OOOOa Vent Assessment Tool™

August 7, 2017

Why do we need it?

Vent system sizes based on nominal, daily production may be inadequate. The flow of oil through the equipment at a wellsite facility is not always continuous, and a Peak Flow occurs during times that dump/throttle valves are open. The OOOOa rule requires certification that vent systems in place on wells constructed, modified, or reconstructed after September 18, 2015 are suitable to handle the peak flow of the facility.

What are the OOOOa requirements for vent system certification?

Condensed from the EPA at the following link:

https://www.federalregister.gov/d/2016-11971/p-597

4. PROFESSIONAL ENGINEER DESIGN OF CLOSED VENT SYSTEMS

It is the EPA's experience…that closed vent systems and control devices for storage vessels…often suffer from improper design or inadequate capacity that results in emissions not reaching the control device …As a result, we have included… provisions for certification by a qualified professional engineer that the closed vent system is properly designed…

Although the final rule does not include requirements for specific criteria for proper design, the EPA believes there are certain minimum design criteria that should be considered to [ensure]…the closed vent system [is] capable of routing all gases, vapors, and fumes emitted from the affected facility to a control device…The minimum design elements include, but are not limited to…

  1. Review of the Control Technologies to be Used to Comply with §§ 60.5380a and 60.5395a.
  2. Closed Vent System Considerations:
    1. Piping—
      1. Size (include all emissions, not just affected facility);
      2. Back pressure, including low points which collect liquids;
      3. Pressure losses; and
      4. Bypasses and pressure release points.
  3. Affected Facility Considerations:
    1. Peak Flow from affected facility, including flash emissions, if applicable; and
    2. Bypasses, pressure release points.
  4. Control Device Considerations:
    1. Maximum volumetric flow rate based on peak flow, and
    2. Ability to handle future gas flow.

How does the OOOOa Vent Assessment Tool work?

In its current form, the OOOOa Vent Assessment Tool is designed to layer onto an existing, steady state model of a facility. The same ProMax® file used for permitting or reporting emissions may be used. Open your existing ProMax file, open the Tool, and you’re ready to begin.

Estimating the Peak Flow rate can be done one of two ways. If the flow coefficients (Cv) of valves are available, along with known pressures in the facilities, then the Peak Flow rate may be estimated in the tool using engineering principles. Otherwise, a conservative factor may be applied to specify Peak Flow.

Once the liquid Peak Flow rate is known, ProMax predicts the subsequent rate of flash gas that must leave the tanks. The user must give the vent line configuration, lengths, diameters, numbers of fittings, and a pressure drop for the control device. With this information, the OOOOa Vent Assessment Tool will calculate the required back pressure to move the gas through the lines. This back pressure is then compared to the set pressure of the relief devices on the tanks (such as a thief hatch).

What are the results?

The fundamental result of the calculation is either a PASS or a FAIL. PASS means the required back pressure to move the gas is less than the relief set points at the tanks. FAIL is the opposite. Along with an indication of PASS/FAIL, the Tool also reports all flow rates, a pressure profile through the vent lines, and a percent overdesign of the system.

Why use the tool?

It’s fast, simple, and straightforward. All of the required information to perform the calculations is readily apparent and easy to gather. A comprehensive report is even generated, with the required OOOOa certification language and space for stamp/signature application, making record keeping simple as well.