Open Loop Circulation of Crude

Open loop circulation of crude is an activity carried out during commissioning stage of a crude refining plant and subsequently after a long short-down of the #plant for #maintenance or other reasons.

Open loop circulation in a #refinery start up is the first practical circulation of crude oil in a refinery complex. It is a significant moment for any refinery which indicates the gradual progress of fully starting up the refinery for production. Open loop circulation involves the process of circulating the crude from the storage tank to the refinery processing units and back to the storage tank or further redirected to a feeder storage tank.

During open loop circulation, no heat or fire is applied at the furnace and no product is obtained at the moment. Hence, the crude is transported through the pipes which is pressurized by a pump to the heat exchangers which further goes to the inlet of the furnace and to the distillation column. In the distillation column, the crude is directed to flow through the residual line through gravity which is further pressurized by residual pump/hot pump then the crude passes through the heat exchangers, air coolers and back to the storage tank through the unqualified line. Centrifugal #pumps are used to increase flow of the crude in the pipe across all unit operations in the refinery. The pump is sized to according to the distance of pipes along the line, the pressure drop by friction in the pipes, pressure drop in valves, bends, elbows, elevations etc. in which the NPSH(available) is designed to be greater than the NPSH(required) by a sufficient safety margin to avoid cavitation.

Objectives of Open Loop Circulation

Open loop circulation is carried out to achieve the following objectives:

• To make sure that #instrumentations are working e.g flow transmitters, level transmitters, level gauges etc
• To stroke the control valves and ensure they are working perfectly.
• To check the reliability of the centrifugal pumps.
• To further check the mechanical equipment, #pipelines, valves etc for leaks.
• To establish a system circulation for the whole device to balance the required liquid level of each tanks and each flow path.
• ?To displace air and #condensate in the device or flow path.

The objectives listed above are very essential during open loop operation. Prior to the open loop circulation, purging operation were mostly carried out using steam, compressed air and nitrogen. During the open loop operation any debris from these purging medium above will be cleaned.

Open loop operation takes place for couple of days, hence it is a perfect time for checking all instruments and automation for optimum functionality. Instruments such as gas detectors, level gauges, level transmitters, flow meters and transmitters etc. including all instruments connected to #DCS (Distributed Control System) are showing properly in the control room.

During the open loop circulation activities, various department play crucial role, these role varies from department to department. Below are duties of personnel from various departments:

INSTRUMENTATION TEAM

The instrumentation team will be on ground during the open loop operation to confirm that all transmitters are working and the reading or values showing in the field conform to the readings in the #dcs from the control room.

MECHANICAL TEAM

Mechanical team are on stand for any #mechanical tightening as a result of a leak from any equipment, joints, valve as well as maintenance of any equipment e.g pumps.

SAFETY TEAM

The role of the #safety department is to ensure that all personnel on ground for the open loop operation work in accordance to safety guidelines to avoid accidents and near misses.

ELECTRICAL TEAM

The #electrical team ensures that electrical power is sent to all equipment that needs #power for startup and they are on standby in case of power outage, emergency issues related to electricity.

PROCESS TEAM

To commence the open loop operation, the #process field operators will ensure that the lines/route for the open loop operation are properly lined up. The valves on the tank farm for the crude storage is lined up to the pumps which are then lined up all the way through the battery limit, through the heat exchangers, furnace, distillation column and then lined up through the residual line to the air coolers, then converted to unqualified line at the either at the battery limit or at the tank farm and finally to the crude storage tank or feeder tank.

The crude is circulated through the unqualified line because at this point no product is formed and everything leaving the tower is unqualified.

Next, the process team put on the centrifugal pumps to move the crude from the storage tank from the tank farm to the process area via the route explained above. Inside the distillation tower the bottom level of the tower is control by two essential control valves (the inlet flow control valve of crude to the tower and the outlet flow control valve of residual line to the tank farm) depending on the design and configuration of the refinery complex.

The process team ensure the smooth running of all equipment used to carry out the open loop operation. They ensure there is appropriate communication between the control room operators, the mechanical team, the instrumentation team, safety team etc for smooth operation.

The control room operator is a process personnel who monitor changes of the flow on the various equipment during the open loop operation. They check the transmitters and confirm with the process field operators to ensure that all transmitters are working accordingly.

The control room operators test all the control valves by stroking the valves during this process. Stroking involves setting the flow controllers to open at various percentages to confirm that control valves are responding to commands and in good condition.

Also the control room operator carry out material balance analysis on the inlet and out of the tower by controlling the inlet control valve to the tower or the control valve on the crude line and the control valve of the residue line to the storage tank. This analysis is carried out for different percentage e.g 80%, 75%, 6o% etc.

This is carried out by manually controlling the tower control valves, cascading them or using automatic mode of the DCS. Carrying out this exercise will prepare the operator to maintaining the level in the bottom of tower during actual production operation.

The level of the bottom tower is calibrated based on the size of the tower which is specified by the design team. For a 14,600kg distillation tower with internal diameter of 800m having a thickness of 10mm with a height of 20,625mm, the tower bottom level is around 700mm according to ASME Code Section VII, Division I, 2017 edition.

Different refinery complex will have different procedures in carrying out this open loop circulation depending on the production capacity, size of equipment, automation etc. Hence, drop a brief description of the open loop circulation procedure in your plant in the comment section to enrich this content.