Print Logo
Technical Articles

Pneumatically Controlled Pressure Regulator Series PRA / PRAM


  • Higher Flow Rates with Less Pressure Loss than any other product on the market.   airreg.jpg
  • Automatically regulates and prevents downstream pressure from exceeding its set point.
  • Wide range of adjustable settings from 5 to 125 PSI.
  • Rugged construction with inlet pressure to 150 PSI (10.34 bar).
  • Dual rolling diaphragm design assures sensitive operation.
  • Sizes 1/4″ through 3″
  • Body materials in PVC, Natural Polypro, PVDF. Choice of FKM or EPDM Seals
  • Connection types NPT, Socket, Metric, BSP, Spigot, Flare, and custom
  • Use with optional Series PRS Stabilizer® to provide ultimate pressure control

Additional Design Features: 

  • Non-sticking design.
  • One-piece body and double u-cup design prevent leakage and eliminate “creep.”
  • Patented Fail-Dry® safety vent warns of seal failure; permits valve to continue operating and prevents liquid/air contamination.


Designed for highly corrosive and ultra-pure chemicals, this pressure regulator uses compressed air to control pressure with greater accuracy as compared to standard spring-controlled models. Sizes 1/4″, 1/2″ and 3″ are Series PRA; sizes 3/4″, 1″, 1 1/2″ and 2″ are Series PRAM.

Pressure regulators provide protection for valuable downstream instruments and filtration systems by converting fluctuating or excessive inlet pressure to a preset maximum downstream pressure. The PRA/PRAM will routinely handle inlet pressure up to 150 PSI and accurately maintain a downstream pressure from 5 to 125 PSI.

The design of the PRA/PRAM features two opposing large area frictionless rolling diaphragms. Process pressure is applied to one of the diaphragms; air pressure is applied to the other. Because the air pressure can be maintained with more accuracy than the force provided by a spring, the PRA/PRAM offers greater sensitivity and less variance from set pressure than any thermoplastic pressure regulator previously available. The PRA/PRAM can be used in combination with Plast-O-Matic’s patented Stabilizer which measures downstream liquid pressure and provides continuous feedback control for even greater sensitivity and performance. The Stabilizer is described in detail below.

Materials of Construction:

Standard connections are female NPT threaded ports. Socket, flanged and spigot connections are also available. Standard body materials are Geon® Grade 1 Type 1 PVC, Natural Polypropylene, and Kynar® PVDF. Standard seal materials are Viton® and EPDM. Some Kynar PVDF components are used in the Natural Polypropylene models for strength. External fasteners are stainless steel. For materials other than listed, please consult factory.


PRA installed in typical use

A relieving type air regulator is required to load the sensing port; see diagram. Series PRA/PRAM pressure regulators prevent downstream pressure from exceeding the desired setpressure. Regulator will remain closed as long as set pressure is maintained. As equipment or valves downstream of the regulator begin to open and demand flow, the downstream pressure begins to fall and the regulator begins to open. As valves or equipment downstream open further, the pressure regulator continues to open until its maximum opening is reached. As the process is reversed, downstream pressure begins to increase and the regulator starts closing. When the downstream pressure again reaches set pressure the regulator closes bubble-tight.

Like any pressure regulator, the PRA/PRAM prevents downstream pressure from exceeding the set point. To “maintain” a specific downstream pressure, the PRA requires an additional control, such as the patented Series PRS Stabilizer – see below.


Chart shows typical lab tested performance of 1″ Regulators Flow Capacity (GPM) versus Pressure Drop-Off (PSI) Comparison of representative Plast-O-Matic models with three competitors.


Drop-off is the difference between the pressure regulator set pressure and the downstream pressure. Performance curveschart, above, identifies the high flow capacities and the lowpressure drop-off of Plast-O-Matic regulators which result intheir greater sensitivity, finer adjustability and superioraccuracy as compared to competitive models.



Pipe Size


1/4″ 2.0 51.0 4.06 103.0 2.6 67.0 1.43 40.0
1/2″ 2.5 64.0 6.18 157.0 4.18 106.0 2.00 51.0
3/4″ 4.5 114.0 8.31 211.0 5.50 140.0 2.81 71.0
1″ 4.5 114.0 8.31 213.0 5.50 140.0 2.87 73.0
1½” 5.0 127.0 9.56 244.0 6.81 157.0 3.37 86.0
2″ 7.0 178.0 12.75 324.0 7.37 187.0 5.37 136.0
3″ 8.0 203.0 15.06 378.0 8.62 220.0 6.43 168.0

dimensional drawing of series pra

Model Numbers & Maximum Flow Rates

Valve Pipe Size Model# w/EPDM Seals Model# w/Viton Seals

Max. Flow Rate (GPM)

1/4″ PRA025EP-PV PRA025V-PV 5
1/2″ PRA050EP-PV PRA050V-PV 10
3/4″ PRAM075EP-PV PRAM075V-PV 35
1″ PRAM100EP-PV PRAM100V-PV 50
1½” PRAM150EP-PV PRAM150V-PV 70
2″ PRAM200EP-PV PRAM200V-PV 100
3″ PRA300EP-PV PRA300V-PV 200

The suffix “PV” indicates Geon PVC body material. For Natural Polypropylene, change “PV” to “PP”. For Kynar PVDF, change “PV” to “PF”. The flow rate indicated is maximum recommended. If you have any questions, please contact factory at 973-256-3000.

Additional Control Uses:

Plast-O-Matic offers a performance enhancing accessory, Series PRS (U.S. Patent Number 6,418,956) which loads the PRA/PRAM with greater accuracy by reacting to actual downstream pressure. Notice on the performance graph, above, how thiscombination approaches theoretical perfection.

To use Series PRA/PRAM to control other flow related parameters, such as flow rate, pH, viscosity, temperature, etc., a transducer, controller, and E/P or I/P converter isrequired.


This page continues now with information on the optional Series PRS Stabilizer. The Stabilizer is a performance enhancement, but is not required for successful use of Series PRA

Series PRS – 

The Stabilizer® 

For use with Series PRA (above) to provide precise control of downstream pressure with minimal drop-off from set pressure

Series PRS in foreground, and shown in a typical configuration piped downstream of Series PRA, Air-Loaded Pressure Regulator.

The Stabilizer®

The Stabilizer is a closed-loop, proportional controller. It maintains process fluid pressure at a constant, preset value by modulating air pressure to the regulator. The combination of The Stabilizer and an Air-Loaded Pressure Regulator (Series PRA, above) eliminates the need for an exotic, expensive control valve in most applications.


  • Used with an Air-Loaded Regulator, Series PRS reacts instantly to fluctuations in pressure, significantly enhancing the accuracy of the air-loaded regulator – downstream pressure has virtually no deviation from set pressure.
  • Desired pressure is maintained without sacrificing flow. Performance rivals control valves, at much lower cost.
  • Ideal for applications with changing pressure requirements.
  • Can be conveniently teed into piping at desired control point, or may be mounted at a more convenient location and connected with tubing for static pressure sensing.
  • Easy to set; adjustable from 5 PSI to 125 PSI.
  • PTFE diaphragm for critical ultra-pure and corrosive liquid applications.
  • No wetted metal or elastomer parts.

Materials of Construction & Piping Connections:

Standard liquid connection is ½” male NPT threaded. Standard air or hydraulic connections are 1/8″ female NPT.Connection to valve air chamber should be ¼” or larger tubing. Standard body materials are Geon® Grade 1 Type 1 PVC, Natural Polypropylene, Kynar® PVDF and PTFE. Standard seal material is a PTFE diaphragm backed with Viton®. Some Kynar PVDF components are used in the Natural Polypropylene models for strength. Adjusting bolt and external fasteners arestainless steel. A Buna-N rubber seal is used in the air chamber. For connection types and materials other than listed, please consult factory.


Comparison of Air-Loaded Regulator performance with and without the Stabilizer. Note that the pressure axis in this graph shows only 16 PSI of the total range; this was done to better illustrate the effectiveness of the Stabilizer. Within this limited range, competitive regulators are below the scale and therefore are not plotted.

Performance Comparison of 1″ Air-Loaded Regulator with and without The Stabilizer

flow curve comparison of air loaded regulator with and without the optional stabilizer

Temperature/Pressure Ratings:

Temperature, ºF (ºC)           Maximum Air Pressure, PSIG (BAR)

        77 (25)                                                     125(8.6)

       104 (40)                                                    83(5.7)

       140 (60) max. operating                       33(2.5)

        40 (5) min. operating                            125(8.6)



dimensional drawing of the stabilizer



Control Pressure Range: 5 – 100 PSI (.3 – 6.9 BAR)

Minimum Air Pressure: Set Pressure + 15 PSI (1.0 BAR)

Maximum Control Point Pressure: 150 PSI (10.3 BAR)

Air Pressure Ports: 1/8″ NPT, female

Process Liquid Port: 1/2″ NPT, male

Minimum Process Liquid Supply Pressure: Set Pressure + All Downstream Pressure Drops to Control Point + 15 PSI



The location of the Stabilizer should be chosen by the system designer, depending on the application.

the Stabilizer may be installed in a tee adjacent to the PRA

1. It may be installed in a tee adjacent to the PRA to regulate pressure immediately downstream, as shown above.

the Stabilizer may be installed in a tee remote of the PRA

2. It may be installed downstream, remote of the PRA, if precise regulation is desired at that particular location. This is called the control point. In this case, appropriate air tubing will be required from the Stabilizer back to the PRA.

Other types of remote installations may be designed as required by the application. These may require additional components or other considerations; please consult the factory for assistance.

In all cases, the air supplied to the Stabilizer should be filtered, lubricated, and regulated. The air chamber vent should be checked periodically to ensure that it is free from debris and permits continuous outflow.

Part Numbers:

MATERIAL                              PART#

PVC                                               PRS-T-PV

Natural Polypropylene             PRS-T-PP

Kynar® PVDF                           PRS-T-PF

PTFE                                           PRS-T-TF

Above are model numbers for standard PRS configurations with PTFE diaphragm and Geon PVC airchamber. For other materials, please consult factory.

For PDF of Catalog Please Click Here: Series PRS – The Stabilizer – (PDF) 

The Stabilizer, US Patent No. 6,418,956, was designed by Plast-O-Matic Project Engineer Jerry Bloom. Used in combination with the Series PRA pressure regulator, this device is the most cost effective way to accurately maintain downstream set pressure with minimal deviation, also known as “droop.” The combination of these two — the PRA Regulator and PRS Stabilizer — rivals or exceeds the performance of costly, exotic control valves.

Pressure Regulator Main Page