Explosion protected electrical heaters

In a recent incident on an offshore installation, there was an explosion in an open drains tank containing oily water. The tank contained an electric heater which was Ex certified. However, the sheath on the electric heating element had corroded, exposing the conductor. Although the incident is still being investigated, it is thought that this fault led to ignition of flammable material in the tank.

It has also been found that dutyholders do not necessarily have a complete list of such heaters, and so cannot be in a position to manage their inspection and maintenance.

Background

The contents of a drains tank are likely to be somewhat unpredictable but:

1. may be corrosive [eg due to saline water content]; and
2. may contain hydrocarbon slops, leading to a flammable atmosphere if the vapour pressure is high enough and air ingress can occur, eg through tundishes, inadequately sealed hatches, etc.

Where a flammable atmosphere may be present in the tank, its interior should be regarded as a hazardous area, and any relevant apparatus in the interior of the tank should be appropriately Ex certified and maintained.

Action Required

Dutyholders should ensure that they are aware of all Ex certified electric heaters on their installations and should ensure that the purpose of each heater is understood. Redundant heaters should be decommissioned [as this is the inherently safer option] and the risks associated with those remaining in service should be appropriately managed in order to reduce those risks to a level which is as low as is reasonably practicable.

Dutyholders should ensure that the interior of tanks has an appropriate hazardous area classification if it can contain a potentially flammable atmosphere and has any potential source of ignition.

Dutyholders should ensure that any relevant apparatus in a tank whose interior is classified as a hazardous area is appropriately Ex certified. Note that some Ex certified products have special conditions for safe use [indicated by an X on the certificate number]. Dutyholders should address any such special conditions for the safe use of Ex certified electric heaters on their installations. For example, the heated part of the element may require to remain immersed when energised; in order to guarantee this, it may be necessary to provide an automatic interlock on tank level to isolate the heater supply when liquid level falls, and appropriate interlocks for start-up and shutdown conditions, in order to prevent violation of the heater T rating and to avoid damage to the heater sheath. Also, a separate over temperature trip function may be required in addition to any temperature control function. Orientation of the heater may be an issue – a horizontally mounted heater is  more likely to remain immersed and so be (a) less likely to experience corrosion associated with conditions around the liquid surface, (b) less likely to overheat, and (c) less likely to expose any damaged section of the element to any potentially explosive atmosphere.

Dutyholders should address the routine inspection and maintenance activities of:

• The tank internals and, in particular, should check the condition of the heater element sheaths, as mechanical integrity of the sheath is fundamental to safe operation of these heaters and sheathing is likely to be part of the certification conditions.
• The electric elements and, in particular, should test the electrical condition of all elements and should poor readings be obtained then consideration should be given to remove the heater from the tank and inspect its mechanical integrity.
• The instrument protection systems, in particular, the operation of any temperature control function and any protective trip functions should be addressed. Dutyholders should note that for example BS EN60079-17:2007 Reference 1 Table 11 addresses the inspection schedule for Ex "d" apparatus, and mentions checks on automatic electrical protective devices [this could include start-up/shut-down interlocks, liquid level trip functions and high temperature trip functions], and checks on specific conditions of use [this could include the integrity of the heater sheath].

Relevant legal requirements

The Offshore Installations (Prevention of Fire and Explosion, and Emergency Response) Regulations 1995, Regulation 9(2)(d) requires measures to control electrical or other sources of ignition; and the Provision and Use of Work Equipment Regulations 1998, Regulation 12(2)(e) requires measures to control unintended explosions.

References

1 BS EN60079-17:2007 Explosive Atmospheres: Electrical installations inspection and maintenance BSI 2007 ISBN 978 0 580 55823 8

Further Information

Any queries relating to this notice should be addressed to:

OSD 3.5
Health and Safety Executive
Hazardous Installations Directorate
Offshore Division
5.N2 Redgrave Court
Merton Road
Bootle
Merseyside
L20 7HS