Pipe Fabrication & Welding: Effects on Pumps and Valves

14 May 2020

Without external influences, liquids will flow from a high-pressure system to a low-pressure system or from top to bottom (due to gravity). Pumps can be used to move liquids mechanically by adding energy to the liquid. Frequently, technical issues arise with pipes connected to pumps and valves, where the need for sound metal fabrication and welding becomes essential.

Welding has a significant role in modern metal fabrication. However, poor welding can cost significant time and money at the onset or later in the life of a project. 

Many problems with pipework arise as a result of poor welding and fabrication practices. Oxyfuel and plasma cutting are the most common metal cutting processes due to their availability and ease of operation. However, these processes can add a layer of oxide to cut edges. 

Oxides on cut surfaces generally have a higher melting point compared to the parent metal and can remain in the weld, causing porosity and lack of fusion. Weld porosity can be detrimental in pipes containing liquids at pressure, especially when connected to a pump. Upstream piping can leak through pores, creating a pressure drop. 

The problem worsens with ingress of air or other gases through pores. This can allow gases and bubbles into the suction side of the pump, causing cavitation. The pump will use excess energy to compress gas bubbles and will be overworked trying to maintain a specified fluid flow rate. Gas bubbles can also impact metal parts of the pump, generating significant noise and reducing pump life. Cavitation can also have a negative effect on control valves. 

Although control valves are designed to withstand cavitation, they can only do that for a limited amount of time. As gas bubbles impact the control valve's wall, they collapse and cause an implosion. This can cause pitting in the valve walls and wear of metallic parts.

To extend the service life of pumps and valves, appropriate welding and fabrication practices must be followed by professionals working to all relevant standards.

Courtesy of the American Welding Society.