Pump Impeller Vane profile 5

[Trojan]

On a standard Francios Vane the type impeller I have read that the curvature reduces the shock losses of the liquid going though the impeller and by imparting kinetic energy to the liquid, the head is increased toward the OD of the impeller.My question is this... How is the pressure head created ??? Is this the velocity head that increases and is then converted to pressure energy though the volute ?

 

[VenkatV]

Dear Trojan,

You are absolutely correct. All centrifugal pumps impart kinetic energy to the fluid passing through it. This kinetic energy (Velocity head) is converted to pressure energy in the volute casing which has an gradual increase in area. Due to the gradual increase in area the kinetic energy available in the fluid is converted in to pressure energy.

regards,

V.Venkat.

 

[tstead]

Trojan:

You haven't been duped by a pump mfgr's sales literature have you? Paco pumps markets their pumps saying that they use "patented" Francis Vane impellers and talks about the physics of their impellers and tries to sell their pumps based on some supposed engineering marvel. The problem is, their "engineering wonders" apply to just about any radial vane impeller used - not just the Francis vane.

Many impeller vanes run perpendicular to the impeller shroud from the eye to the outlet. What makes the Francis vane (F.V.) different is that it twists along this route. While your description of the kinetic energy is correct for a F.V. impeller, it is equally correct for all other radial flow impellers.

Just my $0.02 worth

 

[rdcowan]

Concerning the conversion of kinetic energy into pressure head, an interesting way to think of a centrifugal pump is as a 'throwing device', not a 'pumping device'. Let me illustrate it this way: Get your basic physics book and look at the formula for a projectile thrown straight up. The formula says that h = V^2 /2g where V is the initial velocity, g is the acceleration of gravity (32.2 ft/sec) and h is the final height where the projectile stops (we'll call it feet of head!). Now, calculate the tangential speed of a point on the diameter of an impeller at some rpm and assume the pump 'threw' a projectile straight up at this speed. (Like a baseball pitcher rotating his impeller..er..I mean arm and throwing a baseball straight up.) How high would it go? Let's see.

Given a 10" impeller, 1800 rpm

speed = (3.14159 x (10/12)) x (1800/60) = 78.54 ft/sec

h = (78.54^2) / (2 x 32.2) = 95.78 feet !!

Take a look at a standard centrifugal pump curve for a 10" impeller and 1800 rpm. You'll see that the shutoff head is not far from this. As a matter of fact the shutoff head from the curve will be slightly higher because we haven't taken into account the additional velocity component resulting from the liquid moving RADIALLY out from the eye of the impeller. So, the shutoff head is how high the pump could THROW liquid before it would begin falling back to earth. This little exercise has always helped me understand centrifugal pumps better. - Enjoy!

 

[BillBirch]

rdcowan,
Nice one! I've always enjoyed those analogies. It all comes back to Isaac Newton!

 

[rdcowan]

To carry the analogy a bit further: If the baseball pitcher (pump) can't get the ball out of his glove (suction) with his pitching arm/hand (impeller) smoothly, then he could experience disruption (cavitation) of his normal delivery resulting in a bad pitch (reduced performance,damage,etc.)...

 

[FEH22]

I'm looking for information on how to design a Francis vane impeller. It sounds like you guys might be knowledgable in this area. Can someone point me to a good reference that gives the specifics of the Francis vane geometry? A search for "Francis vane" brought up your thread. I hope you don't mind me "scavaging" off it.

 

[vanstoja]

FEH22
See thread407-81742
thread407-27928
For more info on impeller design, run a search on "impeller design" to see the latest 100 threads containing info on the subject. You latched on to a 3-year old thread which has been dead for that long. If you want to continue this query further you should start a new thread. In the 2nd thread cited above, from my input I would rate books by Lobanoff&Ross, Stepanoff and A.H.Church as the best for vane development methods. The first two have editions that should be purchasable now but Church's book may be out of print.
In this thread, rdcowan's velocity head equation for shutoff head doesn't work so well for a 3600 RPM, 12 inch impeller of about 4000(RPM-GPM-Ft.) specific speed giving a head rise of about 50% between bep and shutoff which is way too high for a tested powerplant coolant circulating pump and, generally, for the higher specific speed pumps,perhaps as well as low SS pumps (<1000)where head can actually droop toward shutoff.