Tied Diaphragm Outlet Pressure Limitations

Monday, April 5th, 2021

Why are tied diaphragm regulators limited on maximum outlet pressure? Tied diaphragm regulators physically attach the poppet (internal valve) to the sensing element (generally a diaphragm) for positive shut off. The linking of the poppet to the sensing element results in the poppet being pulled closed as the outlet pressure exceeds set point. Please refer to Product Note #402 and gas regulator type training videos for a more detailed explanation as to how the tied diaphragm regulator functions.

The diaphragm has a large surface area which results in a significant force as downstream pressure increases. This is not a problem to the poppet and seat, so long as the force pushing up on the diaphragm from the gas pressure is balanced by the downward force of the range spring. If the force from the gas pressure is not countered by the force from the range spring, the poppet can be pulled hard into the seat, causing the seat to permanently deform. If the seat gets damaged in this way, it will cause the pressure regulator performance to degrade and possibly lead to an across the seat leak. This is referred to as trapping pressure or using the regulator as a shut off valve – neither of which are recommended.

As not everyone understands tied diaphragm regulators and how the seat can be damaged by trapping pressure, the decision was made to limit the pressure range available with a knob adjustable tied diaphragm regulator to 150 psig (10 bar). This was done to limit the potential damage to the seat. As trapped pressure exceeds 150 psig (10 bar), significant damage can be done.

Tied diaphragm regulators from AP Tech are limited to 150 psig (10 bar) outlet pressure with an adjustment knob, but are available to higher outlet pressures if preset. Preset means removing the knob and replacing it with lock nuts to secure adjustment. A preset regulator is still adjustable, but it requires tools to make the adjustment, refer to Product Note #437. We have found that most are reluctant to readjust preset regulators and have found it an effective means to help customers not damage their regulators by trapping pressure.