PD Meter vs Coriolis Meter

We are trying to accurately measure oil coming into our refinery. Currently, we use a coriolis meter and a capacitance probe to measure volume and 5S&W coming into our receiving tanks. However, we are having issues with errors that are introduced due to variance in oil density and %S&W.

We are looking at a system that finds the density of the oil and the density of the brine. The system will automagically perform a NOC calc based on the live density vs these known densitys, and calculate %S&W if the capacitance probe is outside its working range.

We need to decide the type of meter we want to used. Should we use a gear driven PD meter or a coriolis meter. My understanding is a PD meter is more repeatable, but the coriolis meters are more accurate. Any thoughts?

ghettofreeryder said:

My understanding is a PD meter is more repeatable, but the coriolis meters are more accurate. Any thoughts?

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That is intersting. I am by no means an expert, but I believe that if the instrument is not repeatable, it cannot be accurate. At least my understanding is that the repeatability should be much higher than the accuracy. Look at any position or distance measurement system (take a micrometer for instance) if the accuracy is +/- .001mm, then the repeatability will be somewhere around+/- .0001mm. Maybe I misunderstood the statement. It is a good question, hopefully you get some better responses.

A coreolis meter will be more accurate for measuring mass flow.
The PD meter may be more accurate for volume flow.
The coreolis can also be set for volume flow (Mass / density).
In my experience PD meters are more of a maintenance problem.
Check out the new Foxboro coreolis meter, the rep was by our office the other day and claims it will measure two phase flow e.g. Air in Water.
Roy

We sell our fluid by volume at standard conditions, so both accurate density and volume measurement is needed. We find that as our live density varies from the calibration density of the S&W probe, the S&W probe needs to be corrected, something like 0.021 %/kg/m3. However, the previous number is just an average. Each oil source has a different body, different density. Therefore, the correction factor changes per source. We also need to know the water and oil densities of the load as its coming in. This way, if the S&W probe is saturated, we can switch to an NOC calc as stated before. Ive been thinking about assuming the water density to be equal to the density of the fluid at the 10th second of the load, and the oil to be the density at the last 10th second of the load. I believe this is the way to determine the live densities, as sampling the load and testing it in our lab is not feasible. In a nutshell, we dont want to pay for water at oil price.

Good repeatability is no good if it's not accurate. Think of repeatability as firing a series of shots at a target. If there is a tight cluster at the upper right edge of the target, the repeatability is good but not the accuracy. If there is a cluster all around the center, the repeatability may not be so good but are all of the shots within a circle defined as accurate? You need first to define accuracy and then judge if the entire cluster falls within the defined circle.
My experience with custody transfer is that the coriolis meter is the superior instrument.

The Coriolis meters from Endress+Haser measure mass flow and density in real time. Therefore, volumetric flow is readily available at the propagated error of both mass and density measurements (which are very good for Coriolis technology). I don't play in the oil and gas market and really don't know what S&W is (solids and water?) but for a volumetric flow, with the right Coriolis meter, aren't you all set? Use the realtime density to correct your S&W meter every X seconds or whatever - anything between as fast as the meter and PLC controller can update (ms?) to the once per setup as in your example (once at the 10 sec mark).

This is probably obvious stuff, so sorry for my ignorance if I am not understanding this particular problem.

Thanks for the help. My boss keeps telling me we want to go with PD meters, and slipstream a small coreolis meter to retrieve density. I told him we dont need to make any changes to our existing setup, just change the way we calculate volume. We already have a micromotion and the S&W(solids and water) probe. We cant infinitely correct the S&W probe meter as its accurate range is only 0-5%. If we recalibrate for a range higher then that, the accuracy is really bad. Therefore once the S&W reaches say 4.5%, we switch to calculating S&W based on the live density vs the known densities. Im going to tell him to keep our existing meters and either we will write the routine to switch calculations or buy a flow computer that already does it.

Micro Motion input

In addition to accuracy and repeatability, I expect that long term stability of the measurement will be important to you. Typically, a Coriolis meter will not drift over time, even in applications with heavy sand. If you want to take your question to our online forum as well, I invite you to visit our new www.micromotion.com/community where you can gain access to more Coriolis users.

Coriolis mass flow meter is the most accurate type I ever deal with. If you have E+H one, you may also get density and temp. additionally to flow rate. Volumetric flow also can be configured as well as mass flow. Did you look into installation of the meter? As far as I know, coriolis likes some back pressure. Maybe simple line/orientation rearrangement would do the job?

val_99 said:

Coriolis mass flow meter is the most accurate type I ever deal with. If you have E+H one, you may also get density and temp. additionally to flow rate. Volumetric flow also can be configured as well as mass flow. Did you look into installation of the meter? As far as I know, coriolis likes some back pressure. Maybe simple line/orientation rearrangement would do the job?

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With the E&H Coriolis a few basic rules are required in order for them to work propoerly:

- minimum flow rate
- mounted at lowest point in a line
- direction of flow should be UP if mounted vertically
- air or foam should be eliminated before entering meter
- minimum distance from pump or elbow in line