Liquid metal embrittlement by mercury in the refining and allied process industries

Health and Safety Executive - Safety alert

Introduction:

This safety notice describes the potential integrity hazards relating to the presence of mercury in crude oil, the refining process and some downstream industries. The element is increasingly present in crude oil feedstock, and contaminates parts of the process in varying degrees. Some metallurgies are known to be vulnerable to cracking when exposed to mercury, so precautions need to be taken to ensure that the processing plant is not at risk.

Background:

Liquid Metal Embrittlement (LME) is defined in API 571¹ as:

'…a form of cracking that results when certain molten metals come in contact with specific alloys. Cracking can be very sudden and brittle in nature.'

At temperatures above -38 °C, mercury is a molten metal. It is known to contaminate crude oil to various degrees. The element is also known to be responsible for causing brittle cracking in some alloying systems, most notably aluminium and copper, but also in Monel/Alloy 400. The interaction of one metal with another is termed a 'couple'. A recent failure² of ASTM A353 9% Nickel steel in the presence of mercury would indicate that there are still some unknown mercury LME couples.

The presence of mercury in crude oil supplies is increasing. As a result the element may well be progressing through the refining process further than before, and potentially with increased concentrations. The element is known to have an affinity with lighter hydrocarbons such as propane and butane, so can be present in increased concentrations in these streams. Mercury is also known to be present in produced condensates and light oils.

Given the possible increased presence of mercury, and the potential for unknown LME couples, operators of COMAH establishments should take all measures necessary to ensure that the mercury does not pose a threat to the integrity of their plant. Based on a hierarchal approach, this may be achieved by measures to remove mercury to a safe level. Where this cannot be achieved, duty holders should ensure that integrity is ensured through selection of materials of construction that are not susceptible to LME by mercury.

Action required:

COMAH operators of equipment that may encounter elemental mercury should ensure that they take suitable and sufficient precautions against liquid metal embrittlement that this element may cause. Steps should be taken, based on risk assessment; to reduce unacceptable risks arising from mercury induced Liquid Metal Embrittlement. Steps could include treatment of process streams to remove mercury, and selection of suitable materials of construction.

Note: this Safety Notice is purely concerned with the mechanical integrity issues for process plant and does not consider the occupational health aspects of exposure to mercury liquid or vapour.

Relevant legal provisions:

• Health and Safety at Work etc Act 1974
• Control of Major Accident Hazards Regulations 1999

References:

1. American Petroleum Institute, API 571 Damage Mechanisms Affecting Fixed Equipment in the Refining Industry, 2nd edition 2011, Product Number C57102. Back to reference of footnote 1
2. 'Environmentally Assisted Cracking of Nickel Steels in Liquid Mercury, Hydrogen and Methanol', Dale McIntyre, NACE 2013 Conference paper, ISBN 02701 2013 CP. Back to reference of footnote 2

Further information:

Hazardous Installations Directorate
Chemical, Explosive and Microbiological Hazards Division Unit 1
(Mechanical Engineering)
Health & Safety Executive,
BP6301 Alnwick House,
Benton Park View,
Newcastle upon Tyne
NE98 1YX

General note:

Please pass this information to a colleague who may have this Product/ Equipment or operate this type of system/process.