Equipment for extrusion molding of polyvinyl chloride double-wall corrugated pipes

The extrusion equipment used for manufacturing double-wall corrugated PVC pipes is essentially similar to that used for single-wall corrugated pipes; the only difference lies in the die used to form the pipe parison. However, since double-wall corrugated pipes are typically produced in larger diameters and require greater quantities of raw material, it is advisable to employ a twin-screw extruder, which offers improved performance. Furthermore, the die structure for forming the parison of double-wall corrugated pipes differs from that of single-wall corrugated pipes in the following respects: the straight section of the die‑formed parison is longer; there are two melt‑diverting cones and two layers of melt flow channels, with the molten material in each channel forming the inner and outer walls of the double-wall pipe, respectively. Between these two melt flow channels, there is an additional compressed‑air inlet, through which compressed air is injected to inflate the outer wall, causing it to conform closely to the corrugated mold and thereby shaping the pipe’s corrugated profile. Additionally, the central bore of the inner mandrel can accommodate a heater and features passages for introducing compressed air and cooling water. On the other hand, the two die structures share certain similarities: both are equipped with a porous plate at the front of the melt‑diverting cone. Adjustments to the parison wall thickness are achieved by regulating the relative positions of the diverting cone and the mandrel, thereby controlling the gap between the mandrel and the die. Meanwhile, the die itself, being inserted between the two halves of the corrugation mold, remains fixed and cannot be moved.

2023-09-18

Installation Method for Metal Expansion Joints

Comprehensive Guide to Metal Expansion Joint Installation: 1. Before installation, verify that the expansion joint’s model, specifications, and piping configuration comply with design requirements. 2. For expansion joints equipped with an inner sleeve, ensure the sleeve is oriented in the same direction as the fluid flow. For hinged expansion joints, the hinge rotation plane must align with the displacement‑rotation plane. 3. For expansion joints requiring “cold‑tightening,” any auxiliary components used for pre‑deformation should be removed only after the piping system has been fully installed. 4. It is strictly prohibited to adjust pipeline installation deviations by deforming the bellows of the expansion joint, as this can impair its normal operation, shorten its service life, and increase the loads on the piping system, equipment, and supporting structures. 5. During installation, avoid allowing welding spatter to come into contact with the bellows surface, and ensure the bellows is protected from other mechanical damage. 6. Once the piping system is installed, promptly remove the yellow positioning aids and fasteners used during transportation and installation, and adjust the limit devices to their specified positions as required by the design, ensuring the piping system maintains adequate compensation capacity under ambient conditions.

2023-09-18

What are the standard performance characteristics of metal expansion joints?

Metallic compensators have two types of standard performance characteristics: one is the performance guaranteed for service, such as pressure resistance, temperature resistance, fatigue resistance, and elastic compensation; the other includes properties like stiffness and cross-sectional area, which are not directly required for operation but significantly influence piping system design and the application of metallic compensators. Therefore, it is essential to have a thorough understanding of both categories. Pressure-Resisting Capability of Compensators Compensators operate within pipelines and must withstand internal or external pressures. This necessitates that metallic compensators possess adequate pressure-resisting capacity. Through design and testing, appropriate structural parameters are established to ensure this capability. The pressure‑resisting capacity determined during design corresponds to the design pressure. Since compensators are produced in standardized series, they are categorized according to nominal diameter and nominal pressure. In most cases, their design pressures follow a series based on nominal pressures of 0.25, 0.6, 1.0, 2.5 MPa, and so forth. Only a small number of products are designed to meet specific pressure requirements outside these standard nominal pressure ranges. During piping system design, engineers determine the maximum operating pressure of the pipeline based on actual needs and then select a compensator with an appropriate nominal pressure rating. Compensation Capability of Compensators The compensation capability of a compensator arises from the elastic deformation of its bellows, which can involve axial extension, compression, bending, or combinations thereof. The magnitude of this compensation capability is specified by the designer according to operational requirements and is expressed as the rated compensation amount—indicating the maximum compensation achievable under given conditions. Different types of compensators exhibit distinct compensation modes, primarily including axial, lateral, angular, and combined compensation.

2023-09-18

Compensator Classification and Characteristics

1. PTFE-lined bellows expansion joint: resistant to high temperatures, corrosion, and high pressure. This product employs a specially designed PTFE hose and a high‑temperature‑resistant braided stainless steel double‑clasp spiral hose, making it suitable for demanding environments involving high temperatures, corrosive media, and elevated pressures. 2. Bellows expansion joint: utilizes the effective axial and lateral deformation of its corrugated bellows to accommodate dimensional changes in pipelines, ducts, vessels, and other components caused by thermal expansion and contraction, or to compensate for axial, lateral, and angular displacements. It can also be used for noise reduction and vibration damping. 3. Metal flexible hose: compensates for axial and angular displacements in piping systems while providing pressure resistance, seismic protection, and noise reduction. 4. Rotary expansion joint: a displacement compensation device for district heating networks, primarily composed of inner and outer tubes, sealing packing, and a spring‑loaded compression mechanism. Its distinctive feature is the highly precise machining of the clearance between the spring‑loaded flange and the inner tube, ensuring minimal clearance. When two or three units are combined, they effectively absorb both axial displacement and axial thrust in district heating pipelines. 5. Spherical expansion joint: compensates for pipe deflection caused by foundation settlement or thermal expansion and contraction. It offers advantages such as large compensation capacity, compact installation footprint, low fluid resistance, ease of installation, and reduced overall cost. 6. Sleeve expansion joint: designed for straight‑run pipelines, suitable for hot water, steam, and oil‑based media. By allowing the inner sleeve to slide relative to the outer sleeve, it accommodates thermal expansion and provides compensation for thermal movement.

2023-09-18

Structure and Types of Metal Bellows

The performance of a metal bellows depends on its structure; different structures exhibit distinct characteristics, which are reflected in the wave profile—defined as the pattern and shape of the corrugations when viewed in axial cross-section. Bellows typically feature U‑shaped, C‑shaped, Ω‑shaped, or S‑shaped waveforms. To enhance pressure‑bearing capacity and reduce stiffness, multi‑layer bellows and bellows equipped with reinforcing rings have been developed. In general, a circular cross‑section offers superior resistance to high pressures but permits only limited allowable displacement. By contrast, a U‑shaped cross‑section allows greater displacement at the expense of relatively lower pressure‑bearing capability. Under normal circumstances, the U‑shaped waveform is the most widely used domestically. When subjected to higher…

2018-10-23

Application of Metal Bellows in Valves

Metal bellows are characterized by their ability to undergo corresponding displacements under the action of pressure, axial force, lateral force, or bending moment, and they also exhibit properties such as pressure resistance, vacuum tightness, corrosion resistance, thermal stability, and a long service life.

2018-10-23

Imported bellows-type back-pressure balanced safety valve

The imported bellows‑type back‑pressure‑balanced safety valve is a pressure‑resistant device designed for use on pipelines and vessels handling toxic, hazardous, flammable, or explosive media as an overpressure protection system. It employs a formed stainless‑steel bellows to ensure zero external leakage of the process medium while simultaneously balancing the back pressure at the safety valve outlet, thereby maintaining the accuracy of the set pressure.

2018-10-23

Design and Selection of Metal Bellows

In bellows applications, the specified operating conditions of the system (the complete machine) or subsystem (component) serve as the primary basis for bellows design and selection. Typically, the following design parameters are provided: 1. Maximum spatial dimensions or nominal diameter 2. Nature and magnitude of the working load 3. Required working displacement 4. Operating temperature range 5. Properties of the working medium 6. Precision requirements 7. Service life Content and Principles of Metal Bellows Design and Selection The main tasks in bellows design and selection involve choosing the appropriate material, structural configuration, geometric dimensions, and performance characteristics based on the known system specifications. During the selection process, it is essential to meet the system’s functional requirements while striving for an advanced and rational structural design, with optimization pursued whenever feasible. Practical experience demonstrates that proper design and selection are critical; even if the manufactured bellows exhibit high quality, inadequate design and selection will still fail to satisfy operational demands. 1. Material Selection Based on the bellows’ intended application, types and magnitudes of loads, precision requirements, working medium, operating temperature, service life, and other relevant factors—while also considering the material’s formability and weldability—a suitable material should be selected. Additionally, market availability should be taken into account. 2. Determination of Structural Configuration (1) Selection of Corrugation Shape Choose a corrugation shape that best suits the bellows’ application, performance needs, and operating requirements, taking into consideration the performance characteristics and manufacturing features of various waveforms. In most cases, U‑shaped corrugations are preferred. (2) Determination of Number of Layers The number of layers depends on the bellows’ application, operating pressure, stiffness, and working medium. For higher operating pressures, multi‑layer bellows are generally chosen, with careful selection of both the number of layers and the wall thickness of each layer. (3) Preliminary Assessment of Coupling with Other Elastic Elements In certain situations, bellows are used in parallel with helical springs—for example, to enhance measurement accuracy, under relatively high operating pressures, or when subjected to impact loads. (4) Selection of End‑Connection Configurations When determining the end‑connection type, consider the forming and welding processes involved, as well as any structural constraints imposed by the overall system. (5) Consideration of Guiding Devices For bellows with significant effective lengths, guiding devices should be incorporated to prevent column buckling during operation. For instance, valve‑type bellows typically include such guiding mechanisms. (6) Evaluation of Reinforcement Rings To accommodate higher operating pressures, reinforcement rings may be employed, or a multi‑layer structure can be adopted; alternatively, a combination of both approaches may be utilized. Reinforcement rings provide localized strengthening, whereas multi‑layer structures offer comprehensive reinforcement. 3. Design Calculations and Selection of Structural Parameters The performance of a bellows is largely determined by its structural configuration. Key structural parameters include inner diameter, outer diameter, wall thickness, pitch, amplitude, number of corrugations, number of layers, dimensions of the end‑fitting sections, effective length, and overall length. Generally, users should select standard bellows from applicable industry standards or product catalogs provided by manufacturers rather than designing entirely new products. This approach minimizes development challenges, reduces manufacturing costs, and shortens production lead times. 4. Design Calculations for Performance Parameters Once the principal geometric dimensions have been determined through design calculations or preliminary selection, further computations are performed to establish the bellows’ performance characteristics. If the resulting performance parameters fail to meet technical requirements, appropriate adjustments to the geometric dimensions should be made. Since different applications impose distinct technical requirements, tailored design methodologies must be developed accordingly. For measurement purposes, the primary considerations are stiffness, strength, and elastic properties. Although various bellows with differing geometries and dimensional parameters may all satisfy the same stiffness criteria, the stress levels they generate can vary significantly. Solutions that not only meet stiffness requirements but also minimize operational stresses are preferable. Reducing stress not only enhances safety margins but also diminishes the adverse effects of material elasticity—such as hysteresis, creep, and fatigue—thereby improving both the precision and reliability of the bellows.

2018-10-23