OPTA-PERIPH
1
SAMPLE TAKE-OFF PROBE
& VAPORIZER for LNG
SUBJECT:
For LNG custody transfer it is now a common practice to determine the composition,
density and gross calorific value of LNG delivered by direct on line GC plus indirect
lab analysis methods. A precise sampling is prerequisite for both analysis.
I- Sample take-off probe:
The ISOSAMPLE 8100 SAMPLE TAKE-OFF PROBE
has been designed to provide a Thermal insulation of the
sample take-off probe by hight vacuum maintained below
1.10-4 torr.
The conjonction of this ultra-low residual pressure
minimizing heat leak by conduction and convection with
layer of radiation shield cryogenic material constitutes a
superinsulation system providing apparent thermal
conductivity (k) values below 0.4 milliwatt per meter-kelvin
(mW/m-k), therefore the enthalpy rise can be controlled to
be inferior of the subcooling degree and fractionation will
not have occured in the sampling line even for low LNG
pressure in line
II- Sample vaporizer:
The LNG sample is vacuum isolated up to the sample
vaporizer coil exchanger sealed in calorific block
temperature controlled. At this point, sample is flashing to
eliminate any fractionated distillation effect.
Inlet / outlet temperatures and pressures are monitored with
interlock shut-off system
This principle focuses on improving reproducibility between
lab and line GC to a value better than 0.15% (ASTM D1945)
and WI repeatabily beter than 0.01MJ/m3 (ISO 6976) .
1
III- System descriptive:
The sampling probe item1 must be located on the top of horizontal part of pipe to withdraw the LNG from the
center and the sampling tube is usually sized 3/8" OD.
The flange item 2 is supplied as per customer specifications and the vacuum jacket item 15 is welded on flange
and assembled with high vacuum flanged connections.
A first sample isolation core valve item 3 is supplied and encapsulated in the vacuum jacket, this valve is only
accessible by opening the flange cap to isolate the system from pipe-line if any maintenance work is required,
thus, the downstream part of system can be removed and the vacuum re-established in this pre-insulation
section without process service disturbance such as outage of the loading line.
A capillary assembly item 4 is provided for breaking equilibrium between the sample and pipe-line pressures
and the main isolating valve item 5 is supplied with extended bonnet and pneumatic actuator item 9.
This actuator is operated via the pneumatic logic unit item 8 to be installed locally: the valve is normally
closed and manually turned to open/close by pneumatic switch. A pneumatic port inlet controlled by the
interlock shut-off system (when acquired) is provided for switching-off the valve if the temperature at outlet
of vaporizer is dropping below the application set point (vaporizer failure or sample flow abnormal increase)
then new switching to open is manual.
The check valve item 7 is a operating as pressure barrier between sample pressure expansion in the vaporizer
and the process line and two thermocouples type K item 11 are supplied at inlet and outlet of vaporizer with 2
meters coils in the vacuum jacket to avoid heat leak by conduction.
The high purity bellows valve item 6 with vacuum pressure gauge is provided on the bonnet vacuum jacket
for re-establishing the vacuum when required.
The LNG sample is vacuum isolated up to the sample vaporizer coil exchanger sealed in calorific block
temperature controlled.
The vaporizer is supplied with PID temperature controller and inlet / outlet temperature interlock system for
auto-shut-off the main valve item 5 if outlet temperature drops to the application set point or if pressure at
vaporizer outlet exceeds 10 bar-g.
Temperature control set point is automatically changed if temperature at coil inlet warms up.
The sample vaporizer is so constructed that the LNG is flashing at the coil inlet and the heavier components
of the LNG do not remain in the vaporizer.
The capacity of the LNG vaporizer is sufficient to gasify the whole volume of LNG which is being withdrawn
for sampling and keeping the enthalpy rise below of subcooling degree compliance
The vaporizer is provided in a stainless steel box, protecting code is ATEX II 2G EEx de IICT4
.
IV- Main specifications:
Pipe-line pressure: 1.5 to 5 bar-g
Pipe-line temperature: - 160 °C
Volume of gas sample vaporized: approx 1200 Sl/h
Lag time through sampling probe and vaporizer: 30 sec approx.
Thermal conductivity of probe: 0.4 milliwatt per meter-kelvin (mW/m-k)
Heating capacity of vaporizer: 0.5 Kw
Sample outlet connection: 1/4" NPTF
Protecting electrical code: ATEX II 2G EEx de IICT3
Enclosure rating: IP65 or NEMA 4x
VAPORIZER and CONTROL
UNIT
VAPORIZER:
The vaporizer is integrated at the extremity of sample probe spool. It
consists in a coil exchanger sealed in calorific block temperature
controlled electric type.
The coil exchanger item 16 is a 2m x 3 mm ID tubing and the
assembly is so constructed for providing sample flushing to
avoid fractionated distilation effect and the heavier components
of the LNG will not remain in the coil.
Four temperature points are monitored:
TS 1: thermocouple K: measuring the inlet temperature of LNG, a
2 m extension capillary is provided in the vacuum jacket for
thermal insulation and connection is made in the thermocouple
junction box model AD 531 certified EExe ia II C T6.
A compensation cable has to be connected by customer to the
EExi temperaure converter ref 5114B1A integrated in the EExd
control unit enclosure ref: AD 442.
The range of temperature is set to - 160 to +40 °C .
PS1 _ TS3
TS2:PT100 (inserted in well item 16) measuring the vaporizer heat
exchanger temperature controlled .
A 3 wires cable has to be connected by customer to the PID
controller ref 2286B /4114 in the control unit.
TS1
TS 3: Pneumatic temperature switch ref FCNFC on the natural
gas at outlet of vaporizer, set point is -20°C.
.
An high temperature trip switch item 7 ,calibrated at 135°C
for class T4 is integrated in the vaporizer for safety
redundancy.
PS1:A pneumatic pressure switch ref FPNFQX is supplied as well on natural gas at outlet
of vaporizer; set point is 12 bar-g.
The temperature control of TS2 is set at 65°C and the outlet of TS 3 and PS1 are
connected to the Pneumatic Logic Controller interlock system for processing the autoshut-off sample take-off valve if outlet temperature T3 drops to -20°C ( normal is +65
°C) or if pressure at vaporizer outlet exceeds 12 bar-g.
VAPORIZER CONTROL SYSTEM
Temperature controller
TS 1 transmitter/indicator
control unit with 24 VDC power supply
plate assembly
24 VDC power supply
TS 2 and heater
TS 3
PS1
vaporizer enclosure
TS 1
EExi junction box