Is A Surge Tank Required?

You may have noticed, in various pieces of literature, guidelines suggesting when a surge tank is needed. Have you ever
wondered why? Typically, surge tanks are recommended for condensate returns in excess of 50% of the system load. This may
be further clarified as Tray‐type deaerators best suited for less than 50% returns and Spray‐type suggested for no more than
30% returns.


Surge tanks even out the flow of water to the deaerator, you may have heard them referred to as a “wide spot in the road”.
They simply act as a buffer to ensure proper operation of the deaerator and protect it from surge loads. Why is this

If you look at pump information for condensate return systems, you will see that the pumps are often sized to pump 2‐3x the
actual condensate flow rate, depending on where you end up on the curve. A return unit sized for 20GPM of condensate may
actually pump 40‐60 GPM back to the boiler room.

The deaerator’s steam valve is typically, conservatively, sized to heat the flow for a boiler system at 100% makeup, from
source to saturation temperature. Depending on the temperature of the pumped condensate, the high flow could easily
outpace the ability of the steam valve to adequately respond to the incoming water. The incoming water would begin to drop
the deaerator’s pressure, the steam valve would respond to the drop by opening to allow more steam to enter. But, as we
just noted, the valve may be (temporarily) undersized and the pressure would continue to drop.

Remember, one of the main things the deaerator is using the steam to do ‐ heat the water to saturation (inside the spray or
tray section), to drive off dissolved gases. If the steam valve is temporarily undersized, for the surge flow, it will not be able
to heat the makeup flow to saturation temperature and adequately remove the gases. This may require additional chemical
treatment in an attempt to compensate – resulting in additional chemical expenses, while being a less reliable means of
controlling dissolved gasses than a properly functioning deaerator system. Lastly, if the surge is severe enough, the deaerator
could go into vacuum. The vacuum breaker will open to take the tank back to zero pressure, but in doing so, it will also
introduce oxygen into the system when it is activated (making it an temporary‐aerator?). A slightly lesser problem of the
flow surge would be that the tank could occasionally overflow, making the tank larger can account for this issue, but it does
not address the deaeration performance issues.

As mentioned above the surge tank does what its name suggests, take flow surges. Unlike the deaerator, the surge tank’s
operation is not adversely effected by temperature swings. The constant run transfer pumps, between the surge tank and
deaerator, with a modulating valve driven by deaerator level, gives the deaearator a steady flow of water, with a slowly
changing temperature, to heat ‐ just what the deaerator was designed to do.


If you have a deaerator that you think is not performing the way it should, give us a call, we are ready to assist you with a no
cost, no obligation site visit. A thorough survey of your equipment will be made, any suggested actions will be documented
for your review and approval, before any work takes place.

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