GAS DENSITY & S.G. MEASUREMENT

Gas Density and Specific Gravity Definitions GD2 2.3

What Changes Density? Temperature Pressure Composition

Temperature Changes Density Example: The density of air changes from 0.0736 lbs/ft3 to 0.0704 lbs/ft3 with a change of 22 degrees F.

Pressure Changes Density Applicable for light hydrocarbons such as LPG Applicable for supercritical fluids such as ethylene and CO2 Applicable for gases Often ignored for crude oil Example: The density of air changes from 0.700 to 0.793 lbs/ft3 with less than a 2 psi change.

Basic Density Equation r = K0 + K1t + K2t2 where, r = density of the fluid K0, K1, K2 = constants determined by the manufacturer t = periodic time of oscillation

Temperature Correction on Density Dt = D[1 + K18 (t - 20)] + K19 (t - 20) where: Dt= temperature corrected density D = density from basic equation using K0, K1, K2 t = temperature K18, K19 = constants from calibration certificate

Pressure Correction on Density Dp = Dt [1 + K20 (p-1)] + K21 (p-1) where: Dp = temperature and pressure corrected density Dt = temperature corrected density p = pressure K20 = K20A + K20B (p - 1) K21 = K21A + K21B (p - 1) K20A, K20B, K21A, K21B = constants from calibration certificate

Ideal vs. Real Specific Gravity Real S.G. = ratio of gas density to air density at reference P&T Ideal S.G. = ratio of MW of gas to MW of air

Gas Specific Gravity Independent of changes in pressure, temperature or compressibility. Needed to determine energy content.

Gas Density Density is defined as weight per unit volume. Changes when pressure, temperature or composition varies. Is needed for mass flow.

Gas Compressibility Density can be calculated from compressibility or compressibility can be calculated from density when the gas composition, temperature and pressure are known. flowing density is a function of specific gravity, pressure, temperature and compressibility

Spring - Mass - Damper System

Resonant Element Density Transducer

7812 Gas Density Capabilities High accuracy Integral temperature sensor Excellent thermal characteristics Low velocity of sound effect Fast response Long term stability Intrinsically safe Frequency output signal Continuous on-line measurement

7812 Gas Density Transducer Specification Parameter Specification Density range 0 to 400 kg/m^3 (25 #/ft3 ) Limits of error (10 to 100% FS) For nitrogen +/- 0.1% of reading For natural gas, ethylene +/-0.15% of reading Maximum operating pressure 250 bar (3625) psi (using pocket kit) Temperature range -20 to +85 °C ( -4 to +185 °F ) Temperature coefficient 0.001 kg/m^3 / °C (0.00003 lb3 / °F) Safety EEx ia IIC T6 Output signal 6V pk-pk @ 1.5 KHz

7812 Gas Density Transducer Requires a clean, dry sample For mass flow systems, pressure differences between densitometer and flow meter are critical Have no recognized standard for field proving Best guide is published by IP

Typical 7812 Installation

7812 Pressure Drop versus Flowrate GD2 8.3

Gas Density Verification Check density accuracy compared to PTZ calculations Check with sample gas of known density Vacuum check confirms that the calibration has not shifted with time Check instrument stability at different flow rates to insure good temperature equilibrium

S.G. Calculation from Line Density using PTZ NX19 or AGA 8 Method Base Density = Line density Relative density = Specific gravity = Ratio of molecular weight of gas to that of dry air = Relative density Errors associated with Z correction at 100bar (1440psi) and ambient temperature 1. Pressure - 0.4% 2. Temperature - 0.2% 3. Composition - 0.05% Total uncertainty = 0.65% plus T and P errors

3098 Gas Specific Gravity Transducer Line ISOLATION VALVE, D VALVE A VALVE B VENT AND INPUT FOR CALIBRATION GASES VALVE C DIAPHRAGM PRESSURE REGULATOR FILTER RELIEF VALVE OUTLET DENSITY TRANSDUCER NEMA 4X INSULATING COVER REFERENCE CHAMBER CONTROL VALVE CONTROL INDICATOR TO VENT INPUT ORIFICE OUTPUT TO SIGNAL CONVERTER 3098 CHAMBER FILLING VALVE, E VALVE F (PURGING VALVE)

3098 S.G. Transducer Specification SG limits 0.1 - 3 typical Fluid Dry, clean non-corrosive gas Accuracy Up to ±0.1% reading Repeatability ±0.02% reading Temp. coefficient ±0.01%/°C (0.005% / °F) Temp range -30 to +50°C (-22 to +122 °F) Control pressure 1.2 to 7 bar A (17 to 101 psia) Gas flow rate 0.2 to 60 normal cm3 Filter 7 µm Response time <5s upon entry - at 60 normal cm3 Output signal 6V peak to peak (3 wire) 2-3V peak to peak (2 wire) Power Supply +15.5 - 33V dc Operating freq. ~ 2kHz at 0 kgm-3 Enclosure NEMA 4x box

3098 S.G. Transducer Key Features Real-time gravity measurement Simple calibration using two sample gases Low gas consumption No mechanical adjustments Measures ideal or real SG Laboratory accuracy at continuous line conditions

3098 S. G.Transducer Calibration Reference Chamber Gas: This gas should be similar to the sample gas in terms of compressibility characteristics. (actual line samples are commonly used) Reference Chamber Pressure: This should be sent to minimize the overall error, particularly the temperature coefficient. Temperature coeff minimized High pressure Z change errors minimized Low pressure Z effect on calibration Low pressure Calibration with Sample Gas: Two certified gases should be used that adequately represent the characteristics and specific gravity range of the line gas. Typically for natural gas Methane and Nitrogen or Carbon Dioxide are used. GD2 3.3

3098 S.G. Transducer Servicing What to touch and what not to touch : On-site changes Solartron factory changes Inlet filter Diaphragm Orifices (input and output) Diaphragm ‘o’ ring Pressure Gauge valve Reference chamber Transducer gasket Amplifier Cylinder* Spool body* * under emergency situations

Gas Density (7812) or S. G. (3098) Gas Density Advantages and Applications: Continuous on-line density at metering point No Compressibility Z or temperature T problems No errors associated with calculating density Used for mass measurement or line volume with Orifice Specific Gravity Advantages and Applications: Application outside the gas density operating conditions Wet gas High temperatures Contamination problems Can derive density from S.G via PTZ Applications where Wobbe Index or Calorific Value is required If S.G. and Density both require highest accuracy then use both transducers

7950/7951 Signal Converters

7950/51 Signal Converter Software Options Version 1020 Gas applications 7812 3098 Version 2010 Liquid applications

7950/51 Signal Converter 1020 Software Gas Density & Gas Specific Gravity Signal Converter Software Instrumentation Connections: 7812 Gas Density Transducers or 3098 Gas Specific Gravity Transducers (quantity 2) Temperature & Pressure Transmitters Basic Functionality : Gas Calculations for line and base density Density coefficients and live inputs PTZ using either NX19, AGA8 or SGERG with P & T Temperature, pressure and VOS correction Reporting and Event logging

7950/51 Signal Converter Summery Common hardware with application specific software Interfaces with earlier Solartron Density/SG transducers Simple to use configuration menu 4 line display making simple interrogation of data Metric or imperial units of measurement Modular design allows customers to upgrade PC Configuration tool available