Onsite Arithmetic: More O&M Practice – Answers

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Here are the answers to practice problem No. 6

1. If you are evaluating a pump tank and pump system and you measure the dimensions to be 49 inches wide by 68 inches long by 62 inches deep, calculate the gallons per inch.

Answer: There are a couple of ways to calculate the gallons per inch, but the easiest way is to take the length times width in feet, times 1 foot of tank depth to get the gallons and divide by 12 inches to get the answer in gallons per inch.

So to convert 68 inches and 49 inches to feet;

68 in ÷ 12 in/ft = 68 in ÷ 12 in/ft = 5.67 ft

49 in ÷ 12 in/ft = 4.1 ft

L x W x 1 ft = 5.67 ft x 4.1 ft x 1 ft = 23.25 ft3

7.5 gal/cu ft x 23.25 ft3 = 174 gallons ÷ 12 in= 14.5 gal/in

2.  If the pump runs for three minutes and the efﬂuent levels change from 66 inches to 73.5 inches, what is the pumping rate for the system?

Answer: The level of sewage on the tank goes down 73.5 inches – 66 inches = 7.5 inches in 3 minutes

14.5 gal/in x 7.5 in = 108.75 gal in 3 minutes; so 108.75 gal ÷ 3 min = 36.25 gal/min for a pumping rate.

3.  If the system has a check valve and the required dose volume is 75 gallons, how long should the timer be set for the “on” setting?

Answer: Since there is a check valve in this system, to find the run time for 75 gallons we simply divide the required dose by the pumping rate. If there was not a check valve in the system we would have to add the gallons of drainback in the supply pipe to 75 gallons to determine how much needs to be pumped.

75 gal x 36.25 gal/min = 2.1 min for the on time

4.  If the number of cycles required for the system was six times per day, for how long should the “off” time be set?

Answer: If there are 6 cycles for the day, 24 hours ÷ 6 = 4 hours for one cycle. If the on time is 2.1 minutes the off time should be 3 hours 57.9 minutes or converting the hours to minutes results in a 237.9 minutes for the off time.

5.  If the timer was set to pump six times a day and the cycle counter read 3,442 today and read 2,899 on the last visit 120 days ago is this system operating properly?

Answer: To see if the pump was turned on often enough over the 120-day period, take the difference between the current reading and the previous reading and compare this number to 6 cycles/day x 120 days = 720 cycles

The difference is 3,442 - 2,899 = 543 cycles; so the pump according to the cycle counter has not operated as often as it should. The pump delivery rate and the timer settings should be checked and changed as necessary.

6.  The elapsed time meter (ETM) reads 215.8 hours today. On the last visit 120 days ago, the timer read 192.6 hours. What is the total time recorded in minutes?

Answer: To find the total run time from the elapsed time meter, subtract the previous hour reading from the current hour reading.

215.8 – 192.6 = 23.2 hours x 60 min/hr = 1,392 minutes of run time

7.  What is the total ﬂow and average ﬂow for the 120 days?

Answer: To get the total flow multiply the run time by the pump delivery rate.

1,392 min x 36.25 gal/ min = 50,460 gallons over the 20 day period for an average daily total of 50,460 gal ÷ 120 days = 420.5 gal/day. This is somewhat less than the expected average of 450 gallons per day obtained by 6 cycles/day x 75 gallons = 450 gal/day

So this bears watching to see if the pump rate needs to be evaluated and the timer reset. If the system is not keeping up with the flow the settings will need to be changed or it is an indicator the pump is wearing out and needs replacement.

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