Use the Power of the Vapor Recovery Tower
October 22, 2014

Use the Power of the Vapor Recovery Tower

Operators are always looking for ways to improve vapor recovery unit (VRU) performance and to avoid NSPS OOOO (Quad O).  Using a vapor recovery tower along with a VRU, a facility can increase production of natural gas and decrease air emissions.   

 Vapor recovery towers (VRTs) are tall, vertical separators used to recover flash gas emissions that would be generated in a crude oil storage tank. 

Flash gas results from natural gas coming out of solution when crude oil is transferred from higher pressure production separators (or other pressure vessels) into storage tanks.  The higher the pressure drop between a pressure vessel and a fixed roof storage tank, the higher the amount of flash gas generated.  Since most production storage tanks are fixed roof tanks that operate at near atmospheric pressure, this flash gas is either vented to the atmosphere, burned in a flare or recovered by a VRU. 

For typical crude oil and condensate storage tanks, the VOC emissions consist of flash, breathing (standing) and working losses.  The majority of VOCs from such storage tanks are the flash emissions.  

Diagram of a VRT equipped with a HY-BON VRU and Vapor Combustion Device (VCU). 

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VRT Applications

VRTs can be used with large and small vapor recovery units for a vent gas recovery system.  It can be a part of your vent gas management system. 

When a VRT is used as a part of a vapor recovery system, it can recover the majority of flash gas from a pressurized oil stream prior to the oil entering the storage tanks. This reduces the amount of flash gas generated in the storage tanks.  The flash gas recovered by the VRT can then be routed to a vapor recovery unit and then ultimately to the sales pipeline – increasing gas production. 

Some applications utilize a VRT to supply the motive force to send the gas to a candle stick flare or vapor combustion device.  For some flaring applications, more motive pressure is required to send the gas to the combustion device than is available from an atmospheric storage tank.  For this application, the VRT’s operating pressure can be adjusted to a pressure setting high enough to move the flash gas to the combustion device.    

VRTs routing flash gas to a vapor recovery system operate at approximately 1 to 5 psig.  For VRTs sending flash gas to a combustion device greater operating pressure may be required. 

The process for a VRT: 

  1. Liquids enter the VRT through the oil inlet and exit the VRT through the oil outlet.  
  2. The liquid level in the VRT normally remains approximately one to two feet below the oil outlet nozzle due to the pressure in the VRT and the oil is pushed up to oil outlet through the down comer pipe.
  3. The oil outlet down comer keeps vapors from being pushed to the liquid storage tanks.
  4. The space above the normal liquid level is used as a vapor space and a VRU or other pressure control valve is used to maintain a pressure of 1 to 5 psig.  By maintaining a positive pressure in the VRT’s vapor space, the chance of air entering the recovered flash gas is minimized. 

The diameter of the VRT is greater than the oil inlet diameter and this lowers the velocity of fluids and lowers gas entrainment.  Also, the vertical vessel height aids in gravity separation of natural gas and oil. 

Since the VRT lowers the amount of flash gas liberated at the storage tank, storage tank VOC emissions may be low enough that no further emission controls are required.  This can aid in compliance with NSPS OOOO (Quad O).    

Where there is concern that oxygen could enter the vent gas stream, a properly designed and operated VRT can exclude air from entering the recovered flash gas.  For a fixed roof storage tank operating at near atmospheric conditions, air can enter the tank through open or leaking thief hatches and leaking pressure/vacuum relief valves. 

 With the VOC air quality enforcement getting tougher every day, operators want more recovery of vent gas and ways to stay out of the alphabet soup of regulations.  You can contact us now how HY-BON's VRTs and Identify, Quantify, Rectify (IQR) methodology will reduce your liabilities and increase profits. 

HY-BON Vent Gas Management – HANDLED!