Basic principles for handling duct and water pipe crossings
Duct and water pipe crossings are the most common type of conflict in MEP coordinated routing. Reasonable crossing handling should follow these basic principles: Duct priority principle — ducts have large cross-sections, cannot be bent, and are difficult to install and adjust (removing a duct requires dismantling flanged connections, and changing duct routing requires fabricating new duct sections). Therefore, when handling crossings, priority should be given to adjusting water pipe positions (water pipes can flexibly change elevation and routing via elbows), and ducts should only be adjusted when crossings truly cannot be avoided. Selection of water pipe avoidance method — water pipes can avoid ducts in two ways: horizontal avoidance and vertical avoidance. Horizontal avoidance (water pipe turns within the horizontal plane to bypass the duct) is suitable when the water pipe has lateral displacement space (pipe spacing not less than 150 mm); vertical avoidance (water pipe bends up or down to cross over the duct) is suitable when the water pipe has no lateral displacement space but sufficient vertical clear height. Clear height priority principle — after crossing handling, the clear height below the ceiling should be ensured to be no less than the design requirement; for example, in corridor areas the minimum clear height should not be less than 2.4 m. In critical areas with limited clear height (such as corridor ends and pipe shaft entrances), priority should be given to the duct cross-section compression scheme (keeping the duct cross-sectional area unchanged, increasing duct height and reducing width, or keeping width unchanged and increasing height while reducing width), avoiding the scheme of water pipes crossing over ducts (which would reduce clear height). Insulation spacing requirements — the insulation layer thickness of hot and cold water pipes must be considered during crossing handling (air-conditioning water pipe insulation thickness 30–50 mm, hot water pipe insulation thickness 40–60 mm). The clear distance between insulated pipes and ducts should not be less than 50 mm (to prevent the insulation layer from being compressed and affecting insulation performance), and insulated pipes should not directly contact duct surfaces. Valve maintenance space — the maintenance and operation space for valves on water pipes (gate valves, butterfly valves, control valves, etc.) must be considered during crossing handling. The clear distance between valves and ducts should not be less than 300 mm (to ensure valve opening and operating space).
Handling methods for local duct compression and raising
When water pipes cannot avoid ducts and vertical space is sufficient, ducts may be locally compressed or raised to avoid water pipes. Local duct compression — under the premise of keeping the duct ventilation cross-sectional area unchanged (without changing air volume), change the duct width-to-height ratio (increase height and reduce width, or keep width and increase height) so that the duct is locally raised to allow water pipes to pass underneath. Calculation formula: original duct cross-sectional area A=W1*H1; after compression W2*H2=A (W2 is the compressed width, H2 is the compressed height). The width-to-height ratio of the compressed duct should not exceed 4:1 (an excessive width-to-height ratio increases duct resistance and is unfavorable for airflow distribution), and the length of the transition section should not be less than 2 times the duct width difference (to ensure smooth airflow transition). Local duct raising principle — keep the duct cross-sectional area unchanged, and raise the overall duct elevation at the crossing (forming an "arch" crossing) to allow water pipes to pass underneath. Duct raising height: after raising, duct bottom elevation = water pipe top elevation + insulation thickness + 50 mm spacing; the length of the raising transition section should not be less than 3 times the duct height difference (inclination angle not greater than 15 degrees, to reduce duct resistance). Selection between duct compression and raising methods: when there is sufficient space above the crossing (free space above the slab), priority should be given to the duct raising principle (less impact on duct resistance); when space above the crossing is limited, adopt the duct compression scheme (requires accurate calculation of the resistance increment after compression to ensure sufficient fan excess pressure). Resistance calculation at duct compression and raising locations: the local resistance coefficient xi may be calculated as a "reducer" or as a combination of "elbow + reducer" (according to the formulas in the Practical Heating and Air Conditioning Design Manual), and the resistance increment should not exceed 5% of the total system resistance (otherwise fan selection must be recalculated).
Methods for horizontal displacement of water pipes and crossing via vertical elbows
Water pipe adjustment is the most commonly used method for handling duct-pipe crossings, avoiding ducts through horizontal offset of water pipes or vertical elbow crossings. Water pipe horizontal offset method — Applicable conditions: sufficient horizontal space on both sides of the duct (after offset, the clearance between the water pipe and other pipelines shall not be less than 100 mm). Method of operation: add 2 × 45° elbows before the duct (forming an "S" bend in the horizontal direction), offset the water pipe horizontally by a certain distance to bypass the duct, then add 2 × 45° elbows after the duct to return to the original position. Horizontal offset distance: determined by the required offset distance (generally 300–600 mm), and the offset section length shall not be less than 2 times the offset distance (to ensure smooth turning). Advantages of horizontal offset: does not reduce clear height, does not increase vertical space occupation, small increase in resistance (local resistance coefficient of 2 × 45° elbows is approximately 0.4). Water pipe vertical elbow crossing method — Applicable conditions: sufficient vertical space below or above the duct (clear height still meets requirements after the water pipe crosses). Method of operation: add 1 × upward 90° elbow, 1 × short horizontal pipe section and 1 × downward 90° elbow before the duct (forming a "gate"-shaped crossing), with the water pipe bypassing above the duct. Vertical crossing height: water pipe crossing height = duct height + insulation thickness + 2 × 50 mm clearance, generally taken as duct height + 150–200 mm. Notes for vertical crossing: an automatic air vent shall be installed at the highest point of the water pipe (to discharge air from the pipe and avoid air locking); a drain valve shall be installed at the low point after the water pipe vertical crossing (to facilitate pipe flushing and drainage). Selection of the two methods: when horizontal space is sufficient, the horizontal offset method shall be preferred (small impact, simple construction); when horizontal space is insufficient, the vertical crossing method shall be adopted (but attention shall be paid to clear height limitations and air venting/drainage measures).
Key points for design and installation of combined supports and hangers
Combined supports and hangers refer to the practice of integrating and installing multiple pipelines such as ducts, water pipes and cable trays on the same support and hanger, which can effectively save space, reduce the number of supports and hangers, and improve construction efficiency. Design principles for combined supports and hangers: Load calculation — combined supports and hangers shall be able to withstand the self-weight of the pipelines they carry (including water weight in pipes, insulation layer weight, duct flange weight, etc.) and possible additional loads (duct maintenance load 500 kg, pipe water hammer impact load, etc.), with a safety factor of not less than 1.5. Profile selection — the crossbeams of combined supports and hangers generally adopt hot-rolled channel steel (No. 8–12 channel steel, material Q235B) or C-shaped steel (41*41/41*52, material Q235B hot-dip galvanized); the columns adopt hot-rolled channel steel (No. 8–10 channel steel) or angle steel (L50*5–L80*8); bolts adopt hot-dip galvanized bolts (strength grade 8.8). Hanger rod spacing — the longitudinal spacing of combined supports and hangers along the pipeline: not more than 6 m for pipes above DN100, not more than 4 m for pipes below DN100, and ducts not exceeding code requirements (e.g., hanger rod spacing for ducts with long side below 800 mm not exceeding 3 m); the spacing of combined supports and hangers shall be executed in accordance with the most stringent spacing requirement among the pipelines carried. Pipeline layout — when arranging pipelines on combined supports and hangers, the principle of "electrical on top, water in the middle, air in the middle/bottom" shall be followed (electrical cable trays on the upper layer, water pipes on the middle layer, ducts on the lower layer or uppermost layer); the spacing between pipelines shall not be less than 50 mm (to ensure insulation layer thickness), and the spacing between pipelines and support/hanger components shall not be less than 50 mm (for corrosion prevention and maintenance operations). Node design — the connection nodes between combined supports and hangers and the structure (connection of hanger rods to floor slab) shall adopt chemical anchors or expansion bolts (M12–M16), with the bearing capacity of each suspension point not less than 3.5 kN (design value); connection nodes shall be subject to anti-corrosion treatment (hot-dip galvanizing or anti-rust paint). Installation acceptance — after installation of combined supports and hangers is completed, load verification shall be carried out (loading test at 1.25 times the design load, lasting 24 h without deformation), and pipelines may only be installed after acceptance is qualified.