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RP 60-6INERT GAS SYSTEMS

PAGE 6

5.3 Site Factors

(a) Availability of an existing medium.

(b) Onshore or offshore location.

(c) Concentration of inert gas users.

(d) Availability of reliable supplies from an external source, either

in cylinders or piped from an adjacent plant.

5.4 Commercial Considerations

(a) The cost of installation, operation and maintenance of an inert

gas generating system should be assessed against bought-in

supplies.

(b) The use of bought-in supplies for peak demands to supplement

the use of a generating plant for constant demands.

6. STEAM

6.1 Steam is often available in onshore process areas and can be used for

many purging and snuffing duties. When it is readily available it may be

a more economic source than manufactured gasses. When considering

steam as an inert gas the following shall be considered:-

(a) Steam is not combustible nor will it support combustion: hence

it is regarded as an inert gas in so far as combustion is

concerned.

When steam is added to air in a confined space at atmospheric pressure

and temperature the mixture temperature tends towards that of the steam.

As a guide, the percent volume of oxygen in a confined space will be

reduced to 5% when the mixture temperature is about 93°C.

(b) Steam may not be compatible with some process systems, e.g.

catalyst damage, or very high temperature processes above

1700C when steam can decompose.

(c) Steam is effective for purging air during the preparation for

start-up of process plant: it has an advantage in that it warmsthe system in addition to purging unwanted air.

When purging a system of air, additional purging may be required to

eliminate air ingress in the event of steam condensing after a system has

been purged before filling with hydrocarbons.




PAGE 7

Steam is not recommended for blanketing purposes because of the

difficulties of safeguarding the vessel.

(d) The pressure of purge steam must be below the design rating of

the plant, normally low pressure saturated steam should be

used, i.e. less than 3 bar(ga).

(e) Until the process equipment and piping warms through the

consumption of steam will be higher than for a dry inert gas due

to the steam condensing.

(f) Adequate condensate drains and vacuum release shall be

provided should the steam condense.

(g) It may be necessary to dry some systems after steam purging.

7. PRODUCTS OF COMBUSTION

7.1 Products of combustion may be readily available from the exhausts of

furnaces and internal combustion engines. Alternatively a fired inert

gas generator may be provided.

Gas Turbine exhaust gas is rich in oxygen. It would be necessary to fit

an after burner to reduce the concentration to a safe level before the

exhaust gas can be used as inert gas.

7.2 In order to use flue gas as a source of inert gas the level of oxygen in a

flue gas-flammable gas mixture shall not exceed 5% by volume.

Excess air and spurious air leaks into furnace chambers account for free oxygen in

flue gases.

7.3 Elements of sulphur oxides, hydrogen sulphide or carbon monoxide

shall be excluded before flue gases are used for purging.

7.4 Soot, ash, and other particular matter in exhaust gases shall be removed

to prevent troublesome deposits forming in the process equipment.

7.5 Exhaust gases shall be allowed to cool and excess water vapour to

condense.

8. CARBON DIOXIDE

8.1 The concentration of oxygen in flammable gas-carbon dioxide mixtures

shall not exceed 5% by volume.




PAGE 8

Carbon dioxide gas is non-flammable, colourless, has a distinctive taste at

concentrations above 10% and is toxic at high concentration levels. Normal

exposure level shall be limited to 5000 ppm.

Properties of carbon dioxide:-

Molecular weight 44.01Specific volume at 15 C 535.9 l/Kg

Boiling point @ 1013.25 mbar (abs) 78.2 C

Density of Gas @ 15 C & 1013.25 mbar (abs) 1.866 Kg.m 3

S.G. of gas @ 15 C & 113.25 mbar (abs) 1.52 (air = 1)

Volume of C02 gas produced from 1 m3

liquid @ 15 C & 51.5 mbar (abs) 437.3 m3

8.2 Where a high demand rate is supplied by commercial high pressure

cylinders, their storage and handling shall be fully assessed.

9. NITROGEN

9.1 General

Nitrogen generation and storage facilities should be located in a non

hazardous area.

The capacity of the generation plant and/or storage provided shall be

dependent on availability and reliability of supplies and the pattern of

demand.

In instances where there are high and fluctuating peak demands

compared with the normal demand it may not be economically viable toinstall on-site generating capacity to satisfy the peak demands. In this

situation consideration shall be given to providing a standby reserve of

nitrogen to satisfy the peak or high infrequent demands. Options for

providing this reserve supply are discussed in 9.7.

Nitrogen is non-flammable, colourless, odourless and has no taste; it is also an

asphyxiant. It is the most commonly used medium for the inert gas uses covered by

this Recommended Practice. It can be supplied in bulk as a gas or as a cryogenic

liquid, or in manoeuvrable gas cylinders. It can also be generated on site.

Physical properties of nitrogen:-

Molecular weight 28.01 (Air = 28.9)

Specific volume at 15 C 843.6 l/m3

Boiling point at 1013.25 mbar (abs) -195.8 C

Density of gas at 15 C &




PAGE 9

1013.25 mbar (abs) 1.185 kg/m3

Density of liquid at boiling point 0.808 g/ml

S.G. of gas at 15 C &

1013.25 mbar (abs) 0.967 (Air = 1.0)

Amounts of nitrogen gas produced after expanding 1 m3 from source conditions to1013.25 mbar (abs) and 15 C.

cryogenic nitrogen at -195.8 C

and 1013.25 mbar (abs) = 680 m3

onsite nitrogen gas generation at

10 bar (abs) and 15 C = 9.9 m3

cylinder gas at 176 bar (abs) and 15 C = 176 m3

9.2 Sources of Nitrogen

Nitrogen for use as an inert gas shall be supplied from:-

(a) commercial cryogenic nitrogen;

(b) commercial gas cylinders;

(c) on-site nitrogen generators, which may be either:-

(i) membrane units

(ii) pressure-swing adsorption units (PSA),

(iii) cryogenic nitrogen generators.

9.3 Commercial Cryogenic Nitrogen

9.3.1 Bulk supplies are received into double skin type or insulated liquid

storage tanks which may be located either inside or outside the

customer's premises.

Cyrogenic nitrogen is dispatched from a supply depot in bulk by road tanker and

other methods of bulk carriage, or for smaller quantities packaged in transportable

Dewar vessels.

Bulk storage tanks operate at atmospheric pressure or higher depending on the

type of vessel.

When cryogenic nitrogen is stored at equilibrium temperatures below 1.5 bar(abs),

condensation of atmospheric oxygen can occur on the outer surface of the storage

vessel and piping. This results in a localised oxygen-enriched environment and

additional fire hazard. In order to reduce the fire risk from liquid oxygen forming

outside the vessel the insulent shall be a non-combustible, non-porous cellular type,

with a vapour sealing material applied over the insulent.




PAGE 10

The purity of nitrogen received from a cryogenic source is high (more

than 99%).

9.3.2 When cryogenic nitrogen is to be used as an inert gas the liquid has to

first be vaporised. Depending on the site location and demand rate the

vaporiser can be heated by ambient air, hot water, steam or electricity.Vaporising equipment is generally provided by the nitrogen supplier

along with the cryogenic storage facility.

Flow energy is provided by pressurising the ullage volume in the tank. See Figure

1.

Dewar vessels, which are like very large thermos flasks and generally available in

sizes up to 200 litres, are delivered to specially allocated areas inside the

customer's premises.

Dewars are not pressure vessels, and the nitrogen contents are transferred by

pumping.

Nitrogen from storage is metered into the customer's distribution system and

charged for at a contract price plus a service charge.

9.3.3 Pipework likely to be subjected to liquid nitrogen temperature shall be

constructed of steels suitable for low temperature operation.

9.4 Commercial Gas Cylinders

9.4.1 Gas cylinders shall be used as the form of supply when a fixed

distribution system is not justified.

High pressure nitrogen gas is supplied from a bottling plant in transportablecylinders ready for use.

To facilitate handling and storage, cylinders generally are delivered as palletized

batches of twelve (referred to as 'quads'). Quads are suitably manifolded and

valved ready to connect with the customer's distribution system. Depending on the

supplier other pallet sizes may be available.

Typically a cylinder of commercial grade nitrogen contains 6.3 Normal m3 , which

is compressed to about 137 bar (abs).

The main disadvantage of using gas cylinders are the relatively small quantity of

gas available from a cylinder compared with cryogenic nitrogen of similar storage

volume. Consequentially a large inventory of cylinders may have to be stored on

site.

Quality of commercial grade nitrogen taken from cylinders should be

99.9% purity.

Experience shows this quality is not always assured. It is recommended that

random samples are taken on deliveries before use.




PAGE 11

9.5 On-Site Nitrogen Generation

9.5.1 Non-cryogenic on-site nitrogen production shall be provided by

pressure-swing (PSA) or membrane type units.

These both require a feed charge of clean, oil-free compressed air .

The purity of nitrogen received from either source is in the region of

97-99%.

PSA Method

PSA units separate air-gases by the use of a molecular sieve, consisting of a bed of

granular carbon, which selectively adsorbs nitrogen from a stream of compressed

air as this flows through the sieve.

PSA units have twin-adsorption chambers which are operated cyclically from an

automatic timer. During the cycle the adsorption process is performed in one

chamber, while the second chamber discharges its supply of nitrogen product into a

buffer storage vessel. Oxygen, carbon dioxide, water vapour and excess nitrogen

which pass through the sieve during the adsorption process are vented to

atmosphere.

The buffer storage dampens pressure pulsations which occur during the pressure-

swing operation.

Figure 2 shows a simplified flow diagram for the PSA installation.

PSA units are fed with compressed air at 10-11 bar (abs). and discharge their

product into storage at 9-10 bar (abs). Air supply shall be dry and oil free to avoid

contamination of the molecular sieve.

During operation some attrition of carbon occurs and this requires attention to

maintain an adequate bed depth.

PSA units can generate nitrogen product of 97-99% purity. Actual purity for

particular size of unit depends on the desired production level and sieve efficiency

at the time.

Membrane Method

Membrane units utilize a semi-permeable membrane to facilitate air separation

when a flow of compressed air is passed across the membrane surface. Nitrogen,

which has a slower permeability than other air-gases, becomes purer as the process

progresses and the unwanted gases permeate through the membrane.

The membrane method is a continuous process and does not require a buffer vessel.

The unwanted gases oxygen, carbon dioxide, and water vapour, and excess

nitrogen which permeate through the membrane are vented to atmosphere.




PAGE 12

Membrane units require compressed air at 8-10 bar (abs) pressure, and discharge

nitrogen product at 7-9 bar (abs). Air supply quality will depend on the membrane

used, but oil free air is recommended.

Figure 3 shows a schematic arrangement for an inert gas system with the

membrane unit.

Nitrogen quality for a particular unit varies with the production rate and generally falls in the range of 95-97% purity. By connecting several membranes in the series

nitrogen can be refined to 99% purity, but at much reduced production rates and

much higher ratios of compressed air-to-nitrogen product.

9.5.2 Cryogenic on-site nitrogen production shall be from a packaged

cryogenic production unit.

Plant for onsite production of cryogenic nitrogen to suit demands for inert gas

applications are commercially available from several manufacturers.

Basically the process involves liquefaction of atmospheric air which has been

drawn into the system and compressed, and then separation of nitrogen from the

liquefied air by fractionation inside a rectification column.

A typical flow diagram for packaged cryogenic production units is shown in Figure

4. Nitrogen liquid from the cryogenic process must be stored in either a double

skinned or insulated vessel.

Before the liquid product can be used in inert gas service this has first to be

vaporised in a vaporiser using ambient air, steam, hot water or electricity as a

heating medium.

Unlike bought-in cryogenic nitrogen, the costs of installing capital plant and

subsequent operation and maintenance have to be taken into consideration.

The benefits of high purity, economic storage and flexibility to meet peak demands

accredited to commercial cryogenic nitrogen are relevant also to onsite cryogenic

production units.

9.6 Selection of Nitrogen Supply Methods

9.6.1 The decision whether to generate nitrogen on the plant or to import

shall be based on the guidelines in Section 5. If generation is selected,

the choice of generating method shall be governed largely by the size of

plant and the purity of nitrogen required.

9.6.2 For most users covered by this Recommended Practice, a maximum

contaminant level of 5% by volume shall be acceptable, when

membrane or PSA units shall be selected.

When comparing membrane and PSA units, the following aspects should be

considered:-

(a) Membrane units have no moving parts and thus need less maintenance

than PSA units.




PAGE 13

(b) Specific consumption of compressed air is about 40% lower for PSA units

and the required compressor power is accordingly about 40% less than for

membrane units of the same capacity.

(c) To provide standby capacity or to uprate nitrogen production beyond an

existing installed capacity will require the purchase of a complete PSA

unit; whereas, with membrane units this can be achieved moreeconomically simply by adding further membrane cartridges to operate in

parallel with those existing.

(d) In general PSA units are of lower cost than membrane units of equal size

and performance, but the PSA unit will be about double the weight of a

membrane unit.

For high purity nitrogen with contaminant levels in the ppm region,

cryogenic generation systems shall be required.

9.6.3 Generally membrane and PSA units are more cost effective at lower

throughput (<500 Nm3 /hr) and at the lower end of this range

membrane units are usually less costly than PSA. For higher demands

(>500 Nm3 /hr), the cryogenic system tends to be more cost effective

although there is an overlap in this respect between PSA and cryogenic

units of similar size around 500 Nm3 /hr.

For very high demands (say >1000 Nm3 /hr), a small number of

cryogenic units would normally be more cost effective than a larger

number of membrane or PSA units irrespective of nitrogen quality

required.

9.7 Storage and Distribution

9.7.1 Storage or standby reserves should take the form of commercial

cryogenic nitrogen or gas cylinders connected up to the distribution

system and ready for use when it is required. An alternative approach

is to build up a reserve stock using the onsite generators, in which case

this should be taken into account when sizing this plant.

9.7.2 To be effective as a reserve supply, nitrogen gas from PSA and

membrane units shall be compressed and stored at a much higher

pressure. Either regular gas cylinders or a specially installed pressure

vessel shall be used to store the compressed gas.

9.7.3 For the storage of liquid from an onsite cryogenic production plant a

vacuum insulated type container should be used. Storage tanks with

external insulation may also be used to store cryogenic nitrogen.

A vacuum insulated container is a double skinned vessel with a vacuum held

between the inner nitrogen container and the outer protective casing.




PAGE 14

When using external insulation, a non-combustible, non-porous cellular type

insulent is recommended.

9.7.4 The inner tank for double skinned vessels shall be designed for contact

with liquid nitrogen at cryogenic temperatures and will require a steel

suitable for low temperature duty. The selected steel shall be subject to

approval.

9.7.5 For distribution of utility inert gas it is good practice to use a ring main

whenever possible.

9.7.6 All nitrogen-containing vessels and pipework shall be clearly identified

to distinguish them from other duties and systems.

10. REGIONAL ANNEXES

This section is supplemental to the foregoing international requirements, to assist withthe application of this Recommended Practice in the particular regions identified.

10.1 United Kingdom

The following are U.K. Health and Safety regulations and other guides

relevant to the design of inert gas systems:-

(a) HSE 'Pressure Systems and Transportable Gas Containers

Regulations 1989',

comprising two separate codes of practice;

(b) 'Safety of Transportable Gas Containers',which is primarily concerned with the mechanical integrity of

pressure parts of pressurised gas cylinders.

(c) Factories Act 1961: Section 30:,

'Dangerous fumes and lack of oxygen'

HSE Guidance Note EH40/91:,

'Occupational exposure limits 1991'

HSE Guidance Note GS5:,

'Confined space entry' (presently out of print for review)

These regulations are designed to protect persons at work,

where such work has to be done in a confined space in which

dangerous fumes or the lack of oxygen is liable to exist to such

an extent as to involve a risk of persons being overcome

thereby.




PAGE 15

(i) Areas adjacent to a cryogenic nitrogen storage area must be kept

free from traps, such as pits and trenches, where leaked nitrogen

could accumulate.

(ii) Barriers and warning notices shall be provided to protect

personnel from the risk of asphyxiation in the event of spillage or

leakage from cryogenic storage vessels.

(iii) The atmosphere near purge vents is likely to become oxygen-

deficient during the purging operation and precautions must be

taken to ensure only essential and properly equipped personnel

are allowed in these areas.

(iv) Free access into confined spaces which have been purged with

inert gases is permitted only after the air in the confined space

has been certified to contain 21% oxygen.

(v) Until a confined space is certified safe after purging, permits to

enter must be issued and only authorised persons properly

equipped shall be permitted to enter.

(d) The British Cryogenics Council 'Cryogenics Safety

Manual' (Third Edition 1991)

Use of cryogenic liquid is not currently subjected to regulations

in the U.K.. Recommendations contained in 'Cryogenics Safety

Manual' are a widely accepted guide concerning production,

storage and handling of cryogenic fluids.

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