During instrument construction, when does it need to be purged and when does it need to be isolated?
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In many processes, there are often some special media, which makes direct measurement difficult, so purging or isolation is used. I would like to ask you: 1. Under what circumstances is purging necessary and under what circumstances isolation is necessary? 2. Is it enough to use only one of the two methods? Under what circumstances will both be used at the same time? 3. When purging is used, will the pressure of the purging medium affect the measured pressure? Why? (This is what I think. It will definitely have an impact. If the medium is not in direct contact with the sensor, the pressure of the measuring pipeline on the side close to the instrument must be higher than the pressure of the measuring pipeline on the side close to the sampling point. But the key is how much this large amount is, I cannot deduce it.) 4. To give a specific example, what method should be used to measure the pressure of a gas containing solid particles? If purging is used, won't the solid particles pass through at all? If an ordinary pressure gauge is used, once it enters the spring tube, can it still come out? If a diaphragm pressure gauge is used, will solid particles stick to the diaphragm? This post was last edited by war3dd on 2009-4-15 22:27 ]
I can only talk about the on-site situations I have been exposed to: 1. The pressure measurement of flue gas containing solid particles requires a backflush device to prevent clogging of the pressure guiding pipe.； Medium with high viscosity and easy solidification requires the addition of isolation fluid. 2. I have never seen two devices used at the same time. It should be due to different principles. 3. If the pressure of the purge device is too high, it will definitely affect the measured pressure. We measure the pressure to be around several thousand pascals, and the backflush pressure is generally set at 0.05Mpa. The pressure after passing through the flow-limiting orifice plate should be negligible to the measured pressure. The holes in the restrictor orifice plate should be calculated. The pressure of the purge gas should be very low. It feels like it plays a role similar to cleaning the pipe, preventing particles from entering the measurement pipe. 4. The normal use time of a pressure measuring device using a backflush device is still very long, several months or even longer. This requires the filter pressure reducer to be drained during maintenance. It has happened that after the filter pressure reducer was blocked, there was no backflush air, and the pressure guide pipe became blocked after a while. If it is blocked, it will definitely be blocked in the spring tube and diaphragm. It will be a dead end. The more blocked it becomes, the more blocked it will be. At this time, the only option is to change the meter and perform deep cleaning.
Take a look at the following two and you will understand http://bbs.hcbbs.com/viewthread.php?tid=25133 http://bbs.hcbbs.com/viewthread.php?tid=214104
HG/T 20515-2000 Design specifications for instrument isolation and purging SH 3021-2001 Design specifications for isolation and purging of petrochemical instruments and pipelines
Reply to 2nd floor: Thank you for your reply. I will discuss your reply further.: 1. Agree ； 2. I have seen two methods used at the same time, both purging and isolation. However, instead of using isolation fluid, a diaphragm pressure transmitter is used. This is also a method of isolation. ； 3. I think your statement is wrong. The pressure of the purge gas must be higher than the pressure of the measured medium. Otherwise, how can the purge gas or flushing liquid be blown in? It will only be blown back by the medium. ； 4. Judging from what you said, there is still no cure for this kind of dirty media. It is easy to lose the table. Reply to the 3rd floor.: Thanks. Maybe you misunderstood me. I am referring to the treatment method of the instrument when measuring special media, not the purging of the pipeline. Reply to 4th floor: Thanks. I have read the two specifications several times, but they have no substantial guiding significance or examples. They have no guidance for our instrument construction, and there is no explanation or explanation for my problem.
The third floor is for craft majors, but it’s good to take a look :)
I would like to ask if anyone knows the following standards on the basis of the regulations on blowing pressure and flow rate in the blowing measurement method, and what information contains detailed explanations. It would be best if they could provide them. I am still confused about the determination of blowing pressure and flow rate, please give me some advice! HG/T 20515-2000 Design specifications for instrument isolation and purging SH 3021-2001 Design specifications for isolation and purging of petrochemical instruments and pipelines