A weighted-lever safety valve uses a hanging weight to hold a valve against its seat. Pressure beneath the valve can lift it and raise the lever. The animation follows Hiscox figure 218; the separate calculator evaluates a steam-orifice expression without predicting valve settings or certified capacity.
Steam-Orifice Arithmetic and Valve Illustration
Evaluate and invert a legacy steam-orifice expression for selected inputs without treating the result as certified capacity.
Equation Used
- The entered diameter is converted to an assumed circular area.
- The absolute-pressure term uses a fixed 14.7 psi atmospheric offset.
- The fixed coefficient 51.45 is used as an arithmetic assumption, not a certified rating.
- Certification, valve mechanics, steam state, back pressure, installation, jurisdiction and code compliance are excluded.
The formula outputs do not select, set, test, certify or approve a safety valve or pressure system.
Weighted lever and valve motion
The upper cutaway illustrates the weighted-lever valve in Hiscox figure 218. Its left-hand fulcrum supports a lever that bears on the guided valve stem. A hanging weight acts farther from the fulcrum; when the disc rises, the lever and weight rise with it. The opening sequence is illustrative, not calculated from the pressure slider. The lower circles compare the selected and formula-implied diameters.
Actual safety-valve performance depends on the certified device, service, set pressure, rated area, discharge coefficient, back pressure, installation and applicable code. None of those are established by a nominal seat diameter alone.
What the calculator determines
Select a gauge-pressure reference, comparison steam flow, assumed circular diameter and count. The calculator forms an evaluation gauge pressure of 1.10 times the entered value, adds 14.7 psi for its absolute-pressure term, and evaluates W = 51.45APabsN. It also reports the relative arithmetic difference from the selected flow and the inverted diameter.
These are legacy formula results only. They are not certified capacity, accumulation, blowdown, set-pressure accuracy, installation suitability or permission to operate pressure equipment.
Retained legacy expression
A = πd²/4, Peval,g = 1.10Pselected,g, Pabs = Peval,g + 14.7 psi, and Wformula = 51.45APabsN. Inversion gives dimplied = √[4Wselected/(π·51.45·Pabs·N)].
The expression uses the fixed coefficient 51.45 with the displayed units. It includes no valve-specific discharge coefficient, rated nozzle area, steam-state correction or back-pressure correction, so its result is a formula comparison rather than a valve capacity rating.
Selected-input arithmetic
At the defaults, the evaluation gauge pressure is 88 psig and the absolute term is 102.7 psia. An assumed 0.75 in circular diameter gives A ≈ 0.4418 in² and a legacy formula flow of about 2,335 lb/h for one unit. A selected comparison flow of 200 lb/h gives an implied diameter of about 0.220 in by the same expression.
This numerical comparison does not establish a suitable or code-compliant safety valve.
What the illustration and formula cover
In a weighted-lever valve, the pressure force and the closing weight act through different lever arms. The animation shows this arrangement with illustrative proportions. It does not calculate the weight, lever setting, opening pressure or discharge rate of a real valve.
A real pressure-relief installation requires a valve suitable for its service and documented capacity. The arithmetic here does not model valve response, flow corrections, inlet or discharge piping, installation conditions or compliance requirements.
Questions
Is formula flow a certified capacity?
No. Use the certified capacity marked or documented for the actual valve under the applicable code and service.
Is entered diameter the rated discharge area?
Not necessarily. This model merely converts the entered diameter to a circular area.
Does 1.10 times the selected pressure define a valid set or accumulation pressure?
No. The calculator uses that multiplier only as an arithmetic evaluation point; requirements depend on the governing code and equipment.
Can the implied diameter select a valve?
No. It omits certification, discharge coefficient, steam condition, back pressure, inlet and outlet piping, mechanical behavior and jurisdictional review.
Primary references
- ASME Boiler and Pressure Vessel Code — governing-code scope and current editions.
- ASME Boiler and Pressure Vessel Certification — certification program and safety-valve certificate scopes.
- National Board: ASME/National Board versus CSA ratings — match certified capacity rating to the code of construction.
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