Problems associated with the use of PTFE-lined centrifugal pumps
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Currently, there are two centrifugal pumps lined with PTFE, and some problems have arisen during their use: 1. The pumps are used in conjunction with an exhaust gas packing scrubber, and they are designed to handle acidic gases containing HCl. The process is as follows: 30% NaOH solution is added to tap water to form an absorption solution, with the concentration maintained at 3%-5%. As the absorption process proceeds, the pH value of the absorbing solution decreases; when it becomes acidic (around pH 5), it is sent to the previous absorption tower. Add liquid alkali again to prepare an absorption solution of about 3%. Repeat this process. Both the tower and the pipelines are made of PP. Currently, the mechanical seals of PTFE-lined pumps leak easily, and the associated pipelines also often leak at the weld points; even after rewelding, leaks occur again within a few days. This pump runs continuously. 2. The other PTFE-lined pump is used in conjunction with a liquid caustic storage tank. It is used to transport 30% liquid caustic to the filler scrubber tower mentioned in point 1. This pump is also leaking at the mechanical seal location; a new seal was just installed, and it worked fine during the first use. If used again after a few days, it leaks. Tightening the screws of the mechanical seal over and over again doesn’t help either. Use this pump once every week, for a few minutes each time. Logically, with a PTFE-lined pump and PP pipes, there should be no problem transporting alkaline liquids – so why is leakage occurring so easily? Please help find the reason, thank you!
1. The quality of the pump is poor. 2. Sodium hydroxide can corrode the silicon carbide rubbing surfaces of the mechanical seal; we can discuss this in more detail via private message
In your case, a mechanical seal should not be used; opt for another type of seal instead. Where are the weld points on your piping? Is it due to the high head and high pressure of the pump?
There is a type of mechanical seal designed specifically for pumping alkaline solutions (it’s a bit more expensive, but the cost of replacement is much lower compared to replacing the pump itself). You can ask the manufacturer to replace the mechanical seal for you and give it a try
I’m considering magnetic pumps now; it seems that when the pH is around 5 and the absorbing liquid is acidic, there are fewer problems with the pipes. Once it becomes 3% liquid caustic, it leaks easily right away
When the pH is 5, both the pipelines and pumps operate normally. However, once the chemical used becomes 3% liquid caustic soda, leaks occur easily in both the mechanical seals and the pipelines. Therefore, it doesn’t seem to be a problem with the mechanical seals; it’s possible that the silicon carbide sealing surface you mentioned has reacted with the NaOH in the liquid caustic soda
Check the material of the O-ring in the mechanical seal to see if it is rubber. Most rubber O-rings are resistant to alkalis but not to acids. For machine seals that are exposed to both acids and alkalis, it is best to use polytetrafluoroethylene as the material.
The leakage in the first pump may be caused by corrosion at the interface of the mechanical seal, while the leakage in the second pump is due to wear resulting from 30% sodium hydroxide crystals (due to low usage frequency). As for the pipeline leakage, it is caused by welding issues – poor welding quality along with pipeline vibration.
If the quality of the pump is poor, consider using a pump without an axial seal or one with a dynamic seal. As for the piping, the welding technique is too poor; it wasn’t welded properly.