Pressure Drop at Pipe Junction

[mjpetrag]

I calculated the line resistance and matched the system curve with the pump curve, but I found a mysterious 20 psig drop in the system that cannot locate. I'm guessing it is at the junction with the other pipe the line discharges into but I can't find a good equation to calculate the drop.

The line looks like this

-------------------------------> 8", Pstat = 32 psig, 5 ft/s, water
^
|
|
|
3", Pstat = 24 psig, 5 ft/s, water, Velocity pressure ~ 26 psig

Can this junction create a 20 psig resistance in the 3" line? The 3" line comes into the 8" line at a right angle.

-Mike

 

[btrueblood]

I think your arrow for the 3" line is backwards. Or your Pstatic measurements are incorrect.

 

[BigInch]

I don't think velocity-pressure is 26 psig. What is velocity-pressure anyway?

**********************
"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]

Made a mistake...

The pump discharge is 148'head at 120 GPM on the 3" line. After 114' of pipe, a check valve, butterfly valve, a globe valve, 6-90 bends, and a 20' elevation, the pressure drop is 11.1 psig (25.6 ').

-------------------------------> 8", Pstat = 32 psig, 5 ft/s, water
^
|
|
|
3", Pstat = 52.9 psig, 5 ft/s, water, Velocity pressure ~ 26 psig.

The velocity pressure I had to take into account since the flow is going perpendicular to the 8" pipe. Did I do this wrong or am I on the right track?

-Mike

 

[Latexman]

Are any of the valves partially open? Globe valves are usually used for control valves.

Good luck,
Latexman

 

[mjpetrag]

everything is full open

-Mike

 

[katmar]

Did you measure the pressure at the end of the 3" line, or is it just a calculated value? How close to the tie-in have you measured the pressure in the 8" line?

What size hole did you cut/drill into the 8" line where you welded the 3" line?

Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com

 

[mjpetrag]

It is a calculated value. I have no way of measuring the pressure of either line near the junction(it's a not so nice design). The 8" line is the discharge piping from a chiller, and the service tech said that the discharge is typically 30-32 psig. The line needs at least 25 psig to get back into the cooling tower. Originally I thought that maybe the cooling water pressure was much higher, but the pumps discharge around 65 psig and by the time they reach the chillers, there is a large pressure drop.


I'm going to test the check valve for a large pressure drop tomorrow.

-Mike

 

[Latexman]

How certain is the 120 gpm? Sounds like the 32 psig in the 8" chiller line may be more certain. What pump flow would give enough pressure drop to have 32 psig at the 8" line? Is that more reasonable? How does that match to the hp the motor is consuming?

Good luck,
Latexman

 

[mjpetrag]

the pump discharge pressure gauge was used to find where i was on the pump curve. the motor amps verify the flow. the pump is almost deadheaded

-Mike

 

[Latexman]

Is this causing operational problems or is it an academic concern? If the pump is doing all it can and it's getting into the 8" line as intended, then why worry? If it's "fixed" the pump won't flow anymore anyway.

Good luck,
Latexman

 

[mjpetrag]

because we need another line to run into the sump and the pump can't keep up with that capacity so the sump overflows.

my question basically came down to this: can the 8 inch line cause a decrease in flow so that the 3" line is restricted to the point of creating 20 psig extra resistance. Basically think of the 8" line's flow as creating a nozzle at the 3" line's discharge.

i want to make sure that it's possible to create this large of a pressure drop since there seem to be no equations to model this, short of CFD.

-Mike

 

[Latexman]

Unless something is going on you don't know about in the 3" or 8" line, no. There is an explanation for this "mysterious" pressure drop. Is the 8" line running hydraulically full? 5 ft/sec in an 8" line is over 750 gpm.

Good luck,
Latexman

 

[mjpetrag]

the 8" line is full.

-Mike

 

[Latexman]

Then I hope you can get a long enough shutdown to go on a search for the problem. See katmar's post, it has some ideas. Also, malfunctioning valves and debris.

Good luck,
Latexman

 

[FredRosse]

The 3 inch line tee into an 8 inch line would not normally account for any significant pressure drop for this case.

However, the installation may have been bad, for example, if the 3 inch pipe protrudes into the 8 inch pipe by 7-1/2 inches, then you have a big restriction there, and all looks well from the exterior. There are numerous other errors that could cause high pressure drop in the line, about as many as you can imagine. This kind of poor installation work should be rare, but I have seen worse.

 

[JSines]

The new 2009 edition of the Crane TP410 addresses Tees and Wyes in a much more comprehensive method that takes into account the flows in each leg and the sizes of the pipelines in the tee. The method was developed from the Idlechick book with comparison to Don Miller's "Internal Flow Systems" book. The 2009 edition should be released within a few weeks.

You can also model your system in software such as PIPE-FLO or Crane Flow of Fluids to help troubleshoot.

An extra 20 psi drop is pretty large, so you may have a problem with partial pluggage. I once found a large bolt lodged between the seat and disc of a 150# steam pressure control valve at the mill I used to work at, and massive pluggage of orifices on a steam dryer that reduced steam flow by 25%.

You get what you inspect, not what you expect.

Jeff

 

[SNORGY]

JSines:

Do you have or have you used Crane Flow Of Fluids software?

I am just looking into it now. Your suggestion in response to the post is excellent. I am considering getting my own copy, I just want to get a few opinions first.

Regards,

SNORGY.

 

[JSines]

Snorgy: In the interest of full disclosure, I work for the company that developed the software and do engineering support and training. That being said, I've worked as a nuclear engineer in the Navy, and as a production supervisor, training manager, and process engineer at a pulp mill, and I wish I would have had the software to help troubleshoot, train operators, and design system modifications.

I'd recommend you download the demo and play with program to see if you like it. I also recommend looking at other programs to do a comparison to see which one best fits your needs.

Jeff

 

[SNORGY]

JSines,

I will do exactly that this weekend in between activities in my "other line of work".

Considering what I think of Crane TP-410, I have no doubt that the software is good. Buying it sight unseen wouldn't scare me.

Regards,

SNORGY.