What is a HAZOP? HAZOP Methodology and Techniques in a Process Industry

HAZOP Methodology and Techniques - Process Industry

Introduction

The main objective of the HAZOP session is to:

1. Check that any deviation in design parameters will not cause hazards to people environment and/or assets, or company reputation.
2. Check whether the precautions incorporated in the design are sufficient to either prevent the hazard from occurring or reduce any consequence to an acceptable level.
3. Consider any safety interfaces, which exist with other installations.
4. Ensure the plant can be started up, operated, maintained, and shut down safely

HAZOP Terminologies

Abbreviation and Description

HAZOP

The application of a formal systematic critical examination to the process and engineering intentions of new or existing facilities. Potential for mal-operation or malfunction of individual items of equipment are assessed and recommendations for mitigation measures are stated.

 NODE/SYSTEM

A defined section of the process plant, general facilities, storage and handling, utilities, other services, and off-sites are to be examined

 DESIGN INTENT

The way the process and equipment are intended to work under both normal and anticipated abnormal conditions.

 HAZARD

Defined as a deviation having the potential to cause damage, illness, injury or other form of loss.

 GUIDE WORD

A departure from the design and operating intention.

 DEVIATION

A departure from the design and operating intention.

 CONSEQUENCE

A result of a deviation.

 SAFEGUARD

Physical or procedural measures are incorporated to eliminate or reduce the likelihood of a hazard occurring.

 

HAZOP Methodology

The method consists of a systematic study of all process and utility lines along with the equipment included in the project. The study is performed primarily on the P&IDs, which have been released for HAZOP Review.

The first step in applying the HAZOP study technique is to select an element of the system, i.e., a line, vessel, or procedural step. The function of this element should be described by the process design specialist in the team. For each element, the study investigates the appropriate process parameters and guidewords to develop deviations from the design intent.

The list of guidewords appropriate to the system being studied is presented by the chairman and agreed upon by the team at the start of the study. Once agreed this list is then normally

applied to all process elements. Where several systems are being reviewed, then the team may wish to tailor the list of guidewords for one or more of the systems, particularly if batch operations are involved.

For each keyword, the HAZOP team lists the possible causes and the consequences regarding the operating procedures and the safety aspects from both a personnel and material point of view.

The next step is to judge whether the engineering and safety systems as presented in the P&IDs and other engineering information can cope with the consequences of all deviations.

If the team judges that the engineering and safety systems are unlikely to cope with the consequences of all deviations, or an operability feature requires attention, then a recommendation is made. The responsible party for each identified action/recommendation is identified.

Recording of the HAZOP session is done utilizing Excel worksheets and ranking of the risk as per general Risk Matrix is performed.

 

HAZOP Study Techniques

A HAZOP study progresses through the plant node by node. The selection of the node sizes and the route through the plant is made before the study by the HAZOP Chairman.

Nodes

The division of the plant into appropriately sized nodes containing process lines and/or equipment is made by the Chairman before the study.

The guidelines to be followed are based on changing nodes at a major piece of equipment or

where a critical parameter (flow, pressure, temperature) changes.

The objective is to focus on appropriately sized sections of the plant for the team to be able to clearly identify all the design and operation issues.

The node intention defines how the process is expected to perform and it should be recorded for all plant operating cases. It is important to give the full range of operating and design conditions.

Deviations

Deviations are the key to the HAZOP process.

Potential deviations to the design intention are generated by considering a series of guidewords and combining them with the parameters of operation. In some cases, the parameter and guide word when combined make a well-understood deviation, e.g., less flow. In other cases, explanations may need to be developed for the deviation, e.g., as well as flow may more easily be described as an additional component, and part of flow arises when an element of the flow composition is missing.

Causes

There may be many causes identified for each deviation, and all potential causes should be

discussed, as the consequences and recommendations for action may be different.

All the potential causes of the deviation should be identified by the team brainstorming. The Scribe will record each separate cause as it is identified by creating new rows on the worksheet.

Generally, causes will fall into one of three categories:

• Human error
• Equipment failure
• External events.

Consequences

The role of HAZOP is to determine the net effect of the potential consequences and the mitigating effect of all the protective measures.

Having identified the credible causes of the deviation, the team members should analysis and

assess the significance of the consequences.

The consequences can be inside or outside the node being studied.

It is not considered good practice to identify a deviation therefore e.g., if a cause could result in a high pressure, the consequence should be recorded as potential overpressure resulting in vessel rupture, not high pressure.

The consequences of the deviation must be assessed and recorded without controls (safeguards).

Controls (Safeguards)

The team must identify all the existing safeguards that are available. The Team shall discuss and agree on the effectiveness of the safeguard in preventing the consequences from happening.

 
HAZOP Guidewords and Parameters

List of Guidewords

Guide Word Definition

HIGH (more of, higher)

Does more than is specified or required quantitative increase (e.g., opens a valve fully, when the procedure calls for “cracking” the valve)

LOW/NO (less of, lower)

Does less than is specified or requires quantitative decrease (e.g., purges a vessel for 5 minutes when the procedure calls for 10 minutes)

MISDIRECTED

Fluid is flowing to a path other than the design intent path

List of Deviations

LOW/NO Flow

Valve closed, wrong routing, blockage, blind flange left in, faulty non-return valve, filter blinded, burst pipe, airlock, failure of trace heating, water contamination in line freezes, flow transmitter/control valve malfunctions, pump or vessel failure, Partial blockage, vessel or valves failing, leaks, loss of pump efficiency.

HIGH Flow

Control valve sticking open, flow controller malfunction, more than 1 pump operating, reduction in delivery head required, increased suction pressure, other routes, exchanger tube leaks.

 
MISDIRECTED FLOW

High pressure downstream/low pressure upstream, wrong routing, burst pipe, valve failure, emergency venting, pump, or vessel failure.

LOW Pressure

Pressure control malfunction. RV relieves and fails to reseat. Pump-out rate exceeds the capacity of the vessel vent. The vessel vent is blocked when emptying. Cooling in the vessel allows vapor to condense. Blocked pump or compressor suction lines.

HIGH Pressure

Pressure Surge, relief, leakage from HP connection, solar radiation on blocked-in line/vessel, pressure control malfunction, level control malfunction allows high-pressure gas breakthrough, inadequate venting of air/vapor when filling.

LOW Temperature

Lower than expected ambient temperature/freezing, loss of pressure, loss of heating (including trace heating), failed exchanger tubes, temperature controller failure, pressure relief, and cooling.

HIGH Temperature

Temperature control malfunction, solar radiation, high ambient temps, fouled cooler tubes, cooling water failure, failed exchanger tubes

HIGH Level

Level control malfunction, control valve sticking shut, flow into vessel exceeds outflow capacity, flow impeded downstream.

LOW Level

Level control malfunction, control valve sticking open, drain valve passing.