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.

 

[Mark31]

I agree with vesselguy. I have ordered a few safety valves in my time.

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.

 

[a31ford]

Way up here (Central Canada) I have even heard the term "Back Pressure" and if it's here, it's gota be used MANY other places also... I mean if a "Hick" like me knows about it.... "LMAO !"

Greg ;-)

 

[poetix99]

I looked up the definition of "discursive" in my dictionary (I don't need a definition of "back-pressure&quot, and the first entry was:

"Thread 794-33900 of EngTips.com"

 

[quark]

Hehe! is your lexicon an electronic one? (very quick updation, ain't it?)

 

[JOM]

Good Lord - 7 repsonses to my last post in one day!

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.

 

[JOM]

Mark31,

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.

 

[TBP]

"Back Pressure - In a steam engine, the pressure on the opposite side of the piston, which opposes a resistance to the working stroke of the steam. This back pressure may be due to the pressure in a receiver, as in the high pressure cylinder of a compound engine; to that of the atmosphere, as in a non-condensing engine; or to that of an imperfect vacuum, as in a condensing engine."

Hawkins Mechanical Dictionary - 1909

 

[JOM]

Thanks TBP. Shows the term has a history longer that well and truly precede my training.

I'm probbaly being a bit too pedantic worrying about its definition. Cheers,
John.

 

[JOM]

TBP,

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.

 

[TBP]

John - steam engines had (have) double acting pistons. Steam is admitted/exhausted from both sides. The inlet steam on one side acting against the piston forces the exhaust steam from the other side of the piston. Near the end of the stroke, the slide valve mechanism moves so the process is reversed. The back pressure in this case is any pressure (even atmospheric) that exists to oppose the flow of the exhaust steam. If you can exhaust into a vacuum, you get a LOT more oomph out of your engine (or turbine) than if you are exhausting into a 10 PSIG building heating system. (This is exactly the same as a back pressure turbine.)

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.

 

[spector]

Back pressure....Like in a constant pressure feed water system where you want to maintain a set pressure in the feed header....That pressure is generated by the pump, but regulated by a valve that bypasses excess feed water back to the DFT thereby creating a 'back pressure'. When the boiler calls for water, the boiler feed valve opens and the resulting drop in pressure causes the back pressure valve to close reducing the amount of water returned to the DA tank. Several manufactures make valves designed for this application and they happen to call them "Back Pressure Regulators"

 

[JOM]

spector,

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.

 

[PEIAI]

The definintion of back pressure is in the Canadian Plumbing Code as "means pressure higher than the supply pressure". As an example a water main into a 5 story apartment buliding. The municipal pump pressure falls and there is no backflow prevention on the building the result is that the municipal system cannot supply against the head pressure.

 

[JOM]

Dear PEAIA,

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.

 

[spector]

Sorry JOM, but it happens everyday....Take a hotwater heater where the overtemp control fails....pressure in the tank will build up to where it overcomes city supply pressure, but is still below tank relief valve setting. Hot water produced in that tank can be used across the street(it has happened).

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.

 

[athomas236]

back pressure is the pressure at the exhaust of a turbine, sometimes used for steam turbines but more usually it is at the exhaust of a gas turbine.
athomas236

 

[JOM]

Dear Spector,

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 &quot;what is backpressure?&quot; 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.