What is "back pressure"? 2

[JOM]

Intersting how a simple question like "What is the difference between psig and psia?" can trigger off much discussion.

I've never been comfortable with the term "back pressure".

To me, a pressure just exists and doesn't point forwards or backwards. Does anyone have a clear definition of the term?

John. Cheers,
John.

 

[svanels]

Back pressure is the pressure measured at the discharge side of the pump, it is caused by the resistance the system (piping) generates agains the passage of fluid. Steven van Els
SAvanEls@cq-link.sr

 

[TD2K]

It also refers to the pressure at the outlet of a safety relief valve. It can refer to constant back pressure or built-up back pressure.

I suspect it can refer to a lot of other areas, I would take it typically to mean the downstream pressure. It can be less than the upstream pressure, in the case of a control valve or PSV, or it can be greater, in the case of a pump as JOM used.

 

[quark]

http://www.sensidyne.com/News/Technical Papers/UnderstandingBackpressure.htm

A good description is given here. The 'backpressure' is a fancy term for me right from the time I was studying engineering. Particularly when it was told that Positive Displacement equipment work on backpressure(?).

Subsequently I developed some reason (or logic) for understanding this subjectively(!).

Note: If I feel like I understood this concept totally, it infers I don't know anything about it.

And oneway I agree with JOM. (regarding direction of pressure, whatever it may be)

Regards,

 

[quark]

On a second thought....

If a pump is connected to a piping system and the pump discharge pressure gauge reads 1Kg/sq.cm then it is the pressure delivered (very crude, isn't it?) by the pump or equally it is the back pressure created by the system on the pump.

Pardon my ignorance for it is bliss

Regards,

 

[Flareman]

Back pressure is the term used to describe the pressure immediately downstream of a component when that pressure is the result of the down-stream losses. That is the pressure "which "backs-up" against the component.
It's not a real technical term but it's so commonly used that it might as well be one.

 

[poetix99]

Or it could be thunk of as the pressure on the "back-side" of a piece of equipment.

 

[JOM]

Dear respondents.

Great range of answers. Check out the paper quark mentioned:

http://www.sensidyne.com/News/Technical Papers/UnderstandingBackpressure.htm

I asked for a definition and get a paper on it! Love the guy's little trailer a-huffin n a-puffin pulling the big caravan (the "back pressure&quot.

He says:"When a pump is working to move air, it must overcome the forces that resist air movement....The sum total of all these forces is called backpressure, and it is a measure of how hard the pump has to work. Any time a pump is working, it is always working against some level of backpressure"

I don't agree with any of that. You have a piping system with a pump, valves, bends then the system will settle into a particular pressure profile governed by the laws of fluid flow. Change something, like a valve position, and a new pressure profile develops. You might have to dumb it down a bit for hygienists, but surely not for engineers.

Anyway, not the most pressing issue we face is it?

John.
Cheers,
John.

 

[JOM]

Hi All.

Great range of replies. Check out quark's link:-

http://www.sensidyne.com/News/Technical Papers/UnderstandingBackpressure.htm

I asked for a definition and got a paper! Love the guy's little car pulling the big trailer (the "backpressure&quot.

I still don't like the term. It's vague and unscientific. Not the world's most pressing problem though.

Cheers,
John. Cheers,
John.

 

[a31ford]

JOM,

Back Pressure: The reverse (inverse) pressure reversely exerted on a moving mass, by passing that moving mass through a confining area or space.

(Note: ex: The air around moving air, can be the confinement, not just piping or fixtures).

Does that sum it up ?

Greg

 

[JOM]

Greg,

"The reverse (inverse) pressure reversely exerted on a moving mass, by passing that moving mass through a confining area or space."



I wonder if the cause and effect is being confused. Surely whatever pressure exists at any point arises from the pump or fan - that's the "cause". I know the downstream conditions influence the actual values, but they aren't the cause. I guess people who regularly use the term know what they mean by it.


John
Cheers,
John.

 

[a31ford]

John, what I ment: (a mass at rest tends to stay at rest), remember ? Anyhow, the pump creates the "moving mass" that would be defined as the "forward pressure source" since this mass is now moving, ( a mass in movement, tends to keep moving) ( second, of the laws, of mass and movement), Any cause that would impeed the movement of that mass would be construde as "back pressure" as it is resisting the movement of said mass. Therefor, anything that will tend to slow a said mass down from it's current speed, would be construde as "back pressure" because it is oposing the forward movement.....

(thought you wanted a simple scientific explanation.)

 

[JOM]

Greg,

I see you're from the Computer Group. Is there an equivalent in electrical studies - a "back voltage" ?



John.

PS - Thanks for the simple scientific explanation. Now, about E=mc2....

Cheers. Cheers,
John.

 

[a31ford]

John,
Why yes, back EMF, resistance, inductance, and capacitance, (among others), all play a roll in "back pressure" in electrical/electronics, The computer heading is simply a "place holder", just call me " mister science" LMAO !

have a great one !

Greg

 

[vesselguy]

This sounds fun. I guess I have to throw my hat in the rink too.

I interpret the all encompassing term "back pressure" as a head/pressure that an equipment has to discharge against and must overcome in order to prevent a back flow. I have never heard anyone consider friction head and static head as part of the term "back pressure". Friction head and static head are separate from back pressure.

So for example, a pump discharge line is tied into a main line with a flow and defined pressure of 100psig (say SG=1, then head =231 ft ). The pump discharge head is calculated as 50 ft. In this case the back pressure is the 100 psig that the pump has to overcome, in addition to the static & friction head. This means the discharge head is 231ft + 50 ft = 281ft.

Right? Please comment on my interpretation. Please be kind.

 

[a31ford]

OMG ! John is playing with us ! all of our answers are somewhat correct, but I am thinking, that John knows something, the rest of us don't.... Cough it up John..

 

[JOM]

Dear a31ford, vesselguy et al,

a31ford, you said:

"OMG ! John is playing with us ! all of our answers are somewhat correct, but I am thinking, that John knows something, the rest of us don't.... Cough it up John.. "

I am not playing with you! I'd sooner poke a crocodile in the eye with a sharp stick.

But I admit the comments have left me bemused - and quite convinced that no-one really knows what the term officially means. I don't see a need for it. If you want to refer to the pressure at the discharge of a pump then it's the pressure at the discharge of the pump. Why does it need a special name? I think I understand why the term is used - downstream obstructions cause resistance and the pressure increases and that is called "back pressure". More than one respondent has explained that.

Did you read the article by the hygienist - the link is in an earlier post from quark. He tries to explain to non-engineers and throws science out the window. The downstream obstructions don't cause the pressure - the pump does - but I know what he's trying to say.

I'm happy to accept that persons who use the term know what they mean and probably the people they talk to have the same undersatnding (boy, is that a big assumption). I just don't like the term, don't see why it's needed - but it's not a big deal.

But I find there are many situations where terminology is used without ensuring we have a common understanding of its meaning. And sometimes politics and hidden motivations exploit this misunderstanding. Now that is important.

For instance, how do you like these two news reports?

"A truck lost its load of caustic soda and hydrochloric acid when crossing the Elwood Canal in Melbourne. The ABC reported that emergency crews managed to mop up the spillage “before the toxic mix reached the bay”. "
(For non-chemists, caustic soda plus hydrochloric acid produces salty water.)

"Victoria Water Police have introduced laser speed guns to book speeding boats. The ABC reported that a special feature of the new guns was that they could measure speed in knots. "
(Such a clever country.)

These are just humerous. There are other examples that have deeper consequences. So, I'm not playing with you, but you're right, I have something else on my mind. Well spotted. (But "back pressure" still annoys me.)

So what is the concensus? Is there an official definition of "back pressure" - one we can find in a book? Or is it a colloquiallism? Or who cares?



Cheers,
John.

 

[a31ford]

Hey... I just found out something the Very hard way.... "shift + esc." together will erase the entire contents of a reply before it's sent (dern) (I recently lost a finger to a table saw, and it's somewhat hard to type.... guess the bandages got in the way, and I had a very good response for John.....) Anyhow, 2nd time around... John, I was using the term loosly "Playfully even !" (pun intended), but just as I thought...

Anyhow, I see your point and I'm in 100% aggreement with you. Wars have started simply over a missunderstanding (history proves this), It's to bad that with all our wisdom & technology that we still see times of "assumpion" (eg: person "a" has to assume that person "b" understands the terms being used, IN the same manner that person "a" uses them... (is that clear ?, are we on the same set of terms ??) See ! lol....

Greg

 

[vesselguy]

JOM,
I totally agree with you on not using the term "back pressure" as if it is an offical engineering term. I have never seen this term used in any of my University text books. I encountered this term only when come out to work. At first, I thought what hell does it imply, but then I gotten to "understand" it by investigating the P&ID and nearby equipment and then the light bulb turned on. I guess the use of this term is faster than saying "overcome an existing pressure at xxx of xxx psi", but far less exacting.

Sad to say, we Engineers are inventing new terms every year. Just this month, I was taking over a simple double wall tank package from a young Project Engineer. I looked into the NDE he specified to the Vendors and I saw "Incis test". I thought what the hell is this. In my 14 years working on tanks, vessels and exchangers I have never heard of such test. But inside, I have stinking suspicion that he means Interstitial Vacuum Test. Sure enough that's what it is. He made it up and the young lad thinks I don't know and starts explaining it in detail to me.

So, whatever terms we get hit with in this bussiness, we should ask the person who threw the term at you and find out exactly what he/she means. Asking anybody else could leave you in a worst state than you started off at.

 

[vesselguy]

JOM,

I was just filling data sheets for buying PSVs and I find that "back pressure" is a required data to supply to the valve manufacturer.

In the Crosby Vavle Engineering Handbook, back pressure is defined as;
"Back pressure is the static pressure existing at the outlet of a pressure relief device due to pressure in the discharge system."

This is the same as what I defined in my initial posting. So, it seems this term is an official industry terminology after all.