The use of manual tank gauging is still common in the oil and gas industry, but it is a method which can compromise the safety of the workers making the measurements. In February, 2016, the U.S. National Institute for Occupational Safety and Health (NIOSH) and U.S. Occupational Safety and Health Administration (OSHA) jointly issued a hazard alert 1 for workers involved in manual tank gauging and fluid sampling at oil and gas extraction sites. This alert identified the health and safety risks posed by exposure to concentrated hydrocarbon gases and vapors, and exposure oxygen-deficient atmospheres—and the potential for fires and explosions when hatches are opened on production and flowback tanks, or other tanks containing process fluids.
The alert states how opening tank hatches, often known as “thief hatches,” can cause high concentrations of hydrocarbon gases and vapors to be released. For workers performing this task, these exposures can have immediate damaging health effects and can even prove fatal. NIOSH and OSHA identified nine worker deaths from 2010-2014 that occurred while workers manually gauged or extracted samples from production tanks. Exposure to hydrocarbon gases and vapors and/or oxygen-deficient atmospheres was believed to be either a primary or contributory factor in those deaths.
What happens when a thief hatch is opened?
When a thief hatch is closed, hydrocarbon gas and vapors in the tank are often at a pressure higher than atmospheric. When the hatch is opened, a large volume of gas, mostly propane and butane, rush out. These gases can displace oxygen in the immediate work area, possibly asphyxiating workers in the vicinity. As the hatch remains open, heavier hydrocarbon molecules (pentane, hexane, heptane) also begin to leave the tank and enter the workspace. The rate of release is high, and these gases and vapors may reach toxic or flammable concentrations. As the hatch remains open, the gases and vapors in the tank approach equilibrium with the environment, significantly slowing the rate of emission.
When workers suffer acute exposure to hydrocarbon gases and vapors, their eyes, lungs and central nervous system can be affected. The NIOSH/OSHA alert states such exposure can sensitize the heart to stress hormones, such as catecholamines, causing abnormal rhythms and ventricular fibrillation, which can lead to sudden death. Even an exposure of less than 30 seconds can result in fatal cardiac arrhythmias, the alert adds.
Operators can take various measures to prevent workers from putting their health and safety at risk through exposure to hydrocarbon gases, vapors and flammable atmospheres, including:
- Engineering controls such as fitting tanks with sampling taps and thief hatch pressure indicators
- Improved training and work practices, and
- Use of protective equipment such as flame-retardant clothing, respiratory protection and impermeable gloves.
However, number one on the NIOSH/OSHA list of recommendations for improving safety is the implementation of alternative tank gauging and sampling procedures, which enable workers to monitor tank fluid levels and take samples without having to open the thief hatch.
Automatic tank gauging systems
Automatic tank gauging (ATG) systems perform a range of measurements within a tank (Figure 1), and can also issue an alert to an operator when action is necessary, for example in the event of the fuel level being too high or low. Critically, these fully automatic systems eliminate the need for workers to climb tanks and open thief hatches, and therefore provide significant safety benefits.
Figure 1: Automatic tank gauging systems avoid potentially fatal hazards for workers while typically delivering greater accuracy than manual measurements.
Although a number of measurement technologies, such as servo and hydrostatic instruments, can be used to perform the principle level measurement, by far the most prevalent is radar technology. Guided wave radar instruments (Figure 2) such as Emerson’s Rosemount 5300, and wireless Rosemount 3308 or non-contacting radars (Figure 3) such as the Rosemount 5400, provide highly-accurate and reliable level measurements largely unaffected by process conditions such as temperature, pressure, density and turbulence. Guided wave radar also offers the benefit of interface level measurement, which enables determination of water and oil levels inside tanks.
Figure 2: Guided wave radar instruments can detect and measure an interface layer between water and oil in a storage tank.
Figure 3: Radar level instruments are immune to the typical application problems (heavy vapors, foam, etc.) typically found in oil storage tanks.
In addition to the guided wave and/or non-contacting radars for level measurement, ATG systems also incorporate temperature transmitters (Figure 4). On well pads, the wired transmitters communicate directly with a remote terminal unit (RTU), or a wireless gateway collects measurements from multiple wireless devices and communicates with an RTU, which in turn communicates with the control room, which may be at a distant location. Monitoring software enables operators to observe and check level and volume measurements easily and efficiently.
Figure 4: Automatic gauging systems used for custody transfer incorporate automated temperature measurement for density calculations.
Regulations and standards still catching up
Automatic tank gauging is a viable option to keep workers safe on the ground and thief hatches closed, in line with the NIOSH/OSHA recommendations. However, while the hazards of manual gauging are well known, regulations and standards haven’t kept up with technology in the past. Gaps in the standards left manual gauging as the sole option for custody transfer applications on small production tanks.
Until recently, there had been no standard in support of cost-effective automatic tank gauging on smaller production tanks (less than 1,000 bbl), as the existing American Petroleum Institute (API) standard API MPMS 18.1 only covers manual gauging for custody transfer, while API MPMS 3.1B covers automatic tank gauging for custody transfer in inventory storage tanks over 1,000 bbl. Level instruments used for custody transfer, according to API MPMS 3.1B, are a non-viable option for small production tanks as the cost is assumed to be too high. However, a new standard has recently been published which will have a significant impact on how tank gauging is performed in the future.
API MPMS 18.2 standard
The new API MPMS 18.2 standard 2, published on July 1, 2016, allows automatic tank gauging to be used for custody transfer on production tanks. The accuracy requirements for API MPMS 18.2 correlate to what is already installed in the field in many applications, where thousands of radar instruments have been in use for level monitoring, overfill prevention and truck scheduling for years. Operators with existing radar level instruments can immediately start to adopt the new standard, relying on existing units for custody transfer, as long as they can prove the accuracy of the instruments.
As with all instrumentation used for custody transfer, a radar level instrument’s accuracy must be verified periodically. The verification of radar accuracy can be done in just a few minutes and does not require the thief hatch to be opened or any product to be transferred.
In custody transfer applications, the fuel’s mass needs to be established, and this calculation requires both its level and average temperature to be measured accurately. Previously—as with the level measurement—the temperature also had to be sampled by manual means. However, API MPMS 18.2 also covers standards for temperature sampling during custody transfer of crude oil from lease tanks to a truck trailer without requiring direct access to the tank’s thief hatch, therefore keeping workers off tanks and increasing safety.
Operators can now use automatic tank gauging for custody transfer applications on production tanks, as it not only complies with the new API MPMS 18.2 standard, but also enables them to follow the NIOSH/OSHA recommendations for increased worker safety.
Use of automatic tank gauging in custody transfer enables efficiency to be increased because:
- There is no need to gauge manually before and after filling and emptying
- Emissions are lower because the thief hatch remains closed at all times, and
- Worker injuries and fatalities related to manual gauging can be avoided by keeping workers on the ground and avoiding exposure to hydrocarbon chemicals.
This combination of technology, industry standard practices and government imposed safety regulations makes for a highly advantageous situation where operational performance, environmental considerations and worker wellbeing all advance together.
 – NIOSH/OSHA Hazard Alert: Health and safety risks for workers involved in manual tank gauging and sampling at oil and gas extraction sites. Published February 2016.
 – API Manual of Petroleum Measurement Standards, Chapter 18.2. Published July 1, 2016.