Back Pressure vs. Pressure Drop

[Tezoc]

Can someone help me define the difference between the terms "Back Pressure" and "Pressure Drop". I believe they are measured differently and are not the same. Isn't Back Pressure taken as a measurement upstream of flow and measures the shock wave from resistance downstream across the component or system? Isn't Pressure Drop a measurement of pressure before(upstream) and after(downstream) of the component or system?

Any help is appreciated...I know its basic, but I need more opinions. Thanks a bunch.

Tezoc

 

[MintJulep]

"Pressure Drop" is a differential measurement. It is upstream pressure minus the downstream pressure.

"Back pressure", to the best of my knowledge, is a colloquial term that has no precise technical definition. The use of the term should be avoided.

 

[pablo02]

You're close, but...

Pressure drop is the difference in pressure upstream vs downstream (the difference from your two measuring points) -- and, assuming you want the "backpressure" to be associated with your differential pressure (pressure drop) it is the downstram pressure point or in effect, the pressure at the final point your pressure will drop to -- for instance in a condensing steam turbine, the backpressure is the pressure (usually measure in "Hg) of the condenser itself... increasing backpressure will decrease your pressure drop (assuming the inlet pressure is constant) and your flow will decrease also...

 

[TBP]

"Back pressure" is simply the pressure on the downstream side of something like a steam turbine, steam trap, or pressure reducing valve. It will have an impact on the "pressure drop" across the device, and therefore, the flow through it. Back pressure steam turbines are common in many industrial applications, and used in place of, or in parallel with, pressure reducing valve stations.

 

[Deaks]

In the hydraulics industry, pressure drop is the difference in pressure between upstream and downstream of a component, and back pressure is the pressure on the downstrean side of a component that usually returns oil to the reservoir (ie the back pressure is associated with the pressure losses thru hosing, fittings, filters, cooler etc)
In real life, pressure drop is the parameter that produces the torque or force (useful work) and back pressure is a power loss (sometimes a necessary evil)
BR
Ian

 

[zdas04]

It is interesting how far appart the language of various industries can be.

In Oil & Gas, backpressure is measured upstream of a pressure-reducing component and represents the artifical outlet pressure that an upstream device will see. Often you will see a backpressure valve installed on the discharge of a flooded screw compressor in an attempt to keep enough differential pressure across the screw to move oil. The valve is measuring compressor discharge to keep it at a constant value and really doesn't care what the pressure downstream of the valve is.

David

David

 

[Tezoc]

I appreciate all of your comments....this really helps me out.

Let me give you an example...say I am feeding Nitrogen(N2) into a pipe and say several inches downstream I have a pressure port that leads into a pressure gauge...then a couple inches downstream from the pressure port I have the component(s) that I need tested. At the end of the component(s) or system it will be open to atmosphere. Now...what pressure is the pressure gauge reading when I feed at a certain flowrate? I think their exists a bias with the nitrogen feed being only several inches away or am I wrong.

N2 feed ----> pressure port ----> components/system ----> to atmosphere


Tezoc

 

[TBP]

It may help to think of fluid flow as a series of pressure drops. The outlet pressure from one component (pipe, valve, fitting, etc.) is the inlet pressure of the next component. As long as the pressure gauge is as close as practical to the component being tested, it doesn't matter much how far away the source of the N2 is. You only care about the inlet pressure to the component being tested, right? The only thing that might matter, depending upon how precise you're trying to be, is the temperature of the N2.

 

[Tezoc]

Thanks TBP for your responses....I'll try to clarify some more...you really got me thinking.

I'm not so concerned as to the pressure i'm supplying to the component, as long as i'm flowing a certain N2/air rate, but what does that pressure measurement at the port mean. I say this because some, here, believe they are measuring back pressure and I don't think that is true and they use both terms "back pressure" and "pressure drop" very loose. According to others on this forum they suggest back pressure is measured at the downstream. I believe any pressure gauge that is close to the feed source will bias the pressure measurement and at the same time its not a back pressure measurement. Is my reasoning true or not. I hope I'm making sense, but I'll try to be more clear. Thanks for your responses.

Tezoc

 

[MintJulep]

Tezoc,

You are correct in that the location of the test port may affect the reading. In fact, not only the location, but the configuration of the test port and the geometry of the fluid entry into the pipe.

A simple hole through the wall of the pipe will give an approximation of static pressure. If the nipple on the gauge extends beyond the ID of the pipe the approximation will be a bit worse.

Better would be a single static pressure probe, and better yet would be an array of static pressrue probes.

A general rule of thumb is that any pressure measurement should be a distance of at least 10 diameters away from any interruption in the flow.

If you are testing a device and want to give a description of the parameter you are testing, why not include a description or schematic of the test set-up rather than attempt to rely on terms such as "back pressure", which as you can see, is "understood" to mean different things by different people.

 

[TBP]

Tezoc - the "backpressure" in your case is atmospheric, or 0 PSIG, so that is constant. The pressure drop - that is the inlet pressure, minus the outlet pressure, will correspond with the flow. The greater the pressure drop, the higher the flow. If the inlet pressure is 10 PSIG, and the outlet pressure (the back pressure) is 0 PSIG, then the "pressure drop" will be 10 PSI. The flow resulting from this pressure drop will be whatever it is. If the inlet pressure goes to 15 PSIG, and the backpressure remains 0 PSIG, the pressure drop is 15 PSI, and there will be a corresponding increase in flow for the same size unit, in the same piping arrangement. The ONLY thing that gives you a flow, is pressure drop. If the back pressure is 100 PSIG, and the inlet pressure is 100 PSIG, there is 0 PSI drop, so there will be no flow.

See if you can get hold of a copy of Crane's "Technical Paper No. 410 - Flow of Fluids". It's a thin, soft covered book that, page for page, is likely the best publication on fluid flow ever printed.

 

[Tezoc]

Thanks MintJulep, TBP & others....

All this information helped me understand...thank you. This is my first time on these forums and you guyz are top notch. I'm gonna stick around these forums whenever possible to assist others as well...it seems I can learn alot hanging around here.

Tezoc, BSME