Thief Hatches are installed on the top of tanks in the oil & gas, chemical, pharmaceutical, biogas, water treatment, and other industries to allow access to the tank. Vance Ray, in his article, Working Wirelessly in the Summer 2018 issue of Tanks & Terminals, says they can be used to take samples of the tank’s contents and determine the level of the tank, and they protect the tank from overpressure and excessive vacuum.
Thief hatches on the tops of tanks at Great Western Oil & Gas Co in Colorado allow access to the tank and protect the tank from overpressure. If left open, huge fines can result.
The Thief Hatch acts as a pressure safety device on the tank. When closed and latched, two separate spring-loaded seals protect against excessive pressure or vacuum. If excessive pressure builds up in the tank, the hinged hatch cover will break its seal, lift and allow the pressure to escape to the atmosphere. When the pressure or vacuum is reduced to the setpoint, the seal is reseated by either spring, sealing the tank.
When a thief hatch closes—either from gravity or from a worker closing it—the hatch may not seal unless it is firmly latched. This allows vapors in the tank to leak into the atmosphere, which can violate existing Colorado regulations, as well as anticipated regulations in other states and countries.
To avoid stiff penalties and protect the environment, Great Western installed wireless thief hatch monitoring systems on its oil tanks in the Denver-Julesburg (DJ) Basin in Colorado. Great Western has more than 600 operating wells there, producing nearly 13,800 barrels of oil per day.
Inside Thief Hatches
A thief hatch is a rugged device designed for harsh environments and handling by user show are often less than gentle. The thief hatch has a latch that locks the lid closed. If an operator opens the hatch to check tank level or take a sample, he or she lifts the latch, raises the lid, performs the necessary function and closes and latches the hatch. If the lid is not properly latched, the tank will not be sealed and will vent to atmosphere. This is the situation the state of Colorado wants to eliminate from production facilities: the unnecessary release of methane and VOCs into the atmosphere.
A thief hatch has a lid that can be opened manually. The latch (upper left) keeps the lid closed and tank sealed when closed.
R3 Automation, an automation services company in Windsor, CO, has been working with Emerson Process Automation and several upstream oil & gas companies to develop a thief hatch monitoring system. Rosemount 702 discrete WirelessHART® transmitters can be installed on existing thief hatches and tank batteries to effectively ensure the hatches are closed and latched.
This thief hatch at Great Western is fitted with a WirelessHART monitoring system developed by R3 Automation. When the latch is not closed, the wireless transmitter sends a signal to the facility’s controller so that it can be logged or an alert can be sent out.
The concept is fairly simple, Vance says:
A switch at the latch detects when the latch is closed. The switch is non-powered, has no magnets, is easy to install on new or existing thief hatches, and no hot work is required. The switch is wired to an intrinsically safe wireless transmitter. The battery-powered transmitter is also easy to install. It requires no power wiring and no signal wiring, so it can be mounted in any convenient location on top of the tank. To conserve battery life, the scan rate is once per minute, more than sufficient for this monitoring application.
At Great Western’s tank farm near Brighton, Colorado, WirelessHART thief tank monitors were installed on 2 water and 12 oil tanks. The tank monitoring system is arranged in banks. Wireless transmitters send their data to a gateway mounted on a DIN rail in the production facility’s main control room.
Great Western’s tank farm has WirelessHART thief hatch monitors on two water and 12 oil tanks.
In addition to the thief hatch monitors, the tank farm has 30 more Rosemount WirelessHART level switches, temperature transmitters and pressure transmitters mounted on the tanks to create a comprehensive tank monitoring system. The data from all transmitters goes to the WirelessHART gateway, which is hardwired to a TotalFlow RTU. The system is programmed to monitor the thief hatch signals and report any open hatches to the operators.
The monitoring system alerts operators to open hatch conditions within one minute, allowing quick resolution of any problems. Field technicians no longer need to periodically monitor and document the status of thief hatches manually, but instead only need to respond to an open hatch alert. This reduces manpower requirements for monitoring thief hatches and demonstrates Great Western is employing the best available technology to protect the environment and comply with regulations.
Vance says Great Western is not only reducing emissions for the benefit of its neighboring communities, it’s containing gases that can be recovered and sold.
Although Colorado is the only state so far to issue tough regulations for monitoring thief hatches, such regulations are probably coming from the U.S. EPA, or from individual states. WirelessHART thief hatch monitoring is a cost-effective solution to avoid fines, reduce product loss to the atmosphere and cut emissions.
You can find more information like this and meet with other people looking at the same kinds of situations in the Emerson Exchange365 community. It’s a place where you can communicate and exchange information with experts and peers in all sorts of industries around the world. Look for the WirelessHART and IIoT Groups and other specialty areas for suggestions and answers.
Posted by Deanna Johnson, Rosemount Measurement & Analytical Global Marcom Manager, Emerson Automation Solutions
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