Bending force at suction inlet?
Bending force at suction inlet?
(OP)
Hello, this is perhaps a stupid question but my mind is playing tricks on me.
At a discharge pipe outlet a stationary force equal to F=m*V creates a bending torque at the support.
If the flow is reversed - what force/torque is developed and in what direction?
Please see attached figure
Thanks
At a discharge pipe outlet a stationary force equal to F=m*V creates a bending torque at the support.
If the flow is reversed - what force/torque is developed and in what direction?
Please see attached figure
Thanks





RE: Bending force at suction inlet?
RE: Bending force at suction inlet?
Pressure inside pipe
Discharge: P1=P2 = P_atm = 0
Suction (smooth inlet): P1=P2 = P_atm - 0.5*rho*V^2 = 0 - 0.5*1000*10^2 = -50000 Pa
Force Fx,Fy
Fx=m*V*sin(90º)+p*A*sin(90º)
Fy=m*V*(1-cos(90º)) + p*A*(1-cos(90º))
Discharge: Fx = 314.15*10*1+0*0.031415*1 = 3142 N
Fy = 314.15*10*(1-0)+0*0.031415*(1-0) = 3142 N
Suction: Fx = -314.15*10*1+(-50000)*0.031415*1 = 1570 N
Fy = -314.15*10*(1-0)+(-50000)*0.031415*(1-0) = 1570 N
Is this correct? Is the force during suction only half for discharge? Somehow I always imagined the force would be equal.
RE: Bending force at suction inlet?
Furthermore, due to the fact that the inflow into the pipe is not perpendicular to the inlet the usual "momentum method" is not straightforward to use.
I had a similar problem myself. I was asked to approximate the force on the suction side of a pump. My conclusion was that there is a outward force (same direction as during discharge) with a magnitude of F=m*V-p*A.
RE: Bending force at suction inlet?
RE: Bending force at suction inlet?
@Sch10Sean: I noticed my aritimetic mistake, thanks. My problem also involves a pump inlet but I wanted to simplify my question. I think I understand how you came up with your force but if you could (or someone else) explain it to me I would be grateful. Thanks again.
RE: Bending force at suction inlet?
If you want to "feel" it, then consider the discharge scenario. Flow "hits" the bend (fy) and there is a force at the exit (fx). Since it is anchored at the bottom in your case, there's a momentum at the anchor point. Now, consider the suction scenario: Flow hits the bend (fx) and there is a force at the exit (fy).
BTW, if you consider the dynamic pressure(0.5*rho*V^2) in your calculations, then you should consider it for the both scenarios. You've considered this for suction condition but not for the discharge. that's why the forces in two scenarios are different in your calculations. In reality, they are the same provided the discharge condition is the same, i.e same pressure (if you are dealing with fluid, same fluid level).
And the final note, Patm is not zero. it is 100kpa. make sure that you keep your pressure reference the same along all your calculations if you assume Patm=0.
Hope this would be helpful.
RE: Bending force at suction inlet?
RE: Bending force at suction inlet?
I have to add that mu assumption was for an identical pipe diameter through the discharge pipe, otherwise the forces differ in the reverse flow direction.
I hope that the following solved problems clear up the case for OP. You can try to solve them for the reverse direction for a better understanding. Both examples are from the book "Fluid Mechanics With Engineering Applications- Finnemore & Franzini".
&