcomers so that no downcomer divides the vapor space from which the vapor is taken.
d. Three-Pass Trays
For three-pass trays, always install two nozzles, one on each side of the downcomer, as close to the horizontal center of the vapor space as possible. Specify symmetrical piping configuration up to the junction point. The cross sectional area of each nozzle must be proportional to the bubbling area it serves.
e. Four-Pass Trays
For four-pass trays, always locate the two draw nozzles below a tray that has the center downcomer. Specify symmetrical piping configuration up to the junction point. Use two nozzles of equal diameter located on each side of the downcomer as close to the horizontal center of the vapor space as possible.
5.6 Liquid Side Draws
Take side draws from columns from a well as part of a contacting tray immediately below a downcomer. Use a sidewell when the drawoff rate is less than 15 percent of the liquid rate that enters the downcomer from the tray above and when surge time is not required. When the drawoff rate is more than 15 percent of the liquid rate on the tray above and where surge time is required, take the side draw from a centerwell (accumulator tray). A centerwell is preferable when a pump takes suction direct from the column. The centerwell shall have a vapor riser(s) with at least 12 percent (20 percent for vacuum service) of the column cross sectional area and provide enough space above the baffle to give good vapor distribution to the tray above. See Procedure, Sections 4 and 9.
5.7 Preferential Once-Through Reboiler
For both single and multi-pass trays, use a collector tray with vapor risers and downcomer(s). Orient the downcomer(s) 90 to the downcomer(s) of the bottom tray to direct liquid from the bottom tray or packed bed, into the drawoff well and preferentially to the reboiler. A blind tray with a downcomer directs all of the reboiler return liquid to the drawoff well. Contour the drawoff well to receive all the liquid from the collector tray and blind tray. See Procedure IPE-TM-300-08 Sections 5, 6 and 7.
Use the following design criteria:
| Vapor Risers(s) : | 12 percent (20 percent for vacuum service) of vessel cross-sectional area |
|---|---|
| Collector Downcomer : | 0.2 ft/sec |
| Blind Tray Downcomer : | 0.2 ft/sec |
| Residence Time | |
| Fired Heater Reboiler : | 1 minute (material vaporized) |
| Thermosiphon Reboiler : | 0.5 minutes (reboiler flow) |
For dimensions refer to Attachment 2, Reboiler Drawoffs.
5.8 Absolute Once-Through Reboiler
Use a collector tray designed the same as for a preferential once-through reboiler, except that the downcomer is the drawoff well to direct all of the liquid from the bottom tray or packed bed to the reboiler.
Use the following Drawoff Well design criteria:
| Velocity, max. : | 0.2 ft/sec |
|---|---|
| Residence Time, min. : | 0.5 minutes (reboiler flow) |
For dimensions refer to Attachment 3.
5.9 Vertical Thermosiphon Reboiler
Use a blind tray with a downcomer to direct all the liquid from trays or packed bed and reboiler return into a drawoff well for the reboiler.
See Procedure IPE-TM-300-08 Sections 6 and 7.
Use the following design criteria:
| Blind Tray Downcomer : | 0.2 ft/sec |
|---|---|
| Drawoff Well | |
| Velocity, max. : | 0.2 ft/sec |
| Residence, min. : | 0.5 minutes (reboiler flow) |
Standard Drawing 3-274 shows details of drawoff wells for reboilers. For dimensions refer to Attachment 3.
5.10 Column Bottom Surge Capacity
a. With Thermosiphon Reboiler
The surge volume, i.e., the float range, shall be the larger of:
- 1 minute on total bottoms (i.e., net plus reboiler flow)
- 2 minutes on material to storage
- 5 minutes on material to feed another column
- 7* minutes on material to feed a reactor section without a surge drum.
b. With Fired Heater Reboiler
Base the surge volume, i.e., the float range, on 1 minute for material vaporized plus the following appropriate requirement(s) for the net bottoms.
- 2 minutes on material to storage
- 5 minutes on material to feed another column
- 7* minutes on material to feed a reactor section without a surge drum
For columns with reboiler drawoff wells, which are an exception to this, refer to Sections 5.7, 5.8, and 5.9 of this procedure. The surge volume for the net bottoms shall be the float range for the above appropriate requirements.
*This number applies to a Stripper upstream of a Platforming Process Unit. For other processes refer to the appropriate Technical Specialist.
Ancillaries
This section covers ancillaries for both vertical and horizontal vessels.
6.1 Vessel Supports
a. Skirt
Most vertical vessels are supported by a skirt. The bottom nozzles must project through the skirt to avoid flanges inside the skirt, reducing the hazard associated with leaks.
For small vessels less than 2'-6" (800 mm) in diameter and 10'-0" (3000 mm) tangent length, if the support length from the base of the support to the bottom tangent line is 5'-0" (1500 mm) or less, provide support legs. If the support length is greater than 5'-0", specify a vessel skirt. Generally, specify straight skirts based on economics. There are exceptions for supports; use good engineering judgment.
b. Legs
Use legs on short drums at low pressures to save cost and avoid flanges inside skirts.
c. Lugs
Use lugs for equipment support when good ventilation of the vessel surface is required or when a small diameter does not allow a skirt or legs to be provided.
d. Table Top
Use table top support where access to the bottom of a vessel is required. It is the most expensive type of support and is generally avoided. Support the vessel on the table with lugs or a short straight or flared skirt.
e. Saddles
Most horizontal vessels are supported on saddles which sit on concrete supports. Saddles are 6 to 10 inches wide and located at a distance from the vessel tangent which is no greater than 0.25 times the vessel diameter.
f. Bracing
The criteria for deciding when bracing may be required depends on the vessel length and diameter. In this case, the vessel length (L) to be considered is the distance from the bottom of skirt to the top of the top head. Vessels with a high length to diameter ratio (L/D) may need bracing to prevent sway. For vessels up to 130 feet in length, bracing may be required when L/D is greater than 20, and for vessels longer than 130 feet, the criterion is L/D greater than 15.
Add the following note to the Project Specification 301 Vessels when these criteria apply:
"Contractor may have to provide a structure or other method of lateral support to prevent harmonic vibration of this vessel due to vortex excitation."
6.2 Stiffener and Insulation Rings
a. Stiffener Rings
Use stiffener rings made from angle irons, channels or tee sections to strengthen vessels for vacuum service and external design pressure requirements.
b. Insulation Rings
Use insulation rings to support block insulation on the walls of vessels. See Standard Specification 9-12, Figure 5.
6.3 Fireproofing and Insulation
a. Fireproofing
Fireproofing is typically applied to all steel vessel supports up to an elevation of 35 feet when located in an area with fire potential.
b. Insulation
Insulation is called for whenever heat loss or heat gain would affect the process. See Standard Specifications 9-11 and 9-12. When insulation is required, indicate a requirement for insulation clips and rings on the vessel project specification. Insulation for personnel protection is left to the Contractor and is not shown in our project specifications or drawings. Some guidelines for personnel protection are given in Standard Specification 911.
c. Weather Shielding
Weather shielding is sometimes provided to minimize thermal stress and differential thermal expansion of nozzles and flanges.
6.4 Davits
Davits are a type of crane installed on fractionators and vessels to facilitate the raising of internals and for handling manway covers.
6.5 Platforms
a. When Required
Platform access is required for manways and equipment, such as controls located at an elevation which cannot be accessed from grade. Require permanent platforms when the elevation of manways is 10 feet (3.3 meters) or greater above grade. Require temporary or portable platforms when the elevation of manways and equipment is less than 10 feet and greater than can be accessed from grade. Platform location is the accountability of the contractor.
Add the following note to the Project Specification 301 Vessels at the data entry for ladder and platform clips when these criteria apply:
“AS REQUIRED BY CONTRACTOR”.
b. P&ID
Platforms are not shown on the Inflection Point Engineering P&ID, but the following note appears on the P&ID legend.
"Contractor is to provide platform access for manways and equipment which cannot be accessed from grade. Permanent platforms are required when the platform structural support members allow a clearance of 6'-8" (2040 mm) or greater above grade. Temporary or portable platforms are required when clearance between platform structural support members and grade would be less than 6'-8" (2040 mm) above grade."
Attachment 1 Minimum Openings Required for Passage of Pipe Fittings
NOM. PIPE SIZE | ||||
|---|---|---|---|---|
| 90 L.R. ELL | 90 L.R. ELL | STRAIGHT | STRAIGHT | |
| (S.R. ELL) | (S.R. ELL) | TEE | CROSS | |
| 6" | 13.25" | 8.5" | 10.25" | 13.25" |
| 8" | 17.5" | 11.0" | 13.0" | 16.5" |
| 10" | 22.0" | 13.75" | 16.0" | 20.25" |
| 12" | 26.25" | 16.25" | 19.0" | 23.75" |
| (20.75") | (14.5") | |||
| 14" | 30.0" | 18.25" | 21.0" | 26.25" |
| (23.5") | (16.25") | |||
| 16" | 34.0" | 20.75" | 23.25" | 29.0" |
| (26.75") | (18.5") | |||
| 18" | 38.25" | 23.5" | 26.25" | 32.5" |
| (30.0") | (20.75") | |||
| 20" | 42.5" | 26.0" | 29.0" | 36.25" |
| (33.5") | (23.0") | |||
| 22" | 46.75" | 28.5" | 32.0" | 40.0" |
| (36.75") | (25.25") | |||
| 24" | 51.0" | 31.25" | 34.0" | 41.75" |
| (40.0") | (27.5") |
Note: Dia. is rounded off to the next quarter inch
22" pipe size is not recommended
Attachment 2 Reboiler Drawoffs
For Preferential Once-Through Thermosiphon Reboiler
For Absolute Once-Through Thermosiphon Reboiler
For Vertical Thermosiphon Reboiler
DV - Vessel I.D. DN - Reboiler Nozzle Diameter
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