Oil and gas production handbook An introduction to oil ... - ABB Group

Emergency valves (EVs) are sectioning valves that separate the process
components and blow-down valves, allowing excess hydrocarbons to burn
off in the flare. These valves are operated if critical operating conditions are
detected or on manual command from a dedicated emergency shutdown
system. This may involve partial shutdown and shutdown sequences, since
the flare may not be able to handle a full blow-down of all process sections
simultaneously.
A 45,000 bpd design production with gas and 40% water cut will give about
10 cubic meters from the wellheads per minute. There also needs to be
enough capacity to handle normal slugging from wells and risers. This
means the separator has to be about 100 cubic meters, e.g., a cylinder 3m in
diameter and 14m in length at the rated operating pressure. This means a
very heavy piece of equipment, typically around 50 tons for this size, which
limits the practical number of stages. Other types of separators, such as
vertical separators or cyclones (centrifugal separation), can be used to save
weight, space or improve separation (to be discussed later).
There must also be a certain minimum pressure difference between each
stage to allow satisfactory performance in the pressure and level control
loops. Chemical additives will also be discussed later.
4.2.3 Second stage separator
The second stage separator is quite similar to the first stage HP separator. In
addition to output from the first stage, it also receives production from wells
connected to the low pressure manifold. The pressure is now around 1 MPa
(10 atmospheres) and temperature below 100ºC. The water content will be
reduced to below 2%.
An oil heater can be located between the first and second stage separator to
reheat the oil/water/gas mixture. This makes it easier to separate out water
when initial water cut is high and temperature is low. The heat exchanger is
normally a tube/shell type where oil passes though tubes in a heating
medium placed inside an outer shell.
4.2.4 Third stage separator
The final separator is a two-phase separator, also called a flash drum. The
pressure is now reduced to atmospheric pressure of around 100 kPa, so that
the last heavy gas components can boil out. In some processes where the
initial temperature is low, it might be necessary to heat the liquid again (in a
heat exchanger) before the flash drum to achieve good separation of the
heavy components. There are level and pressure control loops.
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