Hydronic System - Expansion tank placement

AD7 · May 2016

Hello Everyone,
I work for an energy efficiency retrofit company and have a project that uses hydronic heating system with 5 zones. The zones have circulators (dry rotor) on the return side of the boiler, and the it was designed (ages back) this way to allow the pumps to operate with cooler return water. It's believed that lower water temps prolonged the life of the packing, seal, and motor.

One of my concerns is the placement of the expansion tank which creates issues in system pressurization. The picture attached

shows the schematics of the system. The irregular placement of the expansion tank causes problems in the system with air entering the system and the circulator likely to cavitate. I would like to receive feedback on the following issues:

The current system might create some locations that have subatmospheric pressure when circulator is on. We have put bleeding valves on all radiators to address the situation. Is their a way to avoid this?
The placement of the expansion tank isn't ideal, and i would like to know what would the best placement of the expansion tank? Ideally an expansion tank should be placed before the circulator, however we have 5 zone circulators and placing the expansion tank before them doesn't work out.
I thought about redesigning the system, and placing the circulators on the supply side but the work is construction heavy and with old piping issues crop up that're unforeseen. Any suggestions?

Cheers

Paul S_3 · May 2016

Where is the water feed valve in this setup? What type of system is it 2 pipe , monoflo, etc?? What type of tank compression or diaphragm or bladder type?whats your fill pressure ? How high is the highest heat emitter?....feed valve should be connected in the same location of the expansion tank....because the expansion tank is "the point of no pressure change"....circulators should be on supply side pumping away from the expansion tank this way there will never be "subatmospheric " pressure when a circ is on

Firecontrol933 · May 2016

You mentioned you added bleeding valves at each radiator. What kind (manual or automatic) and what was the indication (actual not expected or assumed) that they were needed?

To actually pull a point in a system below atmospheric pressure when the static pressure in the system is set correctly takes serious issues with the system as a whole, a serious pump curve on the circulator and horsepower. The expansion tank located as it is in your drawing isn't ideal, but there are plenty of systems out there plumbed this way that don't have issues, but that usually is because everything other than that was done right.

hot_rod · May 2016

What system pressure are you running? What type of circulators? What temperatures are you running?

Are you having issues with air or air elimination?

Ideally you want to pump away from the expansion tank, the point of no pressure change. If you are not having issues it may not be worth changing all the piping.

Switching to high head circs could throw a wrench in the works, and high operating temperatures will coax you closer to possible cavitation.

Some good explanation and drawings of expansion tank placement here.

http://www.caleffi.com/sites/default/files/coll_attach_file/idronics_16_na_0.pdf

Adding a few pressure gauges to the piping would show you how it is actually performing.
,

Jamie Hall · May 2016

As others have pointed out, the location of the tank is hardly ideal, and it is certainly possible that there are places in the system where you are getting below atmospheric pressures -- which is quite undesirable.

The inlets to the circulators are some of those places, and it would be a very good first step to determine the pressure and temperature at each inlet. If the pressure is anywhere near the require net positive suction head for the pump at the temperature present, that is potentially a very serious and expensive problem; even a rather small pressure drop or temperature increase -- due to a flow restriction in that circuit, for instance -- could result in cavitation which could easily destroy the pump in a matter of minutes. So. Worth checking.

However, those are not the only possible problem places. You could easily have below atmospheric pressures at high points in the system, even with apparently adequate pressure at the pumps, especially if there are places where the pipe size decreases sharply. All of these should be checked.

As an interim bandaid, however, you might consider increasing the overall system pressure to a much higher value -- in fact, to a value just below your pressure relief valve setting by, perhaps 10 percent. You would, of course, have to change the expansion tank to handle the higher pressure (it should be larger, to limit the pressure change on heating to keep the PRV from popping) but it would help make sure that your pressures throughout the system are adequate.

AD7 · May 2016

@Paul S The water-feeder valve is connected to the expansion tank. It's a one-pipe monoflo system. The tank is bladder type. The fill pressure in the tank is 12 psi. The highest emitter is at 30 ft from the boiler.Do you think a fill pressure of 17 psi is ideal for this system?

@Firecontrol933 The air bleeding valves (automatic) ones were installed as the radiators filled up with air every time the system ran. Yes, I believe the position of expansion tank isn't ideal but old/older systems use this configuration as the circulators worked with lower operating temp. The pressure distribution also depends on the system height (highest emitter), fluid temp., pressure drop, and the system pressurization.

@hot rod The system pressure is at 12 psi. The building uses 1/12 HP, Series 100 NFI Circulator Pump. The system is run at 170F.
The radiators filled up with air every-time we ran the boiler. that was the reason we put automatic bleeding valves on each radiator. We haven't had any problems with the system after we put in the bleeding valves, but i'm worried about some points in the circuit having subatmospheric pressure (near the inlet the pump), which can cause cavitation.

@Jamie Hall Thanks Jamie. The NPSHA across one of the pump (zone having highest emitter) is around 62 ft of head. The NPSHR of the pumps are 8 ft of head. So NPSHA > NPSHR. I'll try to increase the pressure of the system by 10% and see the results

Paul S_3 · May 2016

yes a little more pressure is needed. you would want at least 5 psi of pressure at the top of the system( 30 FT divided by 2.31=13 PSI PLUS 5PSI= 18PSI fill pressure)....that could be why your radiators filled up with air....once all the air is out with the correct fill pressure everything should be fine....circulators are not that big for themselves to cause subatmospheric pressures with the current piping setup

AD7 · May 2016

@Paul S Thanks for the reply Paul. I was wondering the same. Some of the circulators have been making noise (possibly due to cavitation) and was replaced.
Do you know how to increase the system pressurization? Can it be done by changing the location of expansion tank - like putting it higher than at ground level?

SWEI · May 2016

To raise the pressure, you add water.

Hydronic System - Expansion tank placement

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