Air collection in piping

[CBraungart]

Hi, I have a problem with a heat exchanger we are currently working on. There is air collecting at the top of our piping system so we installed 3 high pressure valves and turn on the system and let it run then shut it off and find that it is collecting air at the top of the system. Our pump is spec'd out at 185 ft of head and our highest point is roughly 30 ft. The head shouldn't be an issue and there is no oil leaking out of any of the valves or welds made in the piping. The pump is rated for 100 gpm and we are seeing roughly 60 gpm. After a while the air builds up in the pump and starts to cavitate so we shut down the system, let the oil cool down and bleed off more of the air that accumulates. Any ideas as to why this is occurring? Thanks for the help.

 

[vpl]

A few questions that would help pinpoint things down:

Where is the air coming from? And are you sure it's air?

Why is this a problem with your heat exchanger?

Is the air build-up in the pump or in the piping system?

How long is "after a while"?

How is the system arranged?

What temperatures and pressures are in play?

A diagram of your system might help.

Patricia Lougheed

Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of the Eng-Tips Forums.

 

[unclesyd]

Off topic:

vpl,
Could you take a look at thread292-227332. It comes from Sec III and I assume NCA 2142. This needs a look see by someone versed in the art.

 

[MJCronin]

Is a vacuum somehow created as the system cools down ?

Does the system collect air as it heats and cools ?

Perhaps obvious but have you looked for leaky flanges, old gaskets ???

 

[CBraungart]

I believe the "air" that I mentioned earlier might be vapor pressure from the oil being heated up (temperature will get up to 600 degrees F). This does not affect the heat exchanger but the system going into it because the possible vapor pressure is affecting the pump (the pump is specified by the manufacture to run with the high temperatures and the oil being used along with the head requirements). It starts to cavitate after about 3 hours of continuous running. The vapor pressure is collecting at the top of the piping. We are installing air release valves at the top to see if it eliminates the vapor pressure and brings the flow back where it needs to be. The system is arranged as follows:

Pump followed by discharge piping to the top of the thermal storage tank which is 30 ft tall. The oil then flows through the heat exchanger pipes which shoot down roughly 28 ft and travels through twice. After, it goes back down to where the expansion chamber, oil reservoir, and the suction inlet of the pump.

This is a new system and there are no old gaskets and all the welds are asme certified and there are no leaks there. The heat exchanger was pressure tested with nitrogen and held the pressure in it for roughly a day so there are no leaks there. There are no other signs of oil on any of the piping or pump. The motor is brand new but had a new magnetic linkage replaced due to overheating so this is not an issue this time around.

I will keep this updated and see how the pressure relief valves installed will affect the system. Thanks for any input.

 

[hydtools]

What is the system pressure? Raising it above the vapor pressure at 600F will prevent the vapor from forming.
Is the pump suction below the liquid level of the reservoir? Increasing volume of vapor may push liquid into the expansion chamber forcing the fluid level in the reservoir to go below the pump suction. The pump will then draw in vapor.
Can the oil coming into the reservoir go directly to the pump suction? If the vapor cannot leave the fluid it will go directly into the pump.
Is the reservoir sealed or vented? This will determine whether or not you can increase the whole system pressure above the vapor pressure.

Ted

 

[TBP]

Is there the possibility of dissolved air in the liquid being heated? I experienced a similar event in a domestic hot water system in a 15 story building - we kept wondering where the air at the top of the system was coming from - it was a problem almost every morning. Then it dawned on us that it was dissolved air from the make-up water, that kicked-out of solution as it was heated.

 

[hydtools]

Any concern about fire? What is the 'oil'?

Ted

 

[CBraungart]

THe oil is DowThermA. The system pressure is roughly 200psi on the discharge and 150 on the suction. All the pump system components are on the ground along with the expansion tank. The tank is not vented but pressurized.

What we did find is the strainer was dirty so we cleaned that out. We installed the pressure relief valves on the top and tested it out today. The flow on the discharge is now 94gpm from 55gpm without the new components and is running steadily with the heat(the pump is rated for 100gpm). Thanks for the questions cause they helped out a lot in finding a solution.

 

[unclesyd]

We have a system that at one time used Dowtherm A and our system used no where that your pressures. We used a vapor system operating a 700F and have nowhere your pressures. I think our feed pumps to the vaporizers run around 100 psig.
One reason we keep the pressures as low as possible is that we want to limit decomposition and prevent the fluid from getting over 927F where bad things happen.

My question is why, if you divulge, tell do you have to operate any part of the system at 200 PSI?