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Aksheyac (Mechanical) (OP)
17 Jul 12 10:20
Hi. . Can anyone tell me exactly what back pressure in pumps refers to? What is a high back pressure pump? From what I know back pressure is the resisting pressure so wouldn't high back pressure mean lower discharge pressure?
TD2K (Chemical)
17 Jul 12 11:13
I don't use the term 'back pressure', suction pressure or discharge pressure is less ambigious in my opinion. Looking on the net back pressure seems to be used as another term for discharge pressure.
BigInch (Petroleum)
17 Jul 12 11:22
It's discharge pressure. What else could it be.

If it ain't broke, don't fix it. If it's not safe ... make it that way.

Aksheyac (Mechanical) (OP)
17 Jul 12 12:15
That's what I assumed as well, but when I look up discharge pressure there are lots of varying definitions, mostly referring to pressure drop. Discharge pressure isn't a technical term I've come across anywhere, which is why I'm a little confused. Some places it's an alternate name for discharge pressure, some places it's resistive pressure.
bimr (Civil/Environmental)
17 Jul 12 12:45
It is defined as the resistance to a moving fluid by obstructions or tight bends in the confinement vessel along which it is moving, such as piping or air vents, against its direction of flow. Because it is really resistance, the term back pressure is misleading as the pressure remains and causes flow in the same direction, but the flow is reduced due to resistance. For example, an automotive exhaust muffler with a particularly high number of twists, bends, turns and right angles could be described as having particularly high back pressure.
Helpful Member!  psv1990 (Mechanical)
17 Jul 12 13:04
Back pressure tends to be used in reference to positive displacement pumps. These pumps increase pressure in response to increase resistance to flow.
In centrifugal pumps, the term back pressure tends to refer to the minimum discharge pressure to avoid the pump operating off the curve.
Aksheyac (Mechanical) (OP)
17 Jul 12 13:13
That's the most useful thing I've read yet, thanks. So basically it would depend on the type of pump. So when I say 'high discharge high back pressure centrifugal pump' I basically mean high operating pressure?
psv1990 (Mechanical)
17 Jul 12 13:25
It would mean a pump that has both high discharge presure and a high discharge rate. It also requires a large motor. I try to avoid terms like "high" as in some industries high pressure is 150psi. In others, it may be 5000 psi. The more information you get about a system up front, the less time spent guessing and working on the wrong solution.
BigInch (Petroleum)
17 Jul 12 13:30
Yup. Discharge pressure.

If it ain't broke, don't fix it. If it's not safe ... make it that way.

Aksheyac (Mechanical) (OP)
17 Jul 12 14:07
Thanks guys.
1gibson (Mechanical)
17 Jul 12 14:22
To me, it describes a point on the system curve that the pump operates at. So yes, instantaneously, that is discharge pressure of the pump.

But it could also be used (incorrectly!) as a layman's definition of a system curve. Kinda.
BigInch (Petroleum)
17 Jul 12 14:31
"Backpressure" is more correctly used, not as a pump term, but as meaning the pressure held behind (upstream of) a "backpressure" control valve.

If it ain't broke, don't fix it. If it's not safe ... make it that way.

bimr (Civil/Environmental)
17 Jul 12 16:59
A pump can not develop pressure unless the piping system creates backpressure (ie: Static (vertical height), and /or friction loss). Therefore the performance of a pump can not be estimated without knowing full details of the system in which it will be operating.

Back pressure is not important to positive displacement pumps. Positive displacement pump provides an approximate constant flow at fixed speed, despite changes in the back pressure. Changing back pressure on a positive dieplacement pump results in a minimal flow change.

However, the flow of a centrifugal pump will change considerably with back pressure.

Back pressure is simply the resistance to fluid flow in a piping system. Back pressure is measured by obtaining the discharge head of the operating pump.
Compositepro (Chemical)
17 Jul 12 19:04
The term Back pressure can mean whatever you want, because there is no strict definition. When I use the term it is usually to indicate the the pressure is controlled by something downstream from the pump (like a back-pressure regulator).
Helpful Member!  ciise (Chemical)
18 Jul 12 11:21
I am not an expert, just pump applications guy. I come across this term alot. Say I am pumping into a pressurized vessel(50PSI) which is 20ft above where the pump is. back pressure is 50PSI, elevation is 20ft. Discharge pressure has to overcome 20ft elevation, minor losses and 50PSI back pressure.
BigInch (Petroleum)
18 Jul 12 13:59
So in terms of pumps, the pump's discharge pressure must be 58.6 psig (assuming 20 ft of water), plus minor losses.

If it ain't broke, don't fix it. If it's not safe ... make it that way.

bmw318be (Mechanical)
13 Jan 13 11:45
Is a + suction pressure a back pressurre, I ve encountered some bunch of enginner claimed that we should size a external pressure relief valve by consideing the given suction pressure to be a back pressure as they relief back the pressure to back the suction line.

It is ridiculous how could this be treated as back pressure
Pumpsonly (Mechanical)
13 Jan 13 12:01

If you are relieving back directly to the pump suction, the pump suction pressure become the back pressure of the relief valve. You must size the relief valve base on the differential pressure across it.

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