Stick to the basics

Feb. 13, 2008
Getting immersed in details can doom a basic design contract to financial failure.

For three weeks I’d been trying to get the separator drawings approved. Jesus Christ and the eight disciples, as I called them, had never before had so much work flood into their small company. As a contractor hired to help shore up the team, I could see that the firm would never make money. Before the layoffs began it probably had enough staff to handle the workload. Now only eight remained and they were led by a field engineer with little patience with the design process.

Design should include quality assurance steps to ensure the deliverables meet the customer’s requirements as well as standards for good engineering practices — sometimes these conflict. An independent reviewer should check drawings and calculations for assumptions, conclusions and presentation (logic, flow and readability); then drawings should be back-checked to ensure that the reviewer’s comments make sense. For any of this to work, there must be a schedule and criteria for acceptance. These quality steps are part of every design process whether it’s detailed design or basic design.

Basic design is far more difficult to manage than detailed design. Engineers inherently want to explore problems to their natural ends. Only by sticking to a procedure can a manager hope to make money in basic design.

A basic design should begin with a block flow diagram (BFD) and a simplified material balance. Physical properties should be established for the range of pressures and temperatures required. “Don’t waste time showing all the pumps and other equipment until you have to,” John Eastman, a professor I once worked for, sensibly advised.

Work the BFD as long as you can before going on to the next step, the process flow diagrams (PFD). Remember, each block should show a unit operation such as drying or centrifuging in which material properties change; storage is the exception. A design review is imperative before going to the PFD. Look for ways to minimize the number of steps, inventories, i.e., storage, and the need for high temperature, high energy or high pressure equipment. Consider reliability from the get-go. At the conclusion of the process, the client should expect to receive everything needed for a Phase 1 basic engineering design package.

The PFD will allow development of the control strategy but don’t include loop details at this stage. Describe controls for at least each unit operation in a memo. Complete a detailed material and energy balance and update material properties.

The equipment list, motor list, instrument list, materials-of-construction drawing and tie-point drawing can be developed during this stage. Keep it simple! Without vigilance, you’ll swerve into detail engineering without realizing it.

The two final items for this level of basic design are vessel and equipment sizing. If sizing software isn’t available, this could be where a contract goes sour. This software could be spreadsheets for knock-out pots, heat exchangers, blowers, fans, pumps and relief devices. Movers such as blowers and pumps can be sized on balance information only if a basic plant layout and elevation can be established; otherwise, don’t attempt it. In fact, without such information, don’t add more than labels to the moving equipment and motor list. Line sizes will be required for sizing pumps and similar equipment: smallest is safest and cheapest!

Vessel relief nozzles pose a significant budget risk. Use a simplified fire-sizing calculation assuming a wetted area of 60%. Refer to pp. 16–18 of Carl Branan’s “Rules of Thumb for Chemical Engineers.” Then, increase the size by a pipe diameter. Leave the rest of the relief valves to the vendors. Trying to explore all the potential scenarios in basic design will quickly burn through your budget!

Develop the process description and the operating procedures memo parallel to the equipment sizing but after the first draft of the PFD is issued.

After the procedures are ready, complete the first hazard and operability (HAZOP) study. Include the client and people with operating experience. Operability too often doesn’t get adequate attention. Carefully consider how operators will start and stop the process; identify safety and quality issues and prevent reliability faults from interfering with production. Leave extra money, maybe one-third of your budget, to fix problems the HAZOP finds.

Sadly, my experience with the eight disciples was nothing new. It’s a natural temptation to over-specify. That way leads to ruin! On a job for an Indian company, my company in Pittsburgh made a similar mistake. By the time we were done there was little need for detailed engineering. Our Indian client merely changed the title blocks on the drawings and claimed them for its own. We never did get paid for all that work.

Dirk Willard, contributing editor
[email protected]

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