Calculate Total Head

[ddkm]

This is a very fundamental question which I'm having a tough time explaining to a colleague. Maybe you guys can help.

Question:
What is the total static head in the following piping layout?

To me, the obvious answer is:

Total Static Head = 30m -24m = 6m

regardless of all the bends and fittings etc in the suction or discharge side.

But my colleague insists that as the suction side is not directly to the tank, therefore as a result of all the bends etc (before the pump), the fluid would be at zero velocity and zero potential energy at the suction point. And therefore zero head at the pump, resulting in 30m of Total Static Head.

Can anyone suggest a better way to explain this?

---engineering your life---

 

[BigInch]

It is 6 meters right now as shown in the diagram. If your supply tank drops to the minimum tank level, its 6 plus that much more. If the tank and suction pipe runs out of water, its 30 meters - distance from the reference line to the pump centerline, assuming your 30 m dimension is from reference to the hi-hi level of the discharge tank.

BigInch-born in the trenches.

http://virtualpipeline.spaces.msn.com

 

[BigInch]

When the system flows, don't forget to increase the total head by the equivalent pipe and fitting resistance head.

Your bud is thinking that head is the result of only v^2/2/g. The total head is z1 + p1/rho + v1^2/2/g - Hl = z2 + p2/rho + v2^2/2/g

BigInch-born in the trenches.

http://virtualpipeline.spaces.msn.com

 

[ddkm]

BigInch, agreed on all points. Also, abt your comment about tank being empty, this will be unlikely as there will be an auto cutoff once the tank reach low-level.



---engineering your life---

 

[BigInch]

Maybe when its drained for maintenance then. I think what you really want to figure out is what head the pump must be designed for, right? That is really only the 6 meters, as long as you can ALWAYS fill the suction line up to 24 + meters just before you restart again. If you can do that, no problem.

BigInch-born in the trenches.

http://virtualpipeline.spaces.msn.com

 

[djack77494]

ddkm,
When you say "Total Static Head" it's not totally clear what you mean. When I do pump calcs, I would normally have a static head on the suction side AND a static head on the discharge side of the pump. I consider both as I set up the calculation. Neither is 30 m, because you must always reference them to the pump suction nozzle centerline - not to grade as your diagram is implying. However, the difference is 6 meters, which accounts for the static head portion of the work that is done by the pump.
HTH,
Doug

 

[ddkm]

what head the pump must be designed for, right? That is really only the 6 meters, as long as you can ALWAYS fill the suction line up to 24 + meters just before you restart again.

Exactly my thinking. And in fact, the line will always fill up to the minimum level in the tank before the pump is allowed to start.

When you say "Total Static Head" it's not totally clear what you mean. When I do pump calcs, I would normally have a static head on the suction side AND a static head on the discharge side of the pump. I consider both as I set up the calculation. Neither is 30 m, because you must always reference them to the pump suction nozzle centerline - not to grade as your diagram is implying. However, the difference is 6 meters, which accounts for the static head portion of the work that is done by the pump.

Yes, your points were already considered because:

TOTAL Static Head = (Discharge Static Head) - (Suction Static Head)
= 30m - 24m = 6m

The heights used are referenced to the pump centerline for simplicity of the example. Also, the pressures on both tanks are equal (both at atmospheric).





Regards,
DD

---engineering your life---

 

[WebWizz]

To do the calc's I always use

http://www.engineeringpage.com/engineering/pumps.html

choose pump conditions and just fill out the form, calculates for minimum and maximum conditions as described in the other answers.