My lovely wife of 32 years tells me I am a typical male when it comes to driving: I never look at a map and never ask for directions. She's probably right. But when I get into a client's powerhouse or factory at the start of a new engineering project, the first thing I ask for is a flowsheet of the process I am to work on. You might call this my "map" of the process; so I do ask for directions occasionally.
In my experience, flowsheets are critical to maintain for many reasons:
- They depict the relationship between all of the equipment and controls involved in a given process.
- They are the basis of heat and/or material balances, which, in turn, establish pipe sizes and control logic.
- In emergencies, they tell operators the location of valves and controls so problem equipment or pipes can be isolated.
- For equipment maintenance, they are the basis for "lock-out, tag-out" procedures to safely isolate equipment and/or controls to be worked on.
- They are the basic building block in the training of operators, the passing on of system information to individuals unfamiliar with a process, and the relation of all of the parts of a systemfrom equipment to controls to piping.
A common thread running though all five of these reasons for maintaining up-to-date flowsheets is the word "control." I do not know how I can locate a control device, determine control logic, calculate a control value, determine a control calibration, or even specify a control device that is not shown or is presented incorrectly on a flowsheet.
Over the last three or four years, about half of the powerhouses my firm worked in were using out-of-date flow diagrams. Even a few complete manufacturing plants had outdated piping schematics for the overall facility. When we start an engineering project at such facilities, we end up spending days--even weeks--getting the documents up-to-date.
Two short examples illustrate this point:
* In a powerhouse we are working in, a number of air compressors tie in to a common piping header. Over time, the powerhouse evolved, with new equipment added, but no one went back and ran a mass balance of the piping system. Now we are looking at a major and costly modification because the pressure drops and flow distribution are out of balance, energy is being wasted, and controls are not calibrated correctly.
* Although processes had changed and compressed-air, steam, and natural-gas pipes had been cut and moved, the piping schematics of a 5-million-sq-ft plant had not been updated for 18 years. When plant management was getting ready to install a new process, it did not know where to find all of the needed mechanical utility services.
I am reminded of the old TV commercial in which a mechanic says, "You can pay me now or pay me later," while holding a car's oil filter. Although keeping flowsheets up-to-date requires constant effort, neglecting them may cost thousands of dollars.
A member of HPAC Engineering's Editorial Advisory Board, Glenn M. Showers, PE, is president of Boiler and Burner Systems, specializing in industrial utility systems, with an emphasis on combustion safety and efficiency. He can be contacted at firstname.lastname@example.org.