Minor Losses in Pipes

[Blu1913]

Anyone know why you use the upstream velocity and not the velocity in the new section, at a pipe expansion?

You use the down stream for a constriction...do you just use the high velocity, period?

 

[petergibbons]

I'm not sure what you're asking but it sounds like a continuity equation type problem...Q1=Q2; therefore, A1*V1=A2*V2.

 

[JasonT33]

Yes not sure what you are asking but Peter is right... if flowrate is constant, velocity will decrease at expansion

 

[Blu1913]

Ok, heres the scenario:

if a 12" goes to a 6" (constriction), to calculate the (Hm) minor loss, you use the diameter of the 6" to find the velocity to sub into Darcy. ie. you use the DOWNstream pipe size for velocity

BUT, if that 6" is then expanded to an 18" pipe (expansion), you use the diameter AGAIN for the 6" pipe to get velocity, which in this case is the UPstream pipe...

Why dont you uses the diameter of the 18", the downstream pipe, to find the velocity in this situation?

 

[JasonT33]

Can you just write out the question the way it was given to you and I will show you how to solve it

 

[Blu1913]

THe question is:

Find the minor loss for point 1 (the constriction) and point 2 (the expansion).

Q= 3 cfs

I know how to solve the problem now, I just dont understand why you use the velocity at point 1 using the 6" diameter pipe and then at point 2, you ALSO use the 6" for the velocity again...

Q=VA ----> to get V you use the A for the 6" for both

Then plug into Darcy equation for head loss.

If you are moving upstream, why dont you use 18" for A in the expansion?

(my paint isnt working right and I dont have a scanner, sorry)

 

[petergibbons]

Ok, I could be completely misreading this, but here is my take: Q=3cfs; V1(at constriction) = ???; V2(at expansion) = ???; A1(at constriction = 0.196 ft.; A2 (at expasion) = 1.5 ft. Now take Q=V1*A1=V2*A2. Solve V1 and V2 using what is given. Plug V1 and V2 into Darcy.

 

[JasonT33]

Ok I am still not sure if I am understanding your question but here is my procedure to solving a problem with a change in flow area.

Figure out your K value (loss coefficient) for whether its a sudden enlargement or sudden contraction.

Once you have your K value, multiply this by your velocity head to get your head loss due to minor losses.

Is this how you see it?

 

[JasonT33]

ok for an expansion where v1 is the upstream velocity and v2 is the downstream velocity

hL = (v1-v2)^2 / 2g

for contraction

hL = K (v^2/2g)

K is a function of D2/D1

 

[JasonT33]

i'm sorry the velocity term in the 2nd equation should be v2