Hydrostatic Test With a Relief Valve in Line
A 4-inch Schedule 80 carbon steel line is designed for 285 psi at 100°F and is rated by 150-lb flanges. It has a pressure relief valve set at 300 psi, a pressure gauge, and a drain at the low point. The specification requires a hydrostatic test at 1.5 times design pressure. The available test pump can produce 500 psi, and the only calibrated gauge on site is 0–600 psi with 10-psi graduations. The line has 120 ft of pipe, two flanges, and one gate valve. The relief valve cannot be removed without a shutdown, and the crew wants to leave it in place and bring the line up to test pressure slowly. You are writing the test procedure. The line will be tested with water at ambient temperature.
1. What test pressure and gauge would you specify?
Read the worked answer
Test pressure = 1.5 × 285 psi = 427.5 psi, so specify 428 psi. The 0–600 psi gauge can read 428 psi, but the needle will be at about 71% of full scale; verify the gauge is calibrated and readable at that point. A better choice is a 0–1000 psi gauge so the test pressure is not near the top of the scale, but either can work if calibrated. Use a gauge with graduations fine enough to read 5-psi changes, and install it at the high point or at the test pump where it can be seen. Do not use the line pressure gauge as the test gauge. Record the gauge serial number and calibration date.
2. Can the relief valve stay in place? Explain.
Read the worked answer
No. The relief valve is set at 300 psi, below the 428-psi test pressure. It would open during the test, preventing the line from reaching test pressure, and it could be damaged by chatter or by dirt from the test water. Isolate it by removing it and installing a blind flange or a rated blind, or by removing the valve and capping the nozzle. Do not rely on a closed isolation valve because a leaking valve seat can still pressurize the relief valve. Tag the blind and record it on the test diagram so it is removed after the test.
3. Outline the test procedure and acceptance criteria.
Read the worked answer
Fill the line with water at ambient temperature, vent the high point, and check that all supports are in place. Pressurize slowly in increments, for example 50%, 75%, and 100% of test pressure, and hold each increment long enough to check for leaks. At 428 psi, hold for at least 10 minutes, then inspect all joints, flanges, and the valve. Acceptance is no leakage, no visible permanent deformation, and no drop in pressure after the pump is isolated and the temperature stabilizes. If a leak is found, depressurize, repair, and retest. After the test, drain the line, remove the blinds, reinstall the relief valve, and document the test.