Three key techniques for selecting drain valves and welding stress

Let's talk about the drain valve first.

Choosing it depends not only on the diameter or displacement, but also on the actual working conditions.

For example, equipment such as steam turbines and steam pumps are most afraid of condensed water accumulating inside, which can easily cause vibration and cavitation. In this case, a floating ball drain valve must be used, which can follow the liquid level and discharge the condensed water as soon as it comes out, making it highly resistant to water hammer.

If it is installed outdoors and it is cold in winter, mechanical types such as floating bucket are prone to freezing. It is better to replace it with a drain valve that relies on bimetallic plate action, or to add insulation to the mechanical type.

There are also some high-pressure steam pipelines with large pressure differences between the inlet and outlet, which ordinary drain valves cannot withstand. Switching to a liquid level control valve is actually more worry free. Previously, a chemical plant made this change, and the efficiency of condensate discharge has been significantly improved.

Let's talk about welding residual stress again.

After the equipment is welded, there will be residual stress at the weld seam, which is prone to cracking if left untreated for a long time.

If a newly built large pressure vessel, such as a storage tank of thousands of cubic meters, it is best to use overall heat treatment - heating the entire equipment to a certain temperature, holding it for a few hours, and then slowly cooling it down, which can remove most of the residual stress.

However, complex structures such as pipe bends and flanges cannot be heated as a whole, so local heat treatment is used, only heating around the weld seam. Although it cannot completely relieve stress, it can make the stress distribution more uniform.

If the equipment is already running and cannot be stopped for repair, the hammering method is very practical. Using a round headed hammer to evenly tap the weld seam can offset some of the stress;

For complex welding seams, such as those in long-distance pipelines, explosive methods can be used to induce slight deformation of the weld seam through shock waves, which not only relieves stress but also forms protective compressive stress on the surface. Therefore, it is necessary to hire a professional team to operate.

Actually, these two things need to be coordinated.

For example, when installing a drain valve, do not get too close to the weld seam - the weld seam itself has high stress, and the vibration when opening and closing the drain valve will increase the impact. It is best to be at least two pipe diameters away from the weld seam. If there is not enough space, the weld seam stress needs to be treated first.

When choosing a drain valve, one must also consider its ability to withstand water hammer. Previously, some power plants used pulse type drain valves, which caused pipeline vibration and micro cracks in the welds. 

After switching to a floating ball type, the vibration was reduced and the stress on the welds was also greatly reduced.

Now sensors can be installed to monitor the flow rate of the drain valve and the stress of the weld seam, providing early warning of any problems and making maintenance more proactive.

At the end of the day, the core of steam trap and welding stress control is "targeted" - depending on the type of equipment and the environment in which it is used, and then selecting the appropriate method.

In the future, with intelligence, using digital twins to simulate selection effects, coupled with real-time monitoring of the Internet of Things, problems can be predicted in advance, making equipment operation and maintenance more worry free and secure.