Product guide PG/FLOW/010-EN Measurement & Analytics Wet gas monitoring Economical wet gas metering using non-proprietary correlations Measurement made easy Introduction With many of the most economically attractive gas fields already on stream, the more-marginal fields must now be exploited, including gas condensate fields. Here the gas is the primary resource and only small amounts of hydrocarbon liquid are contained in the gas stream. Separation of the phases for allocation and well testing (for example) is often either not possible or uneconomic and there is a growing requirement for flow meters to either simply measure the gas flow or to measure both the liquid and gas phases. This leads some users to fall back on multiphase meters, which, because of their complexity, can be prohibitively expensive. A more economical solution to metering in many wet gas applications does exist. This guide describes the solution and the public-domain (non-proprietary), well-accepted algorithms / correlations that the solution employs, measuring the flowrates and total flow of both the liquid and gas phases. The metering of this wet gas is a difficult application that requires a special approach in both hardware and data processing. The presence of liquid in a gas stream affects the flow reading of a DP-based meter, giving a value higher than the true gas flowrate. This over-reading becomes greater as the Liquid Volume Fraction, (LVF) the ratio of liquid volume to total volume increases.
Measurement & Analytics Wet gas monitoring Wet gas is defined as a gas stream with liquids present. However, it is treated as single-phase flow measurement, in that it is predominately gas and it is measurement of the gas phase only that is required. The Gas Volume Fraction (GVF) is typically no less than 90 to 95 %, unlike in multi-phase metering where the gas fraction can be very much lower (the liquid fraction can be very much higher). Wet gas metering is differentiated from two-phase flow metering by the value of the Liquid Volume Fraction (LVF). When a gas carrying a liquid phase is measured with a DP flow meter (the most economical technology to use), energy is expended in trying to accelerate the liquid phase up to the gas velocity. This energy loss is manifested as a pressure drop through the meter. The DP measured therefore is higher than ISO5167, for example, predicts for a given meter, resulting in an over-reading of the gas volume. Accurate measurement of the gas phase is only possible using correction factors, that themselves will depend on the LVF and the physical properties of the liquid phase itself. Features and benefits ABB makes wet gas metering easy by leaving out the complications of other wet gas metering solutions: No specialty measurement required No need for flow calibration: ISO5167 meter design No patents, no secrets: everything is in the public domain No uncontrolled software No need for tracer dilution testing for liquid content determination The flow meter is constructed in accordance with ISO-5167 and ISO/TR 11583:2012, so flow calibration is not required, but is offered as an option. Because the basic system is as any other DP flow metering set-up, it is familiar to both operators and technicians and therefore requires no specialists for commissioning or servicing. All calculations of flow rate and PVT properties are contained within ISO documents and are performed by a flow computer. Furthermore, for pure liquids when assuming 100 % oil or water, costly tracer dilution tests are no longer required, as the system is able to calculate the liquid content in real-time, based on the ISO/TR 11583 algorithm. Fig. 1: Panel-mount wet gas flow computer 2 PG/FLOW/010-EN Measurement & Analytics Wet gas monitoring
In addition, the ABB wet gas metering system provides some interesting features: It is the only solution with PVT calculations carried out within the flow computer, so there is no need for a PC. It uses a real (compliant with API MPMS Chapter 21 and MID certified) flow computer: this is highly preferred by users for (fiscal) allocation metering Touch screen operation The degree of over-read of the net gas flow rate is affected by the following: Operating pressure The Lockhart-Martinelli parameter ( ), that represents the Liquid Volume Fraction (LVF). The Gas Froude number that represents the gas velocity in the pipe (Frgas) or meter restriction (Frgas,th) Capable of remote operation from laptops and tablets Fig. 2: External touch screen for flow computer To correct for the over-reading mentioned above, over time a number of correlations have been found. The original correlation was developed by Murdock for orifice plates and then modified by Chisholm to take account of the additional effects of the gas density. De Leeuw found another correlation for wet gas when measured by Venturi tubes. All these correlations involve a coefficient called the Lockhart-Martinelli parameter, a dimensionless number which expresses the LVF in terms of the pressure drops (across the meter restriction) of the liquid and gas phases flowing alone. Further work and corrections to these are detailed within ISO/TR 11583:2012 and the ABB solution uses these latest correlations. Fig. 3: Downstream tapping for overall pressure loss measurement Measurement & Analytics Wet gas monitoring PG/FLOW/010-EN 3
Measurement & Analytics Wet gas monitoring Venturi over-reading calculation The De Leeuw correlation calculates the over-reading ( ) the ratio of the uncorrected gas flowrate to the true gas flowrate: This correlation was established using Venturi meters with a gas stream containing hydrocarbon condensate. Since De Leeuw's original work, the correlation has been updated within ISO/TR 11583:2012 to: = (1 + CCh + 2 ) 1 /2 CCh is a complex function of the densities of the liquid and gas phases, the Beta ratio, Froude Number and a coefficient related to the liquid's surface tension. Venturi Lockhart Martinelli calculation The overall pressure loss across the meter can be used to evaluate the Lockhart Martinelli number ( ) for the application and Venturi tube. The position of the tapping downstream of the meter must be within a range whose limits depend on the Beta ratio and the pipe diameter. However this method is valid only for a certain range of values of the ratio of the actual pressure loss increase due to wetness and the maximum increase. These, in turn, are functions of the overall pressure loss, the meter DP, Beta ratio, gas and liquid phase densities, Froude Number and the liquid surface tension coefficient Note: ISO/TR 11583 applies only for a gas containing a single phase liquid (either hydrocarbon liquid/oil or water not for a mixture). Note that an accurate well stream composition needs to be established and entered in the flow computer, which then performs GERG-2008 calculations in real-time to determine all the necessary gas and liquid properties. ISO/TR 12748:2015 states that fluid composition estimates can be one of the largest source of uncertainty in the output of wet gas flow meters. Although in all other respects the Venturi tube is designed to ISO5167, the discharge coefficient when used on wet gas is calculated according to the ISO/TR standard, that includes modifications involving the values of Throat Froude Number and. Fig. 5: Typical system HMI display Fig. 4: Wet gas venturi tubes 4 PG/FLOW/010-EN Measurement & Analytics Wet gas monitoring
System components The Venturi tube is an ideal primary element to use in wet gas metering as it is less affected than most by the presence of liquid in a gas phase, as any liquid can pass easily through the meter without blockage or damage. A flow computer is the industry-accepted electronics platform for performing the necessary wet gas correction. The ABB wet gas solution incorporates both of these products. The flow computer is available in configurations for mounting in the field (with ATEX zone 2 or Class I Div 2 classification), on a panel, on a DIN rail or in a rack. The pressures and pressure differences are measured using pressure transmitters, differential pressure transmitters and/or multivariable transmitters. The temperatures are measured using RTD elements in thermowells and temperature transmitters. Fig. 7: DIN rail-mount flow computer Fig. 6: Rack-mount flow computer Fig. 8: Ex-certified field-mount flow computer with optional touchscreen Measurement & Analytics Wet gas monitoring PG/FLOW/010-EN 5
Measurement & Analytics Wet gas monitoring Limits of use The limitations of the experimental data used to create the correlations in turn impose limits on the field of application for the correction methods. These can be summarized as follows: Horizontal pipe runs only Venturi Beta ratio ( ) 0.4 0.75 Lockhart Martinelli ( ) 0 < 0.3 Throat Gas Froude Number (Frgas,th) Frgas,th > 4 Ratio of gas density at upstream tapping to liquid density ( 1,gas / liquid) 0.02 < 1,gas / liquid 0.09 Pipe diameter D 250 mm (10 in.) D 50 mm (2 in.) System performance For operation within the limits of use specified by ISO/TR 11583, that standard specifies that the uncertainty of the correction factor for over-reading will be between 2.5 % and 6 %, depending on the value and uncertainty of. Applications Wet gas meters are used in a number of applications, including: Development of marginal oil and gas fields Process metering: where gas contains hydrocarbon liquid or water (or a mixture of the two) Allocation metering: enabling individual well control Well management Well testing: reducing CAPEX in evaluation of new wells Production optimization Mechanical specification Pressure class: ANSI Class to 2500# Process temperature range: 40 to 220 C ( 40 to 428 F) Typical flow meter body material: Duplex UNS S31803 (others on request) End connection: flanged, weld prep or hub type 6 PG/FLOW/010-EN Measurement & Analytics Wet gas monitoring
Certification, testing and documentation ABB wet gas meters are supplied with comprehensive certification and documentation and are subject to stringent testing. Material traceability certificates are supplied to ensure the quality and suitability of the material used in the manufacture of the flow elements. ABB offers testing of the materials used to produce the meter. The complete meter can also optionally be tested on water, gas or wet gas. In the past ABB have used the wet gas test facilities at CEESI (Colorado Engineering Experiment Station) in the USA for wet gas meter testing, but can equally collaborate with the customer's preferred test facility. ABB's extensive range of certification and testing is supported by comprehensive customer documentation packages. The data dossiers are tailored to the requirements of the project but include meter bore calculations, welding qualifications, drawings, NDE certificates & procedures, material certificates and quality plans. Measurement & Analytics Wet gas monitoring PG/FLOW/010-EN 7
Contact us ABB Limited Industrial Automation Salterbeck Trading Estate Workington Cumbria CA14 5DS UK Tel: +44 (0)1946 830 611 Fax: +44 (0)1946 832 661 Spirit Innovative Technologies (a member of the ABB Group) 205613AM Eindhoven The Netherlands Tel: +31 40 236 94 45 Note We reserve the right to make technical changes or modify the contents of this document without prior notice. With regard to purchase orders, the agreed particulars shall prevail. ABB does not accept any responsibility whatsoever for potential errors or possible lack of information in this document. We reserve all rights in this document and in the subject matter and illustrations contained therein. Any reproduction, disclosure to third parties or utilization of its contents in whole or in parts is forbidden without prior written consent of ABB. Copyright 2017 ABB All rights reserved PG/FLOW/010-EN 01.2017 www.abb.com/measurement