Perspectives: Plant InSites

Ponder the Process Design Basis

Deficiencies in the document can undermine the success of a project

By Andrew Sloley, contributing editor

For modifications or new plants, two initial documents set important parameters for the engineering team: the process design basis (PDB) and the basic engineering data document (BEDD). The PDB focuses on the specifics of a particular revamp or new plant. In contrast, the BEDD is more concerned with site-specific parameters such as climatic and seismic conditions, preferred vendors and general engineering practices, and normally is reused many times with different projects. (For more on the BEDD, see: “Rest Easy with a Good BEDD.") The contents of these documents vary in situations and among different plants and companies.

The two may overlap. Because the PDB is specific, when the documents overlap or conflict, it generally takes priority. However, it’s best if the PDB explicitly states that. Regardless, engineers working on a project always should point out disagreements between the documents and ensure all stakeholders recognize the conflicts and agree on their resolution.

A typical PDB pulls together information on a variety of project parameters (Figure 1). Each of these may have multiple subsections. Depending upon the specific job, the subsections may move around. For example, a plant modification to improve safety will treat the safety enhancement as the objective. In another case, say, when the goal is to increase capacity, the safety section may appear within constraints.

Let’s look at what the parts of a PDB should cover:

Objective. What should the plant modification achieve? Is the goal increased safety, higher capacity, changed products, greater energy efficiency, improved reliability, lower pollution or something else? The objective should be clearly defined. For projects with multiple aims, this section should spell out the relative priority of each as well as methods for evaluating tradeoffs among them.

Inputs. What goes into the plant? What is the feed? How does it vary? Often overlooked is that various feeds may have different properties or that feed rate, conditions and compositions may change over time.

Outputs. What does the plant make? What are the product specifications? What ranges of specifications might be required at different times? This section should include specifications that can be quantified and more-general requirements such as odor, haziness, taste, etc.

Constraints. What limits the plant today? What constraints are imposed by regulation, capital availability, schedule or other factors that the engineering team can’t modify? These may include restrictions on the size of the unit, availability of utilities, type of cooling and other general process requirements.

Boundaries. What are the limits on temperature and pressure of all streams entering and leaving the unit?

General Configuration and Special Requirements. Must the engineering team stick with a particular process choice? For example, must a specific option —such as pressure swing absorption versus membranes for hydrogen purification — be used or avoided? What level of reliability is required, e.g., how many days a year must the plant be available or what production target must be met?

Uncertainty. What is unknown or might vary? What risks require mitigation? How are the risks to be evaluated? PDB documents frequently fail to address these issues. Most often, no information is given on what the uncertainties are or how to handle risk evaluation. Risk here specifically refers to the process risk of not meeting a production or economic target. Defined regulatory criteria usually cover environmental and safety risks, so those risks are best thought of as constraints on the engineering team.

The most common approach to uncertainty is to assign a set of ranges to specific plant objectives and limits. The engineering team then checks the capability of the future plant to handle all the possible combinations of results. This approach attempts to ensure that all objectives can be met across an entire operating envelope. The benefit is a highly flexible plant. The cost is that meeting certain extreme cases may require large amounts of capital. A better way to use capital may be to sacrifice some capability, capacity for example, to handle extreme, but rare, situations.

Available tools can provide better insight into economic risk. However, because of tight schedules, they rarely are used. (Not surprisingly, projects with overly shortened schedules often suffer from poor capital allocation.)

Chief among these tools to reduce economic risk are Monte Carlo analysis and carefully conducted test runs. Monte Carlo analysis applies statistical distributions to possible values of selected inputs, outputs and constraints. Multiple evaluations are run using a sample of statistical values for the selected factors. On generally well-understood processes, the analysis can provide impressive insight on how to save capital or reduce economic risk.

Thorough and accurate test runs yield detailed understanding of plant constraints and capabilities. But such runs rarely are done. What most plants call a test run involves gathering barely more than usual operating data plus a few extra process measurements and samples. High-quality test runs include detailed data analysis to provide for independent confirmation of every measurement that affects the likely investment decisions.

The Monte Carlo analysis enables evaluating uncertainty. The plant test run provides information on the constraints of the process. Both are key parts of defining how to handle economic risk in the PDB.



ANDREW SLOLEY is a Chemical Processing contributing editor. You can e-mail him at ASloley@putman.net

More from this perspective...

Title

Plant InSites: Look Beyond the Lore

Don’t rely on recollections about how a unit had performed

10/10/2008

Ponder the Process Design Basis

Deficiencies in the document can undermine the success of a project

08/27/2014

Preheater points out the value of cooling off

Lack of attention to detail in conceptual design can hide many sins. Put the problem aside and then ponder it again. A fresh look may lead to an even better solution.

05/12/2004

Process Engineering: Inspect with your mind, not just your eyes

A thorough inspection of plant equipment following tiny installations can prevent a lot of future problems. Go beyond just checking for compliance with drawings when looking at hardware and physically look at the equipment itself.

03/11/2005

Process Engineering: Poor compressor design puts pressure on pumps

When a poor compressor design put too much pressure on the pumps, a new solution had to be constructed.  Sometimes the ideal solution is just an intricate compromise.

01/18/2005

Properly Assess Energy Recovery Projects

Impact on other operations and transfer prices may alter the economics.

03/09/2011

Properly Position Interlock Valves

The obvious location may not be the most cost effective

10/16/2012

Properly protect centrifugal pumps

Consider various factors when selecting how to guard against low flow, Andrew Sloley says in this month's Plant Insites column.

07/11/2007

Properly Trigger Standby Pumps

A pressure signal works for many but not all situations.

05/06/2010

Put V-cone Meters on Your Short List

06/10/2013

Reliability and Maintenance: Don’t Slight Strainers

These simple but often essential devices deserve adequate attention

05/08/2014

Rethink Reactor Temperature Control

Cascade strategy offers simplicity and fast response.

07/13/2010

Right Vacuum Control Choice Takes the Pressure Off

Choosing the proper recycle stream for regulation is crucial

08/07/2008

Save a bundle solving pressure-drop problems

Pressure drop in compressor-suction and interstage coolers often creates problems. In some cases, just a few pounds of extra pressure drop make a revamp unworkable.

11/04/2004

Select the Right Reboiler

Understand the strengths and weaknesses of various options.

06/04/2012

Shell-and-Tube Heat Exchanger

Allocating fluids in a tubular exchanger demands care.

10/23/2013

Sidestep side-draw control surprises

The simplest approach to control side-draw distillation columns uses flow control on the side product. This solution, however, is not ideal for all side-draw control situations.

07/01/2004

Software: Show Some Skepticism

Calculated results require critical analysis.

06/14/2010

Spot problems with adsorbents

The longer-than-expected life of an adsorbent points up the need to always assess the consequences of system additions. While sometimes this may involve detailed calculations, simply looking to the laws of physics can eliminate potential headaches.

08/14/2006

Steam Systems: Simple Solutions Can Prompt Complex Problems

Steam systems are especially susceptible to developing difficulties

12/23/2008