1. GAS DENSITY & S.G. MEASUREMENT

2. Gas Density and Specific Gravity Definitions
GD2 2.3

3. What Changes Density?
Temperature
Pressure
Composition

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

5. 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 to lbs/ft3 with less than a 2 psi change.

6. 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

7. 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

8. 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

9. 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

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

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

12. 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

13. Spring - Mass - Damper System

14. Resonant Element Density Transducer

15. 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

16. 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 ( lb3 / °F)
Safety
EEx ia IIC T6
Output signal
6V 1.5 KHz

17. 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

18. Typical 7812 Installation

19. 7812 Pressure Drop versus Flowrate
GD2 8.3

20. 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

21. 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 %
2. Temperature - 0.2%
3. Composition %
Total uncertainty = 0.65% plus T and P errors

22. 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)

23. 3098 S.G. Transducer Specification
SG limits 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 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 V dc
Operating freq. ~ 2kHz at 0 kgm-3
Enclosure NEMA 4x box

24. 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

25. 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

26. 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

27. 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

28. 7950/7951 Signal Converters

29. 7950/51 Signal Converter Software Options
Version 1020
Gas applications
7812
3098
Version 2010
Liquid applications

30. 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

31. 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