Quiz Corner: Chemical cost savings

Flowmeter A and Flowmeter B with accuracies of ±0.75% and ±0.50% of the actual flow rate, respectively, are appropriate to measure a flow of 1,000 cc/min of a chemical purchased for USD 2.00 per liter in a process that operates continuously 24 hours per day, 7 days a week. Flowing below 1,000 cc/min fails to adequately protect equipment. Flow above 1,000 cc/min provides no additional benefit and is wasted. Approximately how much money can be saved by purchasing a flowmeter with 0.50% accuracy instead of another flowmeter with 0.75% accuracy?

1. USD 1,000
2. USD 1,500
3. USD 2,000
4. USD 2,500
5. USD 3,000

By observation, the accuracy difference between these flowmeters is 0.25%, which is relatively small. The Flowmeter A and Flowmeter B controller setpoints should be set at 1,007.5 and 1,005.0 cc/min, respectively, to ensure that the flow does not fall below 1,000 cc/min. The potential amount of chemical saved is the difference between the setpoints, which is similarly small at 2.5 cc/min.

However, there are 525,600 minutes in a year, so the potential chemical savings are 1,314 liters per year. At a chemical cost of USD 2.00 per liter, the savings amount to USD 2,628 per year. This is closest to Answer D.   

Additional complicating factors

Knowing that saving 1% of chemicals flowing at 1,000 cc/min saves 5,256 liters (approximately 5,000 liters) per year, one can scale the flow rate and chemical cost to estimate potential cost savings. In addition, downtime tends to reduce potential savings.