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Eliminating downtime caused by lightning strikes

17 February 2015

Control Engineering Europe looks at the journey taken by Tüpras Kirikkale petroleum refinery to ensure its plant is better protected from downtime due to lightning stikes in the future.

Tüpras Kirikkale petroleum refinery was established in 1986. One of the plants within the refinery contains a diesel desulpherisation  (DHP) and continuous catalyst regeneration (CCR) unit.

Since commissioning, in 2007, this plant has suffered a series of electrical trips, caused by lighting strikes in the area.

To help overcome the problem the company asked experts from Yokogawa Italy to help to identify the root cause of the plant shutdowns. Following a site survey in 2013, Yokogawa collected information and measurement data to prepare a report for the company. 

The report concluded that the safety system had performed a safety action because the same event had occured in in many areas of the plant – this consisted of an overvoltage on an input or output channel coming from external devices or cabling, due to lightning strikes.

Analysis of the data did not identify whether the channel overvoltage was caused by equipment installed internally in the system or by marshalling cabinets. This kind of overvoltage is considered to be a major failure of the safety input/output loop, causing the system to drive the plant in a safe conditon (shutdown on emergency).

Mitigating overvoltage
Overvoltage coming from external devices and cabling can be mitigated by an appropriate grounding system which has a specific focus on the grounding of the field devices and the shield of the multicore cable. Grounding parameters can be measured by specific tools and instruments. However, Yokogawa recommended that checks be performed during a period of planned shutdown. 

Yokogawa also advised that a surge protector should be installed at the input/output channel level (usually this is installed into the marshalling cabinets) in order to prevent the overvoltage from damaging the input/output module or being detected as a dangerous failure causing the system to trip units or plant. 

Finally, Yokogawa suggested an improvement to field instrument grounding in the plant, through the use of dedicated wiring instead of mechanical connection to piping.

The installation of a surge protector at the input/ouput channel (1,309 units) has not yet been actioned by the company, owing to the long shutdown period that this would require. However, the plant was shut down for five days early in 2014 to allow for a system check and to undertake improvements to field grounding connections for plant instrumentation. 

At the same time that the company originally spoke with Yokogawa, it also invited experts from Shell Global Solution International  (SGSI) to further investigate the root causes of the problems that were being caused by the lightning strikes.  

A site inspection carried out by an SGSI EMC team concluded that the earthing layout of the plant and the control building consists of multiple earth rings and star point arrangements for instrument earthing, whereas the recommended layout for lightning protection, in accordance with IEC, is a meshed earth grid.
Such a meshed earth grid can easily be installed with the existing structures inside the control building. The company is, therefore, set to install additional earth connections between internal earth ring of the rear control room the control building and the plant.

It was further identified that the cable armouring of instrument cables is only bonded at the field end and is not used as a perfect electric conductor (PEC) to protect the wire pairs from lightning induced effects. Also, the cable trenches do not have any PEC or other protective conductors anywhere along their length. To offer lightning protection, the cable armouring and PEC need to be bonded at the protection zone interface, which in this application is the perimeter of the building. 

A lack of bonding of steel structures and only a small number of lightning current paths to earth, results in inadequate direct lightning protection of tall structures and results in an asymmetrical current distribution. The lightning protection of these structures needs to be improved by earthing and bonding.

The problem encountered by Tüpras Kirikkale is typical in oil and gas facilities in lightning prone areas. By focusing on rectification measures in line with IEC requirements it is possible to resolve these problems without the application of surge arrestors. 

Recommendations
As a first measure, it was recommended that the earthing arrangement of the control building be brought in line with the recommended layout by IEC which galvanised steel cable trays and cavity floor supports being used as an earth mesh by implementation of the following measures:

• Installation of an earth ring in the cavity floor of the equipment room/rear control room, consisting of an earth bar or earth wire.
• Cable trays are to be bonded to the earth ring with a typical spacing of 5m.
• The cavity floor supporting structure is to be bonded to the earth ring with a typical spacing of 5m.
•The protective earthing (PE) bars of the instrument panels are to be bonded to the cable tray below the panels.
• The cable trays in the control room are to be bonded to the equipment room earth ring.
• Additional earth connections are to be installed between the building external earth ring and the internal earth grid.
• Verify the combined earthing drawing of the control building and substation and, where necessary, install earth connections between the earth rings of the control building and the substation.

Another measure to be undertaken is the interconnection of the four earth bars inside the equipment room with short earth wires 70mm2, and bonding of the earth wires of the earthing electrodes where they enter the equipment room with the earth ring. Further, bonding the earth wires from the substation to the earth ring, where they enter the equipment room was advised.

Bonding the steel wire armouring of the incoming instrument cables to the meshed earth grid of the equipment room was undertaken by cutting away the sheathing and installing a spring clip around the steel wire armouring. 

Bonding of the cables is applicable to all instrument systems – starting with the 150-200 cables of the electrostatic discharge (ESD) system. It is important to verify the connections of the cable shields and wire pair shields and verify the correct bonding of glands of multicore cables and instrument cables of junction boxes. The threaded glands are directly installed in the junction boxes without additional rings and nuts.

Improving the lightning protection of the tall lightning exposed structures can be achieved by bonding of exhausts across flexible joints and by earthing the steel structure at multiple points around the module.

Conclusion
The earthing and lightning protection modification work was finalised by Tüpras Refinery Kirikkale during a shutdown period early in 2014. The work was then subjected to an inspection by a combined team of Tüpras, Yokogawa Turkey/Europe and Shell. From the inspection results, the following conclusions were drawn: 
• The minimum required earthing and lightning protection measures, consisting of a meshed earthing layout and bonding of incoming field cables are now installed. These measures comply with the Lightning Protection Zoning (LPZ) concept as described in the IEC standards.
• PEC’s – one per trench –were installed in the cable trenches between the process areas and the control building.
• The earthing and lightning protection of exposed structures in the plant have been improved. The star point earthing arrangement of the instrument systems inside the control building has been simplified without ‘daisy chaining’ of earth connections and is now in accordance with Yokogawa’s recommended practice and also with the recommended earthing and bonding of instrument cables as per DEP 33.64.10.33-Gen.
• The unarmoured instrument cables between the co-located substation and the control building present a potential coupling path for lightning induced currents. However, as additional PEC’s were installed between the buildings, this risk is expected to be adequately mitigated.
 
Tüpras facilities in other locations, with installations similar to that found at Tüpras Kirikkale, may also be similarly at risk of disturbance due to lack of EMC measures, even if the lightning frequency risk is lower and it is expected that similar retrospective actions may be necessary at these locations in the future. 


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