Form 4 and the MAWP calculation

Build the boiler specification card, compute MAWP from survey data, and certify it.

FRA Form No. 4 is the boiler specification card: the durable record of what a steam locomotive's boiler is made of and what pressure it may safely carry. Under 49 CFR §230.8 the maximum allowable working pressure is defined as the pressure this card specifies, and §230.17 requires the card to be completed, updated, and verified at every 1472 service day inspection. Every safety valve setting, gauge test, and hydrostatic test pressure keys off it.

The card is reached from a steam locomotive's detail page, under Form 4 →. Diesel and electric locomotives do not have one.

Starting a card

An engine with no card yet shows Start Form 4.

That creates a draft. Only one draft exists at a time; when you certify it, it becomes the active certified version and any previously certified card is kept as superseded, so the history of what the boiler was rated at, and when, is never lost. New version (1472 re-survey / alteration) starts the next draft when the boiler is re-surveyed or altered.

Boiler identity

The header block records the durable identity of the boiler: Boiler number, Boiler built by, Date built, Type of boiler, Dome location, Grate area in square feet, Lowest gauge glass above crown sheet in inches, Fusible plugs, and Low water alarms. Save it with Save header.

The row sections

Survey data is entered as rows, grouped by section:

How MAWP is computed

Four of those sections govern pressure. For each row in them the app computes an allowable pressure from the measured data and the rule that applies:

• Shell courses — the shell plate formula with a factor of safety of at least 4 (§230.24). Where a tensile strength is unknown, the defaults required by §230.26 are used: 50,000 psi for steel and 45,000 psi for wrought iron.
• Staybolts and crown stays — the net-area staybolt formula at a 7,500 psi allowable stress (§230.25).
• Braces — 9,000 psi allowable (§230.25). Note this differs from the ASME figure; the app follows the CFR.
• Longitudinal seam rivets — rivet shear across the seam pitch (§230.27).

The computed MAWP is the lowest allowable pressure across every governed row. A boiler is only as strong as its weakest governed component, so one thin course or one closely-pitched staybolt group sets the number for the whole boiler. The app always names the governing row in Component results rather than reporting a figure without saying where it came from.

Derived pressures

From the computed MAWP the card also shows the values you need for testing:

• Hydro test (1.25×) — hydrostatic test pressure at 1.25 times MAWP (§230.36(b)).
• Steam test floor — 95 percent of MAWP (§230.37).
• Safety valve ceiling — MAWP plus 6 psi (§230.49).

Certifying

Certifying requires certifying authority at your railroad — see Approvals & signatures. The Certify block asks for Certified MAWP, Calculations by, Verified by, Owner signature (name), and Operator signature (name). The FRA form requires both the owner and the operator, so both are required here.

Click Certify — safe for this working pressure. Three things happen:

1. The specification is frozen. The certified version becomes the record of what the boiler was rated at, and further changes need a new version.
2. The locomotive's MAWP is updated to the certified value.
3. The certification — who calculated, who verified, both signatures, and the date — is stored with the card.

The certified MAWP may be lower than the computed value, but it can never be higher.

Printing

Print / Save as PDF renders the card for the boiler book, the inspector, or the file. Related filing deadlines — including the Form 4 that accompanies a 1472's Form 3 — are tracked on FRA filings. The identity block on the locomotive record reflects the certified card.