Engine/Transmission (1998.5 - 2002) Need some fluid dynamics theory (psi loss per length of 3/8" line) help

[rweis]

If I am reading pressure about 6 feet away from a point, how do I adjust my readings from what I am reading to what the actual pressure at the point is? Flowing fluid.



I have a VP fuel pressure sensor in a VP inlet return line (RASP) about 6 feet away from the VP inlet. Lines are AN-6 (3/8"), I have a reading of 13 psi. In the RASP return line there is generally a fluid flow through the check valve back to the tank. The pressure gauge is set in the line on the VP side of the bypass line and just before the bypass valve.



Seems like I read somewhere in the past that there is a . 75 psi drop per 10 feet of 3/8 rubber hose (effectively what AN-6 brained hose is) for flowing diesel due to hose friction. That would make the true reading at the Vp 13. 75 psi IF I have this right.



Is that about correct? The only way I have to verify the theory is to install a shrader valve at the VP44 inlet and I thought I would ask first before I embarked on that project.



Bob Weis

 

[Diesel Freak]

Head loss (pressure drop) is directly proportional to the square of the volumetric flow rate. Without the flow rate through the line, I can help much, but i would not expect more than 3 psi.

 

[SKneeland]

there are a lot of other restrictions in the line to take into consideration, not to mention the VP basically has a built in regulator.



its not as simple as taking a 3/8 hose (wide open end,no restriction), then taking pressure and fluid rate to calculate pressure drop.



likewise, id be surprised if its more than a few psi drop

 

[RKerner]

Head loss (pressure drop) is directly proportional to the square of the volumetric flow rate. Without the flow rate through the line, I can help much, but i would not expect more than 3 psi.

More correctly, it is directly proportional to the square of the velocity of the flow. Considering this is the return line, flows should be pretty small under load. Using up to 20gph, your gage will have a hard time accurately measuring less than 1/2 psi difference between the VP and the RASP return inlet. If the RASP can supply 150gph, then you will see an approximate 3. 4psi drop but you won't be using any fuel either. So, either by experience, knowledge or SWAG, DF and the other dude are close in their estimations. Oo.



This is all assuming laminar flow, no huge height differences between points A and B and smooth line. The other assumption is that the #6 line is NOT rubber and the 6 feet of line you stated is accurate. Bends and other obstacles add 'equivalent lengths' for each occurence and would add to the total head as head is also proportional to internal flow length.



Rick(engineergeek)

 

[rweis]

On the built in VP regulator, that is an old carry over from the P7100 days. The VP pressure regulator is after the vane pump and it's psi is in the 2xx+ range and is not part of the lp feed system. There is another thread discussing the VP at length that goes into the misconception that the VP44 has a lp level of psi (0 - 99) regulator. Surprised me too.



A couple of psi works for me though. Measure it with a micrometer, mark it with a grease pencil, cut it with an ax, Oo. TLAR. (That Looks About Right)



Thanks,



Bob Weis



EDIT: To test the theory of how much difference in the pressures I think I will put a schrader valve in the line at the VP and see what the differences are.

 

[WestTN]

Actually, it is simpler than all that. If you have a line to your sensor that is 6 feet long the pressure drop is nothing since you have no flow. You are just readin the pressure.

 

[Diesel Freak]

I was thinking about that too, but his post looked like he was asking about the head loss in the supply line to the VP44.



But if it is just the gauge line, then there would be no change in pressure from the static fluid.

 

[NoSeeUm]

Bob;



I know this is not what you asked exactly, but I think what you would actually have to do to find this out for yourself. Or find a pump chart explaining it. Which for the FASS I would believe to be Pressure vs Flow. If you can get or have a chart, you must measure the pressure right at the FASS the reasons I have explained below. This chart should give you xx flow at xx discharge pressre. To do it yourself.



Step 1, measure the pressure of the pump right at the pump with the discharged closed off, meaning no flow. This pressure will be the same for any type of discharge tubing configuration that you could possibly conceive of installing. This would be your maximum pressure, with zero flow or the shut off head pressure.



Step 2, measure the pressure of the pump right at the pump with the discharged fully open. Do this with no tubing connected to the pump. In theory this pressure would be close to zero, but in practice would be more. This would be the minimum pressure, but the maximum flow. If you directed the flow to a container of a known size (a 1 gallon bucket), then you could time how long it took to get 1/2 full. Do the math and you would get a gph result.



At this point, you could add piping configurations and repeat step 2. Instead of doing the math you would find your answer by doing experimental results.



Keep reading if you are interested just for general info.



There are engineering methods of computing head loss (or pressure loss), but basically you have to have the data obtained by step 1 and step 2 for the calculation. Most calculations rely on the notion of a more or less constant flow rate or in many cases the maximum flow rate and minimum delivery pressure required is all that is considered. With that concept in mind, a pump must be installed that can do the required flow rate and delivery pressure asked for by the piping components. Generally, not the other way around.



The pump pressure, measured as close to the pump as possible, that you would see at any given time would be the sum of all the pressure drops of all components connected. Like you said, at xx flow 3/8" tubing will drop pressure xx psi per foot. But at what flow rate, what starting pressure and at what delivery pressure? There are similar constants for pressure drop for elbows, fittings and other restrictions. However, everything is pretty much based upon the flow rate at the very instant the calculation is made.



I hope this helps you out.



Good luck;

Jim

 

[rweis]

Yep, that helps.



Thanks, I enjoy reading the concepts behind ideas.



Bob Weis