14 replies to this topic

[Albert Jia]

I have on real case in plant which puzzles me a lot. We pump city water from one sump to another sump, everything looks normal but the up and down piping layout (The highest point is about 14 meters above grade, and there are a couple of high point), we did the hydraulic calculation based on Hazen-Willams equation and the C we used is 100. While it seems that pressure drop across the whole line is 3 to 4 times the DP we calculated. We checked the piping inside which looks fine, there is no scaling or corrosion issues, we added high point vent on all the high points to vent gas trapped, but this did not help. I can not understand why is this happening. Do you have similar experience on this?Do you know if there is any study on this kind of case, or do you know if any guidance on the hydraulic calculation in this case?

Attached Files

•  Sketch.png   158.17KB   6 downloads

[Bobby Strain]

You should show your calculation in detail. We can't offer any help without it. And include a sketch with dimensions.

 

Bobby

[Albert Jia]

Bobby, thanks for the reply, please see attachment for the calculation sheet, the calculated pressure drop is 1.25 bar, while the real pressure drop is 3.2 bar (The pressure drop across flow meter had been excluded)

Attached Files

•  Hazen-Williams.xls   444.5KB   37 downloads

[latexman]

What pressure measurements do you have in the system?  Surely, there is a discharge pressure.  Are the pressures running constant or bouncing?  What type of pump is it?  Could the high points be under vacuum with two phase flow?  That would add a lot of pressure drop (and noise).  Is it noisy?  Where is the flow meter located?  What type of meter?  What is it's estimated pressure drop?  Does the flow rate cycle low to high, low to high or is steady?

[Albert Jia]

We have pressure transtimmer on the line as indicated in the sketch, also we measured it with local pressure gauge, and they were aligned. We measured the pressure at a few of high point with local pressure gauge, there is one point is bouncing, but not so much. The pump used is vertical centrifugal pump, we checked all the high point and there is no vacuum even some point is at low pressure. Both the flow and pressure is not noisy. The flow meter is on the pump discharge (Upstream of the pressure measurement in the sketch), the flow meter is orifice flow meter. The estimated friction pressrue loss is 1.25 bar, while the real one is 3.2 bar. And we can not see a flow rate cycle, it seems it is qute constant.

[latexman]

There is no pressure transmitter indicated on the sketch.  More details, like a dimensional sketch, would help.  What fittings and valves are in the line?  I did check pressure drop using Darcy-Weisbach equation.  2642 gpm, 2822 equivalent feet of 14" Sch. 30 pipe, f = 0.004 resulted in 10 psi or 0.7 bar drop.

[Albert Jia]

Thanks, The pressure measurement is on the discharge of the pump, and your calculation result is similar as what I had, but the real DP is much more than that, which puzzles me a lot. We had ever dismantled flow meter and butterfly valve(API 609B) on the line, and they are in good conditions, also the pipe is in good condition. The 860 meter in the calculation sheet is the equivalent length, it considered all the fittings such as butterfly valves and all the elbows.

[latexman]

I understand all valves and fittings are included in the equivalent length, but that assumes they are functioning properly.  Gate valves can come off the valve stem and check valves can stick partially open.  I've seen these in my experience.  Seeing the details of what you have, may spark an idea on the excessive pressure drop being experienced.  It would help us help you.  With what we know now (or more like, what we don't know), I'm tapped out.

 

Is this a new installation?  Was the new line flushed properly?  Can debris be in the line/fittings?  Is it a new problem in an old installation.  When did it start?  Do the operators know anything?  Are filters or strainers in the line?  The more details you can show/tell, the more help you'll get.

[Bobby Strain]

Your sketch fails to show the elevation of the source and destination. Grade doesn't tell us anything. My guess is that the discharge is at a higher elevation than the source. You might get a clue if you look at the pump datasheet. Or even the pump curves. You never indicated that there is a problem with pumping enough water. So, maybe you should tell the whole story, including the pump design information.

 

Bobby

[Albert Jia]

Here is the story:

The line was in service since 2008 with one pump running, and now we are trying to pump more flow with two pumps running. And we found the flow was less than expected, and we tried to figure out what is the reason for that, and it seems that we checked every thing in mind (Calculation mistakes, plugging, fouling of piping etc), BUT we did not find any thing wrong. So with this piping layout, is it possible the calculation method is wrong (The real friction  pressure loss is much higher than the calculated friction presure loss?

The pressure measurement is 3 meters above grade, and both sump is atm pressure. And discharge point is 3 meters lower than the pressure measurement on the pump discharge..

[Bobby Strain]

You missed the point. What is the elevation (mean sea level is 0 meters) of the source and the discharge? Seems your measures are based on where the ground is at each location. That's not the same as elevation.

 

Bobby

[Albert Jia]

Thanks, here I tried to understand if there is any calculation method which is specialised for the case mentioned (Pipe has a very big change on elevation (The high point and low low difference can be 14 meters)

[Art Montemayor]

It isn't important to note on a simple flow diagram the essential and required venting valves on each of the vertical loops, and I have to assume that they DO exist.  Without an efficient method to vent all the trapped air at the top of each of the vertical loops you will have all kinds of hell trying to get the system to work according to calculations or expectations.  We all have to agree that you must establish a 100% liquid fluid-full system first before applying any calculations to your systems of vertical loops.

 

Could you please verify that your system does contain the essential venting devices and that it operates at 100% liquid fluid full during the pumping operation?

 

Why do you use an inaccurate civil engineering equation like the Hazen-Williams instead of the more accurate Darcy equation?  I can understand that in a municipal water situation, it may be more a matter of tradition linked to civil engineering practices, but the Darcy equation was developed specifically for this type of application - and it is far more specific and accurate.

[Marian1]

Albert,
to understand what happens to the water flowing in your pipeline, you should build the hydraulic gradient of the pipeline.

Draws on the X axis (pipeline length) the following two curves Y (x) expressed in meters:
A) pressure of the water along X calculated with respect to ground level, equal to the head of the pump (assumed to ground level) less the pressure losses along the pipeline (concentrated, by friction, etc,);
elevation of the pipeline on the ground level.
Check that the water pressure at the level of the pipeline (difference of the curves 2, A- is positive along the entire pipeline.

If A>B along the pipeline, no problem.

But in you pipeline it appears that A<B in N points.

In these points, water vaporizes or air enters from the vacuum breakers. In both cases, the flow in all the N downstream descending segments becomes "channel type" at atmosferic pressure. This disrupts the continuity of the Bernoulli equation: the static head of the ascending segments is no more balanced by the static head of the descending segments.
 
the consequence is that the total "static head" of your pipeline (that you consider negligible because calculated as "sump B level" minus "sump A level") is on the contrary significant, as it must be calculated (meters of H2O column) by adding all the differences in height of the above N ascending segments.
 
To restore a normal hydraulic flow, you should install a back-pressure control valve at the end of the pipeline.

Mariano
Senior Process Engineer

[Bobby Strain]

Elevations?