Connection of plastic flanges and metal flanges
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As the title suggests, when connecting metal flanges and plastic flanges, the standards recommend the use of fully flat flanges, namely FF type flanges, along with FF type gaskets. In actual production, how do you guys make the connections? At present, in our facility, all the inlet and outlet flanges of the pumps are flexible PP flanges that are connected to the stainless steel raised-face flanges of the pipes, using 4 mm thick PTFE RF gaskets. As a result, bubbles appear during air-tightness tests at a pressure of 1 kilogram; on-site, the surface of the PP flanges has been deformed around the edges where they are fastened with bolts. May I ask how many issues there are with the above usage?
The specification of the flange isn’t really an issue; what’s important are the gaskets. It’s best to use acid- and alkali-resistant rubber gaskets, or graphite-tetrafluoroethylene-coated gaskets, as they are softer and can be compressed to a greater extent.
One thing is to choose the gasket appropriately; another thing is that if one side of the flange is metal and the other side is plastic, the tightening force of the bolts must be appropriate, otherwise... the plastic flange will definitely deform, let alone ensure a proper seal.
HG20592 states that PTFE-lined gaskets, due to structural and manufacturing defects, are prone to cracking during production and installation, which allows materials to flow into the core layer and erode the gasket; in critical applications, it is necessary to replace them regularly or use modified polytetrafluoroethylene gaskets. Is this really needed in actual production?
There is indeed damage; these gaskets are basically used in the production of salts and acids
Use expanded PTFE gaskets; there are plastic flanges, so be careful when locking them.
This post was last edited by register891 on 2016-5-1 08:53. May I ask whether the inlet and outlet flanges of the pump are made of PP pipes with metal fittings, or is the entire pipe along with the fittings made of PP? If both the tube and the loop are made of PP, then for on-site maintenance we proceed by replacing the loop – that is, we replace the PP loop with a metal one, or we add a metal loop behind the PP loop and use them together. This helps to reinforce the structure and prevents deformation. As for the gasket, it’s better to use acid-resistant rubber sheets, as they have a greater degree of flexibility and make it easier to achieve a tight seal
The pump head is made of plastic; therefore, the inlet and outlet flanges of the pump are made of PP, but all the pipes are made of 316
When connecting plastic flanges (PP/PPR) with plastic flanges (PP/PPR), or plastic flanges (PP/PPR) with metal flanges, it is absolutely necessary to avoid using hard gaskets such as PTFE gaskets; I have learned a painful lesson from this. With a PTFE gasket, even if the bolts tighten the PPR flange to the point of deformation, leakage will occur at the sealing surface ； Moreover, the cold fluid (water) does not leak during pressure testing, but it leaks when hot fluid (process medium) is used, as the plastic undergoes thermal deformation. Soft gaskets must be used, such as rubber gaskets, graphite-tetrafluoroethylene coated gaskets, etc.
There are several solutions for your situation: 1. Purchase directly from the manufacturer, requesting that the flange on the pump head be made of 316 stainless steel. 2. The 316 flange can be cut in half neatly, and then bolts can be used to secure it behind the plastic flange, thereby providing reinforcement. 3. If the distance between the pump casing and the flange is very small, consider replacing the plastic flange with a metal one; the specific material to use should be determined based on the fluid medium
Thank you for the reminder. I’m not sure how it would work to use FF flanges together with FF PTFE gaskets; the gaskets have bolt holes