Point of free discharge 1

[bif]

I have a a scenario where I have to pump up 30m vertically from a datum level to get over a building. The pipe is then ran back to ground level where it discharges atmospherically into an open pit. The discharge point is pretty much at the datum level.

My understanding of static discharge head is that it is the difference between the pump centreline and the point of free discharge.

Where do I assume the point of free discharge is?

Brief details are
The pump will be used intermitently
When the pump is not running the line will be self draining both ways.
The pipe is a fixed diameter

 

[khardy]

Since you are using the pump intermittantly, and the line will drain when not in use, you have to size the pump to get the fluid all the way up to the top. However, once there, the pump's job gets easier and easier as it comes back down the other side untill at the bottom, at which point there is almost no static head. Think of a siphon.

 

[katmar]

Whether you recover the static head on the way down again depends on the flow rate in the pipe. If the pipe is oversized the down leg will not run full and you will not recover any static head. In this case the pump will always "see" 30m.

If you provide flowrates and pipe dimensions I can comment more specifically.

Harvey

Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com

 

[JJPellin]

Even if the pipe is running full, you will not recover the full 30m unless you hold a back pressure at the open end of the pipe. Atmospheric pressure can only support 32 feet of water. You would draw a vacuum at the top and only recover 32 feet on the line down the other side. If you hold back pressure on the free end of the pipe so you don't pull a hard vacuum at the top, you still have that extra head requirement to overcome. The pump has to be designed to produce the full head to push full flow to the top of the high point. And under ideal conditions, you will still need to produce about 20m of head when flowing a full pipe since you put about 30 m up one side and only recover about 10 m down the other side. Also, make sure the piping system can handle full vacuum at the top.

Johnny Pellin