Pressure Compensation: Don’t Blow Your Top Over It!

Certain applications that we run into often require that we feed into a reactor, pressure vessel, high-speed mixer, vacuum/pneumatic conveying line, or that we provide a gas drying blanket (i.e. nitrogen or clean dry air) to a hopper extension in order to keep the material dry.  
     When looking at supplying feeders for these applications, it is of utmost importance that you advise the applications engineer that you are working with what the vacuum or pressure is that the feeder will be required to feed into, or if your application requires that the material be blanketed to keep it dry.
    Most standard screw feeders are not designed to operate under extreme vacuums or pressures and are typically rated to ambient vacuum/pressure. In a gravimetric screw feeding system, any pressure or vacuum differential from atmosphere will result in either an upward or downward force on the load cells while feeding, adding unwanted error into the system. However, with a properly designed pressure compensation system, one can successfully operate in smaller vacuum or pressure conditions of say around -0.5mbar to +10mbar (-.007psi to 0.14psi).  The use of an infeed or discharge rotary airlock or screw conveyor will be required for those applications that exceed these values in order to isolate the feeder from either the upstream or downstream device creating the vacuum or pressure.
     Typically a feeder manufacturer will list both an operating pressure and a design overpressure in the specifications for the feeder. Do not confuse the two. The design overpressure specifications are meant to be considered for out of the norm, or upset conditions that might occur within the feeding system and not meant to be used for day-to-day operating conditions.  
     For an example, let’s take a look at a screw feeder where we want to provide a nitrogen blanket through an NPT fitting in the hopper cover to keep the material dry. For our example let’s say that the feeder we are looking to use has hopper extension cover which measures 34 in. in diameter. To find the area in square inches of the hopper extension’s cover we take the formula A = π x r2 where A equals the area of the cover in square inches, π (pi) = 3.14, and r = the radius of the circle.  We know that the diameter of the cover is 34 in. so the radius will be half of that or 17 in. Plug this into the equation and we find that the area of the cover A = (3.14) x (17)2 or just over 907 sq in. Let’s say we want to add nitrogen to this hopper extension at a setting of 2 psi. Two lb/sq in. will equate to over 1814 lb of force on that cover! The feeder has a design over pressure of -5mbar to +95mbar. 1 millibar = 0.0145 lb/sq in. so 95mbar will equate to 1.38psi; 1.38 psi acting on a cover with a surface area of 907 sq in. equates to over 1200 lb of force acting on that cover. At 2 psi we will either blow off the cover from the cover clamp(s) failing or possibly blow off a flexible connection. Blanketing nitrogen (or clean dry air) should therefore only be added at an amount that will replace the volume of material being fed out of the feeder unless we use a properly designed pressure compensating system that will help balance the vacuum/pressure.
    So what is a pressure compensation system? Without getting into too much detail with respect to designing one in this article, it involves the addition of a pressure compensation vessel that is flexibly connected to the top of the hopper cover and rigidly connected to a support structure in the field. The geometry of the vessel is designed so that it can compensate for the volume of air traveling into and out of the inlet and vent and yet maintain the operating vacuum/pressure of the system.  
    The pressure compensation system may also include a balancing pipe connecting the downturn of the nozzle to the hopper extension to compensate for downstream pressure or vacuums. The photo shows a typical example of a feeder designed with a pressure compensator and balancing pipe.
    Pressure compensation systems will need to be independently designed for each application.  Always consult the feeder manufacturer’s applications engineer if you are unsure of the application, or if you need to confirm if a pressure compensation system is required for your application.
    Todd D. Messmer is the applications engineering manager for Schenck AccuRate, Whitewater, WI. For more information, visit www.accuratefeeders.com.

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