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Jigalow (Chemical) (OP)
2 May 07 19:05
I have a application which requires the upstream tap of a delta pressure transmitter to be purged with water to help eliminate impulse line plugging.  The request is to purge the impulse line with about 1 gpm of process water.  The question is "How would this best be accomplished and how will it affect the transmitter calibration?"  The taps for the transmitter are across a 16" butterfly valve. I currently have Nitrogen purges on both taps but only the high side tap plugs at this time. Soliciting all inputs as to how to make this work.
danw2 (Industrial)
2 May 07 23:31
http://www.controleng.com/index.asp?layout=articleCtl&articleid=CA6427338&;rssid=129

A variety of reasons make purges the least desirable protection. Adding hardware, piping, and proximity of the purge media creates a miniature process. Also, unless purge flow rates are tightly maintained, accuracy and repeatability suffer. Purges have been successful in measuring liquid level changes in open vessels, but opportunities for this application are limited.

------------------------------
Truth be told, I was unaware a purge in an impulse line would work at all.  How do you account for the contribution of the flow's pressure in the impulse line?  

Dan
StevedeBritt (Petroleum)
4 May 07 11:40
Hi,

You can purge this using a rotameter (variable area flow meter) which are quite inexpensive. The instrument should be zero'd on site.

Why is only HP side plugging?

Steve
dcasto (Chemical)
4 May 07 12:45
Switch to liquid seal lines.
Helpful Member!  JimCasey (Mechanical)
7 May 07 9:51
Further to the comment about the rotameter: there are "Purge rotameters" with constant flow controllers that modulate to hod a pretty constant flowrate even if the purge pressure varies.  Example: Brooks model 1350-8900.

If the purge line causes 1" Wc head developed at the flowrate for which it is set, and the instrument range of the monitoring device is 200" WC., then the error introduced by the purge flow can be ignored.    Earlier comment about zeroing the device in situ is alo quite valid.  

I won't really take exception to DanW2's remark but to say that purges have been used since your granpaw was a little boy and they ameliorate a problem.  Diaphragm seals on a filled purge line sometimes work better and sometimes they initiate different problems.  I remember a power plant that indicated a flooding raw water intake  on a frequent basis.  A little documentation identified  that it was about the same time every day.  Somebody took a look, and the sunlight was hitting one (but not the other) leg of a sealed system.  
JLSeagull (Electrical)
7 May 07 13:09
I agree with JimCasey that for low ranges - below about 20-inches water column differential, the capilaries can be significantly affected by temperature.
danw2 (Industrial)
7 May 07 18:45
Thank you Mr. Casey.  You deserve a star for that explanation!

I assume the direction of purge flow is from the transmitter port back into the flow line.  Correct?

Is this technique normally done on both impuluse lines or only employed on troublesome lines as needed?

Isn't a bubbler's flow controller the same concept - constant flow under varying pressure?

Thanks.  Dan

JimCasey (Mechanical)
11 May 07 15:02

I mis-typed when I called it a filled purge line.  It is a filled sensing line when there is a diaphragm seal.  And JLS was right when he referred to the filled sensing line as a capillary.  



I assume the direction of purge flow is from the transmitter port back into the flow line.  Correct?
Yep.  Squirt clean stuff backwards thru the line to keep icky stuff from plugging the line.  

Is this technique normally done on both impulse lines or only employed on troublesome lines as needed?
Both, if there are 2. It would be assumed that the resistance of both lines is equal and the flow effects would then cancel.  

Isn't a bubbler's flow controller the same concept - constant flow under varying pressure?
Usually.  The difference is in indicating the flowrate.  A bubbler is more qualitative than quantitative.  Bubblers are handy if compressed air is the purge medium.  A rotameter with a constanf flow relay works on air, or on varous liquid purge media....clean water, etc.

InCwithoutE (Petroleum)
21 Jul 07 11:21
What exactly is the process?  How are the taps orientated on the line?  Down / 45 Down / side?
gsjhand (Electrical)
23 Jul 07 20:32
Thanks Mr. Jim Casey

Around one year ago I started at my present job and I saw purge line used for pressure measurment in refiner casing. At that time I also raised lot of questions about its accuracy and I hooked up a gauge right next to casing. I was surprised by the accuracy of measurment. As mentioned by you, we are also using Brooks rotameter in purge line and we try to keep flow around 0.8 L/min. But I still don't understand that why this rotameter also have pressure adjustment port.

Gurinder Singh
JimCasey (Mechanical)
27 Jul 07 13:59
>>But I still don't understand that why this rotameter also have pressure adjustment port.<<

A Brooks meter does NOT have a pressure adjustment port.  
The adjustor knob controls the flow.  The relay ( mushroom-shaped part at the end) monitors the differential pressure across the valve.  If the flowrate decreases for any reason, the differential pressure across the valve decreases. Then the relay senses that and opens a bit to restore the flowrate to the setpoint value.  

As long as the purge flow is constant, the correction for the pressure loss in the purge line is constant, and accuracy is maintained.  


Standingback (Industrial)
28 Feb 08 11:50
Following on from Jims comment, purges lines have been used for a long time. But I note that only continious purge and capilary systems have been mentioned.

Continiously purged impulse lines are useful especially in gas measuring, or liquid level allplications(bubbler).

Remote seals on dp transmitters are another useful solution. but I agree that ambient effects can be an issue especially where they are fitted by third parties to standard transmitters. A good solution for this is to use a transmitter  designed with factory fitted capilaries. These can be designed to have greatly reduced volume of fill volume. The less liquid there is in the fill, the less it is effected by temperature.

So the missing link in this thread for me were intermittant purge systems. These are useful to solve the problem posed. By connecting a water solenoid valve that is controlled by a   timer, it is possible to flush the impulse lines periodically.

The effect on the transmitter calibration can be seen before and after the purge. A word of caution the DP transmitter should be capable of taking the full purge pressure on one side of the DP measurement with damage. This is not too big an ask from a quality manufacturer.

The great benefit in this solution is that you can minimise the water being injected into the process and adjust the frequency and duration of the purge to suit the application.

I have used this many times on flue gas flow metering.

ione thing that is not mentioned inthis thread is the differen

I trust this helps,
Mlv
http://www.coulton.com

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