site glass on above ground storage tank 1

[MPKG]

I have a flow problem involving a vented site glass on the suction side of a pump. The system consists of a 10,000 gallon vented AST piped into a distribution pump with 2" bcs pipe. I tee'd into this pump intake line with a piece of clear PVC thinking this measures the static pressure of that point so the height of the liquid in the site glass would be about the same as that in the 10,000 gallon tank at pumping conditions since the tee is only 2 feet downstream of the tank outlet and the frictional losses would be negligible. Instead the pump sucks the site glass dry. Where did I go wrong? I am thinking the answer to my dilemma involves transient analysis but I do not know too much about fluid mechanical transient analysis.

 

[rbcoulter]

The tube should measure the STATIC pressure in the pipe which should be your tank level. That is if your pump is OFF. Turn the pump ON and all bets are off. My guess is that the velocity is so high where your clear pvc connects to the suction that their is some vacuum pressure there (the eductor effect). Consult bernoulli's equation.

 

[Scipio]

I'd expect the liquid level in the site glass to fall dramatically faster than in the tank when you first start the pump, but barring an eductor effect like rbcoulter mentioned, should recover once flow is stabilized. Sort of like one of those coffee urns with a site glass in the front, you fill a cup of coffee from one of those and the site glass will almost empty into your cup than recover. I'd normally expect to see this in a high-acceleration system - in fact installing a standpipe is one means of eliminating acceleration head in a suction system. Easiest thing to do is put a restriction orifice or a valve on your tee so you can restrict flow from the site glass to the pump suction. Pressurized flow's like electricity, follows the path of least resistance, just make sure it's easier for the pump to draw suction from the tank and you're set.

 

[BobPE]

how much are you pumping in this system? I would think your pipe is teeded into the vena contracta formed at the entrance of the suction pipe into the tank during flow. Depending on the flow, this flow disturbance can extend several feet into the suction pipe. I would not recommend putting any flow or pressure device in this regeion as it will see the same effects you are seeing with your device.

BobPE

 

[MPKG]

Thanks for the responses. I followed up with the vena contracta idea and figure with 2" suction pipe and a flow of 118 GPM, it could be 8.6 feet before flow leaving the tank is fully developed -- in this contracted region (with 10' of water in the tank) if the flow diameter contracts to less than 1.38" then vacuum pressure results. With this being the case, I do not think I will be able to restrict the vented site glass branch to make it work with the pump running, but perhaps increasing the diameter of suction pipe or extending its length would do.

 

[TD2K]

I don't know what your suction piping sch but assuming ID = 2" at 118 gpm, your inlet velocity to your pump is just over 12 feet/sec. That's pretty high especially with small bore piping when pressure drops can mount up rapidly at higher velocities.

What are you pumping? I'd aim for a suction velocity in the range of 3 feet/sec and then check to see how much dP I'm taking over the suction piping before selecting a final size.

 

[BobPE]

TD2K is right on in his thinking. Suction velocities should be in the 2.5 fps range. That alone is a great reason to increase your suction pipe diameter. I don't know why, but more times thean not I find elevated suction velocities in industrial applications more so than public applications.

It seems like you have a good understanding of whats going on now, glad we could help....

BobPE

 

[saxon]

Youngguy, Why not just invest in a compound pressure gage (positive/negative press.)within the pressure range expected and install it where the level sight glass is located? You'll get an accurate reading on the one hand and on the other, you'll eliminate sucking air into the pump suction which can cause massive cavitation and premature pump failure.

Hope this helps,
saxon