Plumbing Tubes: Capillary Soldered Joints

Since the introduction of thin walled copper tubes in the early eighties, this method of joining copper tubes has become the most widely used. This has been due to several factors:

a) Its ease of fabrication.
b) No expensive and heavy tools required.
c) Cost effective - up to 70% saving on time and labour.
d) Excellent mechanical properties.
e) Non-bulky fittings makes for a very compact system.

When installed correctly the capillary joint offers one of the strongest system joints available with a shear strength of up to 60MPa (600 Bar). This gives the system the ability to withstand high degrees of expansion, contraction and vibration.

Soldering

The use of the correct solder is extremely important if the completed joint is to maintain the required mechanical properties throughout its working life. Acid or resin core solders should not be used. These normally contain lead which should be avoided for health reasons and they do not have the required shear strength to withstand contraction and expansion.

Solders recommended for plumbing installations are covered by the SABS 24-1971, amended 1987, or DIN 1707 specifications.

Maksal 97/3 (97% Tin, 3% Copper). This is the most widely used of the solders, and it is the most cost effective and readily available.

Soft solder has a melting point of ±240°C. Their use prevents annealing (softening) of the tube and fitting during heating and eliminates the possibility of the formation of deleterious films which could under certain circumstances lead to pitting corrosion.

NOTE:

a) All solders containing Lead should not be used.
b) Resin Core or Acid Core solders should not be used.

The correct amount of solder is important. A guide to minimum quantities is shown in the table below:

TABLE 6:

Capillary Soldered Joints


Fluxes

The function of the flux is fourfold in ensuring the successful completion of a capillary soldered joint. The selection of a correct flux is therefore most important. A satisfactory flux will perform the following functions during soldering:

a) It will remove residual traces of oxide.
b) It will prevent the reformation of oxide.
c) It will promote "wetting" and assist capillary action.
d) It will readily flow out of the joint as solder flows in.

Combined with these are several properties which are of equal importance:
a) The flux should be "self-cleaning".
b) The flux should be easily applied.
c) The flux should be water based and easily flushed out.
d) The flux should contain NO ammonia.
e) The flux should withstand temperatures above 240°C.

A most suitable flux which meets all the above criteria and is highly recommended is Maksal EVERFLUX.

NOTE:

Self-cleaning fluxes will not remove dirt, plaster, tape glue etc. These must be removed mechanically with wire wool or emery cloth. Keep the lid on flux containers when not in use. Dirty or contaminated fluxes should be discarded to prevent poorly soldered joints. Good practice dictates that what is cleaned, fluxed and assembled today is soldered today. If the joint is allowed to stand overnight without soldering, it should be taken apart, re-cleaned - removing old flux and re-fluxed before soldering.

Heat Sources

As the recommended soft solder has a melting point of ±240°C the soldering of all capillary joints requires only a soft non-localized flame such as that produced by LPG Gas.

The use of Oxy-acetylene is NOT RECOMMENDED. The heat from Oxy-acetylene is too severe and concentrated, especially when using thin walled copper (SABS 460/0) grades. This can cause overheating, burning of the flux, annealing and in more severe cases melting of the tube and fitting.

Various electrical resistance soldering machines are also available and can have several advantages when used in areas such as:
a) Thatched roofing spaces.
b) In close vicinities to combustible materials.
c) Workshops, for prefabrication use.

NOTE:

a) The heat should be uniform, not concentrated.
b) Heat should be sufficient but not too hot.
c) Flame size should not be excessive.

Marking the joint

The soft soldering of capillary joins can be split into two methods, one for smaller sizes up to 54mm diameter and one for sizes above 54mm diameter.

Sizes up to 54mm

Soldering of these sizes can be achieved in any position with only one point of entry of solder into the joint.

  1. Cut the tube to the required length. The use of a tube cutter is recommended rather than a hacksaw as a square cut ensures full penetration of the pipe into the fitting. Ensure all burrs are removed and the tube end is reamed if required to prevent water flow restrictions.
  2. Clean tube end and fitting, if required, to remove any dirt, plaster, tape glue etc by means of a ring brush, wire wool or emery cloth.
  3. Apply Maksal Everflux to the outside of the tube for a length at least equal to the depth of the fitting. It is good practice to also apply flux to the inside of the fitting. The flux should be used sparingly, ideally applied by use of a small brush rather than a finger, nail, stick etc. After fluxing assemble joint ready to solder.
  4. The amount of solder used to fill the joint is important if leaks are to be avoided and strength maintained. To assist, refer to Table 6, which shows recommended lengths of solder to be used for the different joint sizes. It is recommended that this table is used and the solder bent at right angles and used as a guide to ensure sufficient solder is applied to each joint whilst also avoiding wastage.
  5. Apply the flame to the assembled joint concentrating the heat at the centre of the fitting rather than the tube. CAUTION: DO NOT MELT THE SOLDER WITH THE FLAME!
  6. The tube and fitting should be heated sufficiently to melt the solder when applied without the use of the flame. The correct temperature can be tested by removing the flame and offering the solder to the mouth of the joint. Take care not to overheat the joint. An indication of overheating is when the flux turns black. If this occurs remove fitting, clean pipe and fitting and re-solder. Ensure sufficient solder has been applied by the appearance of a visible fillet.
  7. After a short cooling period, slightly longer for brass fittings than copper, remove any excess residual flux with a damp rag. This gives the joint a clean, neat and professional appearance. It also stops the area around the joint, which has been in contact with the flux, turning green.

NOTE:

a) Use only the recommended flux and solders.
b) Cut the tube end square and debur.
c) Apply flux sparingly on tube and fitting.
d) Measure correct amount of solder.
e) Do not melt solder with the flame.
f) Do not overheat the joint.
g) Check visually for solder fillet.
h) Wipe off excess flux after soldering.

Good practice dictates whenever possible the completed system should be fully pressure tested before being closed up. Always flush the system as soon as possible after completion to remove flux residues from inside the pipes. The recommended use of water soluble fluxes such as Maksal Everflux will assist in the ease of the operation. If a system is to be inoperative for any extended period after installation, it is recommended that it be completely drained after testing and flushing and left empty of water. Capillary fittings with short sockets should not be used for soft soldering. Standards laid down by SABS 1067, Part 11 and equivalent to international standards are as follows:

TABLE 7:

FIGURE 2:

Diameter

Schematic Stages of Large Diameter
Soldered Capillary Joint
Capillary Soldered Joints Capillary Soldered Joints

Sizes above 54mm

The successful soft soldering of these large diameter joints requires greater skill and specialised techniques. Soldering of these sizes can also be achieved in any position if the correct procedure is followed.

  • Correct joint preparation is important if the joint is to be made successfully. The basic steps of joint preparation (refer to steps 'a' to 'd' page 18) also apply to larger sizes and should be followed every time.
  • Commence by pre-heating the tube until the flux at the mouth of the joint begins to run, then transfer the flame to the fitting. Continue pre-heating the fitting ensuring the heat is applied evenly and uniformly around the fitting. Finally bring the torch to about the “7 o’clock” position, test and heat until the solder melts from the heat of the tube. Whilst preventing the solder from running out the remaining portion is being soldered. Continue up from the "5 o'clock" position keeping the flame slightly ahead of the solder and ensuring the "Dam" remains solid until the "12 o'clock" position is reached.

Move the flame back down the opposite side and following the same procedure apply the solder from the "7 o'clock" position, up to, and slightly overlapping the "12 o'clock" position. Finally remove the flame and continue applying the solder into the joint until it is completely full and will accept no more solder. Once the solder has solidified, the joint area should be wiped with a damp rag to remove any excess flux. (Refer to section on page 18 & 19 for proper procedures).

Brazed joints

The brazing of capillary fittings to SABS 460 Class 0, copper tube on sizes below 54mm OD and SABS 460 Class 1 copper tube on sizes below 35mm OD, is not recommended by Maksal.

Brazed joints would normally be used only where operating service temperatures are above 150°C.

Unlike solder with a melting point of ±240°C, brazing materials melt in the range of 600°C to 750°C. Consideration of this must be taken when designing a system as these temperatures will anneal the tube and fitting. Pressure ratings should therefore be calculated as for annealed tubing and care taken to avoid mechanical damage and consideration be given to adequate and correct tube supports.

There are two types of filler materials in normal use. These are alloys containing silver and alloys containing phosphorus. Fluxes are not normally required when joining copper to copper only when joining copper to brass or cast bronze fittings. Check with recommendations of the filler material manufacturer for the correct flux and application.

The various steps of joint preparation and application of the filler material are the same as for the large size soldered joints.

It is recommended that a slightly reduced flame be used i.e. Excess fuel.

As with soldering the brazing filler material MUST flow into the capillary joint and fill the capillary gap for the full depth of the socket. It is extremely BAD practice to apply only a “fillet” weld around the mouth of the fitting. It is advisable to allow the fitting to cool naturally after brazing. Brass or bronze cast fittings should never be cooled quickly by means of water.

The heat source for brazing would normally be Oxy-acetylene to provide the required temperature. It is important that this flame does not remain on the same point and damage the joint but kept in motion over the section being brazed.

Compression fittings

Brass compression fittings, to avoid corrosion problems, should be dezincification resistant. It is not recommended by Maksal that they be used in conjunction with SABS 460 Class 0, hard drawn tube, but only on the half hard classes i.e. 460/1 - 460/2 and 460/3 tubing. These fittings rely on the compression of the brass olive onto the tube. It is BAD practice to either solder the olive or fitting into the tube or apply any PTFE tape or hemp to the fitting. DO NOT over tighten the cap nuts.

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