In BSP thread with captive seal connections, precise control of the seal ring compression is crucial to preventing leakage. BSP threads are divided into two types: parallel threads (BSPP) and tapered threads (BSPT). The former relies on the seal ring for low-pressure sealing, while the latter achieves self-sealing through its tapered shape, but still requires the seal ring in low-pressure or non-metallic connection scenarios. The seal ring compression directly determines the contact pressure and seal life: insufficient compression leads to excessively low contact pressure, unable to withstand gap changes caused by medium pressure or vibration; excessive compression may cause excessive deformation of the seal ring, accelerating material aging, or even shear failure during thread tightening. Therefore, it is necessary to coordinate control from four aspects: material matching, structural design, installation process, and environmental adaptability.
The hardness and elasticity of the seal ring material are fundamental to compression control. Rubbers with lower hardness (such as nitrile rubber) deform more under the same compression, making them suitable for low-pressure scenarios, but excessive compression leading to creep relaxation must be avoided; materials with higher hardness (such as fluororubber) can withstand higher pressure, but precise control of the compression is necessary to prevent brittle fracture. For example, in BSPP threaded connections in chemical plants, replacing aged sealing rings with fluororubber and applying appropriate tightening torque can completely solve leakage problems. Furthermore, the cross-sectional shape of the sealing ring (such as an O-ring or lip ring) must match the thread type: BSPP threads typically use O-rings, and their compression must cover the gap between the threaded hole and the connector; BSPT threads, when using sealing rings in low-pressure scenarios, require lip rings to utilize their unidirectional sealing characteristics to reduce the impact of compression fluctuations.
Thread machining accuracy and sealing groove design are the physical basis for compression control. BSP threads have a 55° tooth angle, and their rounded tooth tip structure is more wear-resistant than the 60° tapered shape of American NPT threads, but machining errors can lead to out-of-tolerance pitch diameter, thus affecting the uniformity of sealing ring compression. For example, a factory mistakenly used a 24.50mm hole diameter to machine a 3/4-14 specification thread, resulting in a 0.15mm out-of-tolerance pitch diameter. Mandatory first-piece thread gauge inspection has significantly improved the pass rate. The dimensions of the sealing groove must balance the free-state cross-sectional diameter of the sealing ring and its height after compression. A groove that is too shallow will result in excessive compression, while a groove that is too deep will result in insufficient compression. For BSPP threads, the sealing groove is typically located on the outer cylindrical surface or inner hole of the connector. Precision machining is required to ensure that the groove width and fillet radius meet standards, preventing stress concentration and damage to the sealing ring.
Refined installation techniques are crucial for controlling compression. Tightening torque directly affects the compression of the sealing ring: insufficient torque leads to inadequate compression, while excessive torque may cause excessive deformation of the sealing ring or yielding of the threaded connection. For example, in BSPT tapered thread connections, an ultrasonic stress tester must be used to control the preload error within a minimal range to avoid micro-cracks caused by tapered overload. For BSPP threads, chemical plant case studies show that controlling the tightening torque within a specific range can completely solve leakage problems. Furthermore, during installation, it is necessary to avoid twisting or scratching the sealing ring: grease can be applied to the sealing ring surface, or a special sleeve can be used for protection on the threaded section. If necessary, a retaining ring can be installed to prevent the sealing ring from being squeezed out of the gap.
Environmental factors significantly impact the long-term stability of the compression ratio of sealing rings. Temperature changes cause rubber to expand and contract, necessitating the provision of compression compensation space: at high temperatures, ring expansion may increase compression, requiring the selection of high-temperature resistant materials or a reduction in initial compression; at low temperatures, rubber contraction may lead to insufficient compression, requiring compensation by increasing initial compression or using low-temperature elastomers. Fluctuations in medium pressure also affect the dynamic sealing performance of the sealing ring: high-pressure media may further compress the ring, requiring pressure compensation structures to be considered in the design; alternating pressure may cause fatigue failure of the ring, requiring optimization of compression distribution or the use of combined sealing structures to improve fatigue resistance.
The sealing performance of a BSP thread with a captive seal is essentially the result of the combined effects of materials, design, process, and environment. From the successful application of fluororubber sealing rings in chemical plants to the strict control of thread machining accuracy and precise control of installation torque, every step must prioritize "precise compression ratio." In the future, with the popularization of intelligent tightening tools and online monitoring technology, real-time feedback and dynamic adjustment of sealing ring compression will become possible, providing more solid technical support for the reliable sealing of BSP threaded connections under extreme scenarios such as high pressure and high frequency vibration.
