New Watertube Installation: Support, Alignment, and Hydrotest
A contractor is installing a 20,000 lb/h packaged watertube boiler rated 150 psig in an existing plant. The unit is 18 ft long, 8 ft wide, and weighs 31,000 lb empty. It sits on a new 8 in concrete pad with four steel bearing plates. The erector has tightened all four anchor bolts to 300 ft-lb and grouted the full base. The steam outlet is 6 in Schedule 40 and must tie into an existing header 40 ft away; the flanges are 3/8 in out of alignment. A pipefitter proposes using a chain fall to pull the boiler piping into line. The bottom blowdown line is hard-piped with no expansion loop, and the feedwater line is anchored 5 ft from the boiler. The inspector asks for the expansion calculation and hydrotest plan before firing. Ambient is 70°F; design steam temperature is 366°F.
1. Is the as-installed anchor and grouting arrangement acceptable? Explain how the boiler should be supported for thermal growth and what calculation the inspector should see.
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Not acceptable as described. A packaged watertube boiler grows toward its fixed point and must be free to slide at the other supports. Tightening all four anchor bolts to 300 ft-lb and grouting the full base effectively fixes every corner; when the 18 ft shell and nozzles heat from 70°F to 366°F, the linear growth is about 0.5 in (18 × 12 × 7.8 × 10−6 × 296). If that movement is restrained, it goes into the nozzles, blowdown line, and feedwater anchor. The inspector should see the manufacturer's support drawing identifying the fixed point and sliding surfaces, plus the calculated movement at each nozzle. Typical practice is one fixed point, slotted or clearance holes at sliding supports, and anchor bolts that guide rather than clamp. Grout should not bridge sliding plates. The blowdown and feedwater piping need flexibility or loops to absorb the same movement. If the drawing is missing, the erector should not guess; obtain written support details before hydrotest.
2. What should be done about the 3/8 in flange misalignment? Is cold-pulling with a chain fall acceptable?
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No. A chain fall used to pull the boiler nozzle to the header imposes a bending moment on the nozzle and shell; the force is not measurable by the fitter and remains locked in after bolting. The flange should mate freely with the piping supported independently. The correct fix is to rework the piping-cut and refit a spool, add an offset or expansion loop, or adjust hanger elevation-so the flanges align within gasket and bolt-hole tolerance without external force. Cold spring, where specified by the designer, is a planned offset installed with the pipe cold and calculated for a specific direction; it is not a field correction for fabrication error. The inspector should require the pipe to be disconnected and the boiler nozzle checked for distortion. After correction, verify that the boiler can grow about 0.5 in without loading the nozzle and that the blowdown line is not acting as a rigid tie.
3. Outline the hydrotest plan. What pressure, precautions, and acceptance checks apply?
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With a 150 psig MAWP, the hydrostatic test is normally 1.5 × MAWP = 225 psig, unless the code or manufacturer requires otherwise; the safety valve must be removed or isolated and the test pressure measured by a calibrated gauge. Use warm water near ambient to avoid brittle conditions, fill slowly, vent all high points, and inspect before pressurizing. Bring pressure up in increments-about half the test pressure, then the final-and hold long enough for a thorough examination of all joints, tube seats, handhole gaskets, and nozzle welds. Do not use the safety valve as a test plug. Acceptance means no leaks, no permanent deformation, and no visible distortion; any leak is corrected and the test repeated. After the test, drain and dry, reinstall the safety valve with a new gasket, and record the test. The inspector should witness the test and verify the gauge certificate.