Cold Temperature Considerations
Regulators Rated for Low Temperatures
Te c h n i c a l
In some areas of the world, regulators periodically operate in
temperatures below -20°F (-29°C). These cold temperatures
require special construction materials to prevent regulator failure.
Emerson Process Management offers regulator constructions that
are RATED for use in service temperatures below -20°F (-29°C).
Selection Criteria
When selecting a regulator for extreme cold temperature service,
the following guidelines should be considered:
• The body material should be 300 Series stainless steel, LCC,
or LCB due to low carbon content in the material makeup.
• Give attention to the bolts used. Generally, special stainless
steel bolting is required.
• Gaskets and O-rings may need to be addressed if providing
a seal between two parts exposed to the cold.
• Special springs may be required in order to prevent fracture
when exposed to extreme cold.
• Soft parts in the regulator that are also being used as a seal
gasket between two metal parts (such as a diaphragm) may
need special consideration. Alternate diaphragm materials
should be used to prevent leakage caused by hardening and
stiffening of the standard materials.
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Freezing
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Introduction
Freezing has been a problem since the birth of the gas industry.
This problem will likely continue, but there are ways to minimize
the effects of the phenomenon.
There are two areas of freezing. The rst is the formation of ice
from water travelling within the gas stream. Ice will form when
temperatures drop below 32°F (0°C).
The second is hydrate formation. Hydrate is a frozen mixture
of water and hydrocarbons. This bonding of water around the
hydrocarbon molecule forms a compound which can freeze above
32°F (0°C). Hydrates can be found in pipelines that are saturated
with water vapor. It is also common to have hydrate formation in
natural gas of high BTU content. Hydrate formation is dependent
upon operating conditions and gas composition.
Reducing Freezing Problems
To minimize problems, we have several options.
1. Keep the uid temperature above the freezing point by
applying heat.
2. Feed an antifreeze solution into the ow stream.
3. Select equipment that is designed to be ice-free in the
regions where there are moving parts.
4. Design systems that minimize freezing effects.
5. Remove the water from the ow stream.
Heat the Gas
Obviously, warm water does not freeze. What we need to know is
when is it necessary to provide additional heat.
Gas temperature is reduced whenever pressure is reduced. This
temperature drop is about 1°F (-17°C) for each 15 psi (1,03 bar)
pressure drop. Potential problems can be identied by calculating
the temperature drop and subtracting from the initial temperature.
Usually ground temperature, about 50°F (10°C) is the initial
temperature. If a pressure reducing station dropped the pressure
from 400 to 250 psi (28 to 17 bar) and the initial temperature is
50°F (10°C), the nal temperature would be 40°F (4°C).
50°F - (400 to 250 psi) (1°F/15 psi) = 40°F
(10°C - (28 to 17 bar) (-17°C/1,03 bar) = 5°C)
In this case, a freezing problem is not expected. However, if the
nal pressure was 25 psi (1,7 bar) instead of 250 psi (17 bar),
the nal temperature would be 25°F (-4°C). We should expect
freezing in this example if there is any moisture in the gas stream.
We can heat the entire gas stream with line heaters where the
situation warrants. However, this does involve some large
equipment and considerable fuel requirements.
Many different types of large heaters are on the market today. Some
involve boilers that heat a water/glycol solution which is circulated
through a heat exchanger in the main gas line. Two important
considerations are: (1) fuel efciencies, and (2) noise generation.
In many cases, it is more practical to build a box around the
pressure reducing regulator and install a small catalytic heater
to warm the regulator. When pilot-operated regulators are used,
we may nd that the ice passes through the regulator without
difculty but plugs the small ports in the pilot. A small heater can
be used to heat the pilot supply gas or the pilot itself. A word of
caution is appropriate. When a heater remains in use when it is not
needed, it can overheat the rubber parts of the regulator. They are
usually designed for 180°F (82°C) maximum. Using an automatic
temperature control thermostat can prevent overheating.
Antifreeze Solution
An antifreeze solution can be introduced into the ow stream where
it will combine with the water. The mixture can pass through
the pressure reducing station without freezing. The antifreeze is
dripped into the pipeline from a pressurized reservoir through a
needle valve. This system is quite effective if one remembers to
replenish the reservoir. There is a system that allows the antifreeze
to enter the pipeline only when needed. We can install a small
pressure regulator between the reservoir and the pipeline with the
control line of the small regulator connected downstream of the
pressure reducing regulator in the pipeline. The small regulator is
set at a lower pressure than the regulator in the pipeline. When the
controlled pressure is normal, the small regulator remains closed and
conserves the antifreeze. When ice begins to block the regulator in
the pipeline, downstream pressure will fall below the setpoint of the
small regulator which causes it to open, admitting antifreeze into the
pipeline as it is needed. When the ice is removed, the downstream
pressure returns to normal and the small regulator closes until ice
begins to re-form. This system is quite reliable as long as the supply
of the antifreeze solution is maintained. It is usually used at low
volume pressure reducing stations.
Equipment Selection
We can select equipment that is somewhat tolerant of freezing if
we know how ice forms in a pressure reducing regulator. Since
the pressure drop occurs at the orice, this is the spot where we
might expect the ice formation. However, this is not necessarily
the case. Metal regulator bodies are good heat conductors. As a
result, the body, not just the port, is cooled by the pressure drop.
The moisture in the incoming gas strikes the cooled surface as it
enters the body and freezes to the body wall before it reaches the
orice. If the valve plug is located upstream of the orice, there is
a good chance that it will become trapped in the ice and remain in
the last position. This ice often contains worm holes which allow
Freezing
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Te c h n i c a l
gas to continue to ow. In this case, the regulator will be unable to
control downstream pressure when the ow requirement changes.
If the valve plug is located downstream of the port, it is operating
in an area that is frequently ice-free. It must be recognized that
any regulator can be disabled by ice if there is sufcient moisture
in the ow stream.
System Design
We can arrange station piping to reduce freezing if we know
when to expect freezing. Many have noted that there are few
reported instances of freezing when the weather is very cold (0°F
(-18°C)). They have observed that most freezing occurs when the
atmospheric temperature is between 35° and 45°F (2° and 7°C).
When the atmospheric temperature is quite low, the moisture
within the gas stream freezes to the pipe wall before it reaches the
pressure reducing valve which leaves only dry gas to pass through
the valve. We can take advantage of this concept by increasing
the amount of piping that is exposed above ground upstream of the
pressure reducing valve. This will assure ample opportunity for
the moisture to contact the pipe wall and freeze to the wall.
When the atmospheric temperature rises enough to melt the ice
from the pipe wall, it is found that the operating conditions are not
favorable to ice formation in the pressure reducing valve. There
may be sufcient solar heat gain to warm the regulator body or
lower ow rates which reduces the refrigeration effect of the
pressure drop.
Water Removal
Removing the moisture from the ow stream solves the problem
of freezing. However, this can be a difcult task. Where moisture
is a signicant problem, it may be benecial to use a method of
dehydration. Dehydration is a process that removes the water
from the gas stream. Effective dehydration removes enough water
to prevent reaching the dew point at the lowest temperature and
highest pressure.
Two common methods of dehydration involve glycol absorption
and desiccants. The glycol absorption process requires the gas
stream to pass through glycol inside a contactor. Water vapor
is absorbed by the glycol which in turn is passed through a
regenerator that removes the water by distillation. The glycol
is reused after being stripped of the water. The glycol system is
continuous and fairly low in cost. It is important, however, that
glycol is not pushed downstream with the dried gas.
The second method, solid absorption or desiccant, has the ability to
produce much drier gas than glycol absorption. The solid process
has the gas stream passing through a tower lled with desiccant.
The water vapor clings to the desiccant, until it reaches saturation.
Regeneration of the desiccant is done by passing hot gas through
the tower to dry the absorption medium. After cooling, the system
is ready to perform again. This is more of a batch process and will
require two or more towers to keep a continuous ow of dry gas.
The desiccant system is more expensive to install and operate than
the glycol units.
Parallel pressure reducing valves make a practical antifreeze system
for low ow stations such as farm taps. The two parallel regulators
are set at slightly different pressures (maybe one at 50 psi (3 bar)
and one at 60 psi (4 bar)). The ow will automatically go through
the regulator with the higher setpoint. When this regulator freezes
closed, the pressure will drop and the second regulator will open
and carry the load. Since most freezing instances occur when the
atmospheric temperature is between 35° and 45°F (2° and 7°C),
we expect the ice in the rst regulator to begin thawing as soon as
the ow stops. When the ice melts from the rst regulator, it will
resume owing gas. These two regulators will continue to alternate
between owing and freezing until the atmospheric temperature
decreases or increases, which will get the equipment out of the ice
formation temperature range.
Most pipeline gas does not have water content high enough to
require these measures. Sometimes a desiccant dryer installed
in the pilot gas supply lines of a pilot-operated regulator is quite
effective. This is primarily true where water is present on an
occasional basis.
Summary
It is ideal to design a pressure reducing station that will never
freeze, but anyone who has spent time working on this problem
will acknowledge that no system is foolproof. We can design
systems that minimize the freezing potential by being aware of the
conditions that favor freezing.
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