Closed Loop Pressure Analysis

[macmet]

I'd appreciate some help trying to figure out something I've seen in our plant that I have not been able to explain.

We are a district energy plant and have several remote stations. Our system is closed loop. What I've noticed is the pressure on the return line from each of the stations is quite a bit lower than the suction side of the of the main distribution pump.

We have an expansion tank on the suction side of the pump, which I physically isolated, but the pressure on the suction side remained higher than upstream at the remote stations.

Pressure seen at the suction side of the pumps is ~40psi. Return pressure at the remote stations is ~25psi. Elevation of the remote stations is about 5 ft higher than the distribution station.

I am a loss as to how the pressure at the suction can be higher. I thought it was the expansion tank, but isolating it saw no change in the suction side pressure.

I've been through this site but wasn't able to find anything recently that was similar. And I also had some issues finding anything with Google. I'd really appreciate it if someone can point me to a link that could help explain this.

It's probably not a complicated reason, but I do not have a lot of experience analyzing pressures within a system.

Thanks in advance for the help.

 

[quark]

Presuming you have calibrated gauges (better to check again), this is possible if your return header drops down about 10 meters towards pump suction.

In closed loop systems, the expansion tank is installed at lower pressure zone (generally, before the return line drop) and suction pressure being higher is common phenomenon.

 

[MJCronin]

I think that you have discovered a new source of energy and can now go forward with the design of a perpetual motion machine !!

 

[macmet]

Ha. I wish. I could retire and not have to deal with some of the stuff I've had to deal with today.

Quark, our elevation difference is less than 10ft. I am confused about how our suctin pressure could read higher if the expansion tank is isolated. Is it just because the line remains pressurized until the pressure is somehow relieved? i.e. it still applies the same pressure to the system whether isolated or not?

 

[quark]

Macmet,

Closed loop is not that closed to retain pressure. If you are isolating the expansion tank by valve closure, check whether the valve is leaking.

Suction pressure in a closed loop is always higher (if the pump elevation is lower than the users) with or without an expansion tank, but the magnitude you have is a bit confusing.

If you are checking the pressure at individual station return headers, check once at the common return header.

 

[btrueblood]

Bernoulli effect...does the line size change, i.e. are you measuring lower pressure in a smaller line where the velocity is higher?

 

[macmet]

Bernoulli's.... I was thinking that. The remote lines I'm reading have lower velocity than the main line into the suction of the pump.

Is it as simple as that?

Elevation is higher at the remote stations, so the suction line (which is lower) should have some additional pressure because of the height difference.

 

[quark]

macmet,

Elevation does add the pressure but you should atleast have 10 meters height for that pressure increase.

Regarding Bernoulli's it is the other way round. Velocity should be lower in the common header than the individual headers. This phenomenon (called as static regain) is not common in liquid lines but happens in AC duct work. Still, there may be some possibility. That is the reason I asked you to check the pressure at main header after the branch hook up.