Hydrotest Pressure, Flange Rating and Metal Temperature
A shop-built ASME Section VIII Division 1 vessel is ready for hydrostatic test. The nameplate reads MAWP 250 psig at 400°F. Shell and heads are SA-516 Gr. 70, 1/2 in. nominal, MDMT 20°F. The shop procedure specifies test pressure as 1.3 × MAWP. The vessel carries a 2 in. 150# raised-face nozzle that will be blind-flanged for the test. The only relief valve on the rack is set at 300 psig. Fill water comes from a plant header at 48°F; ambient is 72°F. The test will be run in a bay beside an active walkway with no barricade erected yet. Gauges available: 0-150, 0-300 and 0-600 psig.
1. Determine the required test pressure, including any temperature correction, and evaluate the relief valve and gauge selection.
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Test pressure
Per UG-99, hydrotest = 1.3 × MAWP × (allowable stress at test temperature / allowable stress at design temperature).
- SA-516 Gr. 70 allowable is 20,000 psi at both 72°F and 400°F, so the stress ratio is 1.00.
- Required test pressure = 1.3 × 250 = 325 psig.
Relief valve
A 300 psig relief will lift before the 325 psig test pressure is reached, so the test can never be completed, and worse, a lifting relief on a filling vessel hides overpressure. The relief must be set above the test pressure, and the usual practice is a valve set at or above the test pressure but well under the vessel's failure point - a 400 psig valve, or a test relief set for the purpose. A 300 psig valve out of the rack is not acceptable.
Gauge
Use the 0-600 psig gauge. A gauge should read roughly 1.5 to 4 times the test pressure; the 0-300 gauge cannot read 325, and the 0-150 is useless. Calibration within the last six months and a second gauge at the high point are standard.
2. The 2 in. 150# flange is in the test boundary. Check it against the test pressure and state your options.
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The conflict
A 150# Group 1.1 carbon steel flange is rated about 285 psig at ambient temperature (ASME B16.5). The test at 325 psig exceeds that rating by about 40 psi, and the blind you bolt to it does not change the flange rating - the flange itself is the limit.
Options, in order of preference
- Do not test the nozzle at 325. Blank the nozzle at the vessel side of the flange connection (or use a welded test cap on a spool) so the 150# flange is outside the pressure boundary, test the vessel at 325, then test the nozzle assembly separately at a pressure within its rating. This is a design/engineering decision - get it in writing from the engineer and the Authorized Inspector before proceeding.
- Replace the nozzle flange with a 300# class flange, if the nozzle and drawing permit it. This is a rework decision, not a shop-floor one.
- Do not simply test at 285 psig and call it done. Lower test pressure requires the engineer's approval and may not satisfy UG-99 for the vessel as a whole.
What you must not do is bolt on the blind, open the valve and hope. A flange overpressure during a hydro is a brittle, sudden failure with water and steel in the bay.
3. Before pressurizing, what do you check on temperature, venting and the area? Give the numbers.
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Metal temperature
UG-99 requires the metal temperature during the test to be at least 30°F above the MDMT to avoid brittle fracture. MDMT is 20°F, so the minimum metal temperature is 50°F. The fill water is 48°F. That is below the limit and, depending on the vessel's temperature, the metal will be at or slightly under 50°F. Warm the fill water, or let the vessel come up to ambient before testing - do not test on 48°F water in a 72°F bay and assume the metal follows the air.
Venting
Fill from the bottom and vent the highest point until water runs steady with no spitting. Trapped air is stored energy and it also makes the gauge response sluggish and false. Hold at test pressure long enough for the metal and water to equalize, then inspect all joints and seams at the test pressure.
Area and controls
- Barricade and sign the bay; the walkway is active and no one passes through the line of a blind or a gauge glass.
- No personnel on the far side of any head or blind while pressure is on; use mirrors or remote observation where possible.
- Confirm the relief is set correctly and the correct gauge is installed and calibrated.
- Depressurize through a controlled vent before loosening any blind, and never tighten flange bolts on a pressurized vessel.