Incorrect use of FT for a PDT 1

[mbellow]

thread196-122680
Hello, I would like to point out that unless a differential pressure transmitter has a square-root output, it should not be designated as ‘FT’, it should be 'PDT'. I know, 'FT' is still widely used with a linear output, but that doesn’t make it correct…

What do you think?

Mike

 

[JLSeagull]

I worked with one lead engineer Jim B. who felt strongly about this. Not me.

If you use an orifice or other head producing type element to measure flow, the primary element is a flow element. When you transmit the differential pressure it is transmitting the flow signa. You are measuring flow. In olden times, we used a 0-10 scaled square root scale or chart with a scale multiplier for flow units. Those were flow indicators or recorders. Those could have been pneumatic or even mechanical with no transmitter. The square root could be taken using a pneumatic signal converter. This was normally done for more complex control such as three-element boiler feedwater control where a linear flow was desirable for adding, multiplying etc. With the electronic dp transmitters the external computing device could be eliminated. However many do not extract the square root in the transmitter. Regardless it is a flow transmitter.

Tell Jim B. that I said hello.

 

[mbellow]

When I worked in an older plant they had differential pressure transmitters that had square root mA outputs representing flow. Similar to what you mentioned, they used a simple meter display scaled for 0-100% for square root and read the percentage of flow DIRECTLY, no conversion, no calculator needed. You could read the percent flow directly proportional to the square root. You can't do that with the linear DP reading; it is just a DP until you run calculations and convert it.

I know it is from a primary differential producing FLow element/fitting, but the descriptor needs to represent the actual signal, not the element/fitting use.

I appreciate your thoughts on this. I assume Jim B. and I were in agreement. I'll say 'Hi' if I ever meet him.

 

[JLSeagull]

Yes, I failed to mention that Jim and you share the same opinion. For tagging I prefer to address the loop instead of the device function.

 

[mbellow]

Ok, let's address the loop function. You have an FE-301; everything on the flow process 'loop' that is associated with FE-301 will be referenced to '-301', the number of the 'loop'.
FT-301, PT-301, TT-301, and yes, even PDT-301 (I keep wanting to write 'DP'!).

 

[xnuke]

mbellow,

ISA-5.1-1984(1992) is very specific about this in Section 4.2.2. It does not matter what the construction is, the identification is made according to the function. Examples given in the standard include "a differential-pressure recorder used for flow measurement is identified by FR; a pressure indicator and pressure-activated switch connected to the output of a pneumatic level transmitter are identified by LI and LS, respectively."

I have to agree with JLSeagull that it is an FT regardless of whether or not the output needs conditioning.

xnuke
"Live and act within the limit of your knowledge and keep expanding it to the limit of your life." Ayn Rand, Atlas Shrugged.
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[mbellow]

Ok, I agree; thread closed. Thank you!

 

[controlnovice]

Thread closed, yes...but just to add another example:

On tank level readings when using a Pressure Transmitter to read tank head pressure to determine level, the instrument bubble is LT-xxx, not PT-xxx.

The instrument bubble should represent the function you are trying to read.

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