01 March 2003
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Processors all look to reduce environmental impact and save money.
One way is to minimize the pressure losses through flowmeters, which lowers associated pumping costs.
Many flow technologies, in the course of measuring a flow rate, cause a permanent pressure loss (PPL). This PPL can range from 0 to greater than 20 pounds per square inch (psi).
If a variable speed pump or compressor provides the fluid pressure, then any reduction in this PPL will provide direct energy savings.
When measuring steam flow, PPL energy converts to superheat—assuming insulated lines, which reduce the efficiency of heat transfer.
Finally, in some applications, restricting the flow may limit production rate.
To calculate the PPL for a specific flowmeter, use the equation:
where is density, V is velocity, and Kmeter is the meter factor—a correction constant that applies to a meter's indicated value in order to compensate for variations in ambient conditions.
For proprietary flowmeters, the vendor's sizing program typically calculates the Kmeter.
In many applications, the flowmeter is smaller than the pipe size. In these cases, the additional PPL through the pipe reduction and expansion, as well as any required upstream and downstream piping, incorporate into Kpiping.
This spreadsheet calculates PPL for nonproprietary flowmeters and energy costs for any flowmeter.
Energy cost is, of course, only one of many factors that engineers consider when selecting a flowmeter.
That said, the best practice for minimizing energy cost is to either utilize magnetic flowmeters, which boast 0 PPL, when metering conductive liquids or to leverage averaging pitot tubes for gas and steam flows as well as for larger lines.
The savings of averaging pitot tubes vs. numerous technologies are clear in the spreadsheet results.
To access this working spreadsheet, go to www.isa.org/intech/energycalc. A dialog box mentioning Macros will appear. Click on Yes. Click on Tools, then Protection, then Unprotect Sheet.
Then, in the spreadsheet, click the Change Conditions box, and then click Yes to open the spreadsheet in a separate window. Now click Change Conditions again, and the Process Information box will appear.
Use this box to change any and all parameters and to calculate and compare dollar figures.
—Mark Menezes (Mark.Menezes@EmersonProcess.com), Emerson Process Management
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