Re: DP Measurements as it related to column flooding

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Posted by Don Snowden on March 28, 19102 at 09:12:38:

In Reply to: DP Measurements as it related to column flooding posted by Anonymous on March 27, 19102 at 10:05:15:

Before reading the comments below, keep in mind, that it is possible to control a tower dp withing 0.01 psi. I've worked on two applications where if we changed the control point by 0.02 the column would go into a flooded condition.
1) A "properly" installed dp cell is always the preferred instrument. The reason is that using the difference in two meters create additional errors/problems. The problem is that, from my experience, 9 out of 10 times the dp meter is not installed properly. There are lots of guidelines how to install them properly (e.g. at least 3/4 inch tube, free draining, or filled, steam traced, etc..)
2) Two "high resolution" pressure meters will usually work ok if installed properly. For example, I believe Honeywell has 3 different "series" of meters (e.g. 100, 600, 900). I think the ST100 series is the best. This is based on %of span.
- Thats the next thing to be concerned with - %of span. Don't use 0-500psi gauges for 0-20psi application. Instrumentation guys tell me it doesn't make a difference (i.e. can recalibrate them), but for some reason I have a hard time believe a meter able to read up to 500 psi can be as sensitive as one just reading to 20 psi??
Also make sure the pi meters are as close to the tower as possible. The pigtail should be no more than couple of feet from flange. Make sure it is elevate up so it will freely drain back into the column.
3)I would always recommend separate dps for the stripping and rectification section. The tower will usually flood in only one of the sections, and you don't want the "total" column dp to mask the resolution that you would like. One could take this to an extreme and say I only one the dp across the trays I know I'm flooding (not suggesting this but just pointing it out).
Based on the above points, I usually ask for smart pressure transmitters, with tight spans, and properly installed. The success rate is higher than installing true PDIs.

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Comments:
: Before reading the comments below, keep in mind, that it is possible to control a tower dp withing 0.01 psi. I've worked on two applications where if we changed the control point by 0.02 the column would go into a flooded condition.
: 1) A "properly" installed dp cell is always the preferred instrument. The reason is that using the difference in two meters create additional errors/problems. The problem is that, from my experience, 9 out of 10 times the dp meter is not installed properly. There are lots of guidelines how to install them properly (e.g. at least 3/4 inch tube, free draining, or filled, steam traced, etc..)
: 2) Two "high resolution" pressure meters will usually work ok if installed properly. For example, I believe Honeywell has 3 different "series" of meters (e.g. 100, 600, 900). I think the ST100 series is the best. This is based on %of span.
: - Thats the next thing to be concerned with - %of span. Don't use 0-500psi gauges for 0-20psi application. Instrumentation guys tell me it doesn't make a difference (i.e. can recalibrate them), but for some reason I have a hard time believe a meter able to read up to 500 psi can be as sensitive as one just reading to 20 psi??
: Also make sure the pi meters are as close to the tower as possible. The pigtail should be no more than couple of feet from flange. Make sure it is elevate up so it will freely drain back into the column.
: 3)I would always recommend separate dps for the stripping and rectification section. The tower will usually flood in only one of the sections, and you don't want the "total" column dp to mask the resolution that you would like. One could take this to an extreme and say I only one the dp across the trays I know I'm flooding (not suggesting this but just pointing it out).
: Based on the above points, I usually ask for smart pressure transmitters, with tight spans, and properly installed. The success rate is higher than installing true PDIs.

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