Vacuum to Positive Pressure Control

  • Vacuum to positive pressure

Some applications require pressure control that crosses from negative to positive pressure relative to the atmospheric pressure. While this can appear to be confusing, it is actually quite easy to accomplish.

The Equilibar back pressure regulator works the same for vacuum and positive pressures. In all situations, the pressure on the inlet port is maintained in a precise one-to-one relationship to the pressure applied to the pilot port. If a vacuum pressure is applied to the pilot port, then the Equilibar back pressure regulator will control that same vacuum pressure on its inlet port. The key is simply finding the right type of pressure regulator that can cross from negative to positive and apply that signal to the pilot port of the back pressure regulator.

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Electronic Pressure Control in Absolute Pressure

By using an electronic pressure regulator (EPR) with an internal integrated absolute referenced pressure sensor, it is also very easy to control a process stream from the vacuum to positive pressure range. Equilibar’s QPV ultra resolution EPR has an absolute referenced sensor that natively measures pressure in absolute units. The QPV electronic regulator will supply an absolute pressure signal to the pilot port of the Equilibar back pressure regulator. The resulting electronic control of absolute pressure is useful when computer automating a process.

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Schematic #1 – Open Loop Vacuum to Positive Pressure Control

The schematic above shows the simplest, open-loop form of absolute or vacuum through positive control. The electronic pressure regulator is providing an electronic command signal and the accuracy of its internal pressure sensor calibration is relied upon to achieve the desired system pressure. This approach works great when the accuracy tolerance allows. Commonly this open loop control is useful where the chemical compatibility, high flow rates, or high temperature abilities of the Equilibar regulator can be used to full advantage.

By selecting an available 0-30 psia sensor, the QPV can easily pilot the dome of the Equilibar back pressure regulator and provide the seamless transition from negative to positive pressure. Click here for more details about the QPV electronic pressure regulator.

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Schematic #2 – Closed Loop Vacuum to Positive Pressure Control

This schematic above shows how an externally mounted pressure transmitter and a PLC or PID (proportional integral derivative) controller can be used where higher accuracy is required. The pressure transmitter signal is compared to the desired vacuum or absolute pressure setpoint and this information is used by the controller to modify the command signal to the electronic pressure regulator.

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Manual Control

There are several manual vacuum regulators that can control from negative to positive gauge pressures. These require both a positive and vacuum supply pressure, and typically measure referenced to atmospheric pressure. The Bellofram Type 77 vacuum regulator, shown at right, can be used to conveniently pilot the Equilibar back pressure regulator through the negative to positive range (relative to atmosphere).

By connecting a manual to positive pressure regulator to the Equilibar back pressure regulator’s dome, it is possible to control your larger process stream in this same range. For most processes, a positive pressure source must be available upstream of the Equilibar back pressure regulator, and a vacuum supply (pump) must be available downstream of the back pressure regulator.

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Applications

Vacuum through positive pressure applications that are referenced to local atmosphere (gauge referenced) include evacuation and fill. Evacuation and fill is often used when an inert gas must be back filled into a vessel. This occurs when multi pane windows are filled with Argon gas or when food packages have the oxygen containing air vacuumed away and replaced with an inert mixture.

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Absolute Pressure Regulator

The Equilibar ® precision vacuum regulating valve (lower) can be coupled with an absolute range electronic pressure regulator (above) to form a vacuum-to-positive pressure control solution.
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Electronic pressure regulator with absolute pressure reference

The Equilibar ® QPV electronic pressure regulator is available with an absolute pressure sensor, which can cross from negative to positives pressures (relative to gauge).
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Marsh Bellofram's Type 77 vacuum regulator can span from vacuum through positive pressures

The Marsh Bellofram Type 77 Vacuum Regulator can control from vacuum to positive pressures. This can be used to pilot the set-point pressure for an Equilibar back pressure regulator. Available ranges are: Vacuum – 2psig, Vacuum – 10 psig, up through Vacuum – 150 psig. Learn more.
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Cross section of Type 77 vacuum to positive pressure regulator

By supplying both vacuum and positive pressure to the Bellofram Type 77, it is capable of regulating the outlet pressure from through the vacuum to positive range.
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