Vapor Recovery Unit Principles are designed to recover hydrocarbon vapors which for years were allowed to escape from storage tanks. These vapors..
The first efforts at oil storage, which included open pits and barrels, were unsuccessful because of excessive evaporation of the oil. Steel tanks were better, but they still allowed significant evaporation. Covering the tank helped, but evaporation continued to be a major problem until vapor tight tanks were used. Vapor tight tanks have specialized equipment, e.g. thief hatches, vent valves, etc. which maintain a positive pressure in the tank and keep evaporation to a minimum.
Vapor Pressure
As previously stated, part of the crude oil vaporizes and fills the space between the top of the oil and the tank top. As temperature changes, pressure of the vapor also changes. The warmer the crude oil, the more it vaporizes, and the greater the pressure. As the vapor cools, it condenses and pressure inside the tank decreases. Even though pressure changes are generally small, they can increase to a point which makes venting necessary.
Within a tank battery (two or more tanks together) if vapor pressure is high in one tank, vapor moves to another tank with less pressure through an equalizing line.
This occurs until pressure in all tanks is equal and reaches a set point. At this pressure the vapor recovery system begins to operate by drawing vapor from the stock tanks through a gas vent line to the vapor recovery unit.
Vapor pressure inside the tank affects operation of a vapor recovery unit the most. Various pieces of equipment work together to maintain a set pressure. Maintaining this heated pressure helps to ensure the vapor recovery system will operate properly and efficiently. Also, it reduces the probability that tank damage will occur due to implosion or collapse caused from pulling a vacuum on the tank.
Vapor Recovery Unit
Vapor recovery units (VRU’s) are designed to recover vapors which ordinarily would be lost to the atmosphere. Vapor recovery units are installed when they are required by federal regulation or when it is determined that the quantity and vapors of the lost vapors warrant the expense of a VRU. These units are designed to comply with Environmental Protection Agency (ERA) standards regarding emission of pollutants to the atmosphere.
Vapor recovery units have four basic purposes :
- recover valuable hydrocarbons
- prevent air pollution
- reduce fire hazards
- prevent emission of poisonous gases, e.g. hydrogen sulfide (H2S)
PROCESS FLOW AND COMPONENTS
A typical vapor recovery unit is shown here. It consists of the following components: suction scrubber, liquid transfer pump, compressor, lubricator, gas by-pass system, and control panel (not shown).
Vapors enter the VRU through the suction scrubber. It is here that any liquids, which may have condensed in the gas vent line, will settle out and collect in the bottom. If liquids are allowed to enter the compressor damage could result from the liquids diluting compressor lubricant, causing unnecessary wear to compressor components.
Liquid Transfer Pump
When liquid level reaches a set point in the scrubber, a liquid level controller starts the liquid transfer pump. This pump transfers liquid through the outlet line on the side of the scrubber back to stock tanks. While liquids return to the stock tanks, hydrocarbon vapors at the top of the scrubber are discharged to the compressor. The vapors are compressed so they can be injected into the gas gathering system.
Compressor
The main component of the VRU is the compressor. It may either be a single-stage unit or a multi-stage unit depending on the pressure requirements. There are two types of compressors commonly used :
Rotary Compressors :
Rotary compressors are generally used for single-stage compression up to discharge pressures of 50 psig. If higher discharge pressures are needed, dual-stage rotary compressors can compress gas up to discharge pressures of 125 psig.
Reciprocating Compressors :
Reciprocating compressors are generally used for single-stage compression up to discharge pressures of 75 psig. When discharge pressures in excess of 125 psig are needed, multi-stage reciprocating compressors are used.
When dual-stage compressors are used, a second scrubber is installed to remove any liquids from the vapor which may have condensed as a result of first-stage compression. This liquid removal is done to prevent liquid slugs from entering the second stage of compression which could damage the compressor.
Compressor and Vapor Cooling
In a VRU there may be more than one cooling system: one to cool the compressor and one to cool the compressed vapors.On reciprocating compressors, heat from the compressor cylinder is dispersed to cooling fins on the compressor housing where heat is spread out over a larger surface area cooling the compressor and preventing damage to the compressor. The larger the surface area, the more heat will transfer over time. Fins must be kept clean in order to effectively transfer heat and keep the compressor from over heating.
On rotary compressors, heat from the compressor is transferred by water to a cooling system which resembles an automobile radiator. Hot water enters a cooling coil in the radiator where heat is passed through the coil wall to attached cooling fins. Fins on the coil increase _the;surface area as they do on the compressor. In order to increase the flow of air over the fins, a fan can be utilized.
When cooling compressed vapors, the hot vapors from the compressor pass through an aerial cooler. This type of cooling system is made up of a series of tubes with fins which allow heat to transfer from the hot vapors to the atmosphere, thus cooling the vapors.
Cooling systems which utilize water instead of air as a heat transfer medium have special problems. Here are some steps which must be taken to keep a water-cooled system in proper working order:
- maintain proper coolant level
- keep hoses and seals in good condition
- add antifreeze in cold weather
- use corrosion inhibitor
- keep free of rust and dirt
- keep free of any salt or scale forming minerals.
This system is in place to reduce electrical energy usage and compressor maintenance. The gas by-pass system is designed to reduce frequent starting and stopping of the compressor, which is caused by changing vapor pressure inside the stock tanks.
The gas by-pass system is made up of a bypass valve and piping that recirculates compressed vapor back through the scrubber and compressor. This is done to maintain vapor pressure at a level which keeps the compressor running until pressure inside the stock tanks can increase.
Pressure inside stock tanks is usually measured in inches of water. The compressor starts when vapor pressure increases to a predetermined set point, usually two inches of water.
When vapor pressure drops to about one inch of water, the compressor stops. In the gas bypass system the by-pass valve, operated by a control pilot, partially opens and some of the compressed vapor resirculates while the rest is discharged to the gas gathering system.
If vapor pressure drops well below one inch, the by-pass valve fully opens and all compressed gas is recirculated; however, a continued decrease beyond a pre-determined set point will cause an automatic timing device to shut down the unit.
- [accordion]
- 1. Vapor Recovery Unit Principles Video
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- 2. Separation of Water-in-Oil Emulsions
- 3. Standard Guidline Vapor Recovery
- 1. Vapor Recovery Unit Principles Video
- 2. Separation of Water-in-Oil Emulsions
- 3. Standard Guidline Vapor Recovery
