TUBING

Like Piping and Fabrication said in the last post, that will provide another useful post and now we entering chapter of Tubing.

Copper, aluminum, steel, and stainless-steel tubing are frequently used in hydraulic, pneumatic, and sampling systems. Installation is most often concerned with protection of such materials from damage, since they are often associated with control systems. The manner of protection is left to the designer’s judgment. Lighter wall tubing is often bent using small compression-type benders. Tubing is joined to itself and to pipe-size fitting and components with a variety of proprietary tubing fittings.. Some heavier-wall stainless-steel tubing is welded using specially designed socket welding fittings, GTAW welding with filler metal added is used for such applications. Pipe Supports This section offers some thoughts on the installation of piping supports. The design, manufacture, and influence of supports on the system flexibility.

As pointed out earlier, economics and efficiency dictate that it is preferable to install the permanent supports for a system as the first step, thus minimizing the need for temporary supports. In so doing considerable judgment should be exercised, since there can be minor variations between the as-designed and as-installed line location. Resilient and constant-effort support should be locked with stops to preclude change in supporting effort as the line is being installed. Only after the line has been completely welded, tested, and insulated should the stops be removed. Once removed, the resilient and constant-effort supports should be carefully adjusted to their ‘‘cold’’ positions. This may take several iterations, since adjustment of any one will change the loading on the adjacent ones. Systems with multiple constant-effort supports can be especially troublesome. Since the support design is most often based on theoretical values of weight of the pipe, insulation, and the fluid, there will be some difference between the actual and calculated supporting effort. Where rigid supports are involved, this variation will be taken up automatically. Where a system is designed with multiple resilient or constant-effort supports, every effort should be made to incorporate one or more rigid supports in the design to absorb the variation between actual and theoretical loads. Otherwise it may be necessary, with the approval of the designer, to modify the spring load-carrying settings. As the line goes to operating temperature, it should be carefully observed to assure that there are no unforeseen interferences with its required expansion, particularly at nearby structures, floor sleeves, or adjacent lines or by restrained branch connections. Some modification may be required to assure free expansion of the line. All resilient supports and constant-effort supports should be checked during initial start-up to assure that they are functioning properly, and after the line has been at operating temperature for several hours, they should be checked to verify that they are in the required ‘‘hot’’ operating condition. It may be necessary to readjust some units to match the calculated ‘‘hot’’ loading. These settings should be checked on a regular basis for the first few weeks of service, particularly in systems operating in the creep range, since the temperature will begin to relieve locked-in construction stresses, and the line may choose a different, more relaxed location. Readjustments may be required. If after some time in service, the resilient and constant-effort supports still require significant adjustment (i.e., the system cannot be balanced), a complete review of the flexibility analysis, expansion calculations, weight calculations, hanger, design, and installation procedures should be made to determine the cause. Resilient and constant-effort support units which are not functioning in the spring range (i.e., they have become ‘‘solid’’ or ‘‘loose’’) may impose undesirably high stresses in the line if they are not corrected, which can lead to premature failure or significantly reduced system life.

Leak Testing

At one time, complex shapes were pressure-tested to determine their suitability for the service intended. This involved stressing the component to a point above service stresses, but below bursting stress, and was referred to as a pressure test. Currently most codes require some type of test to determine leak tightness rather than service suitability. The most common method of leak testing for piping systems is the hydrostatic test. Usually this involves water at ambient temperature as the test medium. B31.1 requires that the system be pressurized to 1.5 times the design pressure, ASME III, to 1.25 times the design pressure, and B31.3 requires a test pressure of 1.5 times the design pressure adjusted by the ratio of the allowable stress at test temperature divided by the allowable stress at operating temperature. In each case, however, the test pressure of unisolated equipment or some function of the yield stress of the line material may be a limiting factor. See the applicable code for particulars.

The line must be held at test pressure for at least 10 min, but may be reduced as permitted in the applicable code until the examination for leakage is complete. Depending on the specific situation, alternative test fluids may be employed. As an example, in a liquid sodium system, where water could be very hazardous, or in cases where the possibility of freezing exists, a hydrocarbon or other fluid might be used.

In instances where water or other liquids are unacceptable, or where supports may not be adequate to carry the added weight of water, pneumatic tests may be performed. Pneumatic tests are potentially more dangerous than hydrostatic tests, and extreme care should be exercised. B31.1 and ASME III require the pneumatic test be performed at not less than 1.2 times the design pressure, while B31.3 limits the test to 1.1 times design. In each case, the limits regarding equipment and yield strength previously cited for hydrostatic tests also apply. Prior to the test a detailed review of the section of the line to be tested should

be made with the following in mind:

1. Temporary supports for those sections where the permanent supports were not designed to take the additional weight of the test fluid.

2. Isolation or restraints on expansion joints.

3. Isolation of equipment or valves which may be overstressed at test pressure.

4. Location of test pump and the need for additional test gauges if there is a significant head variation due to elevation differential.

5. Location of vents and drains.

6. Location of a relief valve to preclude excessive overpressure due to possible thermal expansion of the test fluid.

7. Consideration of the probable ambient test temperature relative to the expected brittle fracture toughness of the system materials. Heating the water may be a solution.

8. Alternative test fluid.

9. Accessibility to the weld joints for inspection. Some codes require that the weld joints be left exposed until after the test.

10. Assurance that no part of the system will exceed 90 percent of its yield strength.

It is advisable to prepare a written procedure outlining the scope and boundaries of each test to assure that it is performed in a safe manner. The codes vary a bit on the required test pressures, time at test pressure, pressure during inspection for leakage, and whether alternative tests may be performed. It is advisable to look at each one specifically. 

That's all for now about TUBING and thanks for visiting Piping & Fabrication.