Maintenance access towers

Fresh hydrofluoric acid used for catalyst make-up is anhydrous and of 99.5%+ purity. Since the acid in the unit is continually being regenerated, the only hydrofluoric acid consumed in the process is that which is absorbed in the bauxite towers, lost to the acid tar, lost in handling, or lost through venting equipment and lines for maintenance access. Typically, hydrofluoric acid consumption, including losses to all sources, will average about 0.5 to 0.8 pound per barrel of alkylate. 

A sufficient number of access doors, ladders and walkways shall be provided for safe and easy accessibility to internals and mechanical equipment of the tower for inspection and maintenance. Access doors shall be tight, resistant and easy to operate. Access ladders from the ground should be located in such a position to permit future cell addition. 

The process gases enter a forced-circulation-type waste heat boiler at 800-1000 °C. Several changes have been made to the mechanical rapping equipment and cooling pipe bundles of the boiler to improve the performance and maintenance access. The gases exit the waste heat boilers at a temperature of 300-350 °C and pass through the hot cyclone separators and electrostatic precipitators into the mercury removal towers. Two electrostatic precipitators are installed simultaneously in both lines to ensure continuous processing. 

Whether the planned plant is an inline arrangement or housed in a struaure, the plant layout designer must make provisions for operator and maintenance access. The designer must review the items of equipment that are included in the process and plan for their operation and maintenance requirements. For example, towers must be located in a position to allow for the removal of internals, reactors require space for catalyst loading and unloading, shell and tube exchangers require space for bundle removal, and rotat-... 

Within the conventional inline process unit, drums and their related items are generally located on either side of a central pipe rack serviced by auxiliary roads for maintenance access. In certain cases (e.g., for flash drums and deaerators), drums can be located above the pipe rack. In chemical plants, drums are generally located at all levels of enclosed or open-sided structures. For eixample, Exhibit 5-4 shows the drum location in a tower reflux system. Exhibit 5-5 shows the typical location of feed surge and compressor suaion drums, and Exhibit 5-6 shows the drum locations in an enclosed chemical plant stnicture. 

To facilitate specific elevation requirements for level instruments or positioning a maintenance access, nozzles may be located off the main axis. These nozzles are generally referred to as hillside nozzles. A typical hillside nozzle is shown in Exhibit 5-15- Tall vertical drum nozzles should be elevated and oriented using the tower subjea as a guide. It should be noted, however, that nozzle locations are not restricted by internal components, as is die case for the distillation tower. More information on vertical drums is available in Chapter 10, Towers. 

Altliough some reactors (e.g., those u.sed in methanol and catalytic reforming units) are. spherical, most are vertically mounted ve.ssels with elliptical heads. There are fewer connections for the plant layout designer to be concerned with than there are with the dLstillation tower. Generally, connections are limited to inlet and outlet, maintenance access, unloading,. sample, and temperature. Internally, reaaors are furnished with bed supptms, screens, inlet baffles, outlet collectors, catalysts, and inert materials. Exhibit 9-3 depicts a typical reactor and its principal components. 

This chapter highlights the general requirements for 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 operator and maintenance access. 

A maintenance access is usually located in the bottom top, and intermediate sections of dte 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 downcomer sections of the tower. Qtte must be taken at sections of the tower diat contain internal piping to avoid blocking the maintenance, access entrance. Exhibit 10-28 shows typical elevation and orientation requirements for maintenance accesses. 

Platforms are required on towers for access to valves, instruments, blinds, and maintenance accesses. Platforms are usually circular and supported by brackets... 

At maintenance access platforms, adequate space must be provided to swing the maintenance access cover flat e open for storage against the ice of the tower. Tc head-mounted maintenance accesses... 

For example, the platform elevations shown on the process vessel sketch in Exhibit 10-3S> are the minimum requirements for instrument, valve, and maintenance access. Exhibit 10-50 displays a platform arrangement for the tower using the information in Exhibit 10-39 and the guidelines in this chapter. 

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 diat adequate clips can be furnished for support. Exhibit 10-68 shows utility station requirements. Such towers as demethanizers operate under extremely cold conditions and sometimes... 

Large towers often have the motor mounted horizontally connected to a right angle gear drive. The motor can be closely coupled- in the air stream- or connected with a drive shaft with the motor outside the air stream. Maintenance personnel typically prefer the external TEFC motor- when available- for its easier access. Cooling tower fans- like all fans- operate in accordance with the fan laws one of which states that the horsepower required to drive a fan increases to the cube of fan speed. 

The main factors favoring packed columns are (1) very corrosive applications, where plastic or ceramic packings are favored over trays, which are almost always constructed of metal (2) low pressure drop requirement, which is easier to achieve with packings than with trays (3) small-diameter columns, because trays require access for inspection and maintenance and (4) foaming systems, which are easier to handle in packed towers. 

Adequate access around the tower to facilitate cleaning and maintenance. 

This project is unique as it included the construction of North America s largest ground mounted friction hoist. This was a deviation from the standard tower mounted configuration used historically by the industry. The ground mounted option was chosen for a number of reasons primarily that it provided the least interruption to mine operations during the construction phase of the project. It also provides a maintenance friendly layout once in production as most of the major infrastructure is accessible at ground level. 

Similar to towers, drum elevations are diaated by the NPSH, as illustrated in Exhibit 5-7, minimum clearance, common platforming, and maintenance and operator access. Within structures, especially chemical facilities, drums could be elevated above the minimum requirements of accommodate floor space avail-... 

If process requirements permit, shell and tube exchangers can also be mounted in a vertical position, supported by lugs and tower nozzles in a tower-sup-ported installation (as shown in Exhibit 6-17), within concrete or steel structures (as shown in Exhibit 6-18), and by concrete piers (as shown in Exhibit 6-19). The same considerations for maintenance, control, and operator access should be given for vertical installations as are for horizontal installations. 

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. Exhibit 10-58 is a typical instrument vessel sketch. 

Instruments at vessels (eg., level control instruments and temperature, pressure, and level gauges) are positioned in conjunction with the instrument engineer s requirements for the project Examples of diese requirements and information relating to vessel internals and access and maintenance for vessel instrumentation are highlighted in Chapters 5 (Drums), 9 (Reactors), and 10 (Towers). 

Equipment integrated with major towers such as unfired reboilers, overhead exchangers, and feed effluent exchangers may be located closer to the fractionating column that they are associated with than is indicated in the spacing tables. However, access for firefighting and maintenance should be maintained. 

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