Page 312 - Petroleum and Gas Field Processing

no pressure reduction may be required, thus hydrate formation is
also avoided at the surface.
2. Indirect heaters. Both wellhead and flow line indirect heaters are
commonly used to heat natural gas to maintain the flowing
temperature above the hydrate formation temperature. The
primary purpose of the wellhead heater is to heat the flowing gas
stream at or near the wellhead, where chocking or pressure
reduction frequently occurs. Flow line heaters, on the other hand,
provide additional heating if required. They are particularly
used for cases where the conditions necessitate a substantial
reduction in pressure between the wellhead stream and the
next field processing facility. A typical example is an offshore
production field with its treating facilities on land. Heat is utilized
to compensate for the loss of temperature as the gas expands.
Another significant factor that contributes to the reduction of the
gas temperature in a gas line is the ground temperature.

A temperature drop of about 80 F is experienced by a gas
flow line travelling a distance of about 5000 ft.

12.3.2 Chemical Injection
Methanol and glycols are the most commonly used chemicals, although
others (such as ammonia) have been applied to lower the freezing point
of water, thus reducing (or preventing) hydrate formation.
The application of hydrate inhibitors should be considered for
such cases:

A system of gas pipelines, where the problem of hydrate formation
is of short duration.
A system of gas pipelines which operate at a few degrees below the
hydrate formation temperature.
Gas gathering systems found in pressure-declining fields.
Gas lines characterized by hydrate formation in localized points.
Inhibitors function in the same manner as ‘‘antifreeze’’ when added to
liquid water. Thus, the principle underlying the use of hydrate inhibitors is
to lower the formation of the hydrate by causing a depression of the
hydrate formation temperature.

Methanol Injection
Methanol is the most commonly used nonrecoverable hydrate inhibitor. It
has the following properties:

Copyright 2003 by Marcel Dekker, Inc. All Rights Reserved.