PROCESS

DRAINS

EXP-PR-PR120a-ENRev. 0

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In this course you will learn:The functions of drains How drains work Different types of drains Drains and the process Operating parameters Operating drains

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The functions of drains The function of drainage systems is to collect the occasional liquid purges from the production units.

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How drains work Closed drains

The closed drain system is balanced on the LP flare network

The hydrocarbons from the tank are collected by a pump operating under ON/OFF level control and are sent into the oil line

The pump lift lines are fitted with high pressure safeties linked to the Process Safety System (PSS).

The closed drain tank is equipped with a very low LSLL level safety linked to the PSS and with a very high LSHH level safety linked to the Emergency Shutdown System (ESD).

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How drains work Open drains

The open drain system is in balance with the atmosphere The open drain tank is equipped with a baffle The drains manifold feeds the tanks by means of a plunger tube The mixed water / hydrocarbon phase accumulates in the lower part and the hydrocarbons that are lighter than water are carried over to the second part of the receptacle by flowing over the separator baffle. These hydrocarbons are collected by a pump operating under ON/OFF level control and returned to the closed drain tank The water phase is sent to the waste tube Another decantation process takes place in this waste tube and the hydrocarbons floating on the top are returned to the open drain tank

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Different types of drains The closed drain

Closed drains are made up of a network of manifolds and sub-manifolds that collect: Oily bleed-off from processes, such as liquids from flare drums Liquids from the degassing drum skimmer Hydrocarbons from the oily discharge outlet of the hydrocyclones Hydrocarbons from the sump tank The hydrocarbon phase recovered from the open drain

The vessels are linked to the collectors by lines fitted with valves to prevent the hydrocarbons fromp coming into contact with the air. The different collectors are linked to the closed drain drum by a main manifold.

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Different types of drains Open drains or sump tanks

Open drains are made up of a network of manifolds and sub-manifolds that collect the installation's various oily water purges The vessels are not directly linked to collectors and bleed off occurs through lines fitted with valves that open into funnels installed on the manifolds The collectors are connected to a drum by a main manifold, as for closed drains A discharge tube linked to the drain drum discharges the decanted water to the sea, on an offshore installation, or to water treatment on an onshore installation

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Different types of drains Valve set-up for open and closed drains

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Drains and the process Location and criticality

Drain systems have a high criticality: it is the last barrier preventing liquid hydrocarbons from being sent to the environment

It is prohibited to connect the closed-drains networks with the open-drain networks

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Operating parameters Closed drains

A very low LSLL level detected in the closed drain tank leads to shutting down the hydrocarbon collection pumps and closing the SDV safety valves at the pump suctions in order to protect the latter

A very high LSHH detected in this tank triggers a General Production Shut Down

Open drains

A very low LSLL level detected in the open drain tank leads to shutting down the hydrocarbon phase collection pumps towards the open drain and closing the safety valve at the pump lifts

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Operating drains Start-up Before start-up, it is imperative to carry out an inertance of the closed drain system. This inertance can be done with either nitrogen, with an inert gas, or with water from which all traces of liquid hydrocarbons (in emulsion) have been removed Normal operation Carry out a weekly test of booster pumps Check that the open drains are not obstructed On an offshore installation, check the colour of the waste water to the sea (lack of iridescence indicating a hydrocarbon leak) Shut-down The drainage system is shut-down after the installations are shut down.Before maintenance operations of any nature whatsoever are carried out, it is imperative to inert the closed drain system This is done by means of a nitrogen sweep after having purged all the liquid hydrocarbons