Hydraulic Separators

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Figure Number: Hydraulic Separators

Product Function

The Niles Steel Tank Hydraulic Separator is designed to help reduce short cycling of a boiler system and separate building and boiler circulation. The NST Hydraulic Separator uses stored boiler water to buffer the system load when the boiler is producing more BTU’s than what the building can handle. When the building has a minimal demand, it pulls from the tank allowing the boiler to “rest”. The Hydraulic Separator is piped so the building flow is separate from the boiler flow, allowing independent circulation.

The Hydraulic Separator acts as a decoupler allowing independent circulation in both building and boiler loops. When the building’s hydronic system has a demand for heat, the buildings circulation pumps pull from the hydraulic separator (fig A.) until there is enough temperature drop in the tank to activate the boiler. When this occurs both boiler and buildings circulation pumps are activated allowing direct flow through the tank (Fig. B ). As the building demand is satisfied and it’s circulation pumps are stopped, there is still a demand in the tank to return it to it’s operating temperature (Fig. C ) . The boiler will continue to circulate and fire until the tanks temperature is satisfied.

Model ActualGallons NominalGallons NPT/Flg A B C D E
SEP-30-045 120 138 3″ NPT 49″ 30″ 16.75″ 36.25″ 4″
SEP-30-071 200 217 3″ NPT 75″ 30″ 16.75″ 62.25″ 4″
SEP-36-072 285 317 4″ Flg 76″ 36″ 24.5″ 55.5″ 4″
SEP-42-072 385 432 4″ Flg 76″ 42″ 27″ 53″ 4″
SEP-42-083 450 498 4″ Flg 87″ 42″ 27″ 64″ 4″
SEP-48-096 675 752 4″ Flg 100″ 48″ 29″ 75″ 4″
SEP-48-120 860 940 4″ Flg 124″ 48″ 29″ 99″ 4″

Regularly Furnished

120 to 860 gallons Larger sizes available


ASME Sec. VIII, Div 1 125 psi Bare tank 5 year warranty 4” base ring

The Niles Steel Tank Hydraulic Separators are designed for 125 psi (150 optional). Standard sizes range from 120 gallon up to 860 actual gallons. Custom sizes and additional fittings are available. To properly size the capacity of the Hydraulic Separator, 4 variables are needed: The minimum “run time” of the boiler. Check with your boiler equipment manufacturer for their recommended minimum run time. The minimum BTU output (BTU/hr). The minimum BTU output is based on the boiler output at the minimum firing rate, if equipped with a variable input system that modulates the firing rate down as the demand decreases. The minimum system (heat) load (BTU/hr). Based on the smallest zone calling for heat. Tank delta T for allowable temperature rise. For hydronic heating systems typically allow between 10 and 40 degree rise in tank temperature. V = Tank volume T = Min. boiler run time MBO = Minimum Boiler Output (btu/ hr) MHL = Minimum Heat Load Delta T = Tank temp rise (degrees F)

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