The Design Process: Front End Engineering Design

In the second in a series of articles aimed at giving new graduates a better understanding of the current design process, Tom Baxter takes a look at FEED

A FEED is undertaken to provide more certainty on the project outcome. Engineering definition is improved with consequent cost, safety, environmental impact, and schedule assurance.

The chemical engineer is key to providing that assurance. Numerous other disciplines are also required to support that assurance too, including piping, construction, mechanical (static and rotating), civil, structural, electrical, instrumentation, planning, safety, environment, reliability, operations, maintenance, layout, and cost.

There are differing levels of detail of FEED – from basic to enhanced – and these are usually defined by the client.

The enhanced FEED

A key document is the project execution plan (PEP). The PEP will set out what will be done, who will do it, the deliverables, the cost, and schedule for all activities.

The PEP is used by project management to track and report progress, and to identify issues. A key aspect is the management of discipline and activity interfaces. A risk register is prepared, which is a live document to inform management of deliverable risks and how they are being resolved.

The new graduate will provide activity progress reports on a regular basis.

A number of philosophies will be developed by the FEED team. A key philosophy is operations and maintenance, covering aspects such as manning. Other philosophies will include over-pressure protection, control and instrumentation, and construction.

A basis of design will be compiled providing information on fluid compositions, flowrates, interfaces with services (for example electricity and gas), environmental and site conditions, together with emissions, feedstock, and sales specifications. In addition, the design codes and standards to be followed will be stated.

A new engineer should take time to make sure they fully understand the PEP, philosophies, and design basis and seek clarification of any aspect that is not clear.

Imagine that after reading the design basis you think a significant amount of heat from the proposed chemical plant will be lost to the environment. That heat could be used to support a heat network for local households.

In my experience, the response would have been “that’s not what the client asked for, just get on with it”.

However, with the UN asking engineers to do more with less, the heat network option is a good suggestion. To meet net zero, we have to think system wide, including beyond the site perimeter.

The heat and mass balance and the process and utility flow diagrams (PFD/UFD) generated in the earlier concept selection phase will be further developed by the new graduate and others. For example, the PFD/UFD will indicate the need for heat exchangers, compressors, reactors, distillation columns etc but not the type or associated detail. Is it a shell and tube, air-cooled or plate and frame exchanger? Is the compressor screw, centrifugal or reciprocating? Are the columns packed or trayed?

Sparing provision will also be identified. This will cover very large decisions like the number of process trains to individual pump sparing and will most likely require reliability studies and the new graduate might be involved in sourcing reliability data and running the models.

Long lead items will be identified to allow for the preparation of a schedule to first product.

Process and instrumentation diagrams

An underpinning document for any chemical plant FEED will be the process and instrumentation diagrams (P&ID).

The P&ID is developed by the chemical engineering group with input from a variety of disciplines. The key disciplines involved are shown in Figure 2.

The P&ID shows the process flow and interconnection of process equipment. It will also show information regarding the equipment size, design temperatures and pressures, and capacity and sparing.

P&IDs are also used for safety and environmental assessments.

The P&ID is built up in layers. Starting with the basic functionality, the control system and the other layers are then added – trips and alarms, emergency shutdown valves etc.

Figures 3a through to 3e show a first pass at the development of a P&ID for a three-phase separator.

The function (see Figure 3a) is three-phase gravity separation – gas and two immiscible liquids of different densities.