How to test a Gas Skid Meter Run Calibration??

I have a what can only be called a very difficult and ignorant client. We are supplying a number of small gas metering skids for a client. The arrangement is very simple. From a distribution header, a 2” 600# rated line comes out of the ground at each of the skid locations. The pressure is between 350 and 340 psig and this is maintained by a larger custody skid further up the line at the metering station.

The design of the skid uses a 2” 600# Daniels meter tube. Stainless steel Impulse from the Simplex Orifice flow element, run only a short distance to a Solar powered Fisher FloBoss flow computer. At the end of the meter tube, there is a pressure control valve and a restricted orifice sized to meet the pressure and flow demands of each of the customers.

The problem, is that the client does not believe that the meter tube and the flow computer combination will gave an accurate measurement of the quantity of gas flowing through the system, on which they will be billing each of the customers. (Basically he thinks that he is going to be losing money) Each item has been guaranteed that they are accurate and will work correctly by each of the manufactures. This apparently is not acceptable. Calculations and simulations have been conducted by our design engineers, but these are also apparently unacceptable as they are not an actual representation of the flow computer or the meter tube.

Only possible solution that I can come up with, is to actually simulate the flow through the skid with a test medium at a known pressure, temperature, and flow rate, and then compare it to the actual readings being produced by the flow computer and prove that the thing works.

My questions are for anyone interested:

1. How is a meter run usually tested in the vendor’s shop, to prove that it is giving accurate results? Do they use water or gas or does it depend on the application.
2. Is there a particular reference standard that I can refer to with regard to the calibration and testing of a meter run?.
3. Conducting a simulation, using water would not be difficult. However could a simulation be performed, using a gas, like nitrogen or air. If so basically how would it need to be set up? Eg. 4 gas bottles connected to a manifold and pressure regulator, connect to a calibrated instrument at the inlet of the skid that will measure the flow (??) of the gas entering the skid, crack open the valves and let the gas escape to atmosphere and flow through the skid, and theoretically the flow computer should have recorded the same flow as what was recorded on the calibrated instrument at the inlet if ti works properly.

 

[TD2K]

This is not my area of expertise but I strongly suspect using bottles will not work as you'll be unable to get the flow rate to be compatible. Yes, you could get similar velocities perhaps at low pressure but by the time to you correct the meter readings for that fluid at test conditions to operating conditions, will the customer buy it? How are you going to be able to say what the flow rate of the test fluid really is, eg. is the customer going to argue 'prove to me this is the real flow rate?'

The real question here is what is the customer going to be willing to accept as a 'correct' measurement of the flow rate in order to accept the test results of your meter run's accuracy?

From the point of view of minimizing possible errors and later questions, the closer your test conditions are to operating conditions (gas composition, pressure and temperature) the less questions there will be (or can be) on the accuracy. If you have to make large changes, I would suspect your client will argue that the results are not indicative of the accuracy on his fluid.

Here's a link that gives a brief description of options. At the end of the day, it will come down to cost. If they are measuring a lot of gas, the potential $$ are high.

http://oildompublishing.com/pgj archive/april/proving.pdf

 

[BillBirch]

Scott,
Your client may not be wrong. In my experience Daniel meter runs are only used for internal measurement, e.g flow from one or a group of wells or measurement of gaslift usage in oil production. More sophisticated metering is employed at the point of custody transfer with consideration given to temperature correction and BTU value based on in-line component analysis. This yields an accuract one or more orders of magnitude higher.

You need to review the gas sales agreement to see what accuracy that your client is buying (or selling) to, then look at the guaranteed accuracy of the system you have bought - don't waste your time setting up tests (he will probably still argue their validity).

It seems that he may have been with-holding this in order to get a "cheap" quote and is now demanding a full-blown custody transfer-quality system at your cost (they are not cheap).
Regards,
Bill

 

[pmover]

Scott,
You've presented good questions and to answer your questions:
1) How is a meter run usually tested in the vendor’s shop, to prove that it is giving accurate results? Do they use water or gas or does it depend on the application.

Answer: Bear in mind that I am only talking about the meter run itself, and not the associated piping upstream and downstream of the meter tube (run). The vendor's shop should be equipped to conduct test within shop. If not, recommend using another shop to conduct testing. Typically, there is a closed or loop in which the meter tube is tested and depends on the gas used for testing. See answer to question #2.

2) Is there a particular reference standard that I can refer to with regard to the calibration and testing of a meter run?.

Answer: ABSOLUTELY! AGA Report No. 3 - Orifice Metering of Natural Gas is a standard used by natural gas transmission companies throughout the US as a standard for measuring natural gas (i.e. the cash register). These meters are calibrated and checked frequently (i.e. at least 1/yr, if not more frequent). A copy of the report can be purchased at Brown Book Store (Houston, TX) or from AGA. By the way, AGA - American Gas Association. I would also recommend the report Comparison Analysis of Orifice Metering of Natural Gas and Other related Hydrocarbon Fluids as in addition to the AGA-3 Report No. 8.

3) Conducting a simulation, using water would not be difficult. However could a simulation be performed, using a gas, like nitrogen or air. If so basically how would it need to be set up? Eg. 4 gas bottles connected to a manifold and pressure regulator, connect to a calibrated instrument at the inlet of the skid that will measure the flow (??) of the gas entering the skid, crack open the valves and let the gas escape to atmosphere and flow through the skid, and theoretically the flow computer should have recorded the same flow as what was recorded on the calibrated instrument at the inlet if ti works properly.

Answer: A simulation will be expensive to perform and I do not believe it is necessary to conduct.
I am not able to picture in my mind the set-up of the meter runs so described, so difficult for me to digress on. However, the AGA-3 report does outline various recommended piping arrangements for meter runs. I understand your position, but the client is whom you need to satisfy. I would recommend getting these reports (< $100.00) and investigating.
I've got to go to a meeting... will check later..
pmover

 

Thankyou all very much for your assistance it has given me alot more to think about.

 

[visitor]

pmover gives some very good answers to your questions. I'd also recommend that you look into having the Southwest Research Institute (San Antonio, TX) test your meter runs if your client will pay for it. They are one of the few labs in the US that can do full-scale testing in their natural gas piping loop (comes off an interstate transmission line). Some other &quot;independent&quot; lab in Canada also has a similar flow proving loop.

Orifice meters can be very accurate measuring devices and they have stood the test of time, but new technology can have some advantages. Turbine and ultrasonic meters often have better rangeability. Most western US transmission lines have moved away from orifice meters because of the rangeability issue and the cost of maintaining the orifice plates. Unfortunately, it sounds like your flow rates are probably too small for ultrasonics and turbines in that size are limited as well.

Good luck with your project.

 

[DaveEdgar]

Scott,

You have received a number of answers to your query, so I will just jot down a few lines. If you need any further information let me know.

1) Various methods are available for proving a gas system from other inline meters (ultrasonic, turbine, orifice plate, bell prover etc). All these methods are possible although cost is usually inhibitive.

2) The usual method of determining a orifice system's total uncertainty (accuracy, repeatability etc) is by the following method (UK standards quoted, US/ISO standards also available).
i) The installation must conform to the appropriate standard in all areas (e.g. upstream pipework configuration, upstream and downstream diameters, orifice plate, flow conditioners etc). The UK standard is ISO 5167 and dictates all of these areas, if a system does not conform to the standard then other than comparative proving, uncertainty is very difficult to obtain.
ii) Standard uncertainty calculations are performed to ISO 5168. These calculations are accepted as the norm within the UK petroleum sector by both government bodies, operators and third party consumers.

Hope this Helps

Dave Edgar
Flow Measurement Consultant

 

[Bubbarock]

1) The companies that build meter runs do not prove them as this is near impossible and the cost can not and could not be passed on to the buyer.

2)Meter runs (tubes fitting etc) are built to exact specs with very little room for mistakes to codes that apply eg. AGA 3 API 14.3

3)The codes that the runs are built to have been made for the simple purpose of one thing .... The measurment of Gas steam liquids.

4)The meter runs can handle a wide variety of situations ie. wet gas, particles in stream, etc. whereas turbine, ultrasonic etc may not be able to take certain situations.

5) As far as the standards that we follow there is mainly only one AGA report #3 in either the 1991 edition or the 2000 edition.