John M. Foster’s 1888 pressure-regulating valve uses delivery pressure beneath a flexible diaphragm to move the main valve toward its seat. A spring around the stem opposes that movement. This interactive section shows the direct-acting arrangement in U.S. Patent 391,786, with an adjustable stem passing through the diaphragm centre.
Foster Pressure-Reducing Valve Interactive Calculator
Watch the diaphragm, stem and valve move together through an illustrative opening cycle. Change seat diameter and pressure to compare the force on a circular area. The calculator does not determine valve position or regulated outlet pressure.
Equation Used
Diameter d is in mm, area A in in², and pressure P in psi. F is the one-sided pressure force in lbf, assuming zero gauge pressure on the opposite face. Actual valve force also depends on delivery pressure, exposed areas, spring force and friction. The Vs 200 lbf result is only F/200; 200 lbf is an arbitrary comparison, not a valve rating. The opening cycle is illustrative and runs independently of this static load calculation.
The diaphragm moves the valve directly
The diaphragm’s outer edge is clamped to the body. Its centre carries the threaded gland through which the valve stem passes. With the adjustment held fixed, the gland, stem and valve move together. Delivery pressure reaches the underside of the diaphragm through the open body passages.
A rise in delivery pressure pushes the diaphragm upward and moves the valve toward its seat, restricting the opening. When delivery pressure falls, the spring can move the assembly downward and increase the opening. This negative feedback is the regulating principle; the animation exaggerates movement so the connection is visible.
The spring’s upper end bears on a fixed body shoulder. Its lower end bears on a nut carried by the stem. The nut can slide vertically but is prevented from rotating by the surrounding body. The handwheel adjusts the stem relative to the diaphragm; it is held at a fixed adjustment in the animation.
A historical steam-pressure regulator
Foster described the 1888 device for regulating fluid pressure, including steam used to heat railway cars or rooms. This page illustrates that particular patented arrangement. Other Foster regulators used different springs and linkages, so the name alone does not identify an interchangeable valve or a service procedure.
For an actual installation, use the documentation and pressure ratings for its exact model. A pressure-area calculation cannot select flow capacity or replace the system’s required overpressure protection.
Pressure acting on a circular area
Convert the entered diameter from millimetres to inches by dividing by 25.4. Circular area is πd²/4. Multiplying the resulting square-inch area by pressure in psi gives force in lbf. Multiplying lbf by 4.4482216152605/1000 gives kN.
This is one load component, not the complete stem-force balance. The diaphragm has a separate effective area, and both inlet and delivery pressures act on the valve assembly. Spring characteristics and friction are also needed to predict its position.
Example: a 50 mm circular area
At 50 mm diameter, area is 3.0434 in². At 150 psi with zero gauge pressure on the opposite face, force is 456.5 lbf or 2.031 kN. Dividing by the fixed 200 lbf reference gives 2.28.
Halving diameter quarters area and force. Halving pressure halves force. Neither comparison predicts the regulated pressure, flow rate or required spring setting.
What the animation shows
The section shows the fixed body, clamped diaphragm edge, moving central assembly, spring and seat. The drawn seat widens as diameter increases; the drawing otherwise uses illustrative proportions. The repeating cycle shows how the parts move, not how quickly a real regulator responds.
No steam-flow model, spring-rate value or diaphragm stiffness is supplied. The displayed motion therefore does not simulate hunting, pressure recovery, sealing, leakage or a particular operating point. The pressure and diameter controls affect the static force calculation.
Frequently asked questions
Which Foster valve is illustrated?
John M. Foster’s direct-acting pressure-regulating valve in U.S. Patent 391,786, dated October 30, 1888. Other Foster designs have different arrangements.
Why does increased delivery pressure reduce the opening?
It lifts the diaphragm and attached stem toward the valve seat. The smaller opening opposes the rise in delivery pressure.
Does the animation predict outlet pressure?
No. It demonstrates connected movement through a repeating cycle. Outlet pressure requires a complete force and flow model.
Is the calculated force the spring force?
No. It is pressure multiplied by one circular area, with zero gauge pressure assumed on the opposite face. The complete assembly has additional pressure and mechanical forces.
What does Vs 200 lbf mean?
It divides the calculated force by an arbitrary 200 lbf reference. It is not a limit, acceptance test or product rating.
Why do the handwheel and diaphragm move together?
At a fixed adjustment, the threaded gland carried by the diaphragm holds the stem. The stem and its handwheel therefore translate with the diaphragm centre.
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