Sump Pit Design

Wouldn't it be simpler and more effective to replace the pumps with those of similar specifications? It would certainly save you a lot of headache working out all the calculations.

Problem is the original construction was 40 years ago and the pumps have been changed in the past so no verification can be done to show that they were replaced properly. Also with the pipes being so old some assumptions need to be made for the friction loss of the old pipes. They want to use this building for many years to come and do not want to worry about anything with the pumps. With As Built plans we know how the piping runs and with field test we know the discharge locations so the distances are know.

How complete are the As-Builts? Do they show the original pump specs?

What is the size (HP, flow, head, and discharge pipe) of current pumps?

My first instinct is start with one of these numbers for a starting point.

If you want to grind through Hazen-Williams calculations, the roughness factor C for new pipe is 130, for used cast iron 100. Friction factor is an empirical formula with a number of odd powers to calculate.

Yeah, what Reid said - as-builts plus diddle factor. You could confirm that by differential pressure measurement at the pump outlet and at the discharge point. Use delta as your diddle factor

Depending on how often and when sump pumps were replaced, some predecessors may have already added the "fudge" factor to the pump.

"Let's see the original was 15 gpm, so well buy 20 gpm to be safe."

"Well the burned out one is 20, lets go with 25 to be better"

"Gee, 25 gpm is a lot for a 3/4 HP, lets buy a 1HP for the fudge factor"

The more history you know, the better choices you can make. Knowing the "as-built" and current pump size might eliminate the need for complicated math.

What is size of discharge pipe and length?

What size electrical feeds the pumps?

What is physical size of pit?

Agree with your post that the history factor is key. Too often I have seen systems become unwieldy because of jus tthis effect. This is why maintenance of a central engineering file is necessary for a plant or building.

Since the poster indicates he knows the system, one assumes he must also know what it is intended to accomplish. If that is indeed the case, recalculation of pressure drops, etc. using the basis flow equations cited in the blog should provide sufficient information for specifying the pumps.

Replace identical as per spec.tag, else :

Add up all vertical heights + 15% for frictional lossses(else check each from charts)

= Total head. Select pump as per Gpm against head from chart.

P = (h) x (0.433 psi/ft)

100' x 0.433 = 43.3 psi

H = (p)/(0.433 psi/ft)

150' x 0.433 = 65 psi

H = (2.31'/psi) x p

125 psi x 2.31 = 289' of head

Thanks for help. The system is a series of 3 sump pits with duplex sump pumps in a long basement area. The flows are nominal since they are for discharges from uses in the basement and 1st floor area and if the ground water intrudes on the basement. All three pits are tied into a single 2" galvanized pipe using elbows and tees to go around corners and tie into the main trunk line. The discharge pipe runs approx. 300' inside the tunnel then exists the building and runs approx 250' to a storm water catch basin (this was built 40 yr. ago). We have noted they need to verify if a grease trap exists outside the building some where for proper protection. The sump pits are 6" deep and height to the discharge pipe leaving building is at approx. 12' therefore we have been using a ht of 20' for initial head. With physical inspections and looking through the AS Builts they have provided we have deterimined that we should assume 11 elbows (along the entire length of piping inside and out), 1 tee at each pit to tie duplex systems together (3), two tees (where additional pits tie into the main line), and 1 tee where main line along the wall ties into the 2" line that leaves the building. From the site As Builts it seems as if additional elbows and 2 45's are inplace underground to go to the catch basin. We have noted to clean existing pipes out but assume the friction will still be up due to age. Old pumps are 1 hp but have codes changed to require larger pumps? Power at site is 480 3 phase. I have noted a pump sizing service at Auropa and Gould pumps sites and show. When input of system and use 35 gpm is done I received a Total Head of 34.4 NPSHa of 69' The only point of entry into the system that might be realitively quick is the fire line want to discharge into the pits and we are not sure what the entry rate would be so that the discharge rate is greater than that. If you wish to contact me personally I am at John.Sexton@yangenterprises.com. Thanks.

Confirm pits are 6' and not 6" deep as you posted? (I make this typo too!)

35 gpm thru 2" pipe is 3.35 feet per second velocity, which is a bit low but not bad based on length of run. This should be fast enough for some cleaning of the pipe without making too much friction.

Is pit 2' or 3' diameter? Volume of pit will affect how often the pumps cycle, but if you are using a submersible model I have been told they can start 15 times per hour.

How many cleanouts in the 550' 2" discharge? Can you look inhside to verify you can't see any corrosion or deposits in the pipe? I would not expect debris, but corrosion may be starting to get through the zinc after 40 years.

Check if the 3 basement pits are plumbed together as a oil/grease trap, and whether this could satisfy the code requirement? The outlet of each pit should be submerged, so that the oil and grease will float on the top to be cleaned out periodicly. (They do clean this out periodicly, right?)

1HP, 35 gpm and 69' head sound about right. Aurora, Gould, Weil, Flygt would be decent pumps to install. I think you're ready to talk to a couple of pump salesmen, get some advice and some quotes.

Be careful, a pump that is too big will cavitate and burn up. Check the manufactures minimum run time X the capacity = minimum volume of the pit between pump on and pump off. This is a very common thing that is taught in civil engineering foir sewer pumps and is most often missed by the occational pump specifier.

Also be sure that there is a check valve on the effluent pipe. If there is not it will burn the pump up by refiilling the sump each time the pump cuts off.

A very good reference book is "Pump Handbook" by Igor Karassik. You can Google this and purchase it in several places or maybe find it at a local library. I bought my copy in 1976 from a recommendation by my first boss in my first job out of college and have found it useful for years. There is more info in the book than you need to know for your question, but there is a section on sump design that will specifically show you how to design this system.