# Onsite Arithmetic: Pump Math Part 3 – Answers

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Here are the answers for today's practice problem

1. If the pump tank is 49 inches x 60 inches x 70 inches with no check valve, calculate the gallons per inch.
Convert inches to feet and calculate gallons per inch by: Area x 1 foot x 7.5 gallons/cubic feet ÷ 12 inches per foot

So 49 inches ÷ 12 inches/foot = 4.1 feet and 60 inches ÷ 12 inches/foot = 5 feet

4.1 feet x 5 feet x 1 foot x 7.5 gallons/cubic feet ÷ 12 inches/foot = 12.8 gallons/inch

2. There are 80 feet of 2-inch Schedule 40 PVC supply pipe with no check valve. If the “off” float is at 62 inches from the surface reference point and the “on” float is 57 inches from the reference point, what is the volume delivered to the system?
The difference in floats is 5 inches. So 5 inches x 12.8 gallons/inch = 64 gallons pumped, but there is no check valve so you need to determine the volume of 2-inch Schedule 40 PVC pipe and the volume of drainback. You find the volume of the pipe in a table that can be obtained from the pipe supplier or provided in tables provided in education programs. I know that the volume of 2-inch Schedule 40 PVC pipe is 0.174 gallons per foot, so the drainback will be 80 feet x 0.174 gallons/foot = 13.92 gallons rounded to 14 gallons. The actual volume delivered to the system is 64 gallons – 14 gallons = 50 gallons.

3. If the pump runs for 77 seconds, what is the flow rate?
If the pump runs 77 seconds and pumps 64 gallons, the rate is 77 seconds ÷ 60 seconds/minute = 1.28 minutes and 64 gallons ÷ 1.28 minutes = 50 gallons/minute.

4. If a cycle counter reads 46,490 today and 45,850 120 days ago, how many cycles did the pump run?
The difference between 46,490 - 45,850 = 640 cycles.

5. Calculate the total flow and the average flow over the 120 days.
Total flow delivered to the system is 640 cycles x 50 gallons – remember the drainback! = 32,000 gallons (total flow) ÷ 120 days = 267 gallons (average).

6. If the design flow is 300 gallons per day, what does this tell you?
The actual flow is below the design flow but it is close enough to design flow that it would indicate at times the design flow is being exceeded. The rule of thumb I use regarding flows is when the average flow is more than 70 percent of design flow when measured on a monthly (or longer) basis, the system bears close monitoring. Here 267 ÷ 300 x 100 = 88 percent – well above the threshold to be concerned.

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