Why is there a vacuum inlet pressure rating on a positive pressure regulator?

AP Tech positive pressure regulator inlet pressure rating generally ranges from vacuum to a specified positive pressure, despite the outlet pressure rating only being positive pressure.  Why is this? The reason for the vacuum source rating is to note that vacuum can be applied to the device without damaging it. Vacuum is commonly applied to gas systems for purging and leak testing. The vacuum operating rating assures one that it is safe to apply vacuum to the regulator.

If positive pressure regulators have vacuum inlet pressure rating, why don’t you recommend inboard seat leak test?

We do not recommend performing an inboard seat leak test on regulators (pressurize inlet or outlet with Helium and leak detector pulls vacuum on the outlet or inlet). If vacuum is applied to the inlet of the regulator, if the regulator is closed (no force applied to the range spring), the regulator will remain closed but may not remain leak tight.  If vacuum is applied to the outlet of the regulator, even if it is closed, it may open due to the pressure pulling down the diaphragm.  This does not apply to pulling vacuum on the outlet of sub-atmospheric delivery regulators which are designed to deliver gas at sub-atmospheric pressures. For more discussion on recommended regulator seat leak testing methods, please see product note #422.

My desired maximum outlet pressure is 100 psig (6.9 barg), shouldn’t I specify a 100 psig (6.9 barg) outlet pressure regulator?

No. Even though pressure regulators have adjustment stops to limit the maximum pressure one can adjust, the maximum outlet pressure stop is set at maximum inlet pressure.  Supply pressure effect can cause the outlet pressure to rise above the adjusted set point and the regulator’s maximum outlet rating. Care should be taken to not operate below the maximum outlet pressure rating as exceeding outlet pressure rating can damage downstream devices such as pressure gauges and sensors. For example, if an AP 1410T’s 100 psig (6.9 barg) maximum outlet pressure regulator is adjusted to 100 psig with a full 2,200 psig (151.6 barg) source pressure cylinder, as the cylinder is consumed and the supply pressure drops, supply pressure effect will cause the outlet pressure to rise.  In the case of the 1410T, the change is 1.6 psi (0.11 bar) outlet pressure change per 100 psi (6.9 bar) inlet pressure change.  When the cylinder is consumed to 200 psig (13.8 barg), the outlet pressure will have risen 1.6 psi x 20 = 32 psi (2.2 bar).  The 100 psig (6.9 barg) outlet pressure will rise to 132 psig (9.0 barg) total outlet pressure.  Please click here for a video from our training with an example of the AP 1200’s supply pressure effect.  Notice the outlet pressure increases as the inlet pressure decreases. For a general review of supply pressure effect please refer to Product Note #403.  For more information regarding supply pressure effect when using high flow regulators in high-pressure, compressed gas applications, see product note #426

What happens if I operate the regulator near or below the minimum outlet pressure rating?

While the outlet pressure rating is widely known, many neglect to note the minimum delivery pressure.

Even though one can adjust a regulator from zero pressure to a pressure below the minimum outlet pressure rating, one should not expect to have stable regulated pressure with flow below the minimum rating of the device. If attempted, the pressure will be unstable.  Additionally, the flow capacity of a regulator is greatly reduced at low outlet pressures.

Many will mistakenly think that a mini-regulator is ideally suited for very low outlet pressures, however, larger regulators with a larger diaphragm to seat orifice area ratio offer superior low pressure stability.

AP Tech regulators can safely operate within their rated operating pressures.  However, it is advisable to avoid operating at either extremes of the rating. If one needs to operate within 5% of either minimum or maximum rated outlet pressure, please ensure the maximum flow requirement is achievable and maximum downstream component pressures will not be exceeded due to SPE, respectively.

If you are not sure if your application is suitable for your chosen regulator, please contact your AP Tech distributor or contact AP Tech and we will help you find the best regulator for your application.