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What are some considerations to properly select a vacuum furnace surge tank?

Don Bowe Don Bowe
Sr. Applications Engineer

We are increasingly asked about surge tank sizing for vacuum furnaces. The shift towards faster quenching through higher pressure backfills has made surge tank selection – size and pressure rating – more critical.

First, you need to determine the required tank operating pressure that will provide the necessary furnace backfill pressure and time to backfill. There are tradeoffs between the tank size, its pressure rating, the resulting stored volume of gas and the cost of the tank. The gas supply system also must be able to provide adequate pressure to refill the tank. There are natural pressure level break points from standard cryogenic based supply systems, such as 200 psig from a standard 250 psig rated liquid cryogenic tank.

Be sure that the ASME approved surge tank is rated for the pressure that you are using and that it is adequately protected from over pressurization. Also, if you’re using a cryogenic supply system, make sure it has a low temperature alarm to prevent embrittlement of the carbon steel surge tanks.

The volume of a surge tank is usually referred to in terms of its gallons of water displacement. Since there are 0.134 cubic feet (ft3) per gallon, a 1,000 gallon surge tank has a volume of 134 ft3) . Therefore, for each atmosphere of pressure [(14.7 pounds per square inch (PSI)] there are 134 standard cubic feet (SCF) of gaseous volume available for the backfill. For example, 134 SCF of gaseous volume is available at 14.7 PSIG, 268 SCF at 29.4 PSIG and so on.

A surge tank needs to be able to store the proper volume of gas at an adequate pressure level above the backfill pressure of the furnace. For instance, using simple ideal gas laws, if 100 ft3) is required for a 5 barg quench pressure (approx 72 PSIG), it would require 600 SCF of gas for a backfill from full vacuum. That’s assuming a minimum pressure of 6 bar is required to provide an adequate flow rate to backfill within the desired time, The resulting surge tank would need to be about 750 gallons with a minimum operating pressure level of approximately 12 barg (175 psig). A tank with a 200 psig maximum allowable working pressure (MAWP) rating would be recommended and the actual size would be based on how much overdesign might be desired. A smaller tank could be used with a much higher operating pressure.

With this information as a background and a consultation with an applications engineer, you should be able to determine the required pressure and tank size to properly backfill your furnace. To learn more, contact us online or give us a call at 800-654-4567 and mention code 749.
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