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Vent-O-Mat RBX - Air Release Valves

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Product code: RBX-050

Product description: 50mm

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The Vent-O-Mat Series RBX "Anti-Shock" air release and vacuum break valve, is the product of extensive research into the development of an efficient, but cost effective solution to surge problems (both mass liquid oscillation and elastic transient phenomena) associated with any operating pipeline. Automatic dampening, relevant to the pipeline's needs is provided by either one of three design features. These special features are unique in a pipeline component of such compact and economic design.


Vent-O-Mat Advantages & Key Features

  • High capacity air discharge during pipeline filling
  • High volume air intake for vacuum break purposes, both during draining of water pipelines and/or column separations
  • Independent third-party tested
  • Controlled pressurized air discharge releases air when pipelines are fully-charged
  • Surge relief prevents pipeline bursts during pump trip & rapid pipeline filling
  • NSF61 approved; ARRA, Buy America or PA Steel, American Iron & Steel (AIS), if specified

Vent-O-Mat RBX - Air Release Valve
Vent-O-Mat RBX - Air Release Valve


Size Range

  • 1" (25mm) to 12" (300mm)


  • -40degF to 176degF

Max. Operating Pressure

  • Up to 1450psi (100bar) depending on size

Min. Operating Pressure

  • 7.25psi

End Connection

  • MNPT, Flanged/Studded ASME B-16.5

Flange Drilling

  • ANSI 150#, ANSI 300#, ANSI 600#, ANSI 900#

Body Material

  • Stainless Steel AISI 304L or 316L

Flange Material

  • Fusion Bonded Epoxy Powdercoated

Top Cover

  • ABS Polylac®, Stainless Steel 304L or 316L

Float Material

  • High Density Polyethylene (solid)

Bleed Port

  • Yes

O-Rings and Nozzle Seat

  • EPDM

Available Configurations

  • Standard – 4 functions
  • Biased
  • Vacuum-Break Only
  • Vent Only
  • Flanged Threaded Outlet

Surge and Waterhammer Protection

Surge Protection - Initial Filling

The RBX incorporates the additional floating "Anti-Shock" Orifice which is aerodynamically engineered to throttle air discharge when water approach velocity would otherwise become too great and induce an unacceptable pressure rise. The air throttling action increases resistance to the flow of the approaching water which consequently decelerates to a velocity which reduces the pressure rise when the valve closes (see operation of valve on pages 1 & 2). Vent-O-Mat series RBXis an essential precaution for pipeline priming.

Surge Protection -Pump Trip Conditions

In instances where a pipeline experiences water column separation due to pump stoppage, high shock pressures can be generated when the separated water column rejoins.

The Vent-O-Mat series RBX takes in air through the unobstructed large orifice when water column separation occurs, but controls the discharge of air through the "Anti-Shock" Orifice as the separated column commences to rejoin. The rejoining impact velocity is thereby considerably reduced to alleviate high surge pressures in the system (see operation of valve on pages 1 &2).

Other surge control measures may, dependant on pipeline profile, diameter and operating conditions, be needed to provide the primary surge alleviation function with the Vent-O-Mat air- valves forming an integral and valuable addition in a combined strategy for further reducing surge pressures. The benefit of the "Anti-Shock" Orifice can be readily demonstrated by suitable surge modelling software.

Surge Protection - Pipeline Operating

The operation of valves and similar flow control devices can cause high-pressure transients in an operating pipeline.

The unique, single chamber design of the Vent-O-Mat series RBX valve enables a pocket of air to be trapped in the valve chamber. Automatic operation of the small orifice control float regulates the volume of air entrapped.

The volume maintained in the valve will provide a cushioning benefit to the pipeline for short duration transient pressure "spikes". This effect can be modelled by the design engineer using suitable surge software.

Surge Protection - Primary Pipeline Surge Protection Failure

In instances where air vessels or other alleviation measures are utilised as primary surge protection and these devices fail, excessively high surge pressures will be generated. The same is true if pipeline demands are increased with time without the upgrading of initial surge protection equipment.



The operation of a conventional air release valve is such that fast approaching water is almost instantaneously halted by the valve's closure without the shock cushioning benefit of any retained air in the pipeline. Consequently a transient pressure rise or shock of potentially damaging proportions can be generated in a pipeline system, even at normal filling rates.

In addition to venting through the Large Orifice (1) when water approach velocities are sub critical, the Vent-O-Mat series RBX air release valves feature an automatic "Anti-Shock" Orifice (8) device that serves to decelerate water approaching at excessive speed, thereby limiting pressure rise to a maximum of 1.5 x rated working pressure of the valve.

In reaction to increased air flow, Float (6) closes Large Orifice (1) and air is forced through the “Anti-Shock”Orifice (8) resulting in deceleration of the approaching water due to the resistance of rising air pressure inthe valve.


In instances where a pipeline experiences water column separation due to pump stoppage, high shock pressures can be generated when the separated water column rejoins.

The Vent-O-Mat series RBX takes in air through the unobstructed large orifice when water column separation occurs, but controls the discharge of air through the "Anti-Shock" Orifice as the separated column commences to rejoin. The rejoining impact velocity is thereby sufficiently reduced to prevent an unacceptably high surge pressure in the system. In the same way the series RBX valve prevents high surge pressures resulting from liquid oscillation in a pipeline.

Subsequent to the filling of a pipeline, liquid enters the valve Barrel Chamber (2) and the Floats (4), (5) and(6) are buoyed so that the Large Orifice (1) is closed by Float (6), the valve will then become internally pressurized. A minimal working pressure of < 0. 5 bar (7. 3 psi) acting on the relatively large area of the Orifice (1) will lock Float (6) into the closed position across the Large Orifice (1).

Disentrained air rises through the liquid and accumulates in the valve chamber, when the volume of air is sufficient to displace the liquid, Float (4) will no longer be buoyant and will gravitate downwards thereby opening the Small Orifice (7) and allowing accumulated air to be discharged into atmosphere, as air is discharged the liquid raises Float (4) and re - seals the Small Orifice (7) and prevents escape of liquid


Effective discharge by the valve of pressurized air depends on the existence of a 'CRITICAL RELATIONSHIP' between the area of the Small Orifice (7) and the mass of Control Float (4), i.e., the mass of the float must be greater than the force created by the working pressure acting on the orifice area. If the float is relatively too light or the orifice area relatively too great, the float will be held against the orifice, even when not buoyed, and air discharge will not be effected.

To ensure that the correct 'CRITICAL RELATIONSHIP' exists the requisite 'DROP TEST' described under TEST SPECIFICATION on page 17 must be applied to any air release valve which is intended for discharge of pressurized air.


Air enters Orifice (3), travels through the annular space between the cylindrical floats (4), (5), and (6) and the valve Chamber Barrel (2) and discharges from the Large Orifice (1) into atmosphere.


Simultaneous drainage of liquid from Valve Chamber (2) causes Floats (4), (5) and (6) to gravitate downwards onto the Baffle Plate (9), thereby allowing atmospheric air through the valve to rapidly displace draining liquid in the pipeline and prevent potentially damaging internal negative pressure.

Selection and Positioning

Conversion Tablel /sec. to m/sec. of Pipeline Velocity is available in the brochure.

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Vent-O-Mat RBX - Air Release Valves

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