I'm working on installing a orifice type flow meter on a horizontal steam line. The DP transmitter will remote mount to the plate.

I have Rosemount manual that gives mounting options for this application. The way they suggest installing it is to mount the plate with the adapter plate positioned below the pipe. The tube connections to the transmitter will then goto the adapter and include a 90 elbow in its path. Hopefully Im not confusing.

Question is: is there a reason that condensate has to fill up the transmitter legs? Can the plate instead be mounted vertically and then tie into the transmitter such that its vertically above?

Process engineer I'm working with has concerns that steam condensate might freeze up the lines or at least fill it up with impurities if mounted the first way.

 

Hi there,

For orifice steam flow measurements it is very important to install the transmitter below the steam pipe.

The tapping points (up and dwn stream of the orifice) from the orifice needs to be above the steam pipe at about the 2 and 10 o'clock positions. Two catch pots can be installed directly on these tapping points at exactly the same height and the bottom of the catch pots then connect to the tubing that will go to the transmitter mounted below the steam pipe.

Upstream goes to HP side and downstream goes to LP side.
The idea is to have the catch pots and impulse lines filled with steam condensate all the time to firstly form a thermal buffer between the hot steam and the transmitter and secondly to have a constant pressure head on both legs to the transmitter. We normally let the impulse lines go past the transmitter and then we install drain valves at the end. The transmitter is then connected to the impulse lines with T-pieces. This will also eliminate your process engineer's concerns about impurities reaching the transmitter since all contaminants will accumulate below the transmitter before the drain valves.
You also don't have to use the catch pots (use 90 degree elbow) but then you have to make the impulse line a meter or two longer so that enough cooling can take place before the transmitter.

Once you have done the installation as above, you first need to fill both legs up with ambient temp water and then put the transmitter on line and leave it for a couple of hours (preferably with hot steam but no flow in the steam line) so that the lines can reach a stable temperature and fill up with condensate to the overflow level in the catch pots or in the 90 degree elbow.

After this period you need to isolate the impulse lines again and only then do you do the zero calibration (zero trim, then input the LRV) on the transmitter. Once you are sure about a good and stable zero you can input the URV and put it online.

 

If your worried about the condensate lines freezing you can also fill the leg lines with liquid glycol.

 

To add to Sam's excellent description. If you have the transmitter above the line live steam will go up and condense in the transmitter then run back down to be replaced by more steam. Very quickly the transmitter would reach line temperature and virtually cook.
Sometimes you see a 3,000# tee used in place of the condensate pot, these should not be insulated so they radiate the heat thus condensing the steam.

If your tee or condensate pots are insulated you can get a difference in level which throws off the transmitter zero.
The impulse lines down to the drain valves are usually line spec pipe, be aware that these will quickly expand with heat at startup from empty or blowdown so allow enough flexibility
in the lines from tee to transmitter otherwise the fittings might pull apart.

The drain valves are often referred to as blowdown valves, you need to pipe them to a safe location and be aware each time they are used it will throw off the transmitter for a while until the legs fill up again and stabilize.

Roy

 

Hi again,

Yes Roy is right, you should use T-pieces if you do not want to use the catch pots otherwise you will have no place to fill the lines initially. Thanks Roy, missed that one.

 

Thanks for the detailed responses. I checked out a couple of remote steam installations around the plant, and they fit perfectly with your description. We'll be installing these units tomorrow.

Just FYI, there is a publication that came out early this year, from Emerson-Rosemount on "top-mounting" the orifice plate and transmitter in horizontal steam lines...we don't have one installed in this manner...

 

Hi again,

I would like to see this, do you have a link where to find it?

 

> I would like to see this, do you have a link where to find it? <

http://www2.emersonprocess.com/siteadmincenter/PM Rosemount Documents/00870-0200-4809.pdf

 

Thanks, very interesting read although only applicable to certain conditions.

 

Hi,

this is a very good question and explanation. can you please give the link. The below link is not working.

> http://www2.emersonprocess.com/siteadmincenter/PM Rosemount Doc uments/00870-0200-4809.pdf <

 

> this is a very good question and explanation. can you please give the link. The below link is not working. <

> http://www2.emersonprocess.com/siteadmincenter/PM Rosemount Documents/00870-0200-4809.pdf <

You've probably noticed already, but the link had a space ( or just a line break )

I think it is a bug in this site. as when I preview what worked for me (by removing the space) the link gets a space inserted again.
(maybe the moderators will notice.)

http://www2.emersonprocess.com/siteadmincenter/PM Rosemount Documents/00870-0200-4809.pdf

(remove the space between "Doc" and "uments" above)

At any rate, here's a shorter link that
goes to the same place
http://bit.ly/9NENVA