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(Somewhat unexpected) Pressure Rise
2

(Somewhat unexpected) Pressure Rise

(Somewhat unexpected) Pressure Rise

(OP)
I have a parts washer system that uses a vertical centrifugal pump to deliver a heated, 97% water solution up to the top of the washer through a 3” pipe, then distributes (branches) into two 3” pipes. One of these pipe branches delivers water to 21 nozzles, with an equivalent orifice size of 0.203” each, that spray down on to parts passing under the nozzles. The other pipe delivers a similar system that sprays the water on the under side of the part. The pump pressure on the down-stream side of a filter is around 12psi. On the branch that feeds the nozzles spraying downwards the pressure is consistently around 0.25 to 0.5 psi higher. Coming from the pump the water raises around 5 feet, then after passing through a horizontal filter, the pipe drops approximately 18” then branches and drops a further 18” to the downward nozzles and approximately 36” to the upward spraying nozzles. The upward facing nozzles are typically around 10 psi. What could cause a pressure rise, other than gravity, in the branch that feeds the downward facing nozzles?
Thanks,

RE: (Somewhat unexpected) Pressure Rise

A rough calculation makes me believe that the difference you are measuring is too large to be explained by velocity head effects (Bernoulli) and I cannot think of any other reason.  Were all the measurements done with the same gauge, or could it simply be a calibration problem on your gauges?

Katmar Software
Engineering & Risk Analysis Software
http://katmarsoftware.com

RE: (Somewhat unexpected) Pressure Rise

(OP)
Hi,
I have installed 0.25% FS accuracy (30 psi span) transmitters, I also see this effect on the less accurate dial-type gauges that are installed at these locations.
Thanks,

RE: (Somewhat unexpected) Pressure Rise

Interesting.  I'm not surprised at the higher pressure on the downspray header, but at the lower pressure on the upward spray header.

The inaccuracy of the transmitters is only 0.075psi.  Mechanical gauge accuracy depends on model and can be as bad as 3% (0.9 psi on a 30 psi gauge)



It appears that your 12 psi measurement is at the highest elevation point.
The 12 psig is downstream of the filter.  The filter location must be at the top point of elevation since "coming from the pump the water raises around 5 feet, then after passing through a horizontal filter".   Everything else drops from that point.

The downward spray nozzles are 36" below (18" drop + further 18" drop) that top elevation measurement point.

The upward spray nozzles are 18" below the downward nozzles.

Gravity adds a psi for every 27.7" of elevation, and you have a 36" elevation drop to the downward spray nozzles.  So an increased pressure makes sense.

But the upward spray nozzles are 18" lower in elevation than the downward nozzles, yet are 2psi lower than supply.
I'm still scratching my head.   Have I missed something in the sketch?

Dan

RE: (Somewhat unexpected) Pressure Rise


Flow distribution in dividing (as well as in combining) branched manifolds may be nonuniform, affecting static pressure measurements due to the different impedance characteristics of each branch.

RE: (Somewhat unexpected) Pressure Rise

i'm with 25362, in that your piping network balancing is causing the difference.  Put a header between the two nozzle branches sized to give negligible pressure drop and feed it centrally.

RE: (Somewhat unexpected) Pressure Rise

I did not pick up that the first pressure tapping point was at the highest point - thanks to Dan for the excellent sketch which enabled me to see the true setup.

The pressures read at the two spray headers are lower than they would be if static head was the only consideration. Obviously there are friction losses after the first pressure point, and these losses explain why the header pressures are lower. Without knowing in detail the pipe lengths and numbers and types of fittings it is impossible to comment on whether the losses observed are in line with expectations.

Katmar Software
Engineering & Risk Analysis Software
http://katmarsoftware.com

RE: (Somewhat unexpected) Pressure Rise

Hydraulic equations are not as accurate as many people think.  It is not unusual to find differences in the predictions vs actual readings of up to 10% in some cases, even when the exact dimensions, pressures and flows are known in each branch.  Here you only give a few pressure points, so I wouldn't be too worried if I could not explain 2 psi at this stage.

   Going the Big Inch! worm
http://virtualpipeline.spaces.msn.com

RE: (Somewhat unexpected) Pressure Rise

If the velocities in your system are quite high (well over 2 m/s or 7 fps), what you see could be explained by a velocity head effect, but of course you should have about the same pressure in the two sides of the branch, if the flow rate divides more or less equally (this point is not clear in your description).

prex

http://www.xcalcs.com
Online tools for structural design

RE: (Somewhat unexpected) Pressure Rise

(OP)
Thanks all for your excellent posts. Thanks danw2 for the great graphic. The graphic is essentially exact, except for the locations of the gauges. They would be located in the  pipe before transitioning to the nozzle assemblies. I also failed to mention that in the upper assembly (nozzles spraying downward) there is a 3x2" reducer and 2" valve that is very near the transition to the nozzles. This pipe remains 2" until the transition to the nozzles and the PI and PT are located downstream of this reducer. In the lower branch there is a 3x2-1/2" reducer a 2-1/2" valve and then a second 3x2-1/2" reducer (transitioning back to 3") that is around 36" from the transition to the nozzles. The PI and PT are located downstream of this assembly.  I'm not sure if the oem was trying to use up spare valves or if this is all by design.

Thanks

RE: (Somewhat unexpected) Pressure Rise

Please confirm or correct.

The upper PI and PT are in 2" line prior to any spray nozzles, and the lower PI and PT are in 3" line prior to any spray nozzles.

Good luck,
Latexman

RE: (Somewhat unexpected) Pressure Rise

Uh...  
2" valve          2-1/2" valve
2" pipe           3" pipe
3x2 reducer       3x2-1/2 reducer,
and we're looking for 2 psi?  Its in there somewhere.
Adios muchachitos.

   Going the Big Inch! worm
http://virtualpipeline.spaces.msn.com

RE: (Somewhat unexpected) Pressure Rise

(OP)
There are actually three PIs and PTs. PI 103 and PT103 are directly after the filter and PI 104, PT104 is upstream of the upper nozzles, PI 105, PT 105 is upstream of the lower nozzles. I hope this helps.
Thanks,

RE: (Somewhat unexpected) Pressure Rise

Picking a 0.19" free passage Bete spray nozzle at randon, it'll flow 21 gpm at 10 psi.  21 nozzles like this would need about 400 gpm.  400 gpm through 2" line would have significant higher velocity head and lower static pressure than through a 3" line!

This may not be exactly what you have, but it's probably in the right order of magnitude.

Good luck,
Latexman

RE: (Somewhat unexpected) Pressure Rise

This is starting to make sense now.

The gauge after the filter reads 12 psi, and if the 36" drop down to the downward spraying header added its full static head the gauge on this header should read 12+1.3 = 13.3 psi. However, it actually reads between 12+0.25 and 12+0.5 psi.  Call this pressure 12.3 psi. We expect 13.3, but actually get 12.3, so we have losses of 1 psi.

If I calculate the flowrate through a 3x2 reducer and a 2" globe valve to give a 1 psi pressure drop I get about 45 gpm. If the valve is a ball valve rather than a globe the flowrate would have to be 170 gpm to give 1 psi loss. Both of these estimates are way lower than Latexman's 400 gpm, but I suspect that Bete's definition of "free passage" is the largest particle size that can pass through, rather than the orifice size. You would never get 21 gpm through a 0.2" orifice at 10 psi.

Unless we know exactly what the length of 3" and 2" pipe is, the number and size of all the fittings and the type of valve we cannot give a definitive answer. But we can say that the answer looks to be in the realms of the possible.

Katmar Software
Engineering & Risk Analysis Software
http://katmarsoftware.com

RE: (Somewhat unexpected) Pressure Rise

(OP)
Thanks all for your replies. I have suspected gravity or static head, but since I am no expert I value your assistance.

Best Regards,

RE: (Somewhat unexpected) Pressure Rise

Katmar's right.  A 0.22" orifice /0.13" free passage spray nozzle will flow 4 gpm at 10 psi.  It's starting to add up now.

Good luck,
Latexman

RE: (Somewhat unexpected) Pressure Rise

I think I am missing a point here. My basic understanding is that, in liquids, the pressure gauge reading indicates the pressure drop that will occur after the point where the gauge is installed.

The pressure gauge after the filter should indicate the resistance offered to the flow by the pipe section after the gauge. If the elevation is supplementing the the pump pressure, that should replicate in the gauge reading after filter, unless there is siphoning effect. Otherwise, I am not getting any reason for the filter gauge reading to be lower than that of header gauge.

May be I am wrong.

RE: (Somewhat unexpected) Pressure Rise


Indeed the pressure readings on the branches are supposed to show the friction drops through their downstream spray nozzles.

Since the lower upward-spraying header shows 10 psig while the upper downward-spraying header shows 12.25-12.5 psig, assuming type, degree of cleanliness, and number of nozzles, are equal, it could mean that the flow on the upper branch is greater. Which would confirm the original  assumption of flow uneven distribution.

RE: (Somewhat unexpected) Pressure Rise


Quark, on gas-free liquid flows, beside the downstream friction resistance effects, the gage reading after the filter could be greater or lower than those of the downstream gages depending on whether the flows are upward or downward (due to gravity), and the pipe diameters don't change velocity heads into pressure heads. I think we talked about this in past threads.

RE: (Somewhat unexpected) Pressure Rise

Thanks 25362,

Sometimes I become too stupid, may be due to graying hair or becoming too busy with nontechnical work. I just came to correct my post and fortunately saw your comment.

RE: (Somewhat unexpected) Pressure Rise

25362, in order for the pressure drop between the filter and the lower header to be greater than the pressure drop between the filter and the upper header either the flow to the lower header must be higher, or the piping to the lower header must have greater resistance.

Katmar Software
Engineering & Risk Analysis Software
http://katmarsoftware.com

RE: (Somewhat unexpected) Pressure Rise


Katmar, I agree. I'll go for your second option, which could be justified by an increased pipe length and more fittings between PI 103 and PI 105.

Besides, we still don't know whether any dirt -and for that matter what kind and size of dirt- may be present in the system downstream the filter, and in what branch (and fittings) would it tend to collect. The fact that a filter has been installed to protect the nozzles hints to the possible fouling of the system.

RE: (Somewhat unexpected) Pressure Rise

(OP)
Is it possible that the pipe coming from the pump is not full? The pump curve shows 288 GPM @45ft-hd. I have a pressure drop across the filter of around 2.7 psi, which is what the filter manufacturer says is the appropriate head loss at that gpm. The pipe from the pump is a 6' vertical rise before it turns into the filter. The assembly is very much like the drawing in danw2's reply. The reason I ask is because I have been trying to use a ultrasonic flow transmitter to measure flow on this riser, and while the valves are 100% open I have 0 signal strength. Only after I throttle the valves upstream from the nozzle assemblies can I achieve a useable signal. Just thought I would add more confusion to my already confusing questions.

Thanks,

RE: (Somewhat unexpected) Pressure Rise


Ultrasonic flowmeters have their own idiosyncrasies, some need particles or bubbles in the stream, others need turbulent eddies.

They sometimes suffer because the signal gets carried away beyond the receiving sensor; although they are pointed against the direction of flow, they are affected by installation air gaps, the piping material of construction, etc. For details, see, for example,

http://www.omega.com/prodinfo/ultrasonicflowmeters.html

RE: (Somewhat unexpected) Pressure Rise

Based only on the approximate profile appearance of what is drawn, is it possible the pressure at the lower elevation gauge during flow is simply reduced by loss of head through the apparently multiple nozzles (orifices) to atmosphere that appear in front of the gauge (some unlike the other gauges)?  
With regard to the originalquestion-- also particularly/perhaps if the operation of this system is some intermittent, is it possible say that air is in the system, say by water draining out the nozzles as the systemis shut-down for any reason (and be replaced by some entering, ambient temperature air?)  You of course mentioned that the water pumped into the piping to do the washing is "heated" -- if there is cool air at times in the system, is it possible that heating this air without rapid relief of pressure by the orifices might result in some increase in pressure per "Charles Law" (of course at least until the systemreaches equilibrium or the air is scavenged from the system?)

RE: (Somewhat unexpected) Pressure Rise

(OP)
I have calculated the velocity of the water (at 288 gpm) to be around 13.07 fps and the pipe i.d. is 3". The density of water is ~62 lbs/cu.ft with a viscosity of 0.470cp at 140 degrees F. I come up with a Reynolds number of 646,000. Can someone double-check me? If this is the case, isn't this a guarantee of turbulent flow?
Thanks,

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