Static Head in Force Main with Highpoint 1

[BluDev]

How do you calculate the static head in a force main with a high point in the line?

For example, the suction water surface is at Elev. 0', the discharge water surface elevation is at Elev. -10' and there is an intermediate high point with an Elev. of 10'. Note, there is a combination air valve located at the high point.

So, does the static head = discharge water surface - suction water surface

**OR**

static head = force main high point elev. - suction water surface.

Thanks.

 

[rcooper]

BluDev,

You are right on both counts.

When the pump is switched off, the air valve will allow air in at the high point. This will allow the pipe downsteam of the high point to drain to the discharge. The static head is then given by

static head = high point - suction water surface. (1)

As the pump starts to pump, the water will weir over the high point, and the pipe downstream of the high point will run part full. At this point the static head is as above, and the pump is only pumping into rising section of pipe.

At some point, the flow rate may become sufficient for the pipe downstream of the high point to run full. At this point gravity is insufficient to drive the required water through downstream pipe, and the pump will start to pump this water. The whole pipe is now acting as the system. When this happens the static head is given by

static head = discharge water surface - suction water surface (2)

To work out how your pipe will function, draw the long section of your pipe. Calculate the hydraulic gradient in the pipe (f*V^2/(2g)) and working backwards, plot the hydraulic profile on the long section. The hydraulic profile can be above the pipe, but if it falls below the pipe, run it along the sofit of the pipe until the pipe gradient reduces. If the hydraulic profile is above the pipe profile throughout the pipe length, then case 2 applies, otherwise case 1 applies to some degree.

If you have an instance where the pipe is running full, then be careful at pump start up. At this point the pump is running in case 1 conditions, essentially a different system to which it is selected for. It is possible with some pumps for very large loads to be drawn in these circumstances, resulting in the pump tripping when it is started.