DP of two distant points

Hi folks,

From time to time, I face with problem when dealing with a DP transmitter set up to measure the differential pressure of two distant points. Recently, we have installed a DP transmitter to measure the differential pressure of two points on a long tower. The distance between tapping points and the transmitter’s manifold is nearly 40 meters and we have installed a long run of tubing to connect the manifold to the tapping points. The measurement error is substantial.

It is also not possible to use capillary type DP transmitter due to the capillary length limitation. We cannot even use two pressure transmitters and calculate their difference because the operating pressure of the tower is high and the desired DP is small so if we use two pressure transmitters and calculate the DP, the resulting measurement is still erroneous.

Any hint?

Best Regards,
Abdolazim Hasseli
 
Hi AddoLazim,

Can you confirm, please:
- the service of your DP transmitter, gas or liquid.
- the size of the tubing used from the tapping points to the manifold?
- the hook-up of your transmitter according to service
- the range of your DP transmitter
- the average pressure of the column / tower

I had a similar problem a few years ago on DP transmitter in gas service: we had to change the localization of our transmitter above the upper tapping point (on the last platform) and to respect slope on the tubing on a stabilization column to avoid condensate inside the tubbing.

regards
Bruno
 
Hi Bruno,

Thank you for the hint.

The service is gas in my case.

We have used 0.5" tube size.

The DP range is approximately 20mBar.

The average pressure of the column is about 10Barg

Hasseli
 
Hi Hasseli

DP of 20mBar is very low so installation is your first enemy!

- long tubing: the pressure drop and the time response will increase.

Just for example:
flow 0.1m3/min tubing length: 40m, diameter: 1/2", pressure : 10 barG => pressure drop = 20mBar

- On all gas, "it shall be assumed that liquids can be expected, the process connections for such services shall slope downwards to the process line to allow liquids to drain back into the process."

- is there a big difference between the two temperature gas inside the column (taping point) and the ambiant temperature of impulse lines near the DP transmitter ?

regards
Bruno
 
Hi,

I would suggest you get away from the tubing application and definitely not to install 40 meters of tubing for a gas application. Gas is compressible and this factor alone will make it virtually impossible to do proper measurements in this way even with a bigger calibrated span.

What is the actual distance between the two tap off points?
Is it more that 20 meters? (You just mentioned that you have installed your current transmitter 40 meters away from the tap off points.)

DP capillary transmitters will still give accurate readings with two 10 meter capillaries connected to it. If this is a flow application the distance between your tap off points will not be much and short capillaries can be used. What is the problem, is the tap off points far away from each other or why exactly can you not install a DP capillary transmitter close by?

I do agree with you that it will be unwise to use two separate pressure transmitters and try and calculate the DP since the operating pressure of the vessel is so high. This will mean that you will have to use a very big transmitter in order to measure the operating pressure and therefore the natural error of the transmitters will be to much for a small DP application like this. DP with as short as possible capillaries it the way to go on this application.

Personally I would use a small capillary DP transmitter just big enough to measure the DP.

Something interesting to keep in mind with capillary DP transmitters is that pressure equalization in a liquid takes place at the speed of sound so any change in DP at the pad-cells, will register instantaneously on the transmitter. This immediately eliminates two of your problems, compressibility of a gas and tubing lengths that will causes time delays in response time.
 
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