Method for Connecting Valves and Fittings
Connecting High-Pressure Fittings – When to Solder, Weld, or Braze
You’ve done your shopping and you have your high-pressure fittings. Now it’s time to put the system together. We sometimes hear the terms solder, weld, and braze used interchangeably (or even incorrectly). But these are unique processes. Each has its place in the industrial world. And not all methods are suitable for joining fittings used in high-pressure systems.
Selecting the correct method for connecting valves and fittings is based on the materials involved and their end use conditions such as service temperature, pressure, and needed corrosion resistance.
Choosing the Right Method
Using a filler (solder) or flux to join two pieces of metal, soldering is a simple method that melts the solder but not the metals being connected. The two metals may be thin or thick and can be similar or not.
Soldering is done at relatively low temperatures and the solder typically melts anywhere between 361°F and 572°F (183°C to 300°C). A soldered bond is not nearly as strong as one that’s welded or brazed, and it would most definitely not hold up to high pressure. It does, however, fit the bill for some copper plumbing fittings.
In addition to not having high-pressure applications, it is not a good option where high-temperatures may occur or for connecting large pieces.
Welding can only be done using two similar metals. When welding, the parts themselves are actually melted and joined along the edges, although a filler metal may also be used to fill in or seal any gaps.
The welding temperature depends on the materials being welded. It has to be high enough to melt both of the metals in order to fuse them together.
Correctly done, the welded area is as strong as the adjacent metal, and welded seams can cover large areas. Overheating the metals can weaken them, so care must be taken to use the right temperature and not go overboard.
Welding isn’t a good option for thin metals. It can also result in localized distortion or internal stress of the welded area so consideration should be given to that possibility when it may affect fit or performance.
Brazing joins two metals (including dissimilar materials) by bonding them to a filler. Unlike in welding, metals of different thicknesses can be connected via brazing.
By definition, brazing is done at temperatures greater than 840°F (450°C). In the real world, this temperature is usually over 1,000°F (540°C) and can range from between 1,000°F and 2,300°F (540°C and 1,250°C). The key to brazing temperature is that the filler must be melted, but the metals being joined need to remain solid. Some brazing methods heat the entire unit, eliminating or significantly reducing thermal stress and distortion.
Brazed metals get their strength as a result of a chemical reaction between the metals and the braze filler. Brazing forms an alloy creating a permanent bond that is at least as strong as the base materials. In fact, CPV’s Mark VIII O-Seal fittings, when properly brazed, may prove to be more durable than the tubing itself (as demonstrated by tests in the photos shown here).
In high-pressure systems where permanence is required, CPV’s line of low-cost Direct-Weld/Braze Fittings may be the solution.
Capable of handling up to 6,000 PSI (413 bar) of pressure, they are reliable when there is zero room for error.
3 Tips for Leak-Free Fitting Connections
Regardless of the method selected to attach your valves and fittings, you’ll want to follow a couple general rules to make sure they don’t spring any leaks:
- The metal being joined must be properly cleaned ahead of time. Contaminants can interfere with the bonding and create gaps.
- Be sure to use the right filler and flux for the application. A solder flux will not work for brazing and vice versa.
- Make sure all parts are properly sized. This means knowing what method you’ll be using to connect them.
Whenever they’re available the manufacturer’s installation instructions should be followed. Information and step-by-step instructions for brazing your high-pressure CPV fittings can be found here.