PSVs Discharging to Atmosphere: What to Consider

Pressure safety valves (PSV) are used extensively in oil and gas applications to protect vessels, pipes, and other equipment against overpressure. More commonly, the PSV discharge is sent to a flare via a flare header which collects several relief sources through a flare network of headers and sub-headers. Alternatively, the PSV discharge can be directly sent to the atmosphere on some occasions. The setup required to connect the relief sources to a flare system could be too expensive or complex for remote gathering systems and hence the PSVs might be directed to the atmosphere in such cases. Relieving PSVs directly to the atmosphere, however, requires special safety attention and considerations. This article highlights key areas to look for in the design when a PSV is directly routed to the atmosphere.

Safety Guidelines of PSV Relief

The first precaution is the regulatory and safety requirements of the PSV relief, ensuring you can discharge the gas safely and the gas will not return to the grade level. Here are some guidelines:

Air, nitrogen, steam, water vapour, and other fluids considered non-hazardous could be more safely discharged to the atmosphere if the relief does not create safety issues (damaging equipment, injuring personnel, or creating oxygen-deficient environments). One must exercise caution for vents of water vapour as the water might condense and form small water droplets which will fall toward grade and collect on surfaces.
A small amount of finely dispersed liquids (particle diameter < 600 microns) can be discharged safely if the vapour pressure of the liquid is high at ambient temperature (Molecular weight of less than 80 is required for most hydrocarbons). The volatile liquid will evaporate quickly after discharge. Vapours containing mist could ignite well below the lower flammability limit.

Determining what materials and at which locations in the facility the relief can be safely discharged to the atmosphere is a complicated process requiring detailed evaluation of risks and mitigation options. A comprehensive assessment of the process for determining the safety of discharge to the atmosphere is discussed elsewhere [Refer to References].

Sizing and Evaluation of PSVs

Assuming the above study has been performed and discharge to the atmosphere is deemed to be safe, sizing and evaluation of the PSV would include the following steps just like any other PSV:

Determine the scenarios for the pressure build-up and calculate the corresponding relief rate as outlined in API 521. The scenarios include blocked discharge, fire, power failure, control valve failure, tube rupture, etc.
Size the orifice for the scenario requiring the highest relief rate as outlined in API 520.
Size the inlet and outlet lines for the PSVs as per following:

Inlet and Outlet Lines

The inlet line is checked for an inlet line pressure drop of a maximum of 3% of the set pressure for rated flow.

Outlet lines discharging to flare headers are sized to limit the Mach number to 0.7. For PSVs discharging directly to the atmosphere, limiting the Mach number to 0.7 is not practical and a sonic condition could be allowed in the discharge piping. The sonic conditions at the outlet will lead to a significant amount of backpressure.

The pressure drop in the discharge line should be calculated using the “Isothermal” or “Adiabatic” method as outlined in API 521. It is important to make sure the discharge line pressure drop calculations consider the acceleration term as the gas is compressible. Regular pipe operation in most process simulators ignores this term which leads to inaccurate prediction of the backpressure in the system. Keep in mind that when existing PSVs are evaluated, a very large relief load might be sent to a discharge pipe with a relatively small diameter. This could lead to excessive backpressure in the system. The high flow rates in the small pipe would lead to pressures higher than the atmosphere right at the discharge of the pipe.

Evaluation of Existing Systems

When evaluating an existing system discharging to the atmosphere, the key design considerations that must be checked for this PSV application:

Is the installed PSV orifice adequate for the new conditions?

Compare the required relief rate with the installed relief rate. If the required relief rate is less than 25% of the rated capacity of the PSV, the PSV is too large and would chatter if kept in this application. The PSV should be replaced with a smaller PSV.

If the required flow rate is greater than 95% of the rated capacity of the PSV, the PSV should be replaced with a larger size PSV.

Backpressure PSV limitation

Using the rated flow of the PSV along with the existing discharge line size, backpressure should be calculated and checked against the allowable back pressure depending on the PSV type.

The maximum allowable backpressure for the conventional PSV is 10% of set pressure, for balance bellows is 30% of the set pressure. For pilot-operated valves, the allowable backpressure is 50% of set pressure, but depending on the valve manufacturer, the backpressure could be as high as 100% of the set pressure. It is important to note that in this evaluation only built-up backpressure is considered.
Outlet flange class rating: the maximum backpressure should also be checked against the pressure rating of the existing. For example, 150# flange class is limited to 257 PSIG at 100 F.

Minimum outlet pipe exit velocity

For PSVs discharging to the atmosphere, a calculation for 25% of the rated capacity needs to be conducted to ensure the velocity at the discharge is sufficiently high to allow for dispersion of the flow and avoiding the formation of flammable cloud at grade level as per API (6^th edition) section 5.8.2.2.

Outlet pipe momentum

Choke conditions at the discharge point would lead to high forces and hence momentum calculations must be performed for the outlet pipe to determine if reinforcements are needed for the outlet pipe.

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