Page 1 of 9
Refer to Figures: 16 & 17 as you read on.
Wet, natural gas direct from the wellstreams, at about 1600 Psi, is first pre-cooled and separated from liquids in an 'Intermediate Separator' and cooled in a gas/gas exchanger causing more liquids to separate out in the 'Choke KO Drum'. The liquids pass to other processes.
From the Choke KO drum, the gas passes through another gas/gas exchanger - the 'LTS Pre-cooler'.
(The cooling medium for the gas/gas exchangers is cold dry gas from the Low Temperature Separator. The exchangers are of the 'Double-pipe' type).
The cooled feed gas from the LTS pre-cooler passes into the L.T.S. vessel, first passing through a Flow Control Choke valve. This is an angle type valve with a hardened plug to withstand the high velocity, erosive quality of the gas flow and the low temperature. Through the choke, the gas pressure is decreased from 1600 psi to about 700 psi and expanded into the low temperature separator where the temperature decreases to as low as minus (-) 19 °F. (This temperature depends on the gas pressure and temperature before the choke).
The gas enters the LTS 'Spinner' section at a tangent. The tangential flow in the spinner imparts high speed rotation to the gas which decreases its velocity into the vessel and also causes some separation to take place due to the centrifugal force. The heavier components, water and condensate move downwards while the gas passes upwards.
A hot fluid coil in the spinner section ensures that the formation of 'HYDRATES *' does NOT occur at this point. (* Hydrates are complicated molecules of water and hydrocarbon liquid which are combined to form a semi-solid, icy sludge). (Normally, the formation of hydrates is to be avoided due to being a cause of blockages in equipment and piping. However, in the Low Temperature Separation method of dehydration, hydrates are required and are purposely made to form in order to separate the maximum amount of water from the gas.).
The hot fluid flow through the spinner coil is manually controlled by a globe valve in the inlet flowline. This valve is adjusted as required to maintain 100 to 110 °F coil outlet temperature. Further into the LTS vessel, the cooling of the fluids by expansion causes more water and condensate to form together with HYDRATES. The liquids and semi-solid hydrates flow down the length of the vessel bottom.
Warm, wet gas from the 'Classifier', (discussed later), also enters the LTS vessel near to the spinner.
In order to convert these hydrates back into water and condensate, another hot fluid coil runs the length of the vessel bottom.
As the liquids flow along the separator, the hydrates melt and separation of hydrocarbon condensate and water takes place and form an interface.
Near the end of the vessel, an insulating baffle and a surge baffle separate the cold section of the separator from the warmer end. At the end of the vessel, a hydrate screen is installed to prevent unmelted hydrates from passing into the liquid outlet streams.
Figure: 16 shows the LTS internals
(The hot fluid system supply temperature is about 200 °F). The bottom temperature is controlled by a TIC/TICV, with the control valve located in the coil outlet line.
The dry gas leaves the top of the separator before the insulating baffle and passes to the shell side of the two feed gas coolers.
(These exchangers have temperature control valves, TCV's. These are 3 - way valves (control valve and bypass), controlled by a TIC in the exchanger wet gas outlet line).
The dry gas now passes to the Dry Gas Header via a metering unit and on to further processing.
(Downstream of the metering unit Flow Element (FE), is a SDV which will close on a signal from the Shut-down system. Upstream of the FE, a PCV is set to vent excess gas to flare during any operating pressure fluctuations.
This valve however will open to flare on a signal from the Shut-down system).
The separated water is piped from the bottom section of the LTS separator to the drain header, controlled by Interface Controller LCI & LCIV. (LCI = Level Control Interface).
Condensate is piped via a flow element FE/FR and level control system (LIC) to further processing.