LNG process block diagram and key Process

Block diagram of  LNG process:

Key Process Units

SInlet Facilities (onshore):

             Reception and separation of offshore feed wet gas into gas, condensate, and water. 

SAcid Gas Removal (AGR):

             Remove acid gas (H2S and CO2) by amine system                     ( Ucarsol ).

SSulphur Recovery Unit (SRU) / Acid Gas Injection (AGI): 

              Recover H2S and Mercaptans (RSH) into pure liquid Sulphur (Claus process), or inject the acid gas into onshore acid gas injection wells.

SGas Treating (Dehydration & SELEXOL):  

                 Remove mainly water, Mercaptans (RSH) and Sulphur compounds from saturated gas

SMercury Removal: 

                 Remove mercury (Hg) form the dry gas

. 

SNGL Recovery:

                 Recover NGL (mainly C2+ or C3+).

SFractionation:

               Recover Ethane (C2), Propane (C3), Butane 

(C4), also iso-Pentane (i-C5) and plant condensate (n-C5+)

SRefrigeration: 

              Use Propane and Mixed Refrigerant (MR) for   gas      pre-cooling and liquefaction.  

SLiquefaction: 

              Liquefy the dry sweet gas by Main Cryogenic Heat Exchanger (using C3-MR).

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What is purge level system ?

This method is also known as bubbles method of level measurement. A pipe is installed vertically with its open end at the zero level. The other end of the pipe is connected to a regulated air supply and to a pressure gauge or to ^P transmitter. To make a level measurement the air supply is adjusted so that pressure is slightly higher than the pressure due to the height of the liquid. This is accomplished by regulating the air pressure until bubbles can be seen slowly leaving the open end of the pipe. The gauge then measures the air pressure needed to over come the pressure of the liquid.

/\ P  = H  X  D

USE : On for corrosive liquids where the transmitter cannot be directly connected to process eg... Acids, Some organic liquids.

How is D.P. transmitter applied to an open tank ?

On an open tank level measurement the L.P. side is vented to atmosphere. Whatever pressure acts is on the H.P. side which is a measure of level.

Three way manifold valve operation:

Three way manifold valve operation:

• The following photograph shows a three-valve manifold bolted to a Honeywell model ST3000 differential pressure transmitter. 
• A bleed valve fitting may be seen inserted into the upper port on the nearest diaphragm capsule flange.
• In normal operation, the two block valves are left open to allow process fluid pressure to reach the transmitter. 
• The equalizing valve is left tightly shut so no fluid can pass between the “high” and “low” pressure sides.                
• To isolate the transmitter from the process for maintenance, one must close the block valves and open the equalizing valve. 
• The best sequence to follow is to first close the high-pressure block valve, then open the equalizing valve, then close the low-pressure block valve.
• This sequence ensures the transmitter cannot be exposed to a high differential pressure during the isolation procedure, and that the trapped fluid pressure inside the transmitter will be as low as possible prior to “venting” to atmosphere. 
• Finally, the “bleed” valve is opened at the very last step to relieve pent-up fluid pressure within the manifold and transmitter chambers.

Instrument calibration

Instrument calibration

Calibration and ranging are two tasks associated with establishing an accurate correspondence between any instrument’s input signal and its output signal. 

To calibrate an instrument means to check and adjust its response so the output accurately corresponds to its input throughout a specified range.

In order to do this, one must expose the instrument to an actual input stimulus of precisely known quantity.

For a pressure gauge, indicator, or transmitter, this would mean subjecting the pressure instrument to known fluid pressures and comparing the instrument response against those known pressure quantities.

One cannot perform a true calibration without comparing an instrument’s response to known, physical influence or impact.

To range an instrument means to set the lower and upper range values (LRV & URV), so it responds with the desired sensitivity to changes in input.

For example, a pressure transmitter set to a range of 0 to 200 PSI (0 PSI = 4 mA output ; 200 PSI = 20 mA output) could be re-ranged to respond on a scale of 0 to 150 PSI (0 PSI = 4 mA ; 150 PSI = 20 mA).

In analog instruments, re-ranging could only be accomplished by re-calibration, since the same adjustments were used to achieve both purpose (i.e. Flow measurement).

In digital instruments, calibration and ranging are typically separate adjustments (i.e. it is possible to re-range a digital transmitter without having to perform a complete re-calibration), so it is important to understand the difference, As smart Transmitters using HART or FOUNDATION FIELDBUS protocols.

What is LNG?

 •LNG is natural gas cooled to -161o Centigrade, the temperature at which its main 

  component, methane, liquefies.

•Its volume is reduced to around one six-hundredth of its volume  as a gas

•It is stored and transported at atmospheric pressure as a boiling liquid

•It is an odorless, colorless liquid .