Determining the vent set pressure is important when designing an above ground storage tank. This will help avoid undesired leakage, catastrophic events, and ensure the tank complies with industry standards. For spring loaded vents, such as the Model 200 TVTH, Model 300 TVTH, and Series 4000 Marsh Hawk, the stiffness of the spring is adjusted to change the set pressure. For weight-loaded vents, such as the Series 5000 EPRV and Series 6000 PVRV, the set pressure is set by adding or removing weights.

Main Considerations

The set pressure of a valve is the internal tank pressure at which the valve will begin to open and requires consideration of many factors. The four main factors to consider are:

1. The operating pressure of the tank.
2. The set pressure of other devices.
3. The design pressure or MAWP of the Tank.
4. The required relieving capacity.

The Operating Pressure of the Tank

In order to avoid undesired venting and leakage, the set pressure of the vent must be set far enough above the operating pressure of the tank. Conventional valves may begin to leak at 75% of the set pressure. With that in mind, the operating pressure of a tank should be less than 75% of the vent set pressure.

For example, let us consider a tank with an operating pressure of 9 oz/in². The vent installed on this tank should have a set pressure of 12 oz/in² or greater to avoid leakage.

The Set Pressure of Other Devices

It is common to install several vents on above ground storage tanks. Sometimes additional vents can serve as a backup or help achieve the rated relieving capacity. They can also have a secondary function such as emergency venting or allowing tank access. The set pressures of the different devices should be far apart from each other to prevent unintended activation. This is most important for emergency and non-resetting relief devices.

Let’s consider a storage tank that requires a PVRV for normal venting, and an EPRV for emergency venting. Unless the storage tank is in an emergency situation, the EPRV should not activate. Thus, the set pressure of the PVRV should be selected to achieve the required relieving capacity before the tank pressure reaches the set pressure of the EPRV.

The Design Pressure or MAWP of the Tank

The design pressure is the maximum internal pressure a tank can safely withstand by design. The MAWP is the maximum allowable working pressure of a tank relative to atmosphere. The internal tank pressure shall never exceed the MAWP. The MAWP of the tank will depend on the tank design. For example, the MAWP of an API 650 tank is the same as the design pressure. However, for an API 12F tank, the MAWP is 1.5 times greater than the design pressure. Consult the applicable tank design standard for details.

The Required Relieving Capacity of the Tank

The required relieving capacity is the flow rate, set by an Engineer or the tank standard, that is required by the venting device at the design pressure of the tank. If the selected device can not meet this requirement, there will be a risk of explosion in an overpressure situation.

A common misunderstanding in the industry is that the set pressure of a relief vent is equal to the design pressure of the tank. This can be an issue when working with direct acting valves, where the pallet begins to open at the set pressure. When the internal tank pressure rises to the set pressure of the valve, the pallet is in equilibrium. Theoretically, the vent is not relieving at this tank pressure. In order to have the vent relieving by the time the internal tank pressure is at the design pressure, the pressure setting of the vent must be below the design pressure.

Exercise

Using this flow curve determine a reasonable set pressure of a 4″ PVRV for a 400 bbl tank.

• Design Pressure: 8 ozsi/in²
• MAWP: 9 ozsi/in²
• Operating Pressure: 4 ozsi/in²
• Required Relieving Capacity: 55,000 SCFH
• Other Devices: Emergency Pressure Relief Vent set at 8 ozsi/in²