Download EFMA Ammonia Pipeline Guidance 2008 PDF

TitleEFMA Ammonia Pipeline Guidance 2008
Tags Types Math & Engineering
File Size13.1 MB
Total Pages50
Document Text Contents
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Copyright 2008 EFMA

European Fertilizer Manufacturers’


Avenue E. van Nieuwenhuyse, 6

B-1160 Brussels


Page 25

climatic and topographic conditions; and national legislation (if any) are taken into
consideration. It should also be borne in mind that every interruption in a solid line, e.g.
by valves and flanges, is in itself a potential source of leak. Therefore, the number of
such fittings should be limited. Preferably, all fittings should be welded instead of using
flanges, especially in underground systems.

US practice on long liquid ammonia pipelines is to install isolation valves at such
intervals that the volume that can be released between two valves is limited to 400
tonnes [1]. In the (more populated) EU areas these volumes, based on the above
mentioned consequence analysis, range between about 10 and 225 tonnes.

In case of loading and unloading lines to and from ships the provision of a cable
connection is recommended between ship and shore to enable the closure of emergency
valves remotely from the land-based control centre as well as from the ship.

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Operating an ammonia pipeline with ammonia at sub-zero temperatures will bring the
risk of ice-formation, especially if the ammonia is at -33°C or the line is uninsulated.
Ice formation will cause the line to become heavier, giving an increased load on pipe
supports. Furthermore, there is the possibility that the ice formation is such that
equipment situated in the line, like isolation valves, can become iced in. This can be an
unwanted situation because the equipment can become inoperable. If this is the case,
apply appropriate vapour tight insulation, or install a heating system at the critical
places. This can be electrical or steam heating.

With regard to corrosion, ice on the ammonia pipeline is not an issue. Oxygen
diffusion through the ice is very slow and the low metal temperature basically reduces
the corrosion rate to zero. Old ammonia pipelines that have always been covered with
ice, have been known to stay in excellent condition for many years.

Most attention should be paid to corrosion when the line is intermittently operated
with cold ammonia, for example a line between a sea tanker and a storage tank. During
the unloading of the ship, the line is cold and water condensation or ice formation
occurs. When the ship is empty, the line is taken out of operation and warms up again.
The formed ice melts and the wet line is then vulnerable to corrosion.

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Proper precautions should be taken when commissioning and decommissioning liquid
ammonia pipelines. One precaution (attention to corrosion during the warm up and cool
down cycle) has already been described in the previous paragraph (Ice formation).
Other precautions include:

A pipeline containing air should ideally be purged with nitrogen prior to introducing
ammonia in order to avoid the formation of potentially explosive ammonia/air mixtures.
Also, sending the ammonia/air mixture to storage facilities is undesirable because of the


Page 26

possibility of Stress Corrosion Cracking that can occur when oxygen is present in
ammonia storage tanks.

A pipeline should be cooled down and warmed up slowly to prevent thermal stresses
which can cause damage. The rate of cooling down or warming up should be in
accordance with good engineering practices.

The ammonia vapour generated during the cooling process should not be disposed of
by venting to atmosphere or by absorption in water followed by release to water
courses. It should be returned to an ammonia recovery system.

Emptying a pipeline containing liquid ammonia should be carried out by expelling
the liquid ammonia with a gas, preferably ammonia vapour. Purging the liquid
ammonia line with nitrogen can result in the formation of extremely low temperatures
(as low as -70°C) for which the line might not have been designed. This phenomenon
is often not fully appreciated as it is generally believed that the lowest possible
temperature of liquid ammonia is its atmospheric boiling point of -33°C. This is only
the case when the ammonia vapour pressure above the liquid is 1 bara. When the
ammonia pressure is lowered by an inert gas such as nitrogen, the boiling temperature
becomes much lower. The vaporisation heat needed causes such low temperatures to
occur. The liquid ammonia thus expelled should be routed to a proper location e.g. a
storage tank.

An emptied pipeline which only contains ammonia vapour should be purged with
nitrogen prior to allowing air to enter. As explained above, this is to avoid the formation
of potentially explosive ammonia/air mixtures and to prevent Stress Corrosion
Cracking when sending oxygen to storage facilities. The ammonia/nitrogen mixture
leaving the pipeline should be disposed of in an environmentally acceptable way, for
instance by absorbing the ammonia in water followed by recovery.

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Inspection criteria for pipelines in the USA have been defined in the US Code of
Federal Regulations no. 49, part 195 [13]. See Section 6.2 for some more information
about that Code. Another relevant code is the American Petroleum Institute’s API 570,
which describes the inspection, repair, alteration and rerating of in-service piping
systems [23].

In the EU the EU Pressure Equipment Directive [14] is followed. However, as
already mentioned in Section 6.1, this is a Directive that applies to pressurised
systems in general. In most EU countries the industry has (up to now) its own specific
inspection criteria for liquid ammonia pipelines. A summary of these criteria is given


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