Pipe racks

Multielevation piperacks are usually needed to handle all the required services for piping, electrical, utilities, and instmmentation. The two-level rack is one of the most common but three-level ones are also used. The utility lines are usually mn in the upper level and the process lines in the lower levels. The larger-diameter lines are located to the outside of the rack to be closest to the column supports. Access platforms are required at the battery limit to provide operators access to the block valves and blinds. If long mns of hot pipe are required, a portion of the pipe rack needs to be dedicated to an expansion loop. A horizontal space in the piperack is provided for a set of lines to be flat-turned into a set of expansion loops with the large pipes located on the outside. AH of the pipe turns are in the same horizontal plane, which is an exception to normal piping practice. A flat turn takes up and blocks space for other pipes. Flat turns are generally only made from the outside of the rack to minimize this blockage. 

Induced-draft units are less likely to recirculate the hot exhaust air, since the exit air velocity is several times that of the forced-draft unit. Induced-draft design more readily permits the installation of the aircooled equipment above other mechanical equipment such as pipe racks or shell-and-tube exchangers. 

Ground area and. space requirement.s. Comparisons of the overall space requirements for plants using air cooling versus water cooling are not consistent. Some air-cooled units are installed above other equipment—pipe racks, shell-and-tube exchangers, etc. Some plants avoid such inst ations because of safety considerations, as discussed later. 

Pipe-rack-mounted air-cooled heat exchangers with flammable fluids generally have concrete fire decks which isolate the exchangers from the piping. 

The header is normally a 80 mm diameter pipe (50 mm may be adequate for small units) and is routed via an overhead pipe rack (which is generally sloped) to a non-condensible blowdown drum. 

Figure 4.3. Experimental apparatus for investigation of effects of pipe racks on flame propagation (Harrison and Eyre 1986 and 1987).

Van Wingerden and Zeeuwen (1983) demonstrated increases in flame speeds of methane, propane, ethylene, and acetylene by deploying an array of cylindrical obstacles between two plates (Table 4.3). They showed that laminar flame speed can be used as a scaling parameter for reactivity. Van Wingerden (1984) further investigated the effect of pipe-rack obstacle arrays between two plates. Ignition of an ethylene-air mixture at one edge of the apparatus resulted in a flame speed of 420 m/s and a maximum pressure of 0.7 bar. 

Van Wingerden (1984) Two plates 0.5 m x 0.5 m pipe-rack obstacles C2H4 30 —... 

Minimum size of pipe is, sometimes dictated by structural considerations, i.e., l/i-inch Schedule 40 steel pipe is considered the smallest size to span a 15 to 20 pipe rack without intermediate support. 

A flammable hydrocarbon gas that is lighter than air is processed at a small facility. An office building is located 75 ft (15 m) from the processing equipment. Because of the size of the facility, no pipe racks or other significantly sized equipment are in the area that can create confinement or congestion if a release occurs. Further, the flammable gas is processed at low pressures and ambient temperatures. 

Over long runs, between buildings and equipment, pipes are usually carried on pipe racks. These carry the main process and service pipes, and are laid out to allow easy access to the equipment. 

Protection of pipe racks and cable trays from fire. 

Pipe racks (Fig. 6-4) are an elevated collection of pipes that transport utilities as well as raw material, product, and waste streams from one part of the plant to another. They may also be used to transfer information to and from control centers. Placing all the pipes together simplifies their construction and, later, the location of problems. Nothing should be located under pipe racks, since if leaks occur they may damage equipment. 

Propagation Release of rich oil and gas, formation of vapor cloud, ignition of vapor cloud by recompressors, collapse of absorber tower across pipe rack... 

Pipe racks normally divide a facility into major areas. For economy and process efficiencies the piping arrangement is typically a central corridor in any facility. On either side of this pipe corridor or rack the process units are provided buy its nature divides the facility into units or process areas. 

Ancillaries Pipe racks and cable trays that are in place adjacent to the existing building will require special attention. Penetrations in the new wall are necessary for power cables and instrument lines, These openings should not adversely affect the pressure rating of the building. 

Drains and trenches should be centrally located and as far away from equipment and overhead piping and pipe racks as possible. A minimum spacing of 10 ft (3 m) from major equipment is preferred. 

Main pipe rack structures inside process units 1-2 hours... 

Main pipe rack structures in process units that also support equipment 2-3 hours... 

Main process area pipe racks and interconnecting pipe racks (located in process areas and supporting piping only) should be fireproofed from their bases up to and including the first-level pipe support beams for a 1- to 2-hour rating (see Figure 8-6). 

Main pipe racks located in process or other fire potential areas that also support equipment such as fin-tube air coolers should be fireproofed with a 2- to 3-hour rating from their bases up to and including the equipment support legs and any horizontal beams transmitting the... 

Figure 8-8. Pipe Rack with and without Pumps in a Fire-Scenario Area...

Finned-tube, air-cooled heat exchangers are usually placed in elevated locations, often above pipe racks. Finned-tube heat exchangers that are fan-... 

Pumps handling flammable materials should not be located under pipe racks as an ignited spill could rapidly allow fire to rise and envelop the overhead pipework. Finned-tube, air-cooled heat exchangers, often fan-forced, are frequently mounted above pipe rack structures. 

Main banks of aboveground instrument runs in cable trays (such as those coming from the control room and supported on process unit pipe racks) that are located inside process unit battery limits should be considered for fireproofing by one of the following passive methods ... 

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