Hydrostatic design stress

The physical and mechanical properties of the principal thermoplastics of interest for process plant applications are listed in Table 3.42. Table 3.43 gives typical hydrostatic design stresses for different types of thermoplastic pipe. Plastics widely employed in piping systems are described briefly below. 

Hydrostatic design stress n. The estimated sustained hoop stress that can exist in a pipe at expected service conditions and over the life of the pipe with a high degree of certainty that failure will not occur. 

Pipe analysis Hydrostatic design stress Tfensile stress at 5deld Tfensile creep ruptm stress Compression strength Critical stress intensity factor... 

Since the maximum recommended hydrostatic design stress is for continuous water pressure at73 F (23 C), it is up to the designer to determine the extent, if any, by which this stress should be reduced to account for any departure from these conditions and the need for a suitable margin of safely against other considerations. See the discussion on design. 

Chlorinated Polyvinyl Chloride. As implied by its name, chlorinated polyvinyl chloride (CPVC) is a chemical modification of PVC. CPVC has properties very similar to PVC but the extra chlorine in its structure extends its temperature limitation by about 50°F (28°C), to nearly 200°F (93°C) for pressure uses and about 210°F (99°C) for nonpressure applications. ASTM D 1784, the rigid PVC materials specification, also covers CPVC which it classifies as Class 23477-B. By the older designation system, it is known as Type IV, Grade 1 PVC. CPVCs for pressure pipe are designated CPVC 4120 [i.e.. Type IV, Grade 1 CPVC with a maximum recommended hydrostatic design stress of 2000 Ib/in (13.8 MPa) for water at 73.4°F (23°C)]. At 180°F (82°C) the maximum recommended hydrostatic design stress for CPVC is 500 Ib/in (3.4 MPa). 

Once the HDB is established and then reduced to a hydrostatic design stress (HDS) by the application of an appropriate service (or design) factor, the same formula, rewritten as follows, may be used to compute the pipe pressure rating ... 

The last two digits code the maximum lecommended hydrostatic design stress (RHDS), expressed in hundreds... 

Popelar et al. proposed in their paper that times to failure of 650 h at 80°C would be sufficient to establish the 50-year hydrostatic design stress at 20°C. They also stated that these shift functions could consolidate data irrespective of type of MDPE or HOPE gas pipe material and that this signified that these functions are universal for these materials. This work was used to develop accelerated testing requirements for polyethylene piping grades. 

Developing Long-Term Hydrostatic Design Stress Data and Pressure Rating... 

Long-term hydrostatic design stress is obtained by essentially extrapolating the stress-time regression line on data obtained in Section 12.4.2. The following is a summary of procedures used to obtain long-term hydrostatic design stress (5). 

Hydrostatic design stress Working pressnre Average outside diameter Minimum wall thickness... 

The pressure rating of thermoplastic pipe is mathematically calculated from the SDR and the allowable hoop-stress. The allowable hoop-stress is commonly known as the long-term hydrostatic design stress. This is the stress level that can exist in the pipe wall continuously with a high degree of confidence that the pipe wiU operate under pressure for at least 50 years with safety. The American Society for Testing and Materials (ASTM) and the Plastics Pipe Institute (PPl) has adopted... 

Plastics Pipe Institute, Policies and Procedures for Developing Recommended Hydrostatic Design Stresses for Thermostatic Pipe, PPI-TR3-July 1968. 

Although adherence to ASTM appendixes is not mandatory, the PPI procedure was the only industry-accepted mechanism to determine long-term hydrostatic strength and hydrostatic design stress. 

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