Specific Gravity impact on the system Curve

[mzafar00]

Hi,

I am running calculations to build a system curve for a pump. From what I understand the S.G. of the liquid being pumped has no impact on the head loss in the system. As long as the pump can provide enough head, I should be able to push the water out at a certain flow rate (intersection of pump and system curve point). I will obviously take into account the increased HP required.

BUT, I've come across differing calculations.
Flygt slurry pump handbook states you have to use a friction loss correction factor to calculate the head loss when using SG not equal to 1.
On the other hand, other online friction loss calculators (Goulds Pump selector) give the same head loss whether I use SG 1 or SG 1.5.

Could someone clarify this please. And if we do use a correction factor, link to a table. Thank you so much in advance.

 

[someguy79]

[original post deleted]

I've removed my original post. I'll put more thought into it next time. Sorry for presenting incorrect info.

 

[Artisi]

SG has no effect what so ever on pump performance:
Suggest you read the following, plus there is unlimited info on the net regarding this subject.

http://www.pumped101.com/sgviscositypart1.pdf

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[LittleInch]

Flygt are only looking at slurries with water and suspended solids. They can therefore predict the change in viscosity based on SG and percent of solids compared to pure water

Most other systems can't and if the viscosity remains the same then the pressure drop (system curve) will remain the same. This is quite unusual and therefore when the SG changes so does the viscosity.

As SG rises, then so does the pressure for the same head. Pump power also rises in proportion.

someguy79 - I don't believe your first para is correct. Static head is static head, usually a physical thing -height of a hill or end point e.g.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[BigInch]

somegu, following your own logic, "Pump curves are shown in head instead of pressure. This allows them to be independent of specific gravity (i.e. density).". System curves should be in terms of head loss, so therefore they are also independent of SG. Static, velocity and friction head are also obviously in terms of head, so therefore once again independent of SG. Velocity head is V^2/2/g. No SG there. Friction loss is actually head loss. You can convert to whatever pressure loss that is, if you know the fluid's specific gravity. Otherwise to calculate friction loss all you need to know is viscosity. Pressure is whatever pressure is depending on whatever SG the fluid has. Pressures will change if SG changes; however all curves made in terms of head will remain constant for whatever SG. That fact can be very convenient, which is exactly why these curves should always be in terms of head. They won't change no matter what the fluid's SG, as long as viscosity is independent of SG, which for most cases it usually is so.

 

[Artisi]

There are many factors influencing pump performance when handling slurries:
SG, solids concentration, shape- size and grading of the solids (uniformly mixed sizes, all one size), SG of the solids, etc. For true slurry handling applications care needs to be taken along with careful analysis of the effect it will have on the pump performance.
Weir pumps (Warman slurry selection data)and GIW have good selection information available.
Flygt and like companies are usually only interested in sewage or similar light and easily pumped "slurries".

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[LittleInch]

mzafrfar00 "The Darcy Weisbach equation states head loss is directly proportional to the SG of the fluid being pumped in a pipe"

NO it doesn't,

see

https://en.wikipedia.org/wiki/Darcy–Weisbach_equation

The equation where density comes into it is PRESSURE loss, not head loss.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[BigInch]

No Darcy Weisbach does not state that head loss is directly proportional to SG of the fluid. The equation you probably refer to is,

as seen here on Wiki,

https://en.wikipedia.org/wiki/Darcy–Weisbach_equation

That equation does show that PRESSURE LOSS per unit length of pipe DUE TO FRICTION is proportional to the SG of the fluid, however for HEAD LOSS that is NOT SO. Head loss is different than pressure loss because it is independent of SG.
This DW equation,

as seen farther down on the same Wiki page, clearly shows that HEAD loss due to friction is actually independent of SG.

Head loss due to friction only depends on viscosity only. Pressure loss is simply a derived unit of the hydraulic head loss process used by those engineers that for some reason prefer more complexity and want to introduce specific gravity into the equation so that they can speak in terms of pressure loss. Frictional Head loss for all fluids with the same viscosity is the same, regardless of their SGs, however different SGs will cause those equal head losses to show different pressure losses.

 

[BigInch]

LI, I got a phone call and you finished answering before I did. Good that we both said the same thing.

 

[mzafar00]

Oops. I think I accidentally read the pressure loss equation.
Reason why I ask this is our supplier, uses head loss through pipelines for slurry = head loss through pipeline for clean water * SG
I always questioned that and wanted to get concrete evidence before going back to the supplier.

http://www.pumpfundamentals.com/slurry/flygt_slurry_handbook.pdf

Page 42-43 confuses me, because even though flygt isn't in the proper slurry market they their handbook is using a correction factor to calculate head loss for slurry.

I'll read through the weir handbook and update

 

[BigInch]

The method may be valid, if the supplier has devised some way to predict some kind of "effective viscosity" of a fluid slurry based on the specific gravity of the fluid..

We are simply stating that the true parameter on which to base fluid frictional losses is viscosity and that theoretically IN THE GENERAL SENSE there is no theoretical relationship between specific gravity and frictional loss, nor is any necessary to determine head loss. That's not to say that a relationship between specific gravity and viscosity can not be found, or does not exist in what probably is a very large number of fluids.