When most engineers and contractors think about factory-fabricated mechanical systems, they think about skids. Pump skids, boiler room packages, heat transfer skids. The skid is the visible product of the factory fabrication process and the most commonly specified deliverable.

FabPro also fabricates pipe racks and distribution headers that are not part of a skid assembly but that benefit from the same factory fabrication discipline. Pipe racks that distribute steam, heating hot water, chilled water, or process fluids from the mechanical plant to the building distribution system. Headers that connect multiple boilers, chillers, or heat exchangers to a common distribution circuit. Prefabricated piping assemblies that would otherwise be field-fabricated in cramped mechanical rooms or in occupied spaces where field welding and threading is impractical.

What pipe racks are and where they are used

A pipe rack is a structural frame that supports multiple parallel pipes running from one point to another. In mechanical plant applications, pipe racks support the supply and return mains from the boiler or chiller plant to the building distribution system, the connecting piping between multiple pieces of mechanical equipment in the plant room, and the utility distribution piping serving the mechanical plant itself.

For campus and institutional facilities where the mechanical plant serves multiple buildings through underground or overhead distribution, the pipe rack is the structural system that organizes and supports the distribution piping between the plant and the building connections. Factory-fabricated pipe racks arrive at the job site as complete assemblies with the structural framing, the pipe supports, the pipe guides, and in some cases the piping itself already assembled and ready to set.

For mechanical room applications where multiple pieces of equipment are connected to common distribution headers, the pipe rack organizes the interconnecting piping in a configuration that provides adequate support, allows thermal expansion movement, and maintains the maintenance clearances required for service access to the equipment.

Distribution headers

A distribution header is a large-diameter pipe section that collects flow from multiple sources or distributes flow to multiple destinations. In a multi-boiler plant, the supply header collects hot water or steam from each boiler and presents it to the distribution system as a single supply point. The return header collects return water from the distribution system and presents it to each boiler as a single return source.

The header design determines how well the system manages the flow distribution between multiple boilers and how effectively the system controls are able to stage and modulate the boiler plant in response to load. A header that is undersized for the combined flow of multiple boilers creates excessive pressure drop that affects the performance of each individual boiler and the accuracy of the flow measurement instrumentation that the controls depend on.

Factory-fabricated headers are built to the dimensions and connection configurations specified for the system rather than assembled in the field from stock fittings. The connection locations for each boiler, each pump, each bypass, and each instrumentation connection are laid out in the fabrication drawings before the header is built. The welds are made in the shop under controlled conditions and inspected before the header ships. The pressure test is performed at the factory before installation rather than in the mechanical room after the header is connected to the equipment.

Why factory fabrication improves pipe rack and header quality

Field fabrication of pipe racks and distribution headers in a mechanical room is constrained by the access and working conditions in the room. Welders working in cramped mechanical rooms, around existing equipment, in positions that do not allow full access to the joint, produce welds that are more variable in quality than welds made in a shop with full access, proper positioning, and quality control procedures.

Factory fabrication allows the pipe rack and header assembly to be built in a position that gives the welder full access to every joint. The fit-up of each joint can be verified before welding. Post-weld inspection can be performed with the assembly in a position where the entire weld is visible. Pressure testing can be performed before the assembly is placed in its permanent location where access for repair is limited.

For mechanical plants where the pipe rack and distribution header are part of a larger factory-assembled package that includes pump skids and heat exchanger skids, the interconnection between the components can be designed and fabricated at the factory rather than coordinated in the field. The result is a mechanical plant where the major piping interconnections have been assembled and tested before the equipment arrives at the job site.

What FabPro needs to fabricate a pipe rack or header

The fabrication of a pipe rack or distribution header starts with the system flow requirements, the pipe sizes derived from those flow requirements, the structural loads from the pipe weights and thermal expansion forces, and the dimensional envelope available at the installation location.

For campus distribution pipe racks, the routing and elevation are determined by the site conditions and the building connection points. For mechanical room headers, the dimensional envelope is determined by the mechanical room dimensions and the equipment layout.

FabPro works with the project engineer during the design phase to confirm the pipe sizes, the structural requirements, the thermal expansion accommodation, and the connection configuration before fabrication begins. A pipe rack or header that arrives at the job site having been designed around the actual installation conditions fits correctly and connects to the equipment it was designed to serve.

For boiler plant applications where the distribution header connects to GP Energy Products boiler equipment, the GP Energy team coordinates the boiler connection requirements with the FabPro fabrication team. Visit gpenergyproducts.com for more. For pump skid applications where the pipe rack connects the pump skid to the distribution system, the Merion Pump Company team handles the pump selection and can coordinate the header connection requirements with FabPro. Visit merionpump.com for more.

FabPro Systems designs and fabricates pipe racks, distribution headers, and packaged mechanical systems for commercial, institutional, and industrial applications nationally. If you have a project where field-fabricated piping is creating schedule, quality, or access challenges, reach out and we will evaluate whether factory fabrication is the right solution.

References
1. ASME B31.3. Process Piping Code. Governs design, fabrication, and testing requirements for process piping systems including pipe racks and distribution headers. asme.org
2. AISC. Steel Construction Manual. Covers structural design requirements for pipe rack framing in commercial and industrial mechanical plant applications. aisc.org
3. MSS SP-58. Pipe Hangers and Supports. Covers support spacing, load calculations, and thermal expansion accommodation requirements for pipe rack and header applications. mss-hq.org
4. ASHRAE. HVAC Systems and Equipment Handbook, Chapter on Piping. Covers distribution header design, sizing, and connection configuration for commercial mechanical plant applications. ashrae.org