Custody Transfer Systems for Natural Gas and Challenges in Its Measurement and Handling

Home , Custody transfer

flotek.g 2017- “Innovative Solutions in Flow Measurement and Control - Oil, Water and Gas ” August 28-30, 2017, FCRI, Palakkad, Kerala, India CUSTODY TRANSFER SYSTEMS FOR NATURAL GAS AND CHALLENGES IN ITS MEASUREMENT AND HANDLING

AUTHOR

Rajesh Dure GAIL India Limited, GAIL Bhavan, A.V Appa Rao Road, Rajahmundry-533103 Mobile no: +91-9997014770 E-mail ID: [email protected]

ABSTRACT

In Natural gas application, predominantly eliminate acoustic interference through used technologies for custody transfer flow filtering of USM frequencies from external measurement systems are: - (i) Differential noise are some of the advancements made pressure (DP) (ii) Turbine (iii) RPD (iv) to improve USM metering performance. So, Ultrasonic. Perspective of DP based Orifice objective of this paper is to primarily outline flow meters, Turbine Flow meters, RPD flow the challenges in various natural gas meters & USMs has been presented in this metering systems along with mitigation paper with emphasis on their operating criteria for the same & to finally drive out the principle, their limitations, challenges and conclusion that challenges in metering technological advancements that evolved to systems are inevitable which can be mitigate these challenges. One of the major mitigated through employing best industrial challenges in measurement & handling practices & advanced technological natural gas based custody transfer solutions. applications is under such harsh environments such as wet, rich and / or dirty KEYWORDS gas applications. Latest evolution Orifice flow meter, Turbine flow meter, RPD technology-wise in natural gas metering flow meter, Ultrasonic flow meter, with the best possible accuracy so far that limitations, challenges can cater to the redressing of various persisting challenges in natural gas 1.0 INTRODUCTION metering has been the Ultrasonic Flow Meter technology. To counter such hostile Custody Transfer in the oil and gas industry conditions such as wet, rich and / or dirty refers to the transactions involving gas applications, USM transducer probes transporting physical substance from one which are designed for wet gas are in operator to another.Custody transfer in fluid vogue. Also relocating the transducers to measurement is defined as a metering point the upper side of the meter (in case of (location) where the fluid is being measured USM) to avoid their contamination due to for sale from one party to another. In their positioning at bottom, techniques to Natural gas application, predominantly used

1 technologies for custody transfer flow addition, DP meters are sensitive to flow measurement systems are: - (i) Differential profile and require either a fairly long pressure (DP) (ii) Turbine (iii) Rotary straight run or an upstream flow conditioner. They also generate a medium-to-large Positive Displacement (iv) Ultrasonic. Unlike pressure loss, and they are not as accurate Simple volumetric flow measurement as other technologies, such as gas turbine technology is not the only consideration or ultrasonic gas meters. because the ultimate measurement—and what the customer pays for—is energy delivered. For this reason, accurate and 2.2 Challenges in DP Based Flow repeatable measurement of natural gas flow Metering requires simultaneous measurement of Typical abnormal conditions that cause several other variables, including pressure, inaccuracy of orifice measurement but are temperature, density, and gas composition. not limited to: • Rough upstream pipe wall: Gas flow 2.0 DP BASED FLOW METERING is slower along pipe walls due to FOR NATURAL GAS friction causing the entry flow profile to be more arrow shaped DP meters measure volumetric flow through • Improperly designed or installed flow a calibrated orifice (generally a flow plate), conditioner: Entry flow profile is not are inexpensive, and simple in concept. developed properly and may be Orifice flow meter works on the principle altered by abnormality that when there is a flow restriction in a • Swirling gas flow pattern challenges pipe, a differential pressure results which • Partially closed valves or other can be related to the volume flow rate similar type condition upstream of through the restriction. They are accepted meter tube broadly and are not limited in line size. • Liquid in meter tube While not the most accurate instruments • Valve ‘noise’ or pulsation available, they are acceptable for the • purposes for which they are used, and the Orifice with damaged bore • calculations for correcting to standard Obstruction in orifice bore • conditions are widely known. Meter tube shorter than AGA 3 specifications 2.1 Limitations in DP Based Flow • Orifice not centered in tube per AGA Metering 3 specifications • Protrusions in upstream piping DP meters measure only differential head. To measure either mass or volumetric flow, In case of conventional orifice based they must be corrected for density (mass) or metering systems - paddle type orifice temperature, pressure, and gas composition plates designed to be used with orifice to obtain a standard reading. They have low flanges , the often encountered challenge is turndown unless the orifice plate is orifice plate alignment and eccentricity issue changed. In addition, they are subject to with regard to orifice plate i.e. ensuring the fouling, which can partially obstruct the concentricity of the plate with respect to orifice plate and cause the meter to read meter tube & possibility of plate deflection. high. The only way to counter fouling in a With the evolution of single chamber & dual DP meter is to send someone out chamber orifice fittings, this challenge has periodically to inspect the orifice plate, become addressable. Orifice plate which is expensive in terms of labor and can alignment is one of the most significant mean an interruption in production. In factors in reducing measurement

2 uncertainty. Orifice plate holder which designed such that it reduces measurement carries the orifice plate is concentrically uncertainty. (Refer fig.1)

Figure 1: Orifice plate holder

• Single Chamber Orifice fittings: to make plate changing quick and Allows quick and economical easy at installations where line inspection and replacement of orifice movement from flange spreading is plates with minimum downtime. The undesirable. (Refer fig.2) single-chamber fitting is engineered

Figure2: Single chamber orifice fitting

• Dual Chamber Orifice fittings: Allows simple method of changing orifice easy orifice plate replacement with plates under pressure without flow no down time. It provides fast and interruption. (Refer fig. 3)

Figure 3: Dual Chamber Orifice Fitting

3.0 TURBINE & RPD FLOW have the greatest effect on meter life METERING FOR NATURAL GAS expectancy. Meter oil change frequency will depend upon cleanliness Turbine flow meters & RPD flow meters are of the gas being measured. Oil has to used to measure gas flow, particularly for be changed when the color darkens or clean, medium-to-high steady flow of low- when the level increases, indicating an viscosity fluids. The turbine meter accumulation of moisture. measures volumetric flow based on fluid • A change in the meters internal flowing passed a free-spinning rotor, with resistance can affect the rotary meter each revolution agreeing with a specific accuracy. Any significant increase on volume of fluid. PD meters differ from the meter’s internal resistance to flow turbine meters in that they handle medium will increase the pressure drop and high-viscosity liquids well. In the case of between the inlet and outlet of the RPD meter, two figures "8" shaped lobes, meter, thus increasing the differential. the rotors(also known as impellers or Therefore, the meter differential pistons), spin in precise alignment. With pressure appears as a prime indicator each turn, they move a specific quantity of of meter condition. Establishing base gas through the meter. The rotational line curves – developing an original movement of the crank shaft serves as a differential or baseline curve is primary flow element and may produce recommended at the time of meter electrical pulses for a flow computer or may initial installation. At least 3 test points drive an odometer-like counter. Compared are required at gas flow rates between with orifice-type meters, PD meters require 25% to 100% of meter capacity. Plot very little straight upstream piping since the points on a graph and then connect they are not sensitive to uneven flow the points to form a curve. This distribution across the area of the pipe. provides baseline data for comparison to later tests. 3.1 Limitations in Turbine & RPD • In case of both Turbine & RPD meters, Flow Metering operating flow below Qmin can lead to Certainly, both the turbine and RPD meters increased measurement errors; have moving parts which is always a operating flow above Qmax can be limitation, and the user needs to consider detrimental to the metering system. the cleanliness of the gas. There is also the This challenge can be meted out chance of damage because of over- through appropriate sizing of metering speeding these meters. system with due emphasis on process

conditions. 3.2 Challenges in Turbine & RPD • Flow Metering Turbine & RPD flow meters which are flanged end-to-end & misalignment • In both Turbine & RPD flow meters, during installation can pose critical measurement accuracy and life issues such as zero shift. This expectancy can be impeded by challenge can be mitigated by ensuring excessive deposits of dirt or other type proper flange-to-flange alignment. of foreign material present in the gas stream. Proper oil level and cleanliness

The technological advancements achieved • Reduced sensitivity to severe gas in these meters are: conditions by improved protection of the main bearings against dirt and • Increased life span by using an oil dust. system that not only refreshes the oil in the bearings and lubricates all Technological advancements sought in TFM moving parts, but also flushes out all & RPD metering systems will be to know the dirt and dust. friction of bearings used.

Figure 4: 4 inch size Turbine flow meter

Figure 5: 2 inch size RPD flow meter

4.0 ULTRASONIC FLOW METERING FOR NATURAL GAS

An ultrasonic flow meter takes advantage of Despite such initiatives, due to some of the the principle that an ultrasonic pulse travels operational issues such as liquid carryover faster with the gas flow than against the from wells along with natural gas, operating flow. The larger the difference between the under such harsh environments such as upstream travel time versus the wet, rich and / or dirty gas applications pose downstream, the more gas has passed significant challenges in Ultrasonic Flow through the meter. The advent of high Metering. Some of the technological speed computer processors has made it advancements that evolved to mitigate possible to detect very small time these challenges in USM metering are: differences between the upstream and the • Transducer probes which are designed downstream travel times. Because for wet gas application; they offer ultrasonic meters do not depend on the benefits such as – resistance to liquid kinetic energy from the field, very low flows borne dirt, more tolerance to corrosive can be detected. Other advantages include fluids, robust electrical connection & standard volume flow accuracy of 0.35% to less sensitive to process conditions. 0.5%, with 0.25% available, as well as • Development of path failure negligible pressure drop and high turndown compensation routines is some USMs capability. The high turndown makes them which compensate for failure path based useable in applications subject to wide on path relationships deduced under variations in flow, which means that a single normal operating conditions. ultrasonic meter can replace multiple other • Relocating the transducers to the upper meters side of USM to avoid their contamination due to their positioning at bottom. 4.1 Limitations in Ultrasonic Flow • Technique for filtering of ultrasonic Metering frequencies from generated external noise to reduce and eliminate acoustic Limitations of ultrasonic meters include its interference. Regulating valves create high initial cost and the need for sufficient line noise whose frequencies often fall in straight-run upstream or a flow conditioner. a meter's ultrasonic range. Part of the Because the accuracy of an ultrasonic energy created during pressure meter depends on the accuracy with which reduction is transferred into acoustic the flow profile inside the instrument is energy, which can lower the meter known, ultrasonic meters for custody signal-to-noise ratio and resulting transfer applications generally use a measurement accuracy. minimum of four paths. • Intelligent self diagnosis with Condition Based Maintenance (CBM) functionality. 4.2 Challenges in Ultrasonic Flow CBM provides a single point of critical Metering system information to help operators monitor and manage the performance of Ultrasonic flow meters have historically multiple metering stations and system been installed at custody transfer points in devices simultaneously, delivering natural gas transmission and distribution diagnostic information to predict pipelines known for clean dry natural gas maintenance requirements, reduce composition. Today, with new sources of measurement uncertainty and maximize natural gas entering the value chain, gas system uptime. quality managers are working hard to • In case of USMs that use bouncing path maintain quality natural gas in the pipelines. technology, along with improved flow

profile and swirl information, the SNR ratio, path performance etc. are reflective path matrix provides featured in the USMs to evaluate metering diagnostic information about inside pipe performance. The same are elaborated diameter condition. Changes in path- below:- length caused by build-ups in pipe can be alarmed by diagnostics. 4.3.1 Symmetry

4.3 Gauging of Ultrasonic Flow Symmetry is a dimensionless measure of Metering Performance the flow symmetry comparing the upper chords to the lower chords. The symmetry An important feature of many ultrasonic represents the path velocity ratios of upper meters is their built-in diagnostics, which to lower paths. A change of the symmetry enable them to detect the presence of value may be caused by contamination, liquids (two-phase flow), dirt buildup, blockages or deposits in the line that blockage, and other problems. Various change the symmetry of the flow profile. performance gauging indices such as profile Ideally the Symmetry should be 1.0. factor, swirl angle, symmetry, turbulence,

Figure 6: symmetry plot of USM

4.3.2 Cross-Flow one side of the meter to the chords on the other side. Ideally the Cross-Flow should be Cross-Flow is a dimensionless measure of 1.0. the flow symmetry comparing the chords on

Figure 7: cross-flow plot of USM

4.3.3 Turbulence

Chord Turbulence is an estimate of the indicates changed flow conditions (e.g. a turbulence (percentage) at a chord location. blocked flow conditioner). A change of the A value is calculated for each active chord. turbulence value may be caused by A value of 0% indicates no appreciable contamination, blockage or deposits in the turbulence. A change in the path turbulence line that change the symmetry of the flow profile.

Figure 8: Turbulence plot of USM

4.3.4 Profile Factor

The profile factor represents the path velocities. A change of the profile value may velocity ratios of inner to outer paths. In be caused by contamination, blockages or other words, it is a dimensionless ratio of deposits in the line that change the shape of the inner chord velocities to the outer chord the flow profile.

Figure 9: Profile factor plot of USM

4.3.5 Swirl Angle

The Swirl Angle is an estimate of the swirl function of the meter body style and Profile (to the nearest degree). It is calculated as a Factor. A value of 0 degrees indicates no appreciable swirl.

Figure 10: swirl angle plot of USM

4.3.6 SNR Ratio

A measure of the signal “goodness” is the pipeline, valves that are not fully the signal-to-noise ratio (SNR). The open, sources of noise near the higher the SNR, the better the signal. measuring location, or defective Interfering noise caused by fittings in ultrasonic transducers may affect the signal-to-noise-ratio.

Figure 11: SNR plot of USM

4.3.7 Path Performance

The performance is the percentage of the way from the transmitting to the valid received ultrasonic signals for receiving sensor or due to acoustic each path. That means, if 5 of 100 noise, also caused by very high gas transmitted signals do not meet the velocity, the performance can be acceptance criteria for a signal significantly lower. A change in analysis, then the performance of this performance at comparable process path is only 95%. The performance of conditions can indicate a potential each path should be very close to transducer problem (e.g. 100%. Due to signal attenuation on contamination). A visual inspection of the ultrasonic signals and the transducer itself is required.

Figure 12: Snap of USM diagnostics

Summary of comparison of technologies & challenges for upgradation for Custody transfer measurement & handling:

DP Based Flow Meters Turbine & RPD Flow Meters Ultrasonic Flow Meters Inexpensive & simple Accurate measurement for Most accurate, Negligible clean & medium to high pressure drop & high steady flow turndown # Limitations: 1.This measures volumetric 1.Having moving parts 1.Expensive flow, must be corrected for density 2.Low Turndown & Generates 2.Change in meter internals 2.Flow conditioner required medium to large pressure loss can affect accuracy 3.Requires periodic inspection Deposition of dirt or foreign , sensitive to flow profiles & material ,affects Requires long straight runs performance # Challenges : 1.Errors due to rough 1.Reduced life span due to 1.Liquid carryover, wet upstream pipe wall & damaged deposition of dirt ,rich & dirty gas bore , improper design of profiler & Swirling gas flow pattern

2. Meter tube , Plate 2. Errors due to deposition 2. Noise eccentricity & alignment issues of foreign particles # Upgradation in Technology 1.Orifice plate holder: Ensures 1.Use of lubrication 1.Transducer probes alignment enhances life span designed for wet gas 2.Compact flow profiler design: 2.Improved protection of 2.Path failure Reducing straight length main bearing compensation 3. Single chamber: Inspection 3.Relocation of with less downtime transducers on upper side to avoid contamination 4. Double chamber design: 4.Noise filtration, Condition Inspection without flow based Maintenance & built interruption in diagnostics # Technological advancement required 1.Automatic cleaning 1. To know bearing friction 1.Automatic cleaning of mechanism for deposition on to ensure accuracy probes in case of orifice plate deposition 2.Compact design 2. To restrict entry of foreign 2.Proving & validation particles to avoid damage to requirements rotor

5.0 CONCLUSION to provide better tracking of meter performance problems in a timelier manner. Hitherto, with regard to challenges as More optimizations could take place in the highlighted in this paper in case of various metering technology with more robust Metering Systems being employed for methodologies for flow calculations natural gas, whatsoever the type of rendering very lean possibility for metering system employed in natural gas occurrence and recurrence of measurement applications, high moisture/H2S content in errors. Also, new sensing technology will gas can have adverse impact which could likely allow for integrated metering internally erode / corrode the meter run packages that measure energy rate rather causing pitting, denting the surface than gas flow rate and gas composition (or roughness levels of the meter run rendering heating value) independently. Scope for it non-conformant to relevant AGA further improvisation is sought in USM standards. Also carbon dust depositions & metering arena for use in low pressure also debris which get stuck to inner side of metering applications. The applicable metering system in place & that of piping metering standards will continue to be would impact the metering systems. This updated to reflect changes in technology, would significantly contribute to although those documents inherently lag measurement errors. This challenge can be behind the latest technological dealt with by carrying out periodic meter developments. tube inspections along with cleaning of meter run, meter body & profiler plate, ACKNOWLEDGEMENTS checking their conformity to relevant AGA Authors extend their gratitude to GAIL India governing standards & carrying out buffing Limited Management & GAIL Organization of meter tube as deemed required. So, for its support provided for paper review and challenges in metering systems are provision of further facilitations needed for inevitable which can be mitigated through presenting the paper in FCRI Flotek Global employing best industrial practices and Conference. The special thanks to FCRI for advanced technological solutions. giving Global platform for presentation for 6.0 FUTURE PERSPECTIVE / SCOPE continual improvements.

In the years ahead, it is expected that REFERENCES natural gas flow metering technology to [1] Dan Hackett June 2016 “Selecting flow continue to evolve, with greater use of meters for natural gas” microprocessor technology. It is foreseen [2] AGA Report No. 9 “Measurement of gas that more on board diagnostic capabilities to by multipath Ultrasonic Meters “ be added to the various flow metering types [3] Paul G. Honchar “Fundamental Principles of Gas Turbine Meters” [4] Eric Thompson, “Fundamentals of Multipath Ultrasonic Flow Meters for Gas Measurement”