API 510 Pressure-Relieving Devices — Questions and Answers
Question 1: 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?
- The balanced-bellows design uses a pilot valve to control the main valve opening, eliminating back-pressure effects entirely
- 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 (Correct answer)
- The conventional design opens at a pressure equal to set pressure plus accumulation, while the balanced-bellows opens exactly at set pressure under all conditions
- The balanced-bellows design is limited to liquid service only, whereas the conventional design is suitable for both gas and liquid service
Correct 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.
Question 2: 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?
- Blowdown
- Superimposed back pressure
- Accumulation (Correct answer)
- Set pressure
Correct 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.
Question 3: 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:
- 2 years
- 5 years (Correct answer)
- 10 years
- The valve's nameplate service life, with no maximum interval specified
Correct 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.
Question 4: What is the function of a rupture disk when installed in series upstream of a pressure relief valve?
- It increases the PRV set pressure by adding hydraulic resistance upstream of the disc
- It acts as a secondary relief device that opens before the PRV to protect it from seat leakage into the atmosphere
- 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 (Correct answer)
- It reduces the effective relieving area of the PRV to allow the use of a smaller valve for the same flow requirement
Correct 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.
Question 5: 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:
- Estimated from the nameplate data without bench testing, provided the valve is from an approved manufacturer
- Verified by a pop test or bench test at a certified test facility or in the field using calibrated test equipment (Correct answer)
- Accepted based solely on the previous test certificate if the valve was last tested within 10 years
- Set 10% higher than the vessel MAWP to account for accumulation before commissioning
Correct 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.
Question 6: According to API RP 576, what is 'blowdown' as it applies to a spring-loaded pressure relief valve?
- The total pressure above MAWP reached at maximum relieving capacity
- The difference between the set (opening) pressure and the closing (reseat) pressure, expressed as a percentage of set pressure (Correct answer)
- The volume of fluid discharged per unit time at the valve's rated relieving capacity
- The pressure drop across the inlet piping from the vessel to the PRV inlet flange
Correct 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.
API 510 Pressure-Relieving Devices
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?