What is "back pressure"?
What is "back pressure"?
(OP)
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.
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.





RE: What is "back pressure"?
Steven van Els
SAvanEls@cq-link.sr
RE: What is "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.
RE: What is "back pressure"?
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,
RE: What is "back pressure"?
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,
RE: What is "back pressure"?
It's not a real technical term but it's so commonly used that it might as well be one.
RE: What is "back pressure"?
Or it could be thunk of as the pressure on the "back-side" of a piece of equipment.
RE: What is "back pressure"?
Great range of answers. Check out the paper quark mentioned:
http://www.sensidyne.com/News/Technical%20Papers/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").
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.
RE: What is "back pressure"?
Great range of replies. Check out quark's link:-
http://www.sensidyne.com/News/Technical%20Papers/UnderstandingBackpressure.htm
I asked for a definition and got a paper! Love the guy's little car pulling the big trailer (the "backpressure").
I still don't like the term. It's vague and unscientific. Not the world's most pressing problem though.
Cheers,
John.
Cheers,
John.
RE: What is "back pressure"?
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 :)
RE: What is "back pressure"?
"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.
RE: What is "back pressure"?
(thought you wanted a simple scientific explanation.)
RE: What is "back pressure"?
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.
RE: What is "back pressure"?
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 :)
RE: What is "back pressure"?
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.
RE: What is "back pressure"?
RE: What is "back pressure"?
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.
RE: What is "back pressure"?
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 :)
RE: What is "back pressure"?
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.
RE: What is "back pressure"?
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.
RE: What is "back pressure"?
What about a back-pressure turbine? Dont they exist?
If you are confused about back-pressure and dont think it exists, I think you should get out more.
Perhaps it is a term misused rather than a term that doesnt exist.
RE: What is "back pressure"?
Greg
RE: What is "back pressure"?
I looked up the definition of "discursive" in my dictionary (I don't need a definition of "back-pressure"), and the first entry was:
"Thread 794-33900 of EngTips.com"
RE: What is "back pressure"?
RE: What is "back pressure"?
I didn't realise "back pressure" engendered such passion.
I'm not quite sure what points people are trying to make.
First of all - let's all agree that it isn't an important matter.
Next point - I'm sure engineers out there know exactly what they mean when they use the term.
Next point - the term doesn't have a legitimate basis in the science of fliud mechanics. (fluid, not fliud)
Final point - clear communication is very important. We should remember that when the person you are talking to does not understand they will most likely answer "OK".
vesselguy, you wrote:
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."
That isn't sensible. But if Crosby Valves know what they mean then I guess that's OK.
Cheers,
John.
RE: What is "back pressure"?
you said:
>If you are confused about back-pressure and dont think it >exists, I think you should get out more.
Of course I should get out more. We all should.
Please point me to a text book on flid mechanics that defines the term. (fluid, not flid)
To me pressure is pressure - it isn't back or forwards or left or right or red or yellow.
We probably shouldn't continue this thread much more. We all sort of understand each other's point of view.
Now, about E=mc2....
Cheers,
John.
RE: What is "back pressure"?
Hawkins Mechanical Dictionary - 1909
RE: What is "back pressure"?
I'm probbaly being a bit too pedantic worrying about its definition.
Cheers,
John.
RE: What is "back pressure"?
I first took Hawkin's definition to be an unusual instance of what we've all being referring to - downstream obstructions causing pressure losses.
But reading it again, it is something quite different, if I understand it correctly.
Is he talking about the pressure on the underside of the piston? Perhaps that was the original use of the term. That's a completely different concept to the one used today. Interesting.
Poetix said "Or it could be thunk of as the pressure on the "back-side" of a piece of equipment." Is that what Hawkins is saying? There is a "back-side" of a steam piston.
Mark31 - what is a back-pressure turbine, please?
Cheers,
John.
RE: What is "back pressure"?
The same "backpressure" term is also used in steam trap applications. If there is, say, 5 PSIG in the condensate system, then the pressure on upstream side of the trap would have to be greater than 5 for there to be any flow at all.
RE: What is "back pressure"?
RE: What is "back pressure"?
Thanks for that contribution. I think the term is a sensible one if properly understood (perhaps I didn't understand it before.) Several persons have given good explanations like yours.
Cheers,
John.
RE: What is "back pressure"?
RE: What is "back pressure"?
Thanks for that.
I think you've introduced a new interpretation. The plumbers of Canada view is not the one used by process engineers, I think.
I'm splitting hairs, but "pressure higher than the supply pressure" can't occur. I see what it means to say, but the downstream pressure can't exceed the supply pressure - if it does, the downstream becomes the supply and vice versa.
Boy, I'm glad I didn't ask for the meaning of life. <chuckle>.
Cheers,
John.
RE: What is "back pressure"?
Backpressure is also a serious consideration when designing let's say, an exhaust system on a two stroke engine. The ported, not valved, Fairbanks Morse diesel for one, needs exhaust backpressure to produce proper firing pressures. When that engine is used on a submarine(every sub has at least one), the exhaust is discharged underwater(when at snorkel depth), but the intake is above the surface. Wave action causes the exhaust to experience pressure excursions that will make the engine surge. The surging will allow the generator to produce unstable power. Therefore, the engine must be provided with heavy steel blower rotors vice the standard alum ones when used in that application. Steel rotors require the complete blower be redesigned to handle the loads associated with the massive rotors.
Considerable backpressure exists also whenever I insist my woman keep my house clean.
Call it what you want, but don't deny that it has to be considered in most all things. I just figure it as something that absorbs some or all of the energy I'm trying to direct into whatever I'm trying to overcome. :)
RE: What is "back pressure"?
athomas236
RE: What is "back pressure"?
I didn't mean to say that reverse flow cannot occur - the Canadian Plumbing Code warns against it, so who am I to argue? I was being extremely picky about the wording used - best forget it.
In asking a simple question "what is backpressure?" I've been educated about all sorts of processes and machines. From gas turbines to steam engines. From domestic hot water systems to engines on subs. Patient oxygen delivery systems and boiler relief valves. All useful.
And we turned up the Hawkins Mechanical Dictionary - 1909; full of good advice, such as a definition of a fax machine.
All of this has been more interesting than finding a definitive explanation of backpressure. My feeling is that those who use the term know what they mean.
Cheers,
John.