Author: Beijing Guojie Pipeline Co., Ltd.
Abstract: Urban heating pipelines belong to the GB category of public pressure pipelines and are divided into two major scenarios: primary high-temperature steam networks and secondary hot water networks. They implement the CJJ urban heating industry standard system and differ significantly from the control logic of GC industrial process pipelines and GCD power plant power pipelines.In the procurement process, problems often occur such as directly substituting industrial pipe fittings for prefabricated insulated pipe fittings specially used for heating, ignoring the reinforcement requirements for directly buried pipe fittings, disconnection of supporting standards for insulation and anti-corrosion, and acceptance documents not conforming to special heating specifications.This article sorts out the standard framework for urban heating pipe fittings, material selection, special indicators for prefabricated insulated pipe fittings, key points of incoming acceptance and pressure test control, clarifies the regulatory boundaries of special equipment for heating pipelines, and provides standardized guidance for the procurement of pipe fittings for municipal heating and heat exchange station networks.
I. Introduction
Urban central heating networks are urban livelihood infrastructure, covering urban main steam networks, primary high-temperature hot water networks, community secondary hot water networks, and pipelines in heat exchange stations. In the classification of special equipment, they belong to GB public pipelines and are subject to dual constraints from the housing and urban-rural development heating industry specifications and the market supervision special equipment regulations.The media are mostly saturated steam and high-temperature hot water. The pipelines bear cyclic displacement from thermal expansion and contraction, soil loads, and groundwater corrosion over the long term, with a high proportion of directly buried installation. Elbows, tees, reducers, fixed joints, and compensator matching pipe fittings are weak points in the network. Failure of pipe fittings can easily cause water leakage, steam leakage, and heating outage accidents, directly affecting the stability of livelihood heating.
In the procurement practice of municipal heating engineering, many procurement personnel habitually apply the GB/T12459 general pipe fitting standards for chemical GC pipelines and directly use ordinary industrial butt-welded elbows for directly buried heating trunk lines, ignoring that heating pipe fittings need to be simultaneously equipped with insulation layers, outer protective pipes, anti-corrosion structures, and axial reinforcement for tees;In some projects, the temperature and pressure boundaries of the primary and secondary networks are confused: the secondary low-temperature hot water pipe network uses redundant high-temperature pipe fittings, causing cost waste, or the primary high-temperature steam pipe network uses PE outer protective insulation pipe fittings with insufficient temperature resistance;There are also projects where the documentation only provides the steel pipe material quality assurance certificate and lacks special inspection reports for insulation, outer protection, and joint foaming, resulting in rejection during the completion acceptance of heating engineering and the special equipment use registration stage, causing schedule delays.
The core characteristic of urban heating pipe fitting procurement is the integrated control of steel pressure-bearing base pipe fittings plus the insulation and anti-corrosion system, which is different from purely pressure-bearing industrial pipe fittings.Procurement must not only control the working steel pipe body, but must also incorporate the insulation structure, outer protective pipe, joint construction, displacement compensation, and direct-buried reinforcement into the technical agreement constraints, taking CJJ/T34 heating design code and CJJ28 construction acceptance code as the main lines, combined with GB/T12459 and GB/T13401 steel pipe fitting product standards, while also meeting the regulatory requirements of the Special Equipment Safety Law and TSG31 for GB public pipelines, forming a procurement control system specific to heating scenarios.
II. Hierarchical Framework of the Urban Heating Pipe Fitting Standard System
Urban heating standards are divided into a four-tier architecture: the regulatory bottom line for special equipment, heating design industry standards, national standards for pipe fitting products, and special supporting standards for insulation and anti-corrosion.
At the regulatory level, TSG 31-2025 and the Special Equipment Safety Law constrain the basic requirements for manufacturing licensing, traceability, and supervision and inspection of GB public pressure pipeline components. Heating pipeline components likewise require pressure pipeline component manufacturing qualifications, and cannot be exempted from special equipment control merely because they belong to a municipal heating project.
Core standards for the heating industry: CJJ/T 34 "Code for Design of Urban Heating Pipe Networks" is the source basis for selection, specifying medium temperature, pressure classification, material selection, direct-buried pipeline tee reinforcement, compensation setup, and outer protective pipe selection;CJJ28-2014 "Code for Construction and Acceptance of Urban Heating Network Engineering" is the core execution document for procurement acceptance, pressure testing, cleaning, and trial operation. It specifies mandatory provisions and sets out requirements for incoming inspection of pipe fittings, welding, non-destructive testing, and pressure testing. Among these, clauses such as work in confined spaces and installation of directly buried pipelines are mandatory provisions and must be strictly implemented.
The base body of steel pressure-bearing pipe fittings shall comply with general national standards: GB/T12459 Steel Butt-Welding Seamless Pipe Fittings and GB/T13401 Steel Plate Butt-Welding Pipe Fittings. For heating engineering, machine-pushed and hot-bent formed pipe fittings are preferred. The code explicitly requires that elbows, tees, and reducers be preferentially machine-formed in factories; field-welded pipe fittings are subject to special-condition approval and arbitrary field fabrication of miter-welded elbows is not permitted.
The supporting standards for prefabricated directly buried insulated pipe fittings include two systems: directly buried steam and directly buried hot water. Hot water directly buried pipe fittings mostly use polyurethane insulation + HDPE high-density polyethylene outer casing;Steam directly buried pipe fittings use a steel casing-steel structure, with an inner working steel pipe, an insulation layer, and an outer casing steel pipe. The two differ completely in insulation temperature resistance, sealing, and moisture exhaust structure. The procurement technical agreement must clearly distinguish steel-casing-steel steam pipe fittings from polyurethane hot water insulated pipe fittings, and they must not be used interchangeably.
Key boundaries need to be distinguished: GC chemical piping standards focus on medium corrosion and chemical operating conditions;GCD power plant power piping focuses on high-temperature creep rupture performance;GB heating pipelines focus on cyclic thermal displacement, external soil loads, groundwater corrosion, and insulation system integrity. The three cannot be simply benchmarked against or substituted for one another.
III. Material Classification of Heating Pipe Fittings and Key Points for Operating Condition Selection
Urban heating networks are divided by medium into steam primary networks, high-temperature hot water primary networks, and low-temperature secondary hot water networks, with material selection differentiated level by level.
For primary steam networks and high-temperature hot water primary networks, temperatures can reach 130℃~350℃ and pressures up to 2.5MPa; the base steel pipes commonly use 20# seamless steel pipes and 20G boiler steel pipes; thick-walled tees and large-diameter elbows should preferentially use seamless push-formed structures; the branch positions of tees shall be axially reinforced according to CJJ/T34 requirements to resist the thrust generated by thermal expansion of directly buried pipes. This is the procurement indicator most easily overlooked for directly buried heating tees; ordinary chemical tees have no reinforcement design, and when directly buried in operation they are highly prone to tearing failure.
For secondary low-temperature hot water networks, the temperature is generally below 95℃ and the pressure within 1.0MPa; Q235B and 20# welded steel pipes with matching fittings can be selected; in some scenarios of small-diameter secondary networks in residential courtyards, PE-RT Type II heat-resistant plastic fittings may be used, but plastic fittings are strictly prohibited for primary high-temperature steam networks.
Insulation and outer protective casing matching materials are control items unique to heating fittings: for polyurethane-insulated hot water fittings, the insulation density, closed-cell rate, and compressive strength must meet heating-specific indicators; HDPE outer protective casings are required to have ring stiffness resistance and cracking resistance;Steel-casing-steel steam fittings include aluminum silicate and microporous calcium silicate insulation layers; the outer protective steel pipe needs external anti-corrosion treatment and moisture exhaust pipes; during procurement, one must not focus only on the working steel pipe wall thickness while ignoring insulation layer thickness, bonding performance, and moisture exhaust structure.
A common high-frequency procurement misunderstanding: checking only the PN pressure rating of the working steel pipe while ignoring the upper temperature limit of the insulation, and using HDPE outer-protected polyurethane fittings for high-temperature hot water or steam pipelines above 150℃, which will cause the insulation to melt, the outer protective casing to be damaged, and the network to fail in a short period.
IV. Supplier admission, manufacturing supervision and inspection, and factory document control
The qualifications of heating fitting suppliers are divided into two modules: for pressure-bearing base fittings, the manufacturer needs to hold a pressure piping component manufacturing license, with the licensed scope covering the corresponding materials and specifications;Prefabricated insulated pipe fitting manufacturers, in addition to steel pipe qualifications, must have insulated prefabrication production lines capable of integrated fitting forming, sandblasting and rust removal, anti-corrosion, insulation cladding, and joint prefabrication. Suppliers that outsource insulation to subcontractors must clearly define quality responsibility boundaries.
For directly buried steel-casing steel steam pipe fittings and large-diameter main trunk network pipe fittings, special inspection agencies in some regions require manufacturing process supervision and inspection, and the factory delivery must be accompanied by supervision and inspection certificates;Low-pressure hot water pipe fittings for secondary networks in residential communities shall be subject to sampling inspection in accordance with design document requirements.In addition to the conventional steel pipe material certificates, heat numbers, non-destructive testing reports, and dimensional inspection records, factory delivery documentation for insulated pipe fittings must additionally include insulation material test reports, outer protective pipe test reports, and insulation layer adhesion test records; steel-casing steel pipe fittings shall be accompanied by moisture exhaust structure inspection records.
Product marking differs from ordinary industrial pipe fittings: in addition to the steel stamp markings on the steel pipe body indicating specification, material, and heat number, the outer protective layer of prefabricated insulated pipe fittings must be marked with the heating medium type, design temperature, and standard number, to prevent mixing of hot water and steam pipe fittings at the construction site.Documentation must be archived as a complete set; completion acceptance of heating projects and filing for heating franchise operations both require the full set of pipe fitting and insulation documentation.
Non-destructive testing requirements are lower than those for GC1 high-risk chemical pipelines, but there are special heating-specific provisions: thick-walled tees and reducers in primary main trunk networks shall be subject to RT/UT inspection at the proportion specified in the design;Secondary courtyard low-pressure networks shall be subject to sampling inspection in accordance with the CJJ28 specification. The procurement technical agreement must specify the non-destructive acceptance level, and the 100% non-destructive testing requirement for chemical pipelines must not be directly applied, so as to avoid inflated costs.
V. Procurement Technical Agreement, Incoming Acceptance, Pressure Testing, and Substitution Control
The procurement technical agreement for heating pipe fittings is the core of control. In addition to specifications, quantity, PN, and DN, it must specify the pipe network type (steam/primary hot water/secondary hot water), laying method (overhead/direct burial/pipe trench), whether tee reinforcement is required, insulation structure form, outer casing material, insulation thickness, joint type, non-destructive testing ratio, document list, and finished product protection requirements.During transportation and lifting of pre-insulated pipe fittings, the outer casing pipe must be protected; wire ropes are not allowed to directly press against the outer casing layer, and finished product protection clauses must be added to the technical agreement.
Incoming acceptance is divided into four parts: appearance and dimension verification, steel pipe document review, insulation system sampling inspection, and marking verification.Dimension acceptance focuses on verifying the wall thinning rate of elbows, the dimensions of tee reinforcement plates, and the structure of concentric and eccentric reducers;Sampling inspection of insulated pipe fittings verifies that the insulation layer has no hollowing, cracking, or voids; for steel-cased steel pipe fittings, check that the moisture exhaust pipe openings are intact;For incoming direct-buried pipe fittings, focus on inspecting the groove and the sealing protection of the outer casing pipe openings to prevent mud, sand, and groundwater from entering the insulation interlayer.
Pressure testing shall be carried out in accordance with CJJ28-2014. Heating pipe networks are divided into strength tests and tightness tests. The pressure test medium, pressure holding duration, and pressure drop acceptance criteria differ from those in chemical pipeline pressure testing specifications. Pipe fittings shall be pressure tested together with the entire pipe network and must not be separately subjected to the pressure testing clauses for industrial pipelines in GB50235.
Material substitution control for heating pipe fittings: any change to the insulation structure, base material, or reinforcement structure must be verified by the thermal design institute for thermal thrust, displacement, and insulation temperature resistance, and a formal substitution approval document must be issued; implementation may proceed only after confirmation by the supervision unit and the heating construction unit. Suppliers and procurement personnel may not substitute prefabricated insulated heating pipe fittings with ordinary industrial pipe fittings on their own.
VI. Summary of Typical Risks in Urban Heating Pipe Fitting Procurement
First, mixed use of standards: directly procuring GB/T12459 bare pipe fittings to replace prefabricated insulated heating pipe fittings, lacking insulation, outer casing, and reinforcement structures;Second, confusing steam steel-jacket steel pipe fittings with hot-water polyurethane pipe fittings, and selecting PE outer-casing insulated pipe fittings for high-temperature working conditions;Third, direct-buried tees not requiring axial reinforcement, continuing to use ordinary chemical-industry tee structures;Fourth, missing documentation such as special insulation inspection reports, with only steel pipe quality assurance, resulting in failure to pass heating acceptance;Fifth, when assessing supplier qualifications, looking only at insulation qualifications and ignoring the pressure piping component manufacturing license for pressure-bearing steel pipes;Sixth, copying GC industrial piping non-destructive testing and pressure testing standards, which conflict with CJJ heating specifications;Seventh, the lack of finished product protection: the insulation outer protective pipe is damaged upon arrival at the site, and later groundwater seeps into the insulation layer, accelerating corrosion.
VII. Conclusion
The procurement of fittings for urban heating GB public pipelines is an integrated procurement of pressure-bearing base pipe fittings and insulation and anti-corrosion systems, which is different from the procurement logic for purely pressure-bearing industrial pipe fittings.Procurement management personnel need to take CJJ/T34 and CJJ28 urban heating industry specifications as the main line, distinguish the two major product systems of steam steel-jacket steel and hot water polyurethane insulation, focus on controlling the reinforcement of directly buried tees, the temperature resistance boundary of insulation, and the integrity of the outer protective structure, while also meeting the baseline requirements of special equipment for GB pipeline component qualifications, traceability, and supervision and inspection.From supplier admission, preparation of technical agreements, review of factory complete sets of documents, sampling inspection of insulation upon entry, pressure test acceptance, and material substitution, closed-loop control is carried out throughout the entire process to avoid the risks of standard mismatch, structural degradation, and insulation failure, and to ensure long-term safe and stable heating supply of municipal heating networks.
Beijing Guojie Pipeline Co., Ltd. has long been deeply engaged in the supporting business of pipe fittings for urban heating networks, is familiar with the CJJ series of urban heating specifications, and can assist heating general contractors and heating operating units in preparing technical agreements for the procurement of heating pipe fittings, distinguishing the selection of steel-jacket steel steam pipe fittings and prefabricated directly buried hot water insulation pipe fittings, verifying the complete sets of factory documents for the steel pipe base body, insulation, and outer protection, and supplying heating-specific pipe fittings such as machine-made elbows, reinforced tees, reducers, and fixed joints, suitable for municipal main network, community secondary network, and heat exchange station pipeline projects, and meeting the requirements for heating acceptance, special equipment registration, and pipeline pressure test delivery.
References:
[1] Ministry of Housing and Urban-Rural Development. CJJ/T 34-2022 Code for Design of Urban Heating Networks. Beijing: China Architecture & Building Press, 2022.
[2] Ministry of Housing and Urban-Rural Development. CJJ 28-2014 Code for Construction and Acceptance of Urban Heating Network Engineering. Beijing: China Architecture & Building Press, 2014.
[3] State Administration for Market Regulation. TSG 31-2025 Safety Technical Supervision Regulations for Pressure Piping—Industrial Piping. Beijing: China Metrology Publishing House, 2025.
[4] GB/T 12459-2025 Steel Butt-Welding Pipe Fittings—Types and Parameters. Beijing: Standards Press of China, 2025.
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[6] Special Equipment Safety Law of the People's Republic of China. 2014.
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