Towers and Columns Piping And Design

Introduction

Towers also referred to, as columns are one of the principal pieces of equipment of any processing facility. Towers are cylindrical steel vessels that are used for distilling raw materials in the production of such products as gasoline, diesel, and heating oil. The plant layout designer must understand the internal structure of a tower and how it operates to produce a satisfactory design.

This chapter highlights the general requirements for the tower plant layout design. It describes the internal workings of towers and provides the information required to orient nozzles; locate instruments, piping, and controls and provide platforms and ladders for the operator and maintenance access.

The Distillation Progress

Crude oil is of little commercial use; when separated, or broken down, however, oil becomes one of the most valuable commodities in the world. Crude oil is a mixture of hydrocarbon compounds with a wide range of boiling points from 100 0F (38 0C) to 1400 0F (7600 C).

Separation or distillation is a process by which a liquid mixture is partially vaporized. The vapours are then condensed, separating the individual components of the mixture. As the temperature of crude oil is raised, the initial boiling point (IBP) is reached. As boiling continues, the temperature rises. The lightest material, butane, is produced first, at IBP, just below 100 0F (38 0C); the heavier materials are produced below 80 0F (427 0C) The residue includes everything above 80 0F (427 0C) .

The evolution of distillation towers is best explained in three basic steps.

• The batch shell still process 

 
• The continuous shell still process. 

 
• The fractional distillation process.

Batch Shell

In the batch shell still process, the still is partially filled with a set feed called a batch. The feed is then heated to the temperature required to produce a specific product from the overhead vapours. This process is repeated each time for each product until the batch reaches the maximum temperature for the range of products specified. The feed remaining in the still is then pumped out, and the still is allowed to cool. It is then refilled, and the whole process is repeated Not only is this process time consuming but also the product is not always of high quality. The batch sheet still process was one of the earliest used for liquid mixture separation.

Continuous Shell

In the continuous shell still process, several shell stills are linked in series to form a battery. Fresh feed continuously enters the first still, which is kept at the lowest temperature for the lightest overhead product. The bottoms from the first still are fed to the second still, which is kept at the temperature for the next highest boiling overhead product and so on for the number of products needed. If the feed and the temperature of each still remain constant, the finished product is of satisfactory quality. The continuous shell still process, which is an improvement over the batch shell still operation.

Fractional Distillation

Similar to the continuous shell still the fractional distillation process is made up of several stills linked together in series. The main difference is that all the liquid condensate is returned to the upstream still As the feed is partially vaporized in the first still the vapours rise, travel through the overhead line, and come into contact with the liquid in the second still. Because the temperature of the liquid in the second still. Because the temperature of the liquid in the second still. Because the temperature of the liquid in the second still is lower than the incoming vapours from the first still, the vapours partially condense. At the same time, liquid from the second still enters near the top of the first still. As vapours rise in the first still, they meet the incoming liquid from the second still.

Drawing

This causes vaporisation of the incoming liquid from the second still and condensation of the rising vapours in the first still The same reaction takes place in all the downstream stills. This process improves on the previous operations in terms of quantity, quality and a reduction in the energy needed to heat the raw materials.

All three-process arrangements are satisfactor y operations and play an important part in the development of the modern distillation tower. The final step in combining these operations into one single component is achieved by stacking the stills one on top of each other and installing an internal device between each still to allow the liquid to flow down and the vapors to rise. This means that the single unit can function in a way similar to the multi shell unit for less capital and operational cost. The reflux return line controls
the temperature of the fluids in the upper portion of the tower.

Vapour and Liquid Flow

One of the most common internal devices that allows the single tower to function similarly to the multi still unit is the tray. Slots and holes in the trays allow the vapour to rise and the liquid to flow down.

Rising vapours in the tower pass through slotted bubble caps and come into contact with liquid flowing around the caps. Liquid flowing down from trays above falls through the down comers and over and around the bubble caps round to the next drwncomer. In this manner, the lighting boiling fractions in the down flowing liquid are vaporised by the heat from the rising vapour and heavier boiling fractions in the vapour are condensed and flow down the tower. This process of vaporising and condensing throughout the tower allows the feed to be separated into the required boiling-range fraction, which are drawn off from the side of the tower at the appropriate location.

Types of Towers

Towers are named for the service or type of unit they are associated with for example a stripper is used to strip lighter material from the bottoms of a main tower or a vacuum tower. It is generally used in a vacuum crude unit for distilling crude bottoms reside under vacuum pressure.

From the outside, tower configuration are similar in appearance, varying only in dimension. Some towers have swaged top and bottom section. The principal difference among towers is the type and layout of the internal components that controls the vapor liquid contact. This chapter describes the internal and external plant layout requirements for the two most common types of tower: the tray and packed arrangements and a typical tray tower with some of its associated components.

In a packed tower, instead of having trays, the units are packed with beds of metal rings. On entering the tower the liquid passes through a distributor that route the liquid evenly down through the packed beds of metal rings. Rising vapours passing through the beds come into contact with the descending liquid a manner similar to the tray tower operation, the liquid is partially vaporise by the heat and the vapours are condensed by the cooler liquid.

Design Consideration for Towers

Towers are not a standard operation they are usually located within a process unit adjacent to related equipment and in a suitable position for operator close to such related items as pumps re boilers drums and condensers and should be in position to facilitate an orderly and economic interconnection between itself and that equipment.

Within the conventional inline process unit, towers and their related items are located on either side of a central pipe rack serviced by auxiliary roads for maintenance access in plants in which the related equipment is housed, the towers is often located adjacent to the building or structure containing the equipment.

Tower Elevation and Support

Tower elevation is the distance from the grade to the bottom tangent line of the vessel. Support is the means by which the vessel is retained at the required elevation.

Although the tower elevation must satisfy minimum NPSH requirements, it can be set by a combination of the following constraints – whichever produces the minimum tangent line elevation.

• NPSH 
• Operator access 
• Maintenance access 
• Minimum clearance 
• Vertical reboiler 
• Common access

A skirt is the most frequently used and most satisfactory means of support for vertical vessels, It is attached by continuous welding to the bottom head of the vessel and is furnished with a base ring, which is secured to a concrete foundation or structural frame by means of anchor bolts In most cases, the skirt is straight but on tall, small- diameter towers, the skirt could be flared Access openings are required in vessel skirts for inspection and when possible should be oriented toward the main access way a typical skirt arrangement.

The first step in tower layout is setting the bottom tangent line elevation. This step assists civil engineering in foundation design, vessel engineering in support design, systems engineering in line sizing and rotating equipment engineering in pump selection to set the elevation of a tower, the plant layout designer requires the following information.

• Tower dimensions
• Type of heads
• Support details
• NPSH requirements
• Bottom outlet size
• Reboiler details
• Foundation details
• Minimum clearances

For example, the tangent line elevation of the tower has been set using the following information and the guidelines in this chapter.

Configuration -Exhibit 10- 13 (operator access)

Tower dimensions 4ft (1,200mm) in diameter by

60 Ft (18,300mm) in length

Type of heads- 2:1 elliptical

Support – Straight skirt with base ring

NPSH- 6 Ft (1800 mm) minimum.

Bottom outlet size – 6 in diameter

Foundation – Concrete point of support elevation of

101 ft (100,300mm)

Operator clearance – 7 ft (2,100mm)

A freehand sketch should be used for this exercise.

Although the minimum NPSH requirement was a key factor in elevating the tower in this example, the height was finally dictated by operator access clearance , which was the greater of the two dimensions. If the configuration had been used the tangent line elevation would be 108.5 ft 102,600mm).

Tower Internals

Towers have a variety of internal devices for vapour liquid contact and feed distribution that affect the exterior layout of the vessel. There is a wide range of designs for trays, which are the principal internal component of the tray tower. The two most frequently used are the single pass bubble cap trays (e.g sieve or perforated trays) are similar in design to the bubble cap tray and do not affect the layout of the tower, Tray configuration and dimensions are furnished by process engineering and are included in the process release package.

Towers have the same tray configuration for the whole length of the tower. Some towers, however, especially those with enlarged sections could change from single – pass to double pass tray configurations. The chimney tray, if specified is another device that could change the tray configuration. The chimney tray is a solid plate with a central chimney section and is usually used at draw off sections of the tower.

The plant layout designer must orient the trays along with the tower nozzles to suit the best exterior arrangement. The tray can be at any angle as long as the downcomers directly oppose each other.

Two main items that influence tray orientation are maintenance access ways and reboilers. The process vessel sketch that the reboiler draw off nozzle is located directly below the downcomer of tray 2 and the plan arrangement indicates that the reboiler is located on the west side of the tower and that the maintenance road is south of the tower. Therefore, because the tower reboiler nozzle is generally on the same side as the reboiler and the maintenance access way is best located on the maintenance side, the trays are automatically positioned about a north south centreline.

The principal difference between the travel and the packed tower is that the packed tower uses metal rings instead of trays for vapour liquid contact The metal rings are dunped or pacjed into specific sections of the tower, called beds and supported by cross grid bars spaced to prevent the rings from falling through. The supports are designed to allow vapour to rise and liquid to flow down. Liquid is fed into the vessel at the top of each bed through a liquid distributor. Unlike the tray tower, there are no special considerations for orientation of the beds, the distributor, or the packing supports.

Nozzle Elevation and Orientation

Nozzles must be elevated to meet the internal requirements of the tower and oriented for maintenance and operational needs. Their position must also facilitate economic and orderly interconnection of piping between the tower and related equipment.

A maintenance access is usually located at the bottom; top and intermediate sections of the tower and is used to gain entry to the tower during shutdowns for internal inspection and component removal. Maintenance accesses must not be located at the down comer sections of the tower. Care must be taken at the sections of the tower that contain internal piping to avoid blocking the maintenance access entrance.

Feed connections to trayed towers usually must be located in a specific area on the tray by internal piping. Which can restrict nozzle orientation options. The restrictions are minimized by optional routing of the internal piping to facilitate the most economic exterior arrangement Internal feed piping to packed towers is piped directly to the distribution and can be oriented at any angle.

If specified, reboiler connections are usually located at the bottom section of the tower. For the horizontally mounted there mosiphon reboiler the off nozzle is located just below the bottom tray to the vertically mounted recirculating the boiler. The draw off nozzle is located at the Bottom head for both systems, the return nozzles are located just above the liquid level .

The vapour outlet is usually a vertical nozzle located on the top head of the tower. It is usually a single nozzle but in certain cases (e.g. towers with very large diameters) more than one nozzle is specified on large – diameter vapour lines, the vessel connection could be welded instead of flanged. In addition the vent and relief valve could be located on the top head instead of attached to the overhead piping.

The liquid outlet is located on the bottom head of the tower. If a skirt supports the tower the nozzle is routed outside the skirt. As with the vapour outlet, when more than one nozzle may be specified the elevation of the nozzle is dictated by the constraints discussed previously in this chapter. The orientation can be at any angle but generally it is dictated by pump suction piping flexibility.

Temperature and pressure instrument connections are located throughout the tower. The temperature probe must be located in a liquid space and the pressure connection in a vapour space. The preferred location for both connection level instruments are located in the liquid section of the tower, usually at the bottom. The elevation of the nozzles is dictated by the amount of liquid being controlled or measured and by standard controller and huge glass lengths. This information is furnished on the instrument vessel.

When nozzles especially those with internal piping are positioned the plant layout designer must show adequate clearance at tray support steel illustrates approximate tray support beam sizes indouts are measured from internal diameter of the vessel to the face of the flange. To set top and bottom head nozzle elevations. The type of head must be specified. The information is highlighted in the process vessel data. The two most commonly used are flanged and dished and 2:1 elliptical heads.

As an example, the nozzle elevations have been set using the following guidelines.

• Process vessel sketch

• Tray details

• Type of heads – 2:1 elliptical

• Bottom tangent line elevation

• Nozzle summary

• Instrument vessel sketch

• Piping and instrumentation diagram

• Plant layout specification

• Insulation – None required

Platform Arrangements

Platforms are required on towers for access to valves instruments, blinds, and maintenance accesses platforms are usually circular and supported by brackets attached to the side of the tower. Generally access to platforms is by ladder.

Platform elevations for towers are set by the items that require operation and maintenance and by a maximum ladder run of 30 ft (9150mm). Platform widths are dictated by operator access for intermediate platforms with no controls are required and platforms with controls located to the side or the edge of the platforms. The width must be a minimum of 3 ft (915mm) plus the width of the controls or sections for maintenance access platforms, adequate space must be provided to swing the maintenance access flange open for storage against the face of the Top head- mounted maintenance A access must be from three sides for typical maintenance access arrangements. Top head platforms are required for access to vents, instruments, and relief valves and are supported from the head by trunnions. Typical top head platform arrangements. Access between towers, if layout permits. Is provided by common plat forming. The platform elevations can be within a maximum difference of 9 in (230 mm) but must be connected by mechanical joint.

Brackets for side-mounted platforms are evenly spaced around the tower and when possible, straddle both the main axes. Oddly angled brackets can be used for small platform extensions as long as the bracket clip does not interfere with the adjacent support. Exhibit 10- 46 is an approximate guide to bracket spacing.When a common ladder serves two or more platforms, the ladder rungs must be level with the platforms they serve. The platform elevations must be in even increments to suit the standard 12-in (300mm) ladder rung spacing. Ladders at tower transition sections and at flared skirts may be sloped, if required, to a maximum angle of 15 from the vertical. Offsets in ladders should be avoided On very wide platforms or those that support heavy piping loads, knee bracing is required in addition to the usual platform steel. The potential obstruction immediately under the brace must be kept in mind during platform design For example, the platform elevations shown on the process vessel. These are minimum requirements for instrument, valve and maintenance access.

Tower Piping

Tower piping is located in conjunction with tray nozzle and platform orientation. When possible the piping is grouped for case of support and positioned to accommodate interconnection with related equipment and the pipe rack ,The preferred areas of division for piping platforms and ladders.

Adequate space must be provided between piping and between the back of the piping and the tower shell to facilitate the installation of pipe support which are attached to the tower.

Tower piping should be arranged with sufficient flexibility to accommodate tower growth and to allow interconnection to equipment during regular operating conditions.

Relief valve systems that are open to the atmosphere are located at the top of the tower closed systems are located a minimum distance above the relief ledder.

As an example the piping arrangement has been designed using the following information and the guidelines in this chapter

• Process vessel sketch
• Tray details
• Nozzle elevations
• Instrument vessel sketch
• Piping and instrumentation diagram
• Equipment arrangement
• Platform arrangement
• Nozzle summary
• Plant layout specification

Tower Instruments

Level, pressure and temperature instruments control the operation of the tower and must be placed in a position that enhances operation and maintenance without obstructing operator access. Instrument requirements for towers are usually highlighted on an instrument vessel sketch furnished by the instrument engineer.

Level controllers, switches and gauges are either located individually or grouped on a common bridle or standpipe. The controller must be operable from grade or a platform; gauges and switches may be operable from a ladder if no platform is available.

Like level gauges temperature and pressure instruments can be operable from a ladder if a platform is not available at the required elevation. They can be read locally or in the main control room.

Mounted indicators are available in a variety of styles with straight or swivel heads that can be positioned for clear dial visibility.

The instrument arrangement has been designed using the following information and the guidelines in this chapter.

• Nozzle elevations
• Instrument vessel sketch
• Platform arrangement
• Piping arrangement
• Level instrument locations

Maintenance

Tower maintenance is usually limited to the removal of exterior items (e.g. relief or control valves) and interior components (e.g. trays or packing rings) Handling of these items is achieved by fixed devices (e.g. davits or trolley beams) or by mobile equipment (e.g. cranes) when davits or beams are used they are located at the top of the tower accessible from a platform and designated to lower the heaviest removable item to a designated drop area at grade. When mobile equipment is used a clear space must be provided at the back of the tower that is accessible from the plant auxiliary road.

In certain cases, stiffening rings are specified as additional strengthening especially for the tower shell especially For towers in vacuum services care must be taken in positioning the rings to allow adequate clearance at nozzles platforms ladders and clips. Because of size, towers can be shop fabricated in two or more sections for shipment in one piece or in sections for field welding. As with stiffening rings, allowances for clearances must be made between weld seams and attached fittings. The vendor’s vessel fabrication drawings show the location of weld seams.

Utility stations are required at tower platforms that have maintenance accesses. Steam and air risers are the two services required and must be positioned during the tower layout stage so that adequate clips can be furnished for support. Utility station requirement such towers as demethanizers operate under extremely cold conditions and sometimes.

Require increased standout dimensions for nozzles, platforms, and ladders to clear extra-thick insulation and to prevent frost on supporting steelwork. Polyarethance insulators are usually furnished with typical cold service tower requirements.

The dimensions, clearances, and guidelines highlighted in this chapter are an example of those to be used for tower arrangements. The plant layout designer, however, must be familiar with company and client tower standards before proceeding with tower layout and should coordinate the effort with such supporting groups as vessel, systems, process, and instrument engineering.

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