Pressure-Relieving Devices Flashcards

According to API RP 576, what is the primary difference between a conventional spring-loaded pressure relief valve and a balanced-bellows pressure relief valve?

Answer: The balanced-bellows design incorporates a bellows that isolates the spring from the process fluid and minimizes the effect of superimposed back pressure on set pressure

A balanced-bellows PRV uses a bellows assembly to seal the spring bonnet from process fluid and to neutralize the effect of superimposed (constant) back pressure on opening set pressure. A conventional PRV is sensitive to back pressure because back pressure acts on the disc and opposes the spring force, raising the effective set pressure.

Per API RP 576, what term describes the pressure increase above the maximum allowable working pressure (MAWP) of a vessel that occurs during the relief event while the PRV is open and flowing?

Answer: Accumulation

Accumulation is defined as the pressure increase above the MAWP (or design pressure) during a relief event. For ASME Code vessels, accumulation is limited to 10% for a single PRV in fire cases and 3% for process upset cases. Blowdown is the difference between set pressure and closing pressure; set pressure is the pressure at which the valve begins to open.

API RP 576 recommends that the inspection and test interval for pressure relief valves in clean, non-corrosive, non-fouling service should generally not exceed:

Answer: 5 years

API RP 576 states that PRVs in clean, non-fouling service should be tested and inspected at intervals not exceeding 5 years. In severe fouling or corrosive service the interval should be shorter, determined by operating experience. Ten years is not an API RP 576 PRV interval; it applies to pressure vessel internal inspections under API 510.

What is the function of a rupture disk when installed in series upstream of a pressure relief valve?

Answer: It isolates the PRV from corrosive or fouling process fluid and prevents premature seat leakage, while the PRV provides the required relieving capacity after the disk bursts

A rupture disk upstream of a PRV protects the valve's seating surfaces from corrosive, polymeric, or plugging process fluid. When an overpressure event occurs, the disk bursts first and then the PRV opens to relieve at its rated capacity. This arrangement requires monitoring the space between disk and PRV for leakage, which would indicate a failed disk and a blinded PRV.

When a pressure relief valve is removed for inspection and a replacement valve is installed, API RP 576 requires that before the replacement valve is placed in service its set pressure must be:

Answer: Verified by a pop test or bench test at a certified test facility or in the field using calibrated test equipment

API RP 576 requires that every PRV be tested to verify its set pressure before being returned to or placed in service. Nameplate data alone is insufficient—valves can drift, spring fatigue occurs, and seat condition changes. The test must be performed with calibrated equipment and documented.

According to API RP 576, what is 'blowdown' as it applies to a spring-loaded pressure relief valve?

Answer: The difference between the set (opening) pressure and the closing (reseat) pressure, expressed as a percentage of set pressure

Blowdown is the difference between a PRV's set pressure and its reseating pressure, typically expressed as a percentage of set pressure. ASME Code limits blowdown to 7% of set pressure (or 3 psi, whichever is greater) for most services. Excessive blowdown wastes process fluid and can indicate seat damage; insufficient blowdown causes chattering.