Can the pressure rating of the process connection flange for the instrument be different from the pressure rating of the instrument itself?

Instrumentation usually indicates the pressure rating of the process connection flange; does it also indicate the pressure rating of the instrument body? The pressure rating of the body will definitely match the grade of the process connection flange.

The pressure rating of the instrument itself can differ from that of the flange; in principle, all parts of the instrument that come into contact with the medium must have a pressure rating that is at least as high as the design pressure of the medium. This is first and foremost a safety issue, not a sealing issue. Being below a certain level doesn’t necessarily mean there will be problems or that the sealing is poor, and being above that level doesn’t necessarily mean there will be no problems or leaks either; however, you bear no responsibility for the selection. In practice, both cases of being lower and higher occur. Since there is a margin in the instrument specifications, it usually doesn’t cause problems even if it is below the specified value ； Even if it exceeds the design standard, accidents cannot be guaranteed to be avoided due to issues such as quality, installation, or accidents.

For example, if the pressure in my tank is very low, at only 0.5 MPa, but I selected a glass level gauge of type CL300 according to the selection specifications, then can I use a connection specification of 150 instead? So if my tank is rated at 2.0 MPa, my flange must be of a higher standard than PN20; in other words, choosing a 150 flange in this case would not be appropriate, and I should choose a 300 flange, right?

Generally, yes. Based on the designed pressure of the tank, with the actual operating pressure as a reference, if the designed pressure is higher than the actual usage pressure, it is also possible to choose a lower pressure rating.

The poster has no understanding of design at all; go and ask your company’s piping team.

I’m really a beginner and don’t quite understand this. But we opened the ports for our piping based on the data returned by the instruments, so I got stuck thinking about it.

This post was last edited by hfrs on 2017-2-21 at 14:13. 1. Instruments generally do not have their own pressure rating; the flange pressure rating is the same as the instrument’s pressure rating； 2. The pressure rating of the instruments should be no lower than that of the pipes and equipment; before submitting requests for capital allocation related to piping, the process engineering team and other relevant teams should first provide the process specifications for the pipes or equipment, including their pressure ratings ； 3. As for the example you gave, use the same type of flange that is used on the jars in your home. Even if you say these, you might still get it wrong.

It’s like this: when we were studying the approval document for *3005, the specifications stated that for control valves, the pressure rating of the valve body should not be lower than PN50. At that time, a technician said that the connection specification could be set lower; it could be specified as PN40. That’s why I thought that for instruments, there are two pressure ratings: the body rating and the flange rating. I used to think these two were the same.

Do you know what design is? Do you know what the principles of design are? If everyone designs in this way, then I think anyone can do design work – just go straight for PN50, and if that doesn’t work, use PN110. In that case, what’s left of economic efficiency? Go back and carefully review the design specifications; no specification states such a thing.

The instrument flanges all comply with national standards; the matching flange should be of the same type as that of the main body, and the grade difference must not be too large, otherwise installation will not be possible