The Cost of Liquid Flow Errors
Is a $25 Million Measurement Worth an Extra $600 Investment?   
David W. Spitzer,  P.E.

Last month we considered the economic value of steam flow measurements. Liquid flows are typically more common, so let’s take a look at an example of their economic impact here. 
Assuming the average liquid flowrate is one kg/min valued at $1.00 per liter operating all year, the flowmeter would pass approximately $525,600 of liquid per year (1 liter/min * 60 min/hr * 8760 hrs/yr * $1.00/liter). 
Think about this for a minute … a small flow of a reasonably priced liquid results in over $500,000 of liquid passing through the flowmeter each year. In custody-transfer applications, purchasing a flowmeter that performs 1 percent better can reduce the measurement error by $5000 per year. This value is much larger in many custody-transfer applications, so reducing the magnitude of flow measurement error can be used to justify better (and more costly) flow measurement devices. 
While justification for improved (and more costly) flow measurement devices is relatively easy in custody-transfer applications, how many times have you had to fight (tooth and nail) to purchase a more expensive flowmeter for process applications? Did you ever calculate the value of the material passing through the flowmeter? You might be surprised with the results. 
For example, a flowmeter operating year round at 100 liters per minute will pass over $25 million of liquid per year if the liquid is valued at $0.50 per liter. This may not be economically important in many process applications, such as recycle, cooling, or spray flows where the flow need only be higher than a minimum value. However, it can be of extreme importance in other process applications, such as reactor feeds, where addition of the proper amounts of reactants can drastically affect chemical reactions, process yield, and the economics of the operation. 
The majority of process flow measurements typically fall between these two extremes and can exhibit a detrimental effect on process economics. For example, the flowmeter detailed here might be the slave process variable in a cascade level control. There could be a discussion (fight?) as to whether to buy a flowmeter for $2700 or a superior flowmeter for $3300. Put in another perspective — should one spend an additional $600 to better measure $25 million of liquid per year? 
You decide.


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