Pressure Problems 2

[Patassa]

I've worked myself into a bit of a loop here.

A question came up from a younger colleague today that at first seemed a bit silly but after running a model I might see his point too. Basically, the question was if my pump discharge piping immediately sizes up say from 2" to 20" and I sized the line for pump discharge pressure, could I over pressure the line from almost all of my velocity energy being converted into static energy?

What's curious is that I ran a model with a 2" discharge, 3 inches long, into a 20" line for 10ft then back to a 2" into a tank. The software is telling me that my pump TDH is 40.38 feet, discharge pressure of 19.43 psig, in the 2" discharge piping I only have 4 psig of static head pressure. This seems to be telling me that a pressure gauge on this short section of discharge piping would only read 4 psig. The static pressure then jumps up to 19 psig in the 20" line. So it seems to be that discharge pressure gauges on pumps can be incredibly misleading in cases such as mine? That the TDH converted into pressure is total developed pressure also, but not necessarily static pressure or what I'll see at the discharge of the pump on a gauge? (Is this correct?)

Though converting the TDH into pressure still gives me the correct design data and I'm in no risk of over pressuring anything downstream, it does seem that my discharge pressure gauge readings need to be read in the context of my piping diameter, as my downstream pressure gauges could actually show an INCREASE in pressure if my diameter were to increase.

Am I understanding this correctly?

 

[someguy79]

A couple words of caution:

Pitot tubes are good tools for measuring velocity head in a small region. However, the velocity head as typically calculated for a fluid flowing in a pipe or duct is based on the average speed of the fluid along the duct/pipe axis. If you use the velocity head you find with a Pitot tube and attempt to apply it to the full flow in a pipe, you will have errors.

"If the Y column is total developed head and I convert this into pressure, is this pressure my total pressure (dynamic + static)?"
Be careful not to assume static head and static pressure are directly related. If the fluid is not moving, and in a uniform gravitational field (close enough most of the time) then they correlate nicely. If fluids are moving, interaction with velocity head and friction will play havoc in that conversion. If you understand the rest of this conversation, I shouldn't have to explain it much more.

 

[Patassa]

Someguy79, thanks for bringing this back on topic.

I guess what I'm now confused about is what exactly total dynamic head on the pump curve is giving me in relation to the discharge pressure of my pump at some given flow rate. If I stuck a pressure gauge on the discharge piping (as typically seen on an operating unit) of the pump and use my TDH to calculate the pressure differential across the pump, add it to the suction side pressure, will I see this number on the gauge or will I see some number lower than this number bc the gauge is only reading static pressure on the pipe wall and some of my TDH has gone into dynamic pressure?

I suppose I could just calculate it from an actual case on one of my units but I was hoping to clear up the theory first.

 

[bimr]

The dynamic pressure (velocity head) will be the same on either side of the pump assuming your pipe size is the same.

As stated above, the Dynamic (velocity head) will be relatively small at the typical flow rates used for industrial applications and may be neglected in your calculation.

Suction Head = Total Static plus Dynamic, measured at pump inlet.
Discharge Head = Total Static plus Dynamic, measured at pump exit.
Pump Head = Discharge Head minus Suction Head plus correction for the difference in gage elevation.
Static Head is what the absolute gage reads, converted to feet of water.
Dynamic Head is the same as Velocity Head

Review the attached paper.