Piping branch connections

The piping specifications for a refinery or petrochemical plant will often give the allowable sizes permitted for screwed pipe branch connections. For usual piping services, the 2,000-pound or 3,000-pound F.S.S.E. fitting will be specified. 

Thermowell requirements differ from the screwed pipe branch connection in that the threaded thermowell must be removed occasionally for inspection, especially during plant shutdown, and then replaced. This will require perfect pipe threads for the thermowell connections. Good threads can be assured if the 3/4-inch, 1-inch or 1-1/4-inch F.S.S.E. type fitting rated at 6,000 pounds such as a thread adapter, elbow adapter or pipe coupling is welded in the process piping or equipment. 

Tees Tees may be cast, forged, or hot- or cold-formed from short pieces of pipe. Though it is impossible to have the same flow simultaneously through all three end connections, it is not economical to produce or stock the great variety of tees which accurate sizing of end connections requires. It is customary to stock only tees with the two end (run) connections of the same size and the branch connection either of the same size as the run connections or one, two, or three sizes smaller. Adjacent reducers or reducing elbow fittings are used for other size reductions. Branch connections (see subsection Joints ) are often more economical than tees, particularly when the ratio of branch to run is small. 

FIG. 10-166 Typ es of reinforcement for branch connections. (From Kellogg, Design of Piping Systems, Wiley, New York, 1965. )... 

Acceptable comprehensive methods of analysis are analytical, model-test, and chart methods, which evaluate for the entire piping system under consideration the forces, moments, and stresses caused by bending and torsion from a simultaneous consideration of terminal and intermediate restraints to thermal expansion and include all external movements transmitted under thermal change to the piping by its terminal and intermediate attachments. Correction factors, as provided by the details of these rules, must be applied for the stress intensification of curved pipe and branch connections and may be applied for the increased flexibihty of such component parts. 

Figure 15.19 Examples of crack patterns due to stress-corrosion cracking and corrosion fatigue in branch connections and vessel or pipe support attachment welds. (Reprinted with permission from Helmut Thielsch, Defects and Failures in Pressure Vessels and Piping, New York, Van Nostrand Reinhold, 1965.)...

Where a branch connection is connected to a main run of pipe, it is necessary to specify the type of fitting required. ANSI B31.3 provides a... 

Figure 2-10. Branch connections for welding openings into steel pipe. See Figure 2-4C for alternate welding fittings. By permission, Bonney Forge Corp., Allentown, PA.

Vibration Frequency, Hz Note Indicated vibration iimits are for average piping systems constructed in accordance with good engineering practices. Make addi-tionai aiiowances for criticai appiications, unreinforced branch connections, etc. 

Acceptable Detail for Branch Connection of Pipe Fitting. .. Acceptable Details for Branch Attachment Suitable for 100%... 

Precautionary Considerations Alternative Rules for Evaluating Stress Range Recommended Practices for Proof Testing of Pipelines in Place Estimating Strain in Dents Sample Calculations for Branch Reinforcement in Piping Welded Branch Connections and Extruded Headers in Pipeline Systems... 

Fig. GR-3.4.3(i) Preparation and Alignment of Pipe Branch to Pipe Header Connection...

Connections that require the pipe branch to penetrate into the pipe header shall not be permitted unless approved by the engineering design. 

A pipe having a branch connection is weakened by the opening that must be made in it and, unless the wall thickness of the pipe is sufficiently in excess of that required to sustain the pressure, it is necessary to provide added reinforcement. The amount of reinforcement required to sustain the pressure shall be determined in accordance with para. IP-3.4.2 or IP-3.4.3. There are, however, certain branch connections which have adequate pressure strength or reinforcement as constructed. It may be assumed without calculation that a branch connection has adequate strength to sustain the internal and external pressure which will be applied to it if... 

Fabricated branch connections on steel pipe shall meet the design requirements of paras. PL-2.3 and PL-2.4. 

In pipelines, especially those that are highly stressed from internal pressure, uniform and adequate support of the pipe in the trench is essential. Unequal settlements may produce added bending stresses in the pipe. Lateral thrusts at branch connections may greatly increase the stresses in the branch connection itself, unless the fill is thoroughly consolidated or other provisions are made to resist the thrust. Rock shield shall not be draped over the pipe unless suitable backfill and padding are placed in the ditch to provide a continuous and adequate support of the pipe in the trench. 

There is an NPS 8 branch at right angles to an NPS 12 header (Fig. E-l). Both run and branch are of aluminum alloy Schedule 80 ASTM B 241 6061-T6 seamless pipe. The connection is reinforced by a ring 14 in. O.D. (measured along the run) cut from a piece of NPS 12 Schedule 80 ASTM B 241 6063-T6 seamless pipe and opened slightly to fit over the run pipe. Allowable stresses for welded construction apply in accordance with Mandatory Appendix IX, Table IX-1A, Note (28). The fillet welds have the minimum dimensions permitted in para. IP-3.4. A zero corrosion allowance is specified. What is the maximum permissible design pressure if the design temperature is -320°F ... 

Tees Tees may be cast, forged, or hot- or cold-formed from plate or pipe. Tees are typically stocked with both header (run) ends of the same size. In general, run ends of different sizes are not typically stocked or specified however, occasionally run ends of different sizes are specified in threaded or socket-welded sizes. Branch connections may be full size or reducing sizes. Branch reductions two sizes smaller than the header are routinely stocked, and it is not typically difficult to purchase reducing tees with branches as small as those listed in ASME B16.9 (i.e., approximately one-half the header size). Economics, stress intensification factors, and nondestructive examination requirements typically dictate the branch connection type. 

Stress intensification factors for branch connections are based on tests with at least two diameters of straight run pipe on each side of the branch centerline. More closely loaded branches may require special consideration. 

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