PU Direct-Buried Insulated Pipe It has many advantages that other insulated pipes cannot match. PU insulated direct-buried pipe technology was already mature abroad; after more than ten years of development, China has also made great achievements, pushing domestic pipe-network laying technology to a higher level. PU insulated direct-buried pipe is foamed by chemical reaction using high-performance polyether polyol blend and polymethylene polyphenyl polyisocyanate as raw materials. Because of its simple construction, energy saving and corrosion resistance, light bulk density, high strength, thermal insulation, sound insulation, flame retardancy, cold resistance, corrosion resistance, non-absorbency, and other outstanding characteristics, PU insulated direct-buried pipe is widely used for insulation and cold-preservation of indoor and outdoor pipelines, district heating pipelines, central air-conditioning pipelines, and industrial pipelines in chemicals and pharmaceuticals.
Implements GB/T29047-2012 "High-density polyethylene outer protective pipe polyurethane foam plastic prefabricated direct-buried insulated pipe" or EN253 "District heating pipes — Preinsulated bonded pipe systems for directly buried hot water networks — Pipe assembly of steel service pipe, polyurethane thermal insulation and outer casing of polyethylene"
1. Steel pipe may be seamless, spiral, or electric-welded pipe made of Grade 20 and Q235 steel, with performance indices meeting national and petroleum-industry standards;
2. Seamless steel pipe shall conform to GB/T 8163-2008;
3. Spiral steel pipe shall conform to SY/T 5037-2000 and GB/T 9711.1-2011; electric-welded steel pipe shall conform to GB/T 3091-2001.
In general, seamless or electric-welded steel pipe is used for DN20–DN150, and spiral steel pipe for DN200 and above; other types of steel pipe may also be used as required by the project or the customer.
Steel fittings are produced by pushing or pressing.
Applicable standards:
1. Steel butt-welding fittings GB/T 12459-2005; steel-plate butt-welding fittings GB/T 13401-2005; steel bends SY/T 5257-2004;
2. The outer jacket shall comply with GB 13018-91, Limit Deviations of Outside Diameter and Wall Thickness for Polyethylene (PE) Pipes;
Technical Parameters of PU Direct-Buried Insulated Pipe:
Axial shear strength: ≥0.12 MPa (23±2°C), continuous operating temperature: 140°C Max, peak operating temperature: 150°C Max, service life: ≥30 years (insulation layer and outer jacket; service life of the carrier pipe depends on water quality), bare pipe-end length: 200 mm–400 mm, diameter range: DN20–DN1200, average pore diameter: ≤0.5 mm, closed-cell ratio ≥88%, density at any position ≥60 kg/m3, compressive strength ≥0.3 MPa (at 10% deformation), water absorption: ≤10% (100°C boiling water for 90 minutes).
HDPE Outer Jacket:
Density ≥950㎏/m3 (20℃), carbon black content: 2.5%±0.5% (mass percent), thermal conductivity: 0.43W/(m.℃), coefficient of thermal expansion: 180×10-6 (1℃), melt flow index for PU direct-buried insulated pipe: 0.50~0.70g (MFI 190℃/5kg), tensile strength: ≥19MPa, elongation at break: ≥350%, environmental stress-crack resistance: ≥200h.
Storage method:
1. The ground shall be level and free of gravel and other hard impurities.
2. Drainage ditches shall be dug at the storage site; there shall be no ponding on the site.
3. Pipe supports shall be provided at the stacking site; the outer jacket shall be 150 mm above ground level.
4. Stacking height of insulated pipe shall not exceed 2 m.
5. Insulated pipe shall not be exposed to strong sunlight, rain, or immersion. When stored outdoors, it should be covered with tarpaulin, and the stacking location shall be away from heat and fire sources.
Construction features:
1. Lower project cost
According to calculations by relevant departments, dual-pipe heating pipelines can generally reduce project cost by about 25% (using FRP as the protective jacket) and 10% (using HDPE as the protective jacket).
2. Low heat loss, energy-saving
The thermal conductivity of PU insulated direct-buried pipe is: λ=0.013-0.03 kcal/m·h·°C, far lower than other previously common pipeline insulation materials, improving insulation performance by 4–9 times. In addition, its water absorption is very low, about 0.2 kg/m2. The low water absorption is due to the closed-cell ratio of PU foam being as high as about 92%. Low thermal conductivity and low water absorption, together with HDPE or FRP protective jackets that provide a good insulation layer and external waterproofing, have changed the traditional trench-laid heating-pipe condition of “wearing a wet cotton-padded jacket” and greatly reduced overall heat loss of heating pipelines. Heat-network heat loss is 2%, less than the 10% standard requirement.
3. Anti-corrosive, good insulation performance, and long service life
Because the insulation layer of PU direct-buried insulated pipe is tightly bonded to the outer surface of the steel pipe, air and water are shut out and good corrosion protection is achieved. Its foam cells are closed, so water absorption is very low. Both the HDPE jacket and the FRP jacket have good anti-corrosion, insulating, and mechanical properties. Therefore the outer surface of the carrier pipe is rarely attacked by outside air and water. Provided the water quality inside the pipe is properly treated, foreign data indicate that the service life of PU direct-buried insulated pipe can exceed 50 years—three to four times that of conventional trench or overhead laying.
4. Occupies little land, fast construction, and beneficial to environmental protection
PU direct-buried insulated pipes do not require the construction of large trenches; the insulated pipes only need to be buried underground, thus greatly reducing the project footprint, reducing earthwork excavation by more than about 50%, and reducing civil masonry and concrete quantities by 90%. At the same time, fabrication of PU direct-buried insulated pipes and on-site trench excavation proceed in parallel; only on-site joints are required, which can shorten the construction period by more than about 50%.
5. An alarm system can be installed to automatically detect network leakage, accurately indicate the fault location, and alarm automatically
PU direct-buried insulated pipes are equipped with leakage alarm wires. Once leakage occurs somewhere in the pipeline, the signal is transmitted through the alarm wire, and a dedicated detection instrument can display the exact location and severity of water seepage or leakage in the insulated pipeline, so that leak-inspection personnel can quickly handle the leaking section and ensure safe operation of the heating network.
Precautions and Notes:
1. One must truly understand that direct-buried laying of heating pipelines is divided into two methods: compensated direct-buried laying and uncompensated direct-buried laying. In design and construction, truly master the working principles, characteristics, and application scenarios of each of the two methods, so that they can be reasonably selected in design and construction can be safe, reliable, and economical.
2. Carry out thorough inspection after arrival on site. Before construction, the manufacturer of prefabricated direct-buried insulated pipe must be investigated. Unqualified insulated pipe shall be rejected.
3. In the construction of PU direct-buried insulated pipe, welding is a key task that ensures project quality and must be given sufficient attention.
4. The construction quality of various chambers directly affects project quality and pipeline service life, as do fixed supports. Poor waterproofing of a chamber will damage components by water immersion. Construction shall therefore be carried out carefully to ensure quality.
5. When pressure-test conditions are to be met, pressure testing of PU direct-buried insulated pipe must be taken seriously. First fill with water and vent the air, then proceed in two steps: no leakage within 10 minutes of holding pressure. Strength test: after raising the pressure in the pipe to 1.5 times working pressure, tap each weld around the weld with a 1 kg hammer. Tightness test: when the pressure is reduced to working pressure, 30 minutes with no leakage and a pressure drop not exceeding 0.2 atm is acceptable. Pressure-test records shall be kept as required by the code.
6. Because PU direct-buried insulated pipe is buried underground, most of the work is concealed at project acceptance. If final acceptance is not thorough and as-built records are incomplete, subsequent use will be affected.
7. During installation of insulated pipe, joint treatment may be done as installation proceeds, or after all installation is complete and pressure testing has been passed.
(1) To ensure waterproofing, anti-corrosion, and insulation of the joint, it is essential to guarantee on-site joint conditions.
(2) On-site joint temperature shall be kept above 10°C during operation, and shall not be lower than 5°C. Therefore, joints should not be made in winter, in rainy weather, or when the trench bottom is damp or ponded.
8. Field-joint insulation of PU direct-buried insulated pipe may be carried out only after the pressure test has passed. This item is specific to PU direct-buried insulated pipe construction, and construction quality directly affects service life. The insulation layer may be applied by field foaming or by insulation tiles. Whichever method is used, there must be no annular voids, cracking, delamination, or other defects. There are various methods for the protective jacket (for example HDPE and FRP jackets), but all must ensure the integrity, tightness, and water resistance of the joint.
9. Burial depth must ensure safe passage of a 10-ton truck. To minimize heat loss, pipes should be laid below the frost line; specific treatment shall follow local conditions in different regions.
10. Trench excavation dimensions shall leave 200–250 mm clearance between pipes and between pipes and trench walls to allow on-site installation and joint treatment. The trench bottom shall be compacted with brick and soil; fine sand bedding may be used if necessary.
11. When a direct-buried pipe crosses a highway, burial depth remains unchanged; backfill is omitted and a trench form is used, with a reinforced-concrete slab or concrete casing over the trench.
12. After pipes are welded to each other (FRP pipe connection processes and pressure-test standards are set separately) and installed, joints must be fully cured (48 hours) after being completed before the pipeline can be placed at the trench bottom, to avoid placing pipes with unfinished joints at the trench bottom, where water soaking the joint section would reduce waterproofing, anti-corrosion, and insulation performance.
Structure:
Layer 1: The carrier pipe is generally seamless pipe, spiral-welded pipe, LSAW pipe, PPR, or FRP pipe, according to design and customer requirements. (The steel surface may be shot-blasted for rust removal and may also receive anti-corrosion treatment.)
Layer 2: The PU insulation layer is formed by injecting rigid PU foam raw materials in one shot into the annulus between the steel pipe and the outer jacket using a low- or high-pressure foaming machine (commonly known as the “pipe-in-pipe foaming process”).
Third layer: HDPE prefabricated into plastic pipe of the wall thickness required by the buyer. Its functions are: first, to protect the PU insulation layer from damage by hard mechanical objects; second, anti-corrosion and waterproofing. Alternatively, FRP may be selected, which is characterized by light weight, long service life, convenient transportation, low construction cost, no need for maintenance, and low overall cost.
