Abstract: The layout form of a steam pipe network directly affects network investment, operational heat loss, and steam supply safety. This paper compares three layout forms — the branched network, the looped network, and the connecting pipe — across pipe diameter selection, heat dissipation loss, condensate management, and operational safety. In a branched network, steam flow decreases stepwise from the heat source to the users, so the pipe diameter is reduced accordingly; it therefore has the smallest heat dissipation area and heat loss, with a simple structure and good economy. A looped network must accommodate the demands of users served by two heat sources simultaneously and enlarge its pipe diameter to meet the emergency-state load, which brings problems such as increased heat dissipation area, reduced steam velocity, more saturated pipe sections and condensate, and greater heat loss from condensate discharge; at the stagnation point, condensate tends to accumulate, and changes in operating conditions can readily induce steam-water hammer, potentially causing network collapse in severe cases. A connecting pipe shares all the drawbacks of a looped network, only on a smaller scale. Based on this, selection recommendations are proposed: as a principle, looped networks should not be adopted for steam pipe networks, and connecting pipes are not recommended either; for extremely critical users with special steam supply reliability requirements, a dual-line steam supply is preferable.

Keywords: steam pipe network; branched network; looped network; connecting pipe; heat dissipation loss; steam-water hammer

Steam pipe networks are responsible for transporting steam generated at the heat source and distributing it to individual heat users; they are an essential component of both industrial production and urban central heating systems. The network layout form determines the network's topology, pipe diameter configuration, and operating mode, and directly affects engineering investment, operational heat loss, hydraulic conditions, and steam supply reliability — making it a key link in steam pipe network design.

The selection of a network layout form should be based on technical and economic comparison. The Code for Design of Urban Heating Pipeline Network (CJJ/T 34—2022) stipulates that the layout of a heating network shall comprehensively consider factors such as heat load distribution and heat source location, and a reasonable scheme shall be determined through technical and economic comparison [1]. Article 18.2.2 of the Code for Design of Boiler House (GB 50041—2020) further clarifies that branched pipe systems are preferable for steam pipe networks [2].

Common layout forms of steam pipe networks include the branched network, the looped network, and the connecting pipe, which combines features of both. The branched network, with its simple structure and low investment, is the most commonly used layout form for steam pipe networks; looped networks and connecting pipes aim to improve steam supply reliability, but the heat loss and operational safety costs paid for this are often underestimated. Drawing on engineering practice, this paper conducts a systematic comparison of the operating characteristics of the three layout forms, focuses on the problems of looped networks and connecting pipes, and proposes selection recommendations for reference in steam pipe network design.

A branched network draws a main pipe from the heat source, with branch pipes diverging along the way to each heat user. Steam flow decreases stepwise from the heat source to the users, and the pipe diameter is reduced accordingly, giving an overall tree-like spread.

The main advantage of the branched network lies in its economy: the pipe diameter decreases stepwise along the flow direction, so the pipe metal consumption, insulation area, and heat dissipation area are all at their lowest levels, saving network investment; the structure is simple, and hydraulic calculation and operation management are relatively convenient. The national standard Code for Design of Boiler House (GB 50041—2020) explicitly states that branched pipe systems are preferable for steam pipe networks [2], which is consistent with the choice in engineering practice.

The shortcoming of the branched network is its limited steam supply reliability: when a certain pipe section fails, the users downstream of it will lose steam supply. To address this, sectionalizing valves and standby pipelines are usually combined in engineering to improve steam supply assurance under fault conditions.

A looped network is generally used in dual-source or multi-source heating systems, connecting the main lines of each heat source into a loop so that steam can be supplied to users from multiple directions. The purpose of a looped network is to improve steam supply reliability: when a certain heat source or pipe section fails, steam can continue to be supplied to users through the other direction of the loop.

Unlike a branched network, a looped network must simultaneously consider the demands of users served by two heat sources; the pipe diameter cannot be reduced stepwise along the flow direction, but must instead be enlarged to meet the load under emergency conditions. This pipe diameter configuration is the root cause of a series of problems in looped networks.

A connecting pipe is a connecting pipe section that links two or more main lines, used to achieve mutual support between heat sources or main lines under fault conditions. Functionally, a connecting pipe has some characteristics of a looped network and can be regarded as a smaller-scale looped connection; it therefore shares all the drawbacks of a looped network, only on a smaller scale.

Pipeline heat dissipation loss is directly related to the heat dissipation surface area. After a looped network enlarges its pipe diameter to meet the emergency-state load, the heat dissipation surface area increases accordingly, and the heat exchange between the network and the environment rises. Engineering experience shows that each step of diameter enlargement increases heat dissipation loss by about 10%. For long-distance steam pipe networks, this loss accumulates along the entire line and its impact cannot be ignored.

Insulation is the basic means of controlling heat dissipation loss. The Code for Design of Industrial Equipment and Pipeline Insulation Engineering (GB 50264—2013) stipulates that equipment and pipelines with an external surface temperature above 50 °C that require reduced heat dissipation for process reasons shall be insulated [3]. Steam pipelines operate at high medium temperatures and have strict insulation requirements; diameter enlargement enlarges the insulation area synchronously, and the insulation engineering investment and maintenance workload increase accordingly, further raising the economic cost of looped networks.

After diameter enlargement, the in-pipe velocity under the same steam flow drops markedly. The reduced velocity increases the residence time of steam in the pipe, making the heat dissipation process more complete; more steam loses heat and undergoes phase-change condensation, forming condensate, and the number of saturated pipe sections in the network increases markedly.

Compared with the cold water originally used to generate steam, the condensate discharged from the network is hot water at saturation temperature. Condensate discharged outside the network at a sufficiently high temperature carries away considerable heat. Engineering experience shows that the heat carried away by condensate discharge is roughly equivalent to 30% of the pipeline heat dissipation. Compared with a branched network, a looped network has markedly more saturated sections and condensate, and condensate discharge is itself a heat-discharge process, further amplifying the heat dissipation loss. Therefore, a looped network not only directly increases heat dissipation loss through diameter enlargement, but also incurs additional heat loss through condensate discharge.

Condensate management is an important element of steam pipe network design. The Code for Design of Urban Heating Pipeline Network (CJJ/T 34—2022) sets out provisions for the installation of steam pipeline drainage devices, requiring startup and continuous drain traps at low points of the pipeline and ahead of upward-rising sections, and specifying spacing requirements for drain traps on downward- and upward-sloping sections [1]. A looped network has a large condensate volume and many drainage points, markedly increasing the burden on the drainage system.

A looped network necessarily contains a stagnation point — a point where steam velocity is zero. In the pipe section at the stagnation point, steam velocity is zero and the section is in a saturated state, so condensate tends to accumulate. Discharging condensate at the stagnation point creates additional heat dissipation loss, again amplifying the heat dissipation loss.

More important is the operational safety issue. After condensate accumulates at the stagnation point, once the network operating conditions change — such as a fault, a switching operation, or a sudden load change — the steam that was previously at the stagnation point may flow rapidly; the suddenly accelerated steam entrains the accumulated condensate to form a water slug, i.e., steam-water hammer. Steam-water hammer is extremely destructive: it can damage pipes, supports, and fittings, and in severe cases lead to network collapse. Among the various operating accidents of steam pipe networks, steam-water hammer is the most harmful and can be called the number-one killer of safe network operation.

A connecting pipe shares all the drawbacks of a looped network: the relatively enlarged diameter brings increased heat dissipation loss, and during operation or switching there are risks of increased condensate and steam-water hammer. A connecting pipe is only smaller in scale, so the impact of the above problems is relatively limited, but its installation complicates the network topology and should still be treated with caution.

A looped network is not entirely without value; its only advantage lies in improving steam supply reliability under fault conditions through interconnection of dual heat sources — the "one benefit" among its many harms.

The Code for Design of Urban Heating Pipeline Network (CJJ/T 34—2022) sets out provisions for the interconnection of multi-source heating systems: when technically and economically justified, transmission and distribution trunk lines may be connected into a looped network, and connecting trunk lines may be provided between transmission trunk lines; the connecting trunk lines or main loop shall consider switching means for different fault conditions and meet the minimum heating guarantee rate requirement [1]. The explanatory note to the code also points out that looped networks involve larger investment, and whether to adopt them shall be determined by technical and economic comparison [1].

From the analysis in Section 3 of this paper, the costs paid by a looped network for its reliability benefit include: increased heat dissipation loss from diameter enlargement, heat loss from condensate discharge, the operational safety risk of stagnation-point condensate accumulation and steam-water hammer, and the corresponding increase in investment. In most steam pipe network projects, these costs are difficult to fully offset by the reliability benefit. In contrast, a branched network combined with sectionalizing valves and necessary standby measures can meet the steam supply requirements of general users at lower heat loss and investment levels. Therefore, the "one benefit" of a looped network is insufficient to offset its "many harms," and as a principle it should not be a conventional choice for steam pipe networks.

Table 1 gives the comparison results of the three steam pipe network layout forms across the main dimensions.

As shown in Table 1, the branched network has clear advantages in economy and heat dissipation loss; the looped network is superior only in steam supply reliability, at the cost of significantly increased heat dissipation loss and operational safety risk; the connecting pipe's characteristics lie between the two but are closer to the looped network. Selection should be based on a technical and economic comparison between reliability needs and economy and safety.

This paper compares the three layout forms of steam pipe networks — branched, looped, and connecting pipe. The main conclusions and recommendations are as follows.

(1) In a branched network, steam flow decreases stepwise from the heat source to the users, the pipe diameter is reduced accordingly, and the heat dissipation area and loss are minimal; with a simple structure and low investment, it is the layout form that should be preferentially adopted for steam pipe networks, consistent with current standards.

(2) A looped network must simultaneously consider the users of two heat sources and enlarge the pipe diameter to meet the emergency-state load; each step of diameter enlargement increases heat dissipation loss by about 10%. Diameter enlargement reduces steam velocity, markedly increasing saturated pipe sections and condensate; the heat carried away by condensate discharge is roughly equivalent to 30% of the pipeline heat dissipation, further amplifying the heat dissipation loss.

(3) A looped network contains a stagnation point where condensate tends to accumulate; when operating conditions change, suddenly accelerated steam entrains the accumulated condensate to form a water slug, posing a prominent steam-water hammer risk that can cause network collapse in severe cases — an important threat to the safe operation of steam pipe networks.

(4) A connecting pipe shares all the drawbacks of a looped network, only on a smaller scale, and as a principle is not recommended for installation.

(5) As a principle, looped networks should not be selected for steam pipe networks, and connecting pipes are not recommended either. For extremely critical users with special steam supply reliability requirements, a dual-line steam supply is preferable, meeting high-reliability requirements at a smaller cost. Current standards also have corresponding provisions for users with special heating reliability requirements being supplied by two or more heat sources [1]; when steam use has special requirements, steam mains should adopt a dual- or multi-pipe system [2], which is consistent with the direction of the recommendations in this paper.

[1] Ministry of Housing and Urban-Rural Development of the People's Republic of China. Code for Design of Urban Heating Pipeline Network (CJJ/T 34—2022) [S]. 2022.

[2] Ministry of Housing and Urban-Rural Development of the People's Republic of China. Code for Design of Boiler House (GB 50041—2020) [S]. 2020.

[3] Ministry of Housing and Urban-Rural Development of the People's Republic of China. Code for Design of Industrial Equipment and Pipeline Insulation Engineering (GB 50264—2013) [S]. 2013.