Multiple Sump Pumps - Discharge Pipe 2

[civilman72]

Here's the situation: 6-unit condominium complex with sump pumps in the crawl space of each unit. Sump pumps all pump to one 1-1/2" discharge pipe (flowing south to north). This existing discharge pipe is about 180' long and installed with a negative (uphill) grade. The sump pumps at the south end of the line fail often and the discharge pipe clogs and needs cleaning annually. It's assumed that the most southerly sump pumps are burning out due to the distance they have to pump. It's also assumed that the pipes are clogging (mostly with sediment) due to the negative grade and lack of flushing velocity.

Solution being discussed: Replace existing discharge pipe to allow gravity flow through pipe, install cleanouts along pipe, and add new pump in north crawl space to pump discharge pipe fluid up to existing building outlet. There are openings in the wall between each crawl space, so re-installing the discharge pipe with a positive grade should be simple to accomplish.

Questions: Is the efficiency of the system improved by having a gravity-fed discharge pipe? Should we anticipate that the future pump replacement and maintenance will decrease enough with this new configuration to potentially justify the costs for a new discharge pipe and new pump? Any better solutions?

 

[bimr]

Piping of all the sump pumps into the same 1-1/2-Inch pipe is not appropriate because the 1-1/2-Inch pipe is only sized for one sump pump. One would expect that all the sump pumps will operate at the same time during a storm event and the capacity of the 1-1/2-Inch pipe will be exceeded.

In the selection of pumps for parallel operation one should take into account many factors, paramount of which is called equality of pressures. Parallel pumps should ideally have the same pressure and flow, otherwise one of the units, with less pressure, will be forced to overcome the resistance of the combined pressure. The pumps at the south end are pumping against the highest pressure causing the pumps to back up on the pump curve and the pipe to plug from low velocity.

The first option is to manifold multiple pipes having aggregate cross sectional area equal or greater the cross sectional area of the individual sump pump discharge pipe. The manifold pump should be sized for 4 ft/sec velocity at the combined pump capacity. Maintaining a 4 ft/sec velocity will flush the debris from the pipe. You can probably get by with one 4-Inch manifold pipe.

The second option is to install 2 smaller manifolds instead of one larger manifold. You can probably get by with two 3-Inch manifold pipes.

The gravity pipe is probably not a good option.

1. It will have to be sized much larger. You will probably need a 6-Inch gravity pipe.
2. Should the gravity pipe foul, you will have water squirting from the vents.
3. At low flows, the velocity will be low in the gravity pipe allowing solids to drop out and foul the pipe.
4. It will be inconvenient to maintain the slope through the building.

 

[bimr]

Here is a typical sump pump curve.

 

[cvg]

pump all the sumps into a manhole located outside the building. install a new, larger pump in the manhole to pump from the manhole to the outfall. Forget about headers.

 

[civilman72]

bimr - thanks for the response.

I'm concerned with designing for 4 ft/sec "at the combined pump capacity." I do not believe the amount of water entering the crawl spaces is close to the total pump capacities. The existing discharge pipe for the whole building is 1-1/2” and when I was out there it was not running full-time and when it did flow it was flowing out half-full and was not spitting out, which would indicate a lack of velocity. In order to maintain 4 ft/sec within the discharge pipe with only 20-50 gpm, it would appear that a 1-1/2” pipe would be reasonable (a 3” pipe would need 90 gpm to generate 4 ft/sec.).

The maximum inflow during high groundwater may be 50 gpm for the entire crawl space (with 5 pumps) and the overall head is only 6’-7’. So, I’m not sure the pump capacity is the issue, but maintaining equal flow and pressure might be. The most southerly crawl space has significantly more inflow than the other four crawl spaces (about 2x). If I attempt to keep flow and pressure equal, I’m assuming the pump curve for the south crawl space sump pump (with higher flows) should indicate a head equal to the head of the other pumps at their lower anticipated flow rate? And as the flows equally fluctuate from min. to max., the heads should also stay relatively equal? Maybe that’s too simple…

cvg - I can't put a manhole outside (too cold) and I'm not sure I understand how this proposed setup promotes self-cleaning and less pump replacements?

Thanks.

 

[cvg]

your last response indicates the uncertainty of flow rates which affects your design. Also, the fact that you had low flow coming out of the pipe may be because the pumps are unable to overcome the friction in the line and thus are unable to pump at a higher rate. With 180 foot long discharge line flowing at 50 gpm, you will have about 36 feet of head loss plus the 7 feet of head is 43 feet of headloss total. Using bimr's pump curve, your pumps will shutoff completely at that head and pump nothing at all. That explains why you have low flows and are burning out pumps.

eliminating a header allows each pump to operate independantly of the others, headloss does not change when another pump starts up. The pump can operate at a good point on the pump curve and not overheat. The discharge line for each pump can be easily sized to maintain proper velocity. The manhole can be insulated, heat traced or inside a shelter. Temperature should not be any more of an issue than it already is.

 

[civilman72]

cvg - thank you - good explanation. I originally underestimated my head loss due to friction.

In your second paragraph, does "header" represent a shared discharge pipe?

 

[bimr]

I agree with cvg comments on headloss.

From my observations, I would expect all of the sump pumps to operate simultaneously. Of course, you know the layout of the present installation and if you can predict that some of the pumps will not operate, then you should delete them from the total.

Some additional comments:

1. Add cleanouts for future maintenance to the piping modifications.

2. Consider a partial replacement of only the sump pumps at the south end. You can pipe these sump pump separately and this will relieve some of the capacity issues with the existing pump header.

3. 50 gpm in a 2-Inch pipe for 180 feet will give you a velocity of 5 ft/sec and a total headloss of about 20 feet. A 2-Inch header is probably too small.

4. The pumps are probably burning out because you are overloading the motors. Most pumps and pump stations are overloading. This means that the pump impeller is capable loading the motor beyond the motors motor’s FLA rating into what is called the service factor. UL and NEMA allow this, but NEMA states that the motor will have a shorter life expectancy if the motor runs in the service factor.

5. Note that the sump cycle times are extended (slower sump pumpout and longer pump operating time) when pumping into an undersized force main.

 

[cvg]

yes, a header is for either shared discharge or shared suction. Similar to the headers on your car which combine exhaust from 4 cylinders into one pipe.

 

[civilman72]

bimr and cvg,

Sorry for the delayed response...

Great points. I like the idea of a partial replacement.

Assuming the new header is connected to the last two sump pumps, do you still think a 3" pipe is the most appropriate size? Based on the information below, I think a 2" header makes sense, particularly if I drain this new header to a sump pump in a manhole (less head to pump). Would the lack of velocity in the 3" pipe concern you?

1.5”PIPE~150’
10gpm, v=1.81(ft/s), pressure drop=0.70psi
30gpm, v=5.44, p=4.79psi
50gpm, v=9.07(ft/s), p=16.77psi (38.7’)

2.0”PIPE~150’
10gpm, v=1.02(ft/s), pressure drop=0.17psi
30gpm, v=3.06, p=1.22psi
50gpm, v=5.10(ft/s), p=2.99psi (6.9’)

3.0”PIPE~150’
10gpm, v=0.45(ft/s), pressure drop=0.02psi
30gpm, v=1.36, p=0.17psi
50gpm, v=2.27(ft/s), p=0.43psi (1.0’)

Last question: is there any significance to the distance from the individual pumps to the header? I'm reading about possible turbulence if they are too close.

 

[Artisi]

Just to clarify one point that is wrong in a number of posts and assuming these are centrifugal sump pumps, as the head increases on these pumps the power reduces, therefore running at low flow won't result in the motors "burning-out" - something else is the cause - ie, no water thru the pumps, low voltage, running wrong direction, pumps continually switching on and off (is each pump fitted with a non return valve)?

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[bimr]

While a centrifugal pump will accommodate changes in its operating point as far as it is able within its performance range, a centrifugal pump may operate off the recommended operating range, where its efficiency is low, and its reliability is affected.

Low efficiency results in wasted electricity Poor reliability is caused by higher loading on pump components such as the bearing.

The pump manufacturer should be consulted on the safe operating range of the pump.

Your calculations on pipe size look reasonable.

There is no significance to pumping into a pipe manifold when you are operating within normal pumping velocities.

 

[civilman72]

Thanks bimr...

 

[katmar]

I'm with Artisi on this one. I would not make any changes based on the information given here. There is too much else to discover before you can determine the right solution. We do not have any information on the pump curves, we do not know what control system is in place, we do not know if the non-return valves are in place and have been checked, we do not know the pump duty cycles. We do not even know what the pump failure mode is - is it the pump or the motor that is failing? With the information available you could make as good a case for the line being oversized as has been done here for it being undersized. Do not thow good money after bad.

Katmar Software - Engineering & Risk Analysis Software

http://katmarsoftware.com

"An undefined problem has an infinite number of solutions"

 

[civilman72]

Katmar - I appreciate you taking the time to comment.

A few things I can answer:
The existing pumps are small (probably 1/3 HP) and generally as cheap as they come (<$100), so I doubt the pump curve would be difficult to estimate.
There is no "control system" that I'm aware of.
Every pump does have a check valve - they have not been checked. But considering that at least one of the sump pumps is being replaced annually, I do not believe a faulty check valve is the (ongoing) issue.
Failures involve the motor no longer working. I have not determined the extent of impeller issues with these failures.

What information do I need to gather in order to better size the system? I'm assuming recording the groundwater inflow quantities within each crawl space would be helpful, but I cannot wait until next spring to gather this type of information.

 

[katmar]

Thanks for the extra info. If you do not have the pump curves then a make and model number would allow the curve to be Googled. There has to be a control system of some sort, unless the pumps run 24/7? There should at least be some sort of float switch that works as a simple on-off control. Do you have the size of the pump outlets?

Katmar Software - Engineering & Risk Analysis Software

http://katmarsoftware.com

"An undefined problem has an infinite number of solutions"

 

[Artisi]

If they are $100 pumps, think yourself lucky they are not all being replaced on a yearly basis.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[bimr]

Artisi, it is possible for a centrifugal pump to fail with low flow. Pumps are designed to operate at the Best Efficiency Point. While one does not know the BEP for inexpensive sump pumps, it is reasonable to assume that moving to the left of the pump curve (which occurs when multiple pumps operate) is off the BEP.

"Operating off the BEP can break the pump shaft because the force is always in the same direction while the shaft is turning. This has the affect of flexing the shaft twice per revolution. In many cases you can easily exceed the endurance limit of the shaft material.

The stresses imposed in reverse bending are cumulative.
Most fatigue failure occurs in one million cycles or less. At 1750 rpm you get 2,520,000 cycles per day."

http://www.mcnallyinstitute.com/11-html/11-04.html

In addition, when multiple pumps discharge into a common outlet that is inadequate for the flow volume, the effect is the same as throttling the pump discharge.

"Throttling a centrifugal pump to get a high head will cause some problems:
The resultant shaft deflection can damage the seal or break the shaft.
Internal recirculation can overheat the volute and cause cavitation problems.
A high differential pressure across the pump can damage close internal clearances.
The power loss can be expensive.
The increase in stuffing box temperature can cause a premature seal failure."

http://www.mcnallyinstitute.com/11-html/11-04.html

 

[katmar]

bimr - that is why I pointed out that it is important to know the pump failure mode. OP has confirmed that it is the motors that are burning out, which indicates high flow and low back pressure.

Katmar Software - Engineering & Risk Analysis Software

http://katmarsoftware.com

"An undefined problem has an infinite number of solutions"

 

[bimr]

katmar, how would you make a good case that the discharge line is oversized?

The foundation drain sump pumps that are included in most residential basements typically have discharge connections of from 1-Inch to 1.5-Inch.

civilman72 states that he has 6 pumps tied into one 1.5-Inch manifold.

A reasonable assumption is that when it rains, multiple pumps will operate at the same time.

Note that sump pumps are self controlled as the 0.3 HP pumps are sold off the home depot shelf in a package with on-off float switches.