Determination of PVRV Capacity

A relief device opens when the upward force exerted due to the system pressure exceeds the downward force exerted due to the weight of the pallet or due to spring constant if a spring is used in lieu of weighted pallets. Springs are used in low pressure relief devices when the set pressure exceeds approximately 1 psig (70 mbarg). All relief devices that protect pressure vessels and storage tanks require overpressure for achieving full lift. A few exceptions are pilot operated pressure relief devices which have the capability to achieve full lift at set pressure. In general, PVRV capacity increases proportional to the lift and reaches an asymptote when it reaches full lift.  The low-pressure relief device capacity increases after full lift due to increase in system pressure as the flow is subsonic. The device capacity reported by the manufacturer is often not at the set pressure, rather at an overpressure beyond the set pressure of the relief device. Due to this opening characteristic of the low-pressure relief device, clause 3.6.2.2 from API 2000 [3], seventh edition, specifies that the user ensures that the set pressure be lower than the design pressure of the tank to ensure adequate capacity in excess of the required flow for each identified overpressure scenario. The same is applicable for set vacuum and has to be set higher than the design vacuum of the storage tank. Designers must be cautious and set the pressure/vacuum setting lower than the design pressure and design vacuum of the storage tanks designed to API 650 [2] code of construction as they have 0% accumulation. In summary, stable operation of a PVRV is achieved when there is sufficient pressure differential between the inlet pressure at the nozzle of the PVRV and the back pressure across the device.

Relief Device Capacity certification as per clause 5.3 of API 2000, seventh edition using two methods, (1) Flow-curve method and (2) Coefficient of discharge method. Both the methods require the manufacturer to test devices with a combination of pipe size configuration, orifice size, and at different pressure/vacuum settings. The devices are to be tested at the minimum pressure / vacuum and the maximum pressure / vacuum setting. Since these extrapolations are to be verified, it is imperative that the manufacture be consulted for the capacity of the relief device. Most major commercially available PVRV manufacturers have proprietary software which is accessible and is shown below alphabetically. Using vendor software to determine the capacity of the PVRV is the best practice.

NOTE: Flame arresters can reduce the capacity of the PVRV

Flame arresters are often combined with PVRVs as a means to protect storage tanks from internal deflagrations and to reduce the likelihood of flame propagation back in to the tank. It is important to consider the reduction of capacity of PVRVs due to the installation of a flame arrester. The manufacturer has to be consulted to determine the capacity of a PVRV with an integrated flame arrester design. OEM flow curves or OEM software can provide the capacity when a PVRV is used in combination with a flame arrester.

Vendor software provide ready solution and capacity for PVRV. The DiERS method [4] or the Fisher-Forrest [5] methodology can be incorporated to obtain the capacities of PVRV. The DiERS method is published below and is limited to the specific PVRV (model, size, and set pressure). If users have devices in the same size with similar set pressure, then the model is powerful to determine capacities of all PVRV. The coefficient of discharge is first generated for each overpressure increment. The pressure increments can be 1.1, 1.2, 1.5, 1.75, and 2 times the set pressure. For each overpressure increment, the coefficient of discharge can be computed using the following equation:

All units are expressed in US Customary Unts

SCFH, standard cubic feet per hour (measured at 60 °F and at one atm)

∆P, Differential pressure across relief device

A_PVRV, If nozzle area is not available, then use pipe inside area

The following proposed equations was develop

Ps, is set pressure of PVRV

∆P, pressure differential between inlet pressure of relief device and back pressure

A,C,D, and E are regression parameters till both Kd are the same

Sum of least squares and other regression techniques can be used for development of these parameters. The user can multiple the theoretical flow with regressed coefficient of discharge expressed as   to obtain the device capacity. Fitting parameters can be developed for the make, model, and set pressure for each manufacturer. This can be used for preliminary sizing estimates and the user can confirm the final sizing using OEM curves or OEM software.