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
648
Te c h n i c a l
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
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