Turing a non priming pump into a self priming/automatic priming pump

[DieselTwitch]

Hello,
I have never posted on this site before but find many answers to many questions i have on here.

No I have one to ask directly.

I have the need to install a pump for pond pumping and sump pumping. Im using 2" Motorized valves to select the source... not really part of my question i just wanted to give some background.

For a while I've been looking at 2HP Self Priming pumps.... Having that self priming is a very important thing however the pay off is a dramatic loss in pump efficiency. Best i can hope for with a Self Primer is 50% however with a regular centrifugal pump is 85%. So my idea is this. I want to use a bypass valve,

This valve

http://www.omega.com/Green/pdf/SV3500.pdf

to turn this pump:

http://www.wegotpumps.com/pdfs/87-88.pdf

into a self priming pump

Take a look at my drawing to see what I am thinking.

http://www.greenteksolutions.us/pumpidea.png

I need to know if this has any chance of working. the basic theory of this is that once the pump is primed i will see a dramatic rise in outlet pressure (measured by a pressure sensor) and the system will shut off the valve returning the pump to its high efficiency mode of operation.

Thank you for any help you have to offer.

 

[Artisi]

First-up, what makes you think that the pump selected is 85% efficient. I would think that maybe 50 -55% would be more like it.

 

[DieselTwitch]

After reviewing my math you are right the eff goes between 45 - 60 % but the Self priming pump works out to about 35% at best.

I used a formula I found and have kept around for some time

Power out/Power in = Pump Eff%

The formula works as this: ( I left out SG of water and just used 1 as it only varies slightly )

(Head (ft) X Flow (GPM X 8.33)/33000 = HP Out

Picking a point that I will use regularly on the flow curve can get me Head & Flow.

(60' x 70 GPM x 8.33)/33000 = 1.06 HP

1.06/2 = 53%

Still much better then the 35% of the self priming

In this application the contractor is all about gaining every efficiency point possible . Normally I wouldn't care but....

 

[DieselTwitch]

Also I am driving the pump with a VFD to allow me to slow the pump as needed. so every point in gained efficiency translates to a lower speed I can run the pump at before it loses its ability to pump.

 

[Artisi]

The power formula using 3300 is for UKGPM - if USGPM then use 3960.
What is the term GPM x 8.33?

 

[DieselTwitch]

ummm the 33000 is the number for 1 HP. 1HP means to move 33,000 lb/ft in 1 min.
the 8.33 is the weight of one gallon of water, roughly. the GPM is just the units of the number that precedes it. 70 GPM or gallons per min

anyways the math really doesn't matter as part of my initial question. Im looking to know if this system will work. no matter what the math is, the non-selfpriming pump is more efficient then the self priming pump

 

[Artisi]

Just to clarify for your, the formula for calculating power is

Flow IGPM x H ft x SG / 3300/ pump efficiency (decimal)
if USGPM subsitute 3300 with 3960.

Yes it should work - you could replace the inverted U pipework on the inlet with a small holding tank.

 

[DieselTwitch]

I think the Inverted U is cheaper then a small tank. its all done with PVC.

The 3960 just combines the weight of water (8.33) and the 33000 into on. if you use the number the answer is the same.

Here is my source

http://www.mcnallyinstitute.com/06-html/6-01.html

Thank you. you seam to be very knowable about pumping, would you mind looking at my other post about NPSHR and NPSHA?

 

[MikeHalloran]

Beware of PVC; many fittings have a relatively small bore that will cause problems. Otherwise a P-trap will work fine to provide self-priming; just make it big.



Mike Halloran
Pembroke Pines, FL, USA

 

[DieselTwitch]

Hmm, would over sizing the PVC help to keep the problem at a minimum.
The pump has a 1-1/2 suction and a 1-1/4 outlet.

So maybe 2" on the inlet and 1-1/2" outlet?

How much higher should the top of the p trap be? I was thinking 18" over the pump base? And also the same for the discharge just to help keep more water in the pump for the next cycle. I'm also think I will put way over sized section on the discharge right at the pump for 8" or so to make sure the water and air have time to seperate. I'm thinking a 3" section for this.

Thoughts?

 

[SlurryPumps01]

Have you consider to use a foot valve?

 

[DieselTwitch]

There will be foot valves in the system, one at the base of each inlet ( I have a 2" valve that switches between two different sources.) But should they fail or there is a even a small leak anywhere in the system, all the piping would me drained.

 

[SlurryPumps01]

The self priming system also will not work on leaked suction pipe.

A small leak in the system, especially in suction side, will give big problem, no matter you use, self priming, or modified priming system, vacuum assist priming or just standard foot valve.

Find good quality foot valve

 

[Artisi]

One good reason for having vacuum assisted priming is to account for leaks on the inlet side, operating on snore and to reprime if the sump or water source in lowered enough to expose the inlet.

 

[DieselTwitch]

True, but lets say the leak is below the surface of the water, as much of the piping is in this case. The system will drain slowly out of that hole. once i reengage the pump and use the auto prime it will still run.

that and after the system in winterized or there is maintenance performed on the system i don't want to leave it up to the grounds crew to prime the pump... with this system i can assure that i will alway be able to prime the pump automatically.

Another note. Im plumbing city water fill into the suction side of the pump piping so i can turn on the city water to fill the system if the water in the pump is lost for one reason or another.

 

[Artisi]

"after the system in winterized or there is maintenance performed on the system i don't want to leave it up to the grounds crew to prime the pump... with this system i can assure that i will alway be able to prime the pump automatically"

If your inverted U has been drained at anytime, the system will not re-prime automatically, the inverted U will need filling and any air in the pump case purged.

 

[DieselTwitch]

Actualy I will... Here's how.

I have city water plumbed into that suction side of the plumbing. On a valve by the way. The grounds crew will have a 3 way switch, on, off and maintenance/winterize. If at any time the system is turned off it will trigger a start up subroutine in the controller. When the system is turned on again it will fill the pump and all other pipes with water from the city for a given time before enabling the pump to run again. Thus automatically priming the system with no human interaction. I will know when the system is filled as I have a flow meter on the outlet pipe to calcite flow as well as pressure reading from a pressure transducer.

Automatic is a term I used if I can wirte a program to process.

 

[DieselTwitch]

Also the air in the case is not a huge problem as it might nroamly be. Because of the low hight bypass valve it will separate the water from the air and return it to the intake of the pump during priming mode. Thus creating the vacuum needed for self priming. This is the same way normal self priming pumps works. The only difference is that once the pump has pulled the water from the source and is pumping normally I can shut off the bypass valve and return the pump to a higher efficiency state.

 

[SlurryPumps01]

Based on the figure; i suggest to have vacuum assist priming, you just need to have small vacuum pump or compressor.

The controller will be much simple, just run the vacuum pump for a while (depends on the suction pipe volume and vacuum pump capacity) before run the main pump.

Install a level switch that will activate the solenoid valve on certain level to prevent water ingress. Water trap will be advantage. Once the solenoid valve is activated, the level switch should also cut the power of vacuum pump.

This system will need 5-10 second for primed; while the self priming pumps may require more than 1 minutes before primed.

 

[DieselTwitch]

I hesitate to install another pump just to pull vacuum and save 45 seconds. That and I worry that if get any water in the vacuum pump at all I have another problem to fix. This just eases the cost of the system as I would need another float switch, and still need a solenoid valve to close off the vacuum pump once charged. For the most part the system will stay 99% full of water, I should only need the auto prime mode if I either get a leak or during the system start in the spring.

As for being complex control really it's simple, any complex operational stuff will just be a few lines of code. That part is easy.

In the end if i'm not sure the auto self priming pump operation will not work I plan on just going back to the other inefficient pump.