Page 87 - 4822

Emergency Valves (EV) are sectioning valves that will
separate the process components and blow-down valves that will
allow excess hydrocarbons to be burned off in the flare. These
valves are operated if critical operating conditions are detected or
on manual command, by a dedicated Emergency Shutdown
System. This might involve partial shutdown and shutdown
sequences since the flare might 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 this gives 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 3 m in diameter and 14
meters long. At the rated operating pressure this means a very
heavy piece of equipment, typically around 50 tons for this size.
This limits the practical number of stages. Other types of
separators such as vertical separators, cyclones (centrifugal
separation) can be use to save weight, space or improve separation
(see later) There also has to be a certain minimum pressure
difference between each stage to allow satisfactory performance in
the pressure and level control loops.

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 will also
receive production from wells connected to the Low Pressure
manifold. The pressure is now around 1 MPa (10 atmospheres) and
temperature below 100 degrees C. The water content will be
reduced to below 2%.
An oil heater could be located between the first and second
stage separator to reheat the oil/water/gas mixture. This will make
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 cooling medium placed
inside an outer shell.

87