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Boiler Tubes and Pipes

Heat transfer in a boiler is almost exclusively by the tubes. Accordingly, every boiler employs tubes of different sizes, thicknesses and metallurgy.

Boiler Tubes

To state it crudely, a tube or a pipe is a rod with a hole. In boiler industry, a tube is characterised by the following features:

Tubes are specified by their outside diameter and thickness (indicated by wire gauge SWG or BWG).

  • The maximum outside diameter of tubes rolled is 127 mm (5″).
  • Tolerances on thickness are much closer than in pipes.
  • The carbon content and hardness are lower as tubes are to be bent to close radii.

Even though tubes and pipes look alike, there are serious differences, not permitting interchangeability:

  • Tubes are for conveying fluids and simultaneously transfer heat, while pipes are only for conveying.
  • Tubes are limited to 127 mm outside diameter, while pipes come in much larger size of 1220 mm outside diameter and above.
  • Tubes are always specified by od (outside diameter) while pipes are designated by nominal bore (NB) up to 305 mm (12 in) and by od beyond that.

In a boiler, HSs are made of tube while integral and external piping, large-bore downcomers and headers are made of pipe materials.

Tube Classification

Based on the method of manufacture, all boiler tubes fall into two categories:

  • ERW tubes made by folding and welding steel strips (skelp).
  • Seamless tubes produced by piercing a red-hot billet with a plunger.

The advantages of ERW tubes are

  • The inside diameter (id) and od are perfectly concentric, making it ideal for expanded tubes.
  • The inside surface is very smooth, thereby reducing the pressure losses.
  • Lower cost.

ERW tubes

As ERW tubes are made by longitudinally bending and welding of skelp, tube thickness is limited to 6 mm from welding consideration. The acceptability of ERW CS tube is very high, but AS is low. Often, it is not selected for BB tubes which require flaring during tube expansion for fear of opening up of the weld seam. This hesitation may not be justified today after the modern mills have established that the welding process has been thoroughly upgraded and made very reliable.

Seamless tubes

Seamless tubes are made in two qualities, namely hot finished seamless (HFS) and CDS. HFS tubes, when reheated to appropriate temperature and re-rolled to closer finish, CDS tubes are produced. CDS tubes have closer tolerances, comparable to ERW, but are more costly than HFS.

The advantages of the seamless tubes are that thicknesses >6 mm are available and there is no fear of opening of weld seam. However, the concentricity of inside and outside diameters is not as good as in ERW. Generally, seamless tubes are marginally more expensive.

ERW or Seamless?

This is largely a matter of practice in each market. The European market has an overwhelming preference for seamless tubes while in the other markets, both are accepted on merit. Generally, ERW tubes find acceptance for LP and MP boilers <70 mm. In many markets, ERW tubes are preferred wherever tubes have to be expanded because of better concentricity. Membrane panels made with ERW tubes are also gaining wider acceptance as the ERW tube-making practices have been perfected over the years.

Tube Materials

The metallurgy of tubes is temperature dependent. Accordingly, tubes are made in the following execution:

  • CS
  • Low AS
  • High AS
  • SS

Following gives the details of all these classes of tubes along with the limiting temperature.

Pipes

Similar to tube, pipe is also a rod with hole, but tubes and pipes are quite different as explained in the topic on Tubes. Pipes are used where conveyance of fluids is required with no heat transfer.

In boilers, pipes are used for

  • Downcomers and headers which are placed inside or outside the gas stream
  • Integral piping, namely, drain, vent, blowdown, SB, attemperator and interconnecting piping
  • FW, main steam and RH steam pipes

Pipes of only seamless class of execution are employed in boilers because of the combination of pipes & tubes. Pressure pipes of HC industry, such as line pipes and submerged arc (welding) (SAW) pipes, are not used.

The following table lists the popularly used BQ pipes of American Specs.


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