Pressure & flow 3

[eagertknow]

I have heard people saying "When flow increases presssure decreases".I cannot understand it as, I believe that Pressure=Force/Area and when flow increases pressure should increase because force will increase.

Please help.

 

[25362]

You are right, the original statement "when flow increases pressure decreases" is mistaken. It should read "when (linear) velocity increases pressure decreases" and flow rates stay constant.

Look for Bernoulli's equation for incompressible fluids that equates static, positional and velocity heads (pressures).

When considering constant flow rate of a liquid in a pipe of varying diameters, pressure gages located on narrower sections (where velocities are higher) would should show lower pressures than gages installed on wider pipe sections where the velocity is lower.

 

[katmar]

Like just about everything in engineering the answer is "it depends". It would be very easy to imagine a situation where "When flow increases pressure decreases". It just depends on where you measure the pressure.

Imagine a pump connected to 100 ft of piping and at the end of the pipe there is a valve discharging to atmosphere. The pressure gage is mounted just upstream of the valve.

If the valve is completely closed the gage will read the full available head from the pump. This is usually termed the "closed delivery" head. As you slowly open the valve and let the liquid discharge to atmosphere the flow will gradually increase and the pressure on the gage will slowly decrease. The pressure decreases because there is the friction loss between the pump and the gage, plus the curve of the pump will most likely result in a fall in the delivery head as the flow increases.

So the statement CAN be true, but isn't always ;-)

regards
Harvey (Katmar)

 

[25362]

Eppur si muove. BTW, katmar's example doesn't negate my statement that "when pressure decreases velocity increases" and viceversa. Whether the flow rate increases, decreases or stays unchanged is wrong, irrelevant, or as katmar says: "it depends".

You are right in that pressure is needed to create, increase or mantain a liquid flow against a given resistance.

Resistances depend on both the reaction of a "passive",
physically unchanging system, to an increasing flow
(friction), or to the changing properties of an "active" system, for example, a throttling valve, which also means added friction.

If the resistance increases, as by closing a valve in a pipe, the increasing "pushing" pressure may or may not suffice to increase or even keep the previous flow rate. When the resistance becomes extremely high, say, a closed valve, no practical pressure increase will surmount it and zero flow will be the result. "Pressures up and flows down" is the result, confirming katmar's comment and exercise.

BTW, katmar's example, refers to a common fact when opening a household faucet. Water flow increases and static pressure, ahead of the valve, drops.

The centrifugal pump -a dynamic machine- is a good example of a fixture generating velocity in exchange of head. The expanding volute converts the velocity-energy into pressure-energy. At shutoff (maximum flow resistance) we have zero flow and the developed pressure may reach a maximum.

One (myself included) tends to speak of pressures and velocities, and should however, better think in terms of velocity (kinetic)- energy, and pressure-energy.

Thus when we see the characteristic curve of a centrifugal pump with head dropping with increasing flow rates, we should think of the BHP, that may have to increase with flow rates, as the pump loses efficiency.

For compressible fluids, not even the "pressure up, flow up" or "pressure up, velocity down" statements always hold, even for what I call "passive" systems. Examples would be the inlet diffuser of steam vacuum ejector where the velocities are supersonic, or a gas flowing through an orifice under critical flow conditions.

 

[nagarajh]

What the statement means is, when the flow velocity increases, the static pressure decreases and vice versa. This is the basic Bernoullis' principle. The total pressure = Static pressure + Dynamic Pressure is always constant.

 

[ercanbaser]

To understand the nature of this statement think as
pressure+flowrate="total energy" like the statement;
potential energy+kinetic en.=total energy so,

if the total energy is constant when the flowrate increases, the pressure decreases and vice versa.

In other words, at a point of flowing fluid if we know the pressure and flowrate of a fluid ,it means we know its energy capacity in static and dinamic views.

Another way of think is, if i want to increase the flow where can i find the energy to do this? of course I have to decrease pressure and to increase stat. pressure I have to decrease flowrate.

regards,

 

[martintw]

is your question in relation to a centrifugal fan or pump. If so ( for most impeller types) the pressure will decrease as flow rate increases but the motor power will increase. This is why you start centrifugal pumps/fans against a closed outlet - but re-confirm this with the vendor as some equipment has modified characteristics.
n general, along a pipe or duct the total pressure (excluding minor losses remains constant, as Nagarajh and Ercanbaser have pointed out. One of the classic examples is the venturi tube where velocity is increased then decreased. The dynamic pressure increases at the throat but the total pressure remains constant, so when the fluid slows down at the downstream end of the venturi, the dynamic and static pressure are the same as at the entry, apart from minor losses due to friction, eddies etc.

 

[rmw]

Eagertknow,

Upon reading your post, I immediately associated the comment with pumped flow from a centrifugal pump. A quick look at the typical pump curve will show that the pressure (head) of an operating pump decreases as the flow increases. I have found that plant personnel unschooled in nuances such as pump operating characteristics have a hard time understanding this intuitively inverse relationship, and have made the same comment multiple times in my career.

RMW

 

[Hookem]

Think of it this way: without any differential pressure, there is no flow. with differential pressure, flow begins; with increasing differential pressure, flow continues to increase (up to certain limits). Flow and velocity and pressure are all INTER-RELATED. Can't have one without the other.

 

[25362]

You can look at it as with the Ohm law on electricity: flow F depends on two factors: pressure differential P, and resistance to flow R, in a relation as

F=f(P)/f(R)​

Where f( ) means function of.

So, an increase in F can be caused by an increase in P or a decrease in R, or both. Under any given conditions an increase in f(P) by, say, 50% may not result in an increase in F at all, if f(R) increases by more than 50%.

f(R) depends on the system, and the fluid characteristics.
f(P) is not a linear function.

Is this presentation easier to grasp ? I hope so.