Our industry uses the term “data sheet” to refer to a description of what equipment is supposed to do. Yet, such sheets often list specifications or estimates rather than hard data. Nevertheless, the term “data sheet” leads many engineers to automatically assume the numbers on it accurately represent what the equipment should do. It’s difficult to avoid jumping to this conclusion even after years of experience — indeed, some people never learn to question entries. Unfortunately, this leads to confusion and errors that can undermine equipment troubleshooting and plant modifications.
A more-correct term is “specification sheet” because that conveys the idea that some doubt exists about equipment performance. Regardless of what the sheet is called, it’s crucial not to blindly accept what appears. You must evaluate the nature and accuracy of the numbers listed.
Let’s start by defining what actually constitutes data. They are values resulting from direct measurement or counting. A temperature reading taken from a thermometer is a datum. If you count the tubes in a shell-and-tube heat exchanger, that number is a datum.
If that heat exchanger had thousands of tubes, you might not want to count them one-by-one. Instead, you might weigh all the tubes and then calculate the number of tubes by dividing that value by the average weight of an individual tube. However, the result is not a datum because it isn’t a direct measurement. (The datum is the overall weight of tubes.) Instead, your calculation provides an estimate; some calculations may be very precise, others not so much.
Figure 1 shows part of a specification sheet for a shell-and-tube heat exchanger. The tube count is a datum. However, the duty isn’t a datum but the result of a calculation. Actual exchanger performance will vary.
When troubleshooting or modifying a plant, far too many conversations include the remark: “The data sheet shows the heat exchanger will do so many BTUs per hour.” You can substitute tower capacity, pump head, filtration rate, NOX release and lots of other parameters for equipment performance. However, unless the equipment has been put through a test, the performance values shown on the data sheet only are estimates. Modern pumps, compressors and burners tend to have reasonably good estimates. Heat exchangers, distillation equipment, separators, filters, centrifuges and many other types of equipment often perform much differently than “expected.”
So why does performance vary from that listed on the specification sheet? Figure 1 highlights some issues:
Age. The sheet was filled out in 1977. We do have better methods for estimating exchanger performance today — especially for complex systems with phase changes.
Guesstimates. At the specification stage, estimating the fouling factor is guesswork for many heat exchangers. For troubleshooting, you must know the real fouling factor; it may be much higher or lower.
Other potential issues are less apparent:
Fitting the bid. Many equipment items only come in standard models. So sometimes, to minimize risk of losing the job, a vendor will make the specification match what’s expected rather than what the standard equipment really delivers.
Maintenance. The equipment now may differ from what was delivered. Wear, use of non-OEM parts and cumulative “insignificant changes” may markedly affect equipment operation.
Safety margins. Some calculations have enough uncertainty that engineers, vendors and operators all may add safety margins. This may result in equipment that’s far from a best fit to the process requirements. Vastly over-sized equipment may operate completely differently than expected.
Many other factors may come into play as well. In any case, never forget that “data sheets” often show numerous things that aren’t data. Don’t fall into the trap of thinking that equipment operation won’t deviate much from what’s on paper.
ANDREW SLOLEY is a Chemical Processing contributing editor. You can email him at ASloley@putman.net.