5　Layout of Ducts

5.1　General Requirements

5.1.1　Layout of ducts shall comply with the following requirements：

a）Ducts shall be arranged for uniformly distributing flue gas and air；

b）Proper provisions against vibration and load transfer shall be made for the ducts connected to equipment；

c）Ducts shall be arranged to satisfy thermal compensation；

d）Ducts shall be routed as short as practical to reduce the pressure loss of fluid flow，and the varieties and quantities of components and parts should be minimized for easy construction；

e）Ducts shall be arranged in a symmetrical way for good esthetics，integrity and consistency without interruption of access，operation and maintenance of adjacent equipment and piping；

f）Components and parts required for operation and maintenance shall be installed at readily accessible locations；

g）The locations of sampling points for operation and calibration purposes shall be considered.

5.1.2　Waterproof and drainage provisions shall be made for the surfaces of outdoor ducts.

5.1.3　For ducts erected above the access，the vertical clearance from its lowest point to the grade，platform surface or handrail shall comply with the following requirements：

a）The minimum clearance shall be 2500mm for maintenance vehicle passage；

b）The minimum clearance shall be 2000mm for general passage；

c）Ducts erected above the stairway shall be arranged as illustrated in Figure 1，where the vertical distance from the external insulation surface of ducts to the inclined surface of stairway shall not be less than the value indicated in Table 2.

5.1.4　Except the ducts constrained by locations（e.g.，air inlet ducts of forced-draught fans，ducts adjacent to tubular heaters），the clearance between adjacent flue gas and air ducts，and the clearance from the ducts to equipment，piping and buildings shall be selected from Table 3（For insulated ducts，the clearance shall be calculated from the external insulation surface）.

5.1.5　Ducts shall be joined by welding，and bolted joints may only be used if the equipment and components connected are provided with flanges or the duct sections are required to be disassembled for maintenance.

5.1.6　The distance between inner sides of two Z-shaped bends，non-planar bends or-shaped bends shall be as specified in Table 4，otherwise，baffles or guide vanes should be provided.

5.1.7　Where the ducts turns a corner and then immediately followed by sudden contraction，a contraction bend should be installed at the turning point，and where the ducts turns a corner and then immediately followed by sudden enlargement，a bend with equal cross section should be installed at the turning point，followed by an diffusion duct.A straight duct run with sufficient length should be installed downstream these bends at turning points，and the length of straight duct run shall comply with Article 5.1.6.

5.1.8　Expansion joints shall be installed in any of the following cases：

a）Where ducts cannot compensate the thermal expansion and additional movements at the terminal；

b）Where vibration and load transfer is required to be controlled in some duct sections，e.g.，duct section at fan inlet and outlet.

5.1.9　Installation of expansion joints in ducts shall meet the following requirements：

a）The radial offset of duct sections at both sides shall be within the range of expansion joints；

b）Expansion joints should not carry and transfer the load of ducts；

c）Supports and hangers shall be arranged at both sides of the expansion joints for horizontal ducts，and the clear length of the duct section between the edge of support or hanger and the expansion joints should be not larger than 3 times the equivalent duct diameter or 4m，whichever is smaller；

d）Consideration shall be given to thespace for erection，cold-stretching and maintenance；

e）Corrugated expansion joints in adjacent parallelt ducts should be installed side by side，or their distance should not be less than 300mm if they are staggerly installed；

f）For the expansion joints installed in vertical ducts，if it is installed above the platform decking（or floor slab）or the grade，the headroom clearance should not be less than 2000mm，and if it is installed below the platform decking（or floor slab），the clearance from the expansion joints to the beams or decking（slabs）shall not be less than 300mm.

5.1.10　Layout of dampers and their driving assemblies shall meet the following requirements：

a）Dampers shall be arranged to facilitate operation and installation of their driving assemblies；

b）Dampers operated by electrically or pneumatically controlled drive assemblies or remote controlled drive assemblies shall be installed in duct sections with minimum thermal movement whenever possible；

c）Dampers installed in tandem and located adjacent to irregular duct sections shall be fully openable，and shall not interfere with installation of their driving assemblies；

d）Manual dampers actuators should have a centralized arrangment if synergic operation is required；

e）Driving assemblies for manual dampers required for frequent operation should be installed at readily accessible locations.

5.1.11　Where the handwheel of the damper is horizontally arranged，the distance from the handwheel surface to the operating floor should be 900mm，and where the handwheel of the damper is vertically arranged，the distance from the handwheel center to the operating floor should be 900mm to 1200mm.

Where the handwheel of the damper is located beyond the operation platform，the distance from the handwheel surface or handwheel center to the platform handrail should not be larger than 300mm.

Where several handwheels are arranged side by side，the clearance between handwheel edges should not be less than 150mm.

5.1.12　The outlet flue gas ducts and the inlet cold air ducts of the regenerative air preheater should be provided with ash handling hole（or it is also served as manhole）.Where flushing facilities are provided，both outlet flue gas ducts and inlet cold air ducts shall be routed in a slope of 0.05 for drainage，descending towards the lowest point of the ducts where drain piping is installed.

5.1.13　Where the ducts passes through building walls or floor slabs，the clearance from the interior of the hole in the building wall to the ducts surface（including reinforcement rib and insulation）should be 30mm-50mm，but it shall be otherwise considered for the ducts with large radial thermal movement.

5.1.14　Adequate number of manholes for internal inspection，maintenance and cleaning purposes shall be designed in the ducts with large cross section，and platform ladders required for accessing manholes shall be provided.

Inspection holes or handholes should be designed in the ducts with small cross section if required.

Internal grids shall be installed at the connect position to prevent personnel dropping，where horizontal ducts with manholes is connected with lower vertical ducts.

5.1.15　Maintenance platforms or vertical ladders should be provided at the locations where ducts components and parts are required for frequent operation or maintenance，such as manholes，dampers and handholes.