Fluid pressure question due to static head

[jmk407]

Hello,

This should be a relatively easy question, but it's been racking my brain for a little - more of a though experiment that developed at a meeting where a coworker asked about connecting a separate pump to the pipe at a vertical storage tank outlet. One thing I needed to look at was the backpressure due to the static head in the tank, and my brain decided to frustrate me.

The storage tank is about 30' high. The tank outlet is at the bottom, which is piped to a transfer pump. This pipe also has a solenoid valve upstream of the pump, for the sake of this question assume it is normally open, and the pump is off. The initial pressure at the pump inlet is a function of the height of fluid in the storage tank. If only the valve is closed, and the pump is not running, will the pressure downstream of the valve be what it previously was, even though it is hydraulically separated from the tank?

thanks!

 

[pmover]

if the column of fluid between the pump and solenoid valves remains in place and at same elevation, the pressure will remain the same at the pump. however, there are likely thermal effects and any fluid leakage most likely will impact the pressure value.

hope this helps.
-pmover

 

[TD2K]

It's hard to say. Pmover brings up good points.

Another factor is what is the pump connected to and how is it blocked in when not running? Is it blocked it or is discharge pressure left sitting on the check valve? Is the system heat traced and how? Valves leak and usually not the one you want to leak.

 

[rmw]

The way I read this, the valve is between the tank and the pump suction, right? So if the valve is normally open, and the pump is not running, the tank is wanting to drain itself through the pump unless there is something downstream of the pump to prevent flow.

Neglect leakage and temperature effects for the moment. (TD2K, I never wanted any valves to leak, frankly. grin)

Assuming something downstream to prevent flow, then the pressure on the system is the static height of the fluid in the tank. Now close the solenoid valve and the pressure remains the same as before it was closed. The pressure is now trapped in there.

If you then opened another valve somewhere upstream of the solenoid valve and completely drained the tank, the upstream pressure of the solenoid valve would be zero but the downstream pressure would remain at the 'trapped' pressure of the original static head.

Now you have to consider temperature effects because the 'trapped' fluid will expand if heated up any at all and the pressure will be more than the original static head.

Hope this helps.

rmw

 

[BigInch]

Depends on if you closed the solenoid valve while the pump was running, in which case there may be no pressure remaining between valve and pump, or perhaps somewhere near the vapor pressure of the fluid, or if you closed the valve when the pump was off, in which case it would be somewhere close to the pressure from the tank head at the time (given there has been no valve leakage, or leakage from pump discharge back through the pump into its suction since then).

What would you be doing, if you knew that you could not fail? Ans. Bonds and derivative brokering.

 

[SEP87]

I would suggest that the pump will not provide a tight seal (hence the purpose of the solenoid valve). When the solenoid valve is closed and the pump is off, the liquid downstream of the pump will leak back through the pump therefore pressurising the gap between the solenoid valve and the pump to the same pressure as that downstream of the pump. So the pressure between the solenoid valve and the pump will rise to higher than what it is under operating (flowing) conditions.

If the pump didn't leak then there would be no requirement for the solenoid valve. So it is safe to assume the pressure will equalise across the pump.