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Multiple Sump Pumps - Discharge PipeHelpful Member!(2) 

civilman72 (Civil/Environmental) (OP)
26 Jul 11 10:28
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?
Helpful Member!  bimr (Civil/Environmental)
26 Jul 11 12:04
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 (Civil/Environmental)
26 Jul 11 12:11
cvg (Civil/Environmental)
26 Jul 11 13:55
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 (Civil/Environmental) (OP)
29 Jul 11 12:12
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 (Civil/Environmental)
29 Jul 11 13:59
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 (Civil/Environmental) (OP)
29 Jul 11 15:18
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 (Civil/Environmental)
29 Jul 11 15:24
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 (Civil/Environmental)
29 Jul 11 16:49
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 (Civil/Environmental) (OP)
9 Aug 11 10:02
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.
Helpful Member!  Artisi (Mechanical)
9 Aug 11 10:44
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 (Civil/Environmental)
9 Aug 11 13:33
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 (Civil/Environmental) (OP)
9 Aug 11 20:35
Thanks bimr...
katmar (Chemical)
10 Aug 11 8:55
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 (Civil/Environmental) (OP)
10 Aug 11 9:44
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 (Chemical)
10 Aug 11 9:51
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 (Mechanical)
10 Aug 11 10:30

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 (Civil/Environmental)
10 Aug 11 11:25
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 (Chemical)
10 Aug 11 11:33
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 (Civil/Environmental)
10 Aug 11 11:42
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.
katmar (Chemical)
10 Aug 11 12:06
bimr, Your conclusions were all logical, what I was saying is that there was insufficient information to be certain. You obviously had these particular pumps in mind, and it seems that these are indeed what is installed, but no detail had been given to confirm that.

Based on the actual information given it would be fair to use the following logic:

Lines are clogging up implies low velocity = line too big.
Pumps are burning out implies insufficient back pressure = line too big

Katmar Software - Engineering & Risk Analysis Software
http://katmarsoftware.com

"An undefined problem has an infinite number of solutions"

bimr (Civil/Environmental)
10 Aug 11 13:52
Regarding "Lines are clogging up implies low velocity = line too big."

These pump applications do not pump continuously. The solids tend to settle out when the fluid stops as the pump shuts off. So you need a high velocity to resuspend the solids when the pump is restarted.  Otherwise, the pipes will gradually foul over time.


Regarding "Pumps are burning out implies insufficient back pressure = line too big"

civilman72 has stated the pumps with the longest pipe length (and presumably the largest head loss) are the pumps failing. So this is really not plausible. If it was true, the pumps with the shortest pipe length would fail first.

 
katmar (Chemical)
10 Aug 11 14:53
With the extra info now available I am starting to see it bimr's way, but there is still information outstanding to be certain of a proposed solution.

If these pumps are similar to those in bimr's data sheet (and I must confess I did not envisage such small pumps) then it looks to me as though each pump will be operating in the top left corner of the curve and bimr is probably correct that these pumps are simply rattling themselves to death by being too far from the BEP. Especially if they are "cheap and nasty" pumps.

One thing that has not been mentioned is that the crawl spaces are flooding, so presumably this means that when the pumps are actually working they do have sufficient capacity.

We do not know what the static head is on the discharge pipe - 7 ft appeared somewhere but it is not clear if this is the static head.  By making a few assumptions (dangerous), I come to around 36 gpm flowing through the 180 ft of 1.5" pipe and giving a friction head of 16 ft and a velocity of 5.6 ft/sec.  This means that each of the 6 pumps is delivering only 6 gpm and the velocity in the pipe from the southmost pump will be only 1.0 ft/sec.  The northmost pump would be a bit above 6 gpm and the southmost pump would be below 6 gpm.  This would explain the silting up of the line at the southern end. The section of the header between the 1st and 2nd pumps could probably be only 3/4" and then from the 2nd to the 3rd a 1" pipe and only use 1.5" after the 3rd pump.

To do a proper design we would need the actual pump curve of the installed pumps, plus we would need the distances between each pump and the static head up to the discharge point.  If the pumps are coping with the flowrates and the only real problems are the silting up at the southern end and the premature failure of the pumps then perhaps the answer is to use smaller pipes on the south end as described above and to invest in better quality pumps when replacing failed units.

Katmar Software - Engineering & Risk Analysis Software
http://katmarsoftware.com

"An undefined problem has an infinite number of solutions"

civilman72 (Civil/Environmental) (OP)
17 Aug 11 10:58
Katmar - I finally tracked down the exact type of pumps that are currently being used in the crawl spaces.  It sounds like the condominium management company has been pulling the sump pumps off the shelf at a local Ace Hardware. The claim they use the 3/4 HP type - I have included a link to the 1/2 HP Ace Sump Pump.  Based on the customer reviews, this is not the most reliable pump.

I could not find a pump curve, but specs for 1/2 HP Ace include:
65 gpm @ 5'
52 gpm @ 10'
35 gpm @ 15'

It appears the Wayne pumps may be similar.  The performance specs for 3/4 HP Wayne include:
71 gpm @ 5'
58 gpm @ 10'
43 gpm @ 15'
18 gpm @ 20'

So, it looks like this pump (if placed in the south crawl space) cannot overcome the friction loss in the 150' long, 1.5" discharge pipe, when trying to pump over 18 gpm.  

Also, more specifics on pumps and pipe configuration:
There are five (not six) crawl spaces and pumps.  Each unit is 30' wide, so total building length is 150'.  Each sump is placed in the middle of the crawl spaces, so the total distance between the south and north pumps is approximately 120'.  The discharge pipe takes a 90 degree turn in the north crawl space and discharges on the east side of the building, so total length of pipe from south sump pump to discharge location is 150'. The discharge pipe elevation is just below the finished floor, the crawl space is approximately 4'-5' in height, and the sumps are buried 2'-3', so I've been assuming a total head of about 7'.

Katmar – I was thinking about upsizing the pipe in increments as it heads north, just as you suggested.  I'm trying to determine if this would be a better long-term option than installing a separate header (2'-3") for the two south pumps and separating them from the three north pumps.
 
DubMac (Petroleum)
17 Aug 11 12:06
Consider the effects of parallel pumping in this combined header; they cause many unforseen problems. You could be pumping some of the check valves shut.

Heat buildup due to running at shutoff can transmit back through motor shaft since this is most likely a small close-coupled pump and the impeller is attached to the motor shaft.
 
katmar (Chemical)
18 Aug 11 12:16
Thanks for the requested info. With all the facts on the table we can now see where the problem is, but the solution is not easy.

To make the description easier, and hopefully clearer, let me give the pumps the item numbers P1 thru' P5 with P1 being at the South and P5 at the North.

With 5 off of these 3/4 HP pumps connected to a single 1.5" manifold there is a serious mismatch between their duties. It is aggravated by the fact that the line from P5 to the eventual discharge is only 30' and not the 120' I originally took it to be.  If the discharge line was long relative to the distance between the pumps it would make the individual pump duties more equal.

My estimate is that with all 5 pumps working the total flow will be about 70 gpm, but the individual flows vary between 30 gpm from P5 and less than 5 gpm from P1. This puts P1 way up on the left top corner of the curve and it will be running for long periods under very stressful conditions. It partially explains why the line is silting up but by my calculations when all the other pumps have emptied their sumps and shut off and P1 is still running its flow rate should pick up to around 30 gpm. I'm surprised this isn't enough to clear out the line.

The proposal to install a 4" header will certainly equalize the pressures, but it means that the velocity on the south end will be very low. I suppose it would take a long time to silt up with such a big diameter. One factor to bear in mind if you do use this method is that the pressure drop will be very low when only one pump is running and without the 7' of static head these pumps could run off the curve on the right hand side and possibly burn out more quickly than they already are. Check on the static head and make sure the pumps are OK there.

If you are unhappy with cvg's proposal of running individual pipes from each pump to a new sump with a new larger pump there is a halfway method that I believe would also work. You could run separate 1.5" lines from each pump through to the position of P5, and then joint them all there into a single 3" line which runs to the discharge point. This could be done with no new pump. All the pumps running together would give about 200 gpm with the flows varying from P1 at 30 gpm to P5 at 60 gpm and giving velocities in the 1.5" sections of between 4.5 and 9.0 ft/s.

Because P5 will discharge directly into the 3" line the concern remains of it running off its curve if the static head is too low. A disadvantage of this scheme is that when only one pump is running the velocity in the 3" section will be low - between 1.4 and 2.4 ft/s depending on which pump is running. The pumps would be pumping at much higher rates than they are now, so changing to this arrangement will mean that the pumps pump for a much shorter fraction of the time and therefore there will be a lower chance of them running at the same time and achieving an increased velocity. With this arrangement the pumps, particularly P1/P2, will be running for a fraction of the time they currently run and should therefore last proportionally much longer.

All these calcs are based on the pump curves being the same as the 3/4 HP Wayne pump data given.

Katmar Software - Engineering & Risk Analysis Software
http://katmarsoftware.com

"An undefined problem has an infinite number of solutions"

cvg (Civil/Environmental)
18 Aug 11 13:54
the take away from this is that each section of the header should be sized to match the assumed flow and the pump curve(s) that are producing the flow. Sizing should maintain minimum velocity while keeping the head loss to a minimum. There may be numerous options for your piping system, you need to evaluate them to determine the recommended one. Don't forget minor losses which in these types of systems can be significant.

An additional issue which has not been discussed is the potential for air in the force main. Since these types of systems are rarely profiled, it is not uncommon to have high and low points in the pipes. The high points collect air which may impede flow and increase head loss. The low points collect sediment also resulting in increased head loss.  
civilman72 (Civil/Environmental) (OP)
19 Aug 11 14:17
Lots of good discussion and explanations here...  Thanks again.

It appears there is no cookie-cutter, easy answer here, without having actual groundwater inflow information in each crawl space.  This type of monitoring is overkill as far as I'm concerned.

It's very difficult to discharge from the crawl space to the exterior of the building under the middle Units (#2-4).  But I can combine the two pumps in the most southerly crawl spaces and create a new exterior discharge point out from Unit 5.  This will decrease the total distance to pump to 50'-60', which should eliminate a lot of the concerns discussed above, and offer a cost-effective solution.
ccfowler (Mechanical)
21 Aug 11 0:32
civilman72,

Since the sump pumps were selected primarily on the basis of availability, convenience, and price, it is reasonable to expect that the check valves were selected for the same set of reasons leading me to suspect that they are simple brass swing check type.  It seems likley that these check valves probably do not seal well due to crud in the flow.  The troublesome pumps may be suffering from having their sumps refilled by significant check valve leakage in addition to the ground water inflow.  Since these pumps are already working against greater effective heads than the others and they may be re-pumping the same water, their actual number of on-off cycles and number of working hours may significantly exceed those of the other pumps.

Another possibility involving the check valves is that consideration may have been given to the presence of crud in the flow, and reasonably inexpensive soft-seated spring-loaded check valves were chosen.  In this case, the cracking pressure is likely to be relatively large since they would likely be a readily available type normally used in a pressurized domestic water system where their 2-3 psi cracking pressure would be relatively insignificant.  Such a cracking pressure represents roughly 4 to 6 feet of head just to overcome the cracking pressure with even more head required to overcome the restriction of the flow due to the configuration of the check valve.

Valuable advice from a professor many years ago:  First, design for graceful failure.  Everything we build will eventually fail, so we must strive to avoid injuries or secondary damage when that failure occurs.  Only then can practicality and economics be properly considered.

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