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The plant maintenance supervisor or the water system manager is diagnosing a pump system to ensure everything is operating properly.

 

He notices an apparent loss of production, which could be a sign of wear. He knows the pump performance data and he is familiar with piping losses, so he knows what his gauge should read downstream near his point of use.

 

The problem appears to be that the pump is not performing to the curve. However, there is more going on than he expected!

 

In this example, a pump was designed to pump 300 gallons per minute into a system of 4” PVC pipe for 100 ft, then reduced to 2” PVC to a point in the plant 60 feet away, before going into the process. The process requires 80 psi. The gauge is reading 75 psi.

 

Total Dynamic Head (TDH) was calculated as 255 ft (80 psi x 2.31 + 4.3 ft friction in 4” + 66 ft friction in 2” and no elevation change). The pump that was selected produced 300 gpm at 255 ft TDH. The maintenance supervisor we described above read only 75 psi on his gauge. Changing the gauge, he found the new gauge read 75 psi as well, so he had the pump diagnosed.

 

When a gauge was placed near the pump it read 110 psi, which is very close to 255 ft TDH. The water was pure, and the piping should not be affected by buildup on the walls, but his gauge was still off by 5 psi. What is going on here is a misunderstanding of TDH.

 

TDH is a measurement of all energy in a flowing system. Most applications consider elevation change, pressure, and friction losses. But there is another component often overlooked we term “velocity head”.

 

When liquid enters a smaller pipe the velocity increases and a gauge on the smaller pipe reads less than the larger pipe. This is due to the change in energy from pressure to velocity, but no energy is lost, only transferred.

 

In the example, 300 gpm was flowing at 28.7 ft/sec. (This number is available in most friction loss charts) Using a formula for calculating velocity head (v2/64.2) the Velocity head was 13 ft or 5.62 psi. Add the 5.62 psi to the 75 psi that he read on the gauge and we get 80 psi as expected. Water entering the process is at 185 ft TDH and easily converts back to 80 psi.

 

The point of this scenario is that TDH is a measurement of total energy in the system and pump requirement. In this case, the process device was furnished with the required 300 gpm at 185 ft TDH. When the pressure gauge was viewed at only 75 psi, the entire energy was still present, but the gauge could only read the pressure portion of the energy. Had the piping been the same diameter all the way through, the original findings would have been accurate. But different diameters mean different velocity and a drop in pressure gauge readings.

 

If you want to dig deeper into this topic, Bernoulli’s Principle details energy within a piping system.

 

 

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