PUMP TDH CALCULATIONS

[allie075]

Hi guys,

I have two questions for you:

(1) Should the static head for a wet well pump be checked from the level of the 'pump-on' float switch or from the 'pump-off' float switch?

(2) If there are points along the pressure main where the pipeline elevation drops below the elevation of the wetwell, should the static head be the maximum difference in elevation along the pipeline or still the difference between the free water surface in the wet well and the maximum elevation along the pipeline?

Thanks in advance.

Regards,
Allie

 

[jgailla]

allie075,
1. I always calc it to the pump off level.
I assume you're asking about a sewage wetwell, in which case the difference is negligible due to the inaccuracy of the friction head calculation.
2. For a long line with fluctuating elevation, you should be looking at the total head at key points in the system,
I had a lift station project last year in which the static head measured from the pump to the highest point on the line was 30 feet.
The high point was at the beginning of the line.
The point of discharge ended up being about 20 feet of static head.
In this case the 20 feet of static head plus the friction head in a full pipe controlled.
If the 30 feet of static was near the end of the line, that head would have controlled.

You have to look at the maximum head condition the pump will see, which could be mostly static or mostly friction.

You also have to worry about vacuum and air release valves, although the combination valves can be placed at changes in elevation and slope.

 

[allie075]

jgailla,

On your second point, forgive me but I'm confused. If the pump is designed to overcome a static head of 20ft, how would it possibly overcome 30ft? If it does, wouldn't that reduce the design life of the pump anyway? Please explain.

On your first point, I have new info from this book by Qasim called Wastewater Treatment Plants - Planning, Design and Operation. He indicates that both (max. and min. static head) should be used to develop system head-capacity curves at either end of the operating range. These curves are then superimposed unto the manufacturer's pump head-capacity & effciency curves to find an appropriate pump operating point which gives the head and capacity of the pump for the given application. So, I'm going with this.

Regards,
Allie

 

[JohnGP]

allie075,

He did explain - the controlling condition was the 20ft of static head at the end of the line plus pipe friction. When the line is empty, the pump had to be able to initially overcome the 30ft of static head near the start of the line. Once fully flowing, line headloss obviously exceeded 10ft.

 

[BigInch]

At low flowrates (ie. startup) the hydraulic grade line (HGL) line is nearly flat, thus pump head (the vertical coordinate at the beginning of the HGL) would have to be above the highest point in the pipeline (30 ft), no matter where that 30 ft elevation was located.

At higher flowrates the HGL assumes a slope equal to head loss due to flow friction per foot length of pipe. Starting at the pump discharge head elevation that sloped HGL line is plotted, which must be above all high points located anywhere on the pipeline (in order to maintain single phase flow).

The "Control point" is found by noting the minimum vertical distance between the HGL and the top of pipe elevations at any and all points along the pipeline. The HGL vertical coordinate above the control point plus the friction head loss from there back to the pump gives the discharge head value that must be supplied by the pump.

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"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

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