3.4 Pressure Parts Construction

Pressure parts are the pieces of a boiler that contain the pressurized working fluid — water, steam, or both. Minnesota DLI exams and inspectors care about pressure parts because that is where a failure becomes a pressure-vessel incident. Operators do not design boilers, but they must recognize what is pressure-containing, what is not, and which construction features show up on nameplates, inspection reports, and repair scopes.

What Counts as a Pressure Part

Typical pressure parts include:

• Shells and drums
• Tubes and tube sheets / headers
• Furnace (in firetube designs where the furnace is waterside-backed)
• Waterwalls and generating banks
• Superheater and economizer coils when in the boiler code stamp boundary
• Manways, handholes, and their covers/gaskets when closing the pressure boundary
• Nozzles for feedwater, steam outlet, blowdown, and instrument connections

Non-pressure parts include outer casings, ductwork, burner windboxes (unless also a pressure boundary), platforms, and refractory that only guides gas. Refractory failure is serious for efficiency and metal overheating, but the code stamp lives on the pressure boundary.

Materials and Joining

Power and heating boilers use code-allowed steels (and cast iron for Section IV sectionals). Tubes may be seamless or resistance-welded per specification. Drums and shells are rolled and welded under qualified procedures. Staybolts and stays reinforce flat surfaces — firetube flat heads, water legs, and some firebox designs — so pressure does not bow the plate. Broken or wasted staybolts are a classic internal-inspection finding.

Watertube drums use ligaments between tube holes; the remaining metal between openings must be thick enough for the design pressure. Firetube tube sheets take rolling or welding of tubes; leaking tube ends, cracked ligaments, or wasted tube sheets drive retubing decisions.

ASME Stamping and Nameplate Data

Every boiler Minnesota regulates should carry identification that includes manufacturer, serial number, year, MAWP, heating surface or input rating as applicable, and the ASME stamp appropriate to construction (Section I power, Section IV heating, and related marks). The National Board number appears when the vessel is registered. Operators should be able to find and read this data during rounds and before any pressure test.

MAWP is not a suggestion. Operating controls and safety/relief valves are set so the vessel does not exceed MAWP under normal fault conditions. Hydrostatic testing for repairs or inspections uses controlled overpressure with safety precautions — never "see how high it will go" with the burner.

Fabrication Features Operators Should Recognize

Telltale holes in staybolts are small axial holes; if the staybolt cracks, boiler water weeps through the telltale and alerts the operator or inspector. Do not plug a weeping telltale and keep firing — that hides a failed stay.

Heat Transfer Surface and Stress

Pressure parts live at the intersection of pressure stress and thermal stress. Scale on the waterside insulates the metal from cooling water, so tube metal temperature climbs even when gauge pressure looks normal — a leading cause of tube rupture. Flame impingement, poor burner setup, or blocked gas passes create local hot spots. On firetubes, overheating can loosen tube ends in the tube sheet; on watertubes, blisters and longitudinal ruptures appear.

Cold feedwater entering a hot drum or shell without proper sparge/dispersion can cause thermal fatigue cracking near nozzles. That is why feedwater introduction details matter on larger steam boilers.

Repairs, Alterations, and Who May Touch What

Minnesota follows the culture of ASME/National Board repair rules: welded repairs to pressure parts are not casual maintenance. Authorized inspectors and qualified repair organizations (often R-stamp holders) govern welding on the pressure boundary. Operators may clean, replace gaskets on designed joints, and perform external care — but cutting in a welded patch on a drum without proper process is both illegal and dangerous.

After certain repairs, a hydrostatic test and inspector acceptance may be required before returning to service. Keep repair records with the boiler file; DLI inspections review history as well as present condition.

Inspection Mindset for Engineers

On external rounds, look for:

• Weeping at tube sheets, handholes, and fittings
• Distorted casings that might hint at overheating beneath
• Safety valve discharge evidence (lift marks, drips) that suggests overpressure events
• Corrosion at supports and waterline exteriors on idle boilers

On internal inspections (when opened), inspectors examine waterside scale, pitting, grooving at waterlines, cracked ligaments, wasted stays, and flame-side wastage or cracking. Your job as the plant engineer is to present the boiler clean and safe to open, with lockout, and to understand findings well enough to prioritize repairs.

Exam trap: thinking the burner or stack is a pressure part. Another trap: ignoring a small weep at a staybolt telltale. Third trap: assuming any welder in the maintenance shop may repair a pressure boundary without code process.

Pressure parts are the boiler's legal and physical identity. Know the boundary, read the stamp, respect MAWP, recognize stays/tubes/drums, and escalate pressure-boundary defects through proper repair channels — that is the construction literacy Minnesota expects on the license exam.