Pressure drop through nozzle 3

[mjpetrag]

If I have a system like this...

1" 0 psig
^
|
3" 80 GPM 60 psig>-----------|----------> ? psig, 3"

How do I find the pressure drop across the tee if the 1" line is dumping water to atmospheric pressure. Basically I need to find an equation that finds the pressure in the right hand pipe taking into account a loss of mass flow. I couldn't find one anywhere.




-Mike

 

[tmacm]

What is the 3" pipe connected to? Is it open to atmosphere or to a closed valve. Both would give very different results.

 

[mjpetrag]

sorry about that...the 3" pipe dumps to a closed tank at 15 psig.

-Mike

 

[BigInch]

Need to know
1" pipe length
3" pipe length

and either, or both of these
1) the flowrate in the 1" and/or the 3"

If you only know one of those, then we need this one too,
2) the end pressure at the outlet of the 3"



**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[mjpetrag]

1" pipe length is 20'. 3" pipe length is 30' up to the Tee. 20' after the tee. flow in the 3" before the Tee is 80 GPM.

Say the flowrate in the 1" line is 40 GPM.

This is a conceptual question so we really don't have to take into account line losses but included them just in case. I just needed some direction on how to calculate the pressure drop.

-Mike

 

[BigInch]

Still need any two of these
flowrate in the outgoing 3",
flowrate in the outgoing 1",
or
pressure at the end of the outgoing 3".

Without those, flow in both pipes is 0 to 80 gpm.

If you just want to try a very rough estimate, assuming the end pressure at the 3" is also 15 psi and pipe roughness is zero, ratio by pipe area and the 3" would have 72 gpm with 8 gpm in the 1".

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[mjpetrag]

I basically need a resource to size a minimum bypass line. I have a pump discharging at 55 psig and 80 GPM with a control valve about 50% closed throttling the discharge. The pump should be discharging 45 psig at 300 GPM. In between the discharge and control valve is a minimum bypass line that is 1".

I'm thinking that the minimum bypass line is too small to handle the extra 220 GPM, since the 1" pipe is 20 ft long and I get a huge pressure drop of about 310 psig at 220 GPM. It looks like the max flow I can get out of the 1" line is around 90 GPM, since the pressure drop is about 55 psig for this flow.

Am I looking at this the right way?

(I realize how bad the design is, the bypass is only a temporary fix).



-Mike

 

[BigInch]

It might not be a bad design. It could be that the small 1@ bypass is only for minimum flowrates, say 20% of BEP flow or about 60 gpm +/- and you should be establishing mainline flow by getting all valves open on that line by the time you get to 80 gpm, so there would be no need to recirculate more flow than that. Does that sound like a possibility?

Anyway... if you want to recirculate more flow than that now, you'll have to size that recirc line for the flowrate you want now using a differential pressure equal to the pump discharge pressure (45 psig) minus the 1" outlet pressure (of the tank at 15 psig) and the 20 ft of length. Which looks like you can have a dP of 30 psi drop to work with. Size the line, let's say that's a 1-1/2". Then find a 1" or 1-1/2" control valve that will do that flowrate with the same pressure drop and at about the 70% open position.




**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[Latexman]

The pump mnfr. should be able to tell you the minimum flow requirement of the pump. This is what needs to go through the 1" pipe at about the dead head pressure of the pump.

Good luck,
Latexman

 

[mjpetrag]

The problem is that the pump is continuously deadheaded. Almost all of the time. The only time it operates correctly is when we get a rain storm and the rising feed level needs to be pumped down quickly. The control valve on the discharge is run at 50% open almost all of the time. I think it was designed for the worst case scenario, and most of the time the pump is throttled back from its BEP of 49% to about 33%. When the level rises, the pump sounds great and the control valve is open around 80%.

Seems to me like 50% throttling continuously will lead to pump problems pretty quickly without a good bypass flow.

-Mike

 

[vpl]

As Latex said, talk to your pump manufacturer. I know for nuclear service, we "recommend" approximately 25% recirc for pumps that are dead-headed for a long time (it's in quotes, because my Agency recommendations have a way as showing up as requirements along the way --- which is my personal opinion and not a statement of fact.)

I've seen values as low as 10% and occasionally run into them as low as 2% -- that one didn't survive.

Patricia Lougheed

******

Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of the Eng-Tips Forums.

 

[BigInch]

2% must have been a typo.

20% is usually a reasonable minimum, 10% might be an absolute minimum, but heating is likely, so only do that if the manufacturer agrees, and neither for extended operation times. If its a large pump, these rates should only be used for startup, or shutdown sequences.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[mjpetrag]

Thanks for the help guys. I'll follow up once I try out the minimum flow requirements on the bypass line.

-Mike

 

[Latexman]

There are "Lo Flo" pumps that also have a very low minimum flow requirement, but these are like 1-50 gpm pumps, much smaller than the OP's pump.

Good luck,
Latexman

 

[Latexman]

If the pump mnfr. has software that generates pump curves, it should spell out the minimum flow required.

Good luck,
Latexman

 

[mjpetrag]

yep, just got the minimum flow required from goulds

thanks again

-Mike

 

[Latexman]

Well, tell us what it is! Don't tease us.

Good luck,
Latexman

 

[BigInch]

20% unless he pushed them into a corner with proof calcs.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[mjpetrag]

no way!




just kidding...info from goulds is

3796 MTX 3x3-13 - 12 1/8" impeller, 1770 RPM
Min hydraulic flow 58.8 GPM

so that would mean that for the 3/4" (went back out in the field and found it was 3/4" SCH 40 NPS and not 1") line with 20' of piping and a minimum flow of 58.8 GPM, I would need about 56 psid to overcome the friction losses.

-Mike

 

[Latexman]

Your delta P seems low to me.

Good luck,
Latexman