.
Our site needs your help!
Site categories

Nitrogen Generator System on Liquefied Natural Gas Carriers

Join Our Telegram (Seaman Community)

Discover how nitrogen generator system play a crucial role in LNG carrier operations. Explore the uses of N2, generator types, O2 content management, setup procedures, maintenance tips, and safety protocols for efficient and secure vessel operations.

Reference: SIGTTOLNG Shipping Suggested Competency Standards”, Sections:

1 Have an awareness of their purpose, the different types and their operating principles:

  • requirements for and uses of nitrogen.

2 Know and understand nitrogen generator operating principles and parameters, the outlet requirements and limitations:

  • dew point;
  • temperature requirements;
  • buffer tank.

3 Know and understand operational requirements and procedures:

  • setting up;
  • starting;
  • shutdown;
  • maintenance.

4 Know and understand bulk liquid nitrogen storage systems:

  • loading;
  • regasification.

5 Know and understand alarm settings and resulting actions.

Uses of N2

N2 is the most common naturally occurring gas. It represents 78 % of the volume of the atmospheric air, with O2 making up 20,9 % and the balance consisting of other trace gases.

N2 as generated on board cannot support life.

N2 is used for its inert properties in circumstances where the carbon dioxide (CO2) in combustion generated IG would freeze in close proximity to an LNG cargo. Large volumes of N2 for inerting can be taken from ashore if required post-refit.

Photo of nitrogen generator
Fig. 1 Nitrogen generator

Generally, N2 is created on board ship to:

  • prevent the formation of flammable mixtures in the event of a leak;
  • allow easier detection of an LNG leak in protected spaces;
  • prevent corrosion.

LNG ships produce N2 for the following purposes:

  • inerting the interbarrier spaces;
  • inerting the cargo tank insulation spaces;
  • inerting the hold spaces;
  • purging cargo pipelines;
  • purging cargo heaters and other cargo machinery;
  • purging boil-off gas lines on the deck/engine room;
  • sealing glands on HD/LD compressors;
  • mast riser fire extinguishing;
  • instrumentation.

Types of generator

Nitrogen generators – membrane type

Generally, nitrogen generators on board ships use an air-separation process. This works by passing compressed air across a membrane, which separates the air into two paths:

  • one containing the N2;
  • the second containing the O2, CO2 and the other trace gases found in the atmosphere.
N2 generator/typical details
Capacity
150 m3/h @ 97 % N2, 3 % O2
No. on board2

The capacity of the air-separation membrane nitrogen generators depends on:

  • the number of membrane modules;
  • air inlet pressure and temperature;
  • the required purity of the N2.

Typically, an air-separation membrane nitrogen generator produces a gas of 95-97 % purity.

Adsorption: This process describes accumulation on the surface of a solid.

Another common type of nitrogen generator is the PSA type. Its carbon molecular sieve retains O2 in the air, allowing the N2 to pass through.

Construction of a membrane separator
Fig. 2 Membrane separator on a nitrogen generator

There are two adsorber beds that are alternatively pressurised then depressurised (for regeneration) in a cyclic process. O2 content as low as 4 ppm can be achieved when using the adsorption type nitrogen generator.

Separation through molecular sieve
Fig. 3 Carbon molecular sieve

N2 produced from the generator must be dust and oil free and meet the following minimum quality requirements:

  • O2 ≤ 3 % volume.
  • dew point ≥ minus 65 °C (-65 °C) at atmospheric pressure.
Physical properties of N2
Colourless
Odourless
Density close to air1,25 kg/m3
Liquid temperature isminus 196 °C (-196 °C)
Liquid SG at boiling point1,81
Vapour SG at +15 °C/1 bar abs0,97
Gas volume/liquid volume ratio at minus 196 °C (-196 °C)695
Flammable limitsnone
Dew point of 100 % pure N2
below minus 80 °C (-80 °C)

O2 content

The nitrogen generator will be equipped with an oxygen analyser, connected before the remotely operated 3-way valve that continually monitors the O2 content in the generated N2. If the O2 level rises 1 % above the desired value an alarm must activate. If the O2 level rises further the high-high alarm must operate. Flow must be re-directed to the atmosphere and the discharge line to the buffer tank closed.

Buffer tank

Two nitrogen generators are normally installed in the engine room. The capacity of these units is such that one will be capable of all normal service requirements within a 20 % margin. For periods of high demand, such as the initial cooldown of the Cargo System – Tank Constructioncargo system, this load may be satisfied by having both generators operating in parallel.

A nitrogen generating system should include a buffer tank with sufficient capacity (approx. 40 m3) to ensure that the system need not start up more than once in a two hour period during normal operations at sea.

Typical N2 generator
Fig. 4 N2 generator

Start-up and shutdown procedures must always be as per the manufacturer’s recommendations for the equipment.

The nitrogen generating plant must operate automatically and allow remote control and monitoring from the cargo control room.

The N2 generated should be diverted/vented if the O2 content exceeds 3 % by volume.

The two nitrogen generators, which are each typically of 150 m3/h, will be able to produce 300 m3/h of N2 at a delivery pressure of 4-6 bar. The highest demand period will be when topping up the insulation spaces/annular space during cargo loading and cooldown.

Setting up

The control panel is fitted with the following:

  • push-buttons for start/stop operation;
  • push-button to acknowledge alarms;
  • electrical heater temperature control;
  • emergency stop push-button.

It also has the following alarms and indicators:

  • system status indications;
  • continuous N2 delivery pressure;
  • continuous O2 content reading;
  • dew point analyser.

An oxygen analyser, after the membrane, monitors the oxygen content and, if it is out of range, redirects the flow to the atmosphere.

Typical nitrogen production system (schematic shown below):

Scheme of N2 production system
Fig. 5 Nitrogen production system

Initial starting/shutdown

Start one nitrogen generator to pressurise the buffer tank (in the event of a large demand for N2 the second nitrogen generator will automatically start).

Adjust the setpoint of the N2 supply regulating valves.

The programmed stopping sequence (shutdown) will be initiated by pressing the stop button.

Maintenance

Maintenance is organised as per the established planned maintenance system (PMS), which must cover the following:

  • filter changes;
  • screw compressor oil changes;
  • screw compressor vibration analysis records;
  • O2 meter calibration checks;
  • humidity meter calibration checks;
  • completion of the relevant section in the alarm test register;
  • performance of the automatic drain traps;
  • membrane performance checks (contamination).

It is recommended that there is a plan for balanced running of the two nitrogen generators. It is found that if they are not run for about 3-4 hours every week, or if they are not operated for a considerable period, it can take several hours of running before the N2 quality is at an acceptable level when they are brought in to service.

In its liquid state, N2‘s low temperature will damage living tissue. Any spillage of liquid N2 on the ship’s deck will result in brittle fracture of the steel in the same manner as for LNG.

A typical mechanism used on older classes of ship was to load liquid N2 (LN) across the manifold into bulk storage while alongside at the discharge port. This facility would normally be set up as part of the construction of the terminal.

N2 storage tanks onboard
Fig. 6 Nitrogen storage tanks

The innovation of N2 generating plants, and the ever increasing requirement for the International trade of Liquefied Natural Gas in maritime industryLNG trade to extend beyond long term liner trade projects and toward spot market trading has brought about the demise of onboard LN bulk storage systems.

Alarms and shutdowns

The following are the usual trip setpoints on an N2 generator:

AlarmTrip point
N2 Generator feed air temperature, high-high
+65 °C
N2 Generator filter differential press high
0,6 bar
N2 Generator air heater temperature high
+120 °C
N2 Generator O2 content high
3,5 %
N2 Generator O2 content high-high
4,0 %
N2 buffer tank pressure low
5,5 Barg
N2 buffer tank pressure high
10,0 Barg
N2 Generator dewpoint level high
minus 60 °C (-60 °C)

Footnotes
Sea-Man

Did you find mistake? Highlight and press CTRL+Enter

Апрель, 08, 2024 70 0
Add a comment


Notes
Text copied