Hello there,

I have an orifice plate installation with flanged tapping on an 8" horizontal pipe for flow measurement. The tapping, which is at 3 o'clock, is connected to a DP cell transmitter via impulse lines below the orifice.

Of late we have been having repeated blockage in the impulse line (service contains Amine, H2S, sour water and a few particulates: iron & salts, etc). This is usually at the HP side.

To ensure accurate measurement I do not envisage we move the tapping point above 3 o'clock. However, following discussion with other engineers we had a number of suggestions which I do not feel too comfortable with, hence the post to a wider audience.

Solution 1: Increase the orifice plate: the rapid drop in DP may have resulted in the buildup of particulates on the HP side. Hence a larger orifice plate would reduce flow resistance.

Solution 2: Re-orient impulse lines; with the 3 o'clock tapping, re-orient the impulse line such the DP cell is above the orifice and gravity would prevent a build up of particulates or blockages. (Wouldn't this affect the accuracy of the measurement?)

Solution 3: Use a new technology. Which one?

Ladies & gents, any contributions?

Many thanks.

Young Engineer

 

You don't say where the impulse lines are plugging, is it right at the orifice connection or in the tubing? If it's the former, I don't think there is much you can do unless you can add a small water purge. If it's the latter, you might try connecting the transmitter directly to the orifice plate with short nipples rather than tubing so the transmitter will now be supported by the 8" pipe.

If the problem cannot be solved, a vortex shedding flowmeter might be a better solution.

Hope this helps,

Roy

 

Dear Young Engineer,

Orifice plates are widely used and are commercially attractive for flow measurement. But if we look at the service which is categorized in slurry service, by using orifice plate for this service is not advisable.

As explained by Roy Matson, using Vortex flow meter or Magnetic flow meter might be a better solution.

Regards,
Huda

 

Having had a similar experience on a refinery in the UK, we moved the tappings above the 3:00 position as you suggested. This worked fine.

Our investigation had shown that samples of fluid in te HP impulse leg was of a much higher density than the LP side. The cause was a larger concentration of solids in the HP.

By rotating the tappings above the 3:00 position the build up of solids was prevented.

A few things to be careful of:-
1. if there is lots of gas component in the above solution may not work and a dp transmitter with remote seals may provide a better solution.

2. Using vortex meters on liquid applications is not a good idea. liquid flow should be between 1 and 5m/sec. Vortex meters prefer velocities of 100m/s. so you end up having to make pipework reductions to match the size of vortex meter.

3. Magflo meters have electrodes that would not like the iron particles. Not a good choice either.

Another alternative might be a clamp on ultrasonic flowmeter, but this will again deepend on how much gas content is entrained in the liquid.

Hope this helps
Malvern

 

Malvern, I think you are confusing liquid with gas for a vortex meter. The velocity for a vortex on liquid is lower. Several of the vendors also market line size meters with built in reducers, which makes it easier to match velocity.

The firm I work for uses vortex as a preference for flow in our petrochemical process.

Roy