Page 155 - Handbook Of Multiphase Flow Assurance

Liquid holdup 151

flow velocity is sufficient, water holdup gets swept by the shear stress exerted on water by the
flowing hydrocarbon. Water may also be mechanically removed from the flow systems which
don't have sufficient flow velocity by scraping.

Condensate in gas lines

Hydrocarbon deposits have various condensate-gas ratio or CGR. Fluids with a higher
CGR are likely to condense more liquid hydrocarbons. Pipelines are designed to maximize
throughput and to reduce condensate holdup. Significant accumulation of condensate in a
low spot may lead to the onset of terrain induced slugging. Therefore lines are predominantly
routed to avoid passing through low spot locations.
In tiebacks to offshore facilities it may be more economic to install a longer pipeline to
avoid a low spot because terrain slugging requires a larger size separator or slug catcher
in the topsides process system. Larger topsides equipment requires more deck space and
weighs more which may lead to a greater increase of the host facility cost compared to the
extra cost of a longer line.

Steam (condensed water in oil sands steam injection lines)
Steam transmission lines are used in one of the costliest methods of hydrocarbon ex-
traction, the steam assisted gravity drainage, or SAGD. Steam generated at a central facility is
distributed to the injection wells through pipelines.
Phase transitions important for this single-component system are condensation and evap-
oration. Condensation occurs as superheated steam gradually cools down and water begins
to condense in the transmission pipelines due to ambient cooling. Any condensed water acts
as a restriction to flow and reduces the throughput capacity of the steam transmission lines.
Majority of steam distribution systems are designed to deliver superheated steam without
condensed water because the hydrocarbon bitumen has to be heated to 150–300 °C to become
mobile and steam has to be hotter than this.
Superheated steam may come in contact with a pool of cold fluid, either condensed wa-
ter or condensed hydrocarbon. A sudden condensation of superheated steam leads to steam
hammer.
Works by Irani and Carlson (Irani, 2013; Carlson, 2012) describe the process.
In some cases the condensed water may exist in various locations of the steam generation
system. It may be a manifold near the plant with a low spot or a pipeline with a low spot used
to collect steam generated from several units to the distribution line. Sequential startup of
multi-unit systems should be prepared for the possibility of condensed water accumulation
near the cold non-operational unit. Valve opening sequence should be planned so as to avoid
the possibility of steam hammer. If a valve to a new unit is opened suddenly, superheated
steam from the operational units may reach the condensed water and suddenly condense
causing steam hammer.
System design should avoid low spots and account for the potential locations of condensed
fluids to reduce the possibility of steam hammer.