BOLT TORQUE FOR POLYETHYLENE FLANGED JOINTS
|Publication Date:||1 January 2019|
Lap-Joint Flanges (LJF) have been used for decades. The typical polyethylene flange adapter with loose LJF is also known as a Van-Stone Flange joint. The HDPE flanged joint assembly is an engineered pressure containment connection subject to diverse forces. While simple in appearance, its design is complex due to the axial shear, radial dilation, disk-bending moments, residual interfacial sealing pressure, bolt-load versus bolt-torque, HDPE flange face creep-relaxation, LJF disc flexure, axial tension from thermal contraction of the pipe-line, some vibration, pressure-surge, pipe bending due to soil settlement, etc. The greatest contributors to flange leakage are insufficient torque, un-even torque, and flange misalignment. Written and correct bolt torque specifications and installation procedures will eliminate these problems. The flange assembly design, and written assembly specifications, are controlled by the pipeline designengineer or project engineer-of-record.
The ideal flange-adapter joint should exhibit Compressibility, Resilience, and Creep-Resistance. The plastic flange-adapter face should be able to compress into any and all surface texture and imperfections of the mating flange. The plastic flange face should be sufficiently and elastically resilient to move with dynamic loadings to maintain seating stress. The flange-adapter face should exhibit sufficient creep-resistance so as not to permanently deform after bolt-up under varying load cycles of temperature and pressure.
The "memory" of pipe-grade HDPE makes it an ideal flange face sealing surface. It becomes its own "gasket flange", and seals well when un-marred and torqued to meet or exceed the HDPE seating stress. When properly torqued with a flexible LJF, the HDPE flange-adapter becomes self-gasketing.
The LJF assembly is typically evaluated as a combined mechanical "spring" assembly. The torqued bolts are elastically stretched to initiate the sealing pre-load. The metal LJF (lap-joint flange) is elastically flexed (bent by the bolt-load) to maintain the pre-load and to transfer the load to the HDPE flange face. At small strains, the HDPE flange-face is elastically and visco-elastically deformed (axial compression and slight radial enlargement) so as to maintain pre-load sealing pressure on the flange-face surface. The HDPE flange face compressibility is the measure of its ability to deflect and conform to the mating flange face. This compressibility compensates for flange surface irregularities such as minor nicks, non-parallelism, metal corrosion, and variation in surface roughness or grooving depth. The HDPE flange face also exhibits Memory / Recovery / Resiliency which are measures of the elasticity of the HDPE material to recover shape and to maintain its deformation sealing pressure under varying loads across broad temperature ranges. Although the HDPE is a visco-elastic material that slightly creeps over time, at sufficient torque the flexure of the LJF and bolt stretch exceed the expected long-term compressive creep of the flange face, such that the residual sealing force exceeds the sum of the operating separation forces. In this way, the sealing pressure is maintained.
The combined "springs" of the stretched bolts, the flexed disc LJF, and the elastic component of the compressed flange-face, all serve to provide an elastic / visco-elastic, resilient "spring-seal" of the hydrostatically pressurized joint.
The key element to an effective sealing HDPE flanged joint, is to torque the bolts to a sufficiently high value to stretch the bolts, so that the LJF is flexurally distorted, and the HDPE flange-face sufficiently and continuously compressed. The joint is at equilibrium, with the compressive sealing force distributed across the sealing face and equal in magnitude to the pre-tension in the bolts. The total bolt tension must be able to constrain the joint assembly against operating pressure, surge pressure, pipe-line axial thermal contraction, and pipe bending strain from soil settlement, and flange angular alignment; all with an applied safety factor.
Caution: The component, Fpipe-bend (forces from pipe beam-bending), in the above equation can sometimes exceed thermal contraction and hydraulic forces. HDPE flange joints are geometrically rigid assemblies, unlike the flexible HDPE pipe ring "hoop". The rigid flanged joint cannot shed stress by ring deformation. Localized HDPE pipe beam-bending at a flanged joint due to soil settlement, water buoyancy or wave action, pipe "snaking" above ground, etc, must be managed so as to isolate the flange from beam-bending strain. External installation measures to protect PE flange joints from beam-bending strain are necessary. While additional torque can maintain the pressure seal, bending strain across the HDPE flange adapter should be limited to prevent flange adapter fracture.
Appendix "A" provides the method for calculation and determination of specified bolt torque at the required seating stress. Proceed to Appendix "A" to perform the required engineering calculations to determine the required target torque to be used in the Checklist following on the next page.