Safety Moment #15: Important HAZOP Guidewords

HAZOP Guidewords


One of the philosophies that lies behind Process Safety Management (PSM) is that each chemical process is unique. Therefore it is not possible to have a prescriptive standard that tells operating companies what to do. Instead, companies have to identify the unique hazards associated with their facility, and then implement corrective actions based on a risk-ranking methodology. For this reason, facilities covered by PSM standards have to conduct a series of Process Hazards Analyses (PHAs), often using the Hazard and Operability (HAZOP) methodology.


Yet, in spite of the differences between facilities, many of the hazards that they face have much in common. Consequently, the discussions to do with various HAZOP guidewords share many features. Discussed here are the HAZOP guide phrases ‘High Flow’, ‘Low / No Flow’, ‘Reverse Flow’ and ‘Misdirected Flow’.

High Flow

Generally, the phenomenon of ‘High Flow’ — in and of itself — is not inherently hazardous. Indeed, high flow rates are often desired because they imply that the facility is maximizing production and revenues. Although high flow can occasionally create hazards, such as erosion of pipe walls or of a valve seat, its main effect in terms of process safety is to create secondary deviations such as ‘High Level’ in a tank. ‘High Flow’ can also lead to a ‘No Flow’ situation; for example, if a pump overspeeds, the sudden surge in motor amperage may result in the motor burning out leading to the flow stopping.

Low / No Flow

As with ‘High Flow’, the phenomenon of ‘Low Flow’ is not usually, in and of itself, hazardous. However, ‘Low Flow’ can create secondary effects. For example a low flow of cooling water in a heat exchanger can lead to ‘High Temperature’ of the process stream. ‘No Flow’ is usually more serious than ‘Low Flow’ because its occurrence implies a sudden cessation of a processing activity. Probably the biggest hazard associated with ‘No Flow’ is the possibility of it being followed by ‘Reverse Flow’ because the upstream and downstream pressures have equalized, or even reversed.

Both ‘Low Flow’ and ‘No Flow’ are usually caused by the inadvertent closing of a valve or the failure of rotating equipment such as pumps and compressors. Because such events occur quite frequently, most facilities have plenty of instrumentation and safeguards to respond to this scenario.

Reverse Flow

‘Reverse Flow’ can create high-risk hazards because it can lead to the mixing of incompatible chemicals or to the introduction of corrosive chemicals into equipment not designed for them. The causes of ‘Reverse Flow’ are usually a pressure reversal in which a high pressure section of the process loses pressure; process fluids then flow into that section back from low pressure sections of the process. (The occurrence of reverse flow almost invariably implies that a check valve and/or safety instrumented system has failed to prevent the event.)

The sketch shows a process consisting of three sections: A, B and C. The chemicals in Sections A and B are non-corrosive, so these two sections can be safely made of carbon steel. When the two chemicals are mixed in Section C they react to form a corrosive product, hence this section has to be made of stainless steel. If a reverse flow should occur from Section C to either A or B, then those sections would corrode, leading to loss of containment.

HAZOP Reverse Flow

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