Diagnostics for Liquid Meters Do they only tell us what we already know? Terry Cousins - CEESI

Diagnostics for Liquid Meters Do they only tell us what we already know? Terry Cousins - CEESI

DIAGNOSTICS FOR LIQUID METERS-DO THEY ONLY TELL US WHAT WE ALREADY KNOW? T. Cousins CEESI CMSI

INTRODUCTION Over the years there have been many papers on the use of diagnostics. However, in most cases the analysis is the result of a combination of very knowledgeable engineers and hindsight. Much of the work with Gas meters with higher uncertainty. Most of the diagnostics are of poor resolution when it comes to small changes in performance.

INTRODUCTION-PROFILE EXAMPLE For example a change in profile in a multi-path ultrasonic flow meter What change in the meter uncertainty, if any does it indicate? It often really only says that possibly something should be done, maybe!!!! Most production flowlines, however, change their properties with time and so the problem is continually how much change requires some action.

INTRODUCTION To a degree the issue can be resolved on liquids by meter proving. Gas measurement has never had the luxury of an onsite calibration of meters, the nearest being check metering So a concept such as diagnostic analysis of the meter operation was accepted with open arms. It at least gives some form of potential indication of a change in the operation or performance of the meter. On the liquid side proving should be the real arbiter, but there is pressure both economically and commercially to stop the use of provers.

PROVING AS A DIAGNOSTIC Proving for particular meters, especially Turbine meters is a true diagnostic. It gives a measure of the change in uncertainty and value of the meter calibration with time and installation. For Turbines it gives a good indication of potential failure by looking at the repeatability.

WHY NOT ALWAYS USE PROVING FOR LIQUIDS With such a good diagnostic why would we want any other diagnostic method? Like all good things in life there are some downsides, both real and imaginary. Provers are still costly items: They require for good operation: Maintenance Good quality engineers/technicians. It is not sensible to keep them running continuously. so there will always be a gap of time when something can be going wrong and cause a mis-measurement. This then has to be resolved retrospectively, never a good situation for selling and buying high value product.

WHY NOT ALWAYS USE PROVING FOR LIQUIDS Perhaps the biggest issue is the lack of compatibility with modern meters such as USMs and Coriolis Meters. Lack of good short term repeatability. Only meet the repeatability criteria with large volumes or large numbers of proves. Size with USMs it is feasible to use one large meter and not a number of small meters and proving is not feasible.

USM & CORIOLIS DIAGNOSTICS OVERVIEW Those that relate to the general operation of the meter. Cable breaks, electronic hardware malfunctions, software glitches, transducer failures, changes in data configuration etc. Those that are influenced by the quality of the fluid. Flow profile (USMs). Buildup of materials in the pipe. Air entrainment. Water mixing. Corrosion/Erosion of meter tubes

DIAGNOSTICS OVERVIEW- TRENDING The most effective method of using internal meter diagnostics is by Trending (Control charts) Absolute values of most diagnostics are very coarse. Some values are obviously wrong: For a USM a SNR of 10dB for a liquid is possibly a sign of signal problems. However if between 20 and 80 it is probably ok. It is the changes with time that are mostly the best indicator of a potential measurement issue. There are usually alarm limits but these tell you something is happening, or has happened they do not tell you the effect on the performance, uncertainty, of the meter.

CORIOLIS DIAGNOSTICS DRIVE GAIN Drive Gain is a measure of the amount of power the transmitter has to use to keep the tubes resonating, as a percentage of the total power available to the transmitter. For a given fluid, it takes a certain amount of power to keep the tubes resonating. If the fluid consistency does not vary, the amount of drive gain does not vary. Changes in fluid density and presence of bubbles in the fluid cause changes in the drive gain. Therefore, the drive gain can be used to monitor fluid consistency by comparing to the known good baseline.

CORIOLIS DIAGNOSTICS DRIVE GAIN Drive gain can therefore indicate: Fluid properties (Gas or mixtures) Damage to the tubes (Erosion and corrosion) Hammer and overpressure will change the physical shape of the tubes.

CORIOLIS DIAGNOSTICS DRIVE GAIN The stability of the Drive gain is akin to the SNR of a USM. It is an indicator of stability of conditions and fluid properties. Mainly Liquid Gas Slug starting in the Line Gas mixture the slug Gas Slug passed the meter

CORIOLIS DIAGNOSTICS OTHER FEATURES Other diagnostics include checking for any changes in: Data Entry Zero error drift Temperature malfunction Cable issues Density Mass Flow Viscosity

USM DIAGNOSTICS METER CRITERIA Updates Samples/Info Tdown/Tup (Transit Times) DeltaT (Time difference) Status Rejects Gain Up & Down SNR Impedance

USM DIAGNOSTICS CACULATION CRITERIA Velocity (Each Path, hence profile) VNorm (Each Path) Sound Velocity (Each path and average) Profile Factor Flatness Ratio (Profile) Swirl Asymmetry Standard Deviation (Turbulence) Although called turbulence it is really a combination of flow turbulence, acoustic noise, electrical noise, flow variations, swirl etc. Reynolds Number

USM DIAGNOSTICS TYPICAL STD DIAGNOSTIC

EXAMPLE: USM DIAGNOSTICS SIMILAR EFFECTS Often a single diagnostic is not sufficient to determine a problem or its effect. Usually a combination of diagnostic events is required Taking a multi-path USM as an example the same issue will appear in different forms in different diagnostic properties, so swirl may show up in flatness ration, asymmetry, swirl calculation, STD, gain and SNR. All facets together would be needed to at least give a high probability of a correct diagnosis.

EXAMPLE: USM DIAGNOSTICS SNR SNR Target > 20 Main Function: Signal Quality Fluid Viscosity too high Transition Laminar Swirl General flow noise Gas in Liquid Water in Liquid Particles in Liquid Transducers Poor transducer seating Transducer failing Build up on transducer housing Connection Poor wiring connections Solder failure Wiring faults Paths wired wrongly Cable broken

USM DIAGNOSTICS COMBINED FUNCTIONS Presence of gas - Gain/SNR/VOS changes often on upper transducers, profile + knowledge of the process. Presence of second liquid phase such as water VOS, Flow profile, SNR variations, the transducers affected depend on the density of both fluids, profile + knowledge of the process. Presence of particulate matter VOS, SNR, Gain, profile + knowledge of the process.. Signal attenuation (High viscosities) Gain/SNR/ Viscosity + knowledge of the process. Transducer malfunction Gain/SNR, profile, transducer number. Connection and Cable problems Gain/SNR/Impedance Wax formation Gain/SNR/Profile + knowledge of the process. Installation profile changes due to changing upstream conditions- Profile, Standard deviation (Turbulence), Swirl + knowledge of the process.

DO THEY ONLY TELL US WHAT WE ALREADY KNOW? The title of the paper asks the question whether diagnostics really tell us in advance of an issue, before we have found it ourselves by some other method. The answer is in theory yes, if the effect is large enough, and we understand the diagnostics sufficiently. The real problem is that most users either: Do not trust the diagnostics. Find them too complex to use to work their way through. Do not bother to use them. Often a combination of all. Unlike proving which should be part of procedures for metering, I have yet to find on liquid measurement a site where review of the diagnostics is part of the measurement procedure.

DO THEY ONLY TELL US WHAT WE ALREADY KNOW? Without this discipline in place there is very little chance of the diagnostics being used before or even during an event. In general you only find that they are used by a technician, or engineer who is interested, they have not become part of the mainstream usage. Alarms are often ignored; you only have to look at flow computer audits to see the reams of alarms left untouched, often because the alarm limits are not sensible. The result is that in the event of a major issue they are also ignored until some other feature, such as proving or a mass balance, highlights a measurement deficiency.

TUBE BUNDLE MISSING

GAS IN TRANSDUCER POCKET?

GAS IN TRANSDUCER POCKET? Calibration Gain SNR Path Velocities

SO WHY ARE THEY NOT USED? It is clear from the preceding that diagnostics could have, if not prevented an issue, at least very quickly pointed out a problem. The diagnostics for both USMs and Coriolis meters will not, unlike proving, predict how much a calibration will change for a given incident, such as profile change, gas in liquid mixture or build up on the meter. Also the diagnostics can be misleading or too coarse in the analysis of an incident or change. However, it is clear that they can, at a minimum in many circumstances indicate that There is a potential measurement issue Further in combination with proving it can be a powerful tool to allow tracking in between proves, Also enables tracking back to the start of a measurement issue.

WHY ARE DIAGNOSTICS NOT USED TO PREDICT EVENTS? For liquid metering the uptake on the use of diagnostics is small, particularly compared to gas measurement. Some of this is the number of smaller companies that find it difficult to find the resource to use this type of activity. They, however, do find the effort to carry out proving, although often done though by external entities. The lack of ability to predict events is therefore more about the process than the actual diagnostics themselves.

WHAT IS NEEDED TO INCREASE THE USE OF DIAGNOSTICS It is therefore the process that needs to be changed to make diagnostics more usable. This would include: Manufacturers making the diagnostics more accessible. There is, like proving, a need perhaps to provide services from external experts There is a need to continue the publicity of the beneficial effects of the use of diagnostics, but based on a realistic and practical set of goals. There is a need to show the potential payback from using diagnostics. Finally there needs to be a set of standards, to allow a user to determine the benefits, needs and operation of diagnostics for the different meters. Having that standard will work through into contractual measurement and push the concept into a similar acceptability as proving.

CONCLUSIONS There is little doubt that diagnostics for many meters are of great benefit in determining the operation and potential performance. In liquid measurement they are not used to anywhere near the degree as in gas measurement, probably due to a combination of proving and smaller companies not having the resources to use them. Often they are therefore used after an event causing measurement error, not because they cannot be physically used to either predict or detect the event as it happens, but because of the way in which they are used, or in many cases not used. What is needed is: Better training, More accessible presentation A set of good standards All would contribute greatly to making this tool more usable and enable it to refute the title of this paper.