Pump Suction

[doubleT87]

Hi, I'm new to Eng-Tips and I'm looking for a bit of advise.

I'm trying to calculate head loss in my suction system and I'm having second thoughts about how to calculate this through a small part of a perforated "stinger". I'm perforating a HDPE pipe with 375-1.25" holes (to meet 3x the area of the suction line which is 14.005" and double as a strainer - solid size pump can handle is 1.31"). I've looked and found K-values for pipe entrance (K=0.78) and used the formula

h_f = K * ( V^2 / 2g )

Which is what I've done for all of the other fittings. The flow rate is 3,000 gpm and I divided that among the 375 holes to find 8 gpm/ea. I used the 8 gpm to find V... 2.093 ft/s... to finally get h_f = 0.053 ft of head loss per hole. Multiplied by 375 holes gives me a head loss of more than 19'... this seems way high?

SO, I tried a second method using the Hazen and Williams Formula. This time I again split the flow to 8gpm per hole. For the length I used the thickness of the pipe, 0.941". This gave me a head loss of 0.00122' per hole - multiplied by 375 gives a much more reasonable number of 0.458'.

These two are so far off I just wanted a bit of advise if anyone has any?

To answer questions that I'm sure will arise - this 16" DR 17 pipe is left over from a previous job and we're trying to reuse them if the NPSHA is sufficient. The stinger is in an open pit and will be placed near horizontal 3' off of sump. Also, perforations will be made 360 deg around circumference of pipe.


Thanks in advance!

 

[LittleInch]

double t87

All your holes are in parallel so you only need work out the head loss from one. It doesn't matter if there is one hole at 8pm or 2000 holes at 8gpm, the head loss will be the same. At three times the surface area of the main pipe, the head loss through the holes will be very small compared to the head loss in the main 14" pipe.

The straight pipe length calculation will not be accurate as the losses are to do with entry and exit and an orifice calculation is probably closer to the truth.

You are over complicating this.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[doubleT87]

LittleInch-

That's what I was afraid of... this is my first time designing a suction system and we're pulling out of a pit that's relatively deep and I want to ensure that I have plenty of NPSH to avoid tearing up the pump. I didn't think about all of them being in parallel. Thanks for the reply.

 

[1gibson]

Always do a qualitative analysis of an unfamiliar problem or equation. More holes would lead to more losses, and that doesn't make sense. More holes means fewer losses, so right there you know that something isn't right with the approach.

What are you trying to keep out with the strainers? Be aware that pumps are rated for max sphere size that can pass through. If you get a 2" long bolt through your 1.25" strainer holes, it will still tear up the pump. Start up with a temporary strainer that is considerably smaller than the max sphere size. Don't think you're 100% safe just because the hole size is smaller than the max sphere size. Most of the stuff you're trying to keep out is not going to be round.

 

[doubleT87]

1gibson-

Thanks for your response. I'm trying to keep out primarily rocks. However, I hadn't considered bolts/studs or anything long with a dia. less than 1.25". This is out in a field near a caliche pit and I'm concerned about rocks blowing into and settling in the sump of the pit. I did look into a temporary start-up strainer, however, the possibility of rocks steered me to a permanent screen/strainer. Would you suggest not perforating the suction "stinger". Again, I'm new to suction design so any pointers are appreciated! Thanks!

 

[1gibson]

If the rocks get into an HDPE hole they may wedge in there and stick, it's a soft material. I don't think that's an effective strategy. I'd go with a big wire mesh basket type strainer, maybe 0.5" square openings.

 

[bimr]

It would probably be more cost effective to buy an intake screen.

http://www.rainbird.com/documents/turf/ts_PumpSuctionScreens.pdf

 

[doubleT87]

Ok great. Thank you guys for all of your help - I appreciate it!

 

[Artisi]

You also need to consider what you are pumping and what could be entrained in the inflow as well as what could be accidently introduced into the sump.
What are you pumping?

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.)