The following wiki contains information regarding how a pressure regulator operates, how a pressure regulator should be selected, and a walkthrough of how a pressure regulator was selected for the STINGER Engine. All numbers provided are subject to have changed, but demonstrate the methodology accurately.
Background knowledge required: General concepts of pressure and force balances.
A regulator is a device which controls pressure using a mechanical system. A reducing regulator reduces an inlet pressure and outputs a pressure that is as constant as possible regardless of variations in flow and inlet pressure.
Limiting of the output pressure is done via a balancing of forces produced by the input pressure load, the desired output pressure, and a spring which is set to a specific position by the operator.
There are 4 major components which manage the balance of forces in the regulator: the inlet spring, the poppet, the loading spring, and the outlet diaphragm. The inlet spring provides an upward force on the poppet, balancing the force of the loading spring in the neutral state. The loading spring can be compressed by the operator to force the poppet further downward into the seat, blocking more flow through the regulator. This is how the outlet pressure is "set" by the operator. The inlet pressure then acts against the spring, and the resulting balance allows some of the medium to "bleed" through.
The process of designing and manufacturing our own regulator involves complicated fluid mechanics and analysis alongside extensive empirical testing, and is thus deemed too difficult and outside the scope of this article. Source (5) is included for reader curiosity. Swagelok's description of the operating procedure can be found on page 6 of (3).
The following information is largely derived from the Swagelok webinar of (1).
The most common number used to describe a regulator is the flow coefficient value of the regulator, given by Cv.
Cv is NOT an appropriate method for sizing a regulator. It CAN work, but it is not the best approach. Cv is the flow capacity in GPM of water flow for a drop of 1 psi. This is calculated when the regulator is in the fully open position, which is not an operating condition that should ever be put into practice, nor is it representative of typical operating conditions.
The best method to size a regulator is by using a flow curve.
Flow curves are derived from testing the regulator under many conditions. Typically this is done with Nitrogen or Air, at standard temperature
To use the flow curve, we need to know the inlet pressure, the target outlet pressure, and the target flow rate (which is often actually an operating range).
The behavior of the flow curve is nonlinear. This is due to a phenomenon called droop. There is a nonlinear decreasing amount of outlet pressure as flow rate increases within the regulator. It is not possible to adjust droop after the valve is manufactured, so it must be accounted for beforehand. In general, a dome-loaded regulator will experience less droop than a spring-loaded regulator, and a piloted regulator can come very close to the true expected value.