Pressure Relief: Low Pressure Vessels

“Pressure comes from within and so must be mastered from within.”  — Ed Jacoby

Ten years ago, on Friday morning, April 24, 2015, a catastrophic still failure at the Silver Trail Distillery in Hardin, Kentucky seriously injured two workers, one who died 17 days later from his injuries. They were about four gallons into a distillation batch when the 300-gallon still exceeded its 1-psig design pressure. The overpressure launched the still, which landed 50 feet away.

The pressure relief valve, set at 150 psig and designed to protect water heaters, failed to protect the propane-fired still when a blockage somewhere in the system created the conditions for pressure to climb out of control.

Even low-pressure and atmospheric vessels need emergency pressure relief.

What Makes a Tank “Low-Pressure” or “Atmospheric”?

A pressure vessel is designed per the ASME Boiler and Pressure Vessel Code to operate at a pressure greater than 15 psig. The rules for pressure relief are now described in Section XIII of the BPVC. These rules, however, don’t apply to vessels designed to operate at 15 psig or lower.

In the Process Safety Management (PSM) standard (29 CFR 1910.119), OSHA defines an atmospheric tank as a storage tank designed to operate at 0 psig up through 0.5 psig.

A low-pressure tank, then, is any vessel designed to operate above 0.5 psig (13.85 inches of water) up through 15 psig. The still at Silver Trail Distillery was a low-pressure vessel.

In each case, it doesn’t matter at what pressure the vessel actually operates; it’s the design pressure that matters.

Regulations and Standards

OSHA requires that the design of equipment in PSM-covered processes comply with recognized and generally accepted good engineering practices (RAGAGEP), although OSHA doesn’t stipulate what RAGAGEP to follow. Where the PSM standard does not apply, for instance in a process that contains flammable liquids but less than 10,000 pounds (and a 300-gallon still contains less than 2,000 pounds of flammable liquid), OSHA turns to the general duty clause, which states in part that workplaces must be “free from recognized hazards.” While OSHA doesn’t enforce RAGAGEPs in non-covered processes, it does use them to demonstrate that hazards are recognized.

When a tank contains a flammable liquid, it’s emergency relief must comply with OSHA’s Flammable Liquids standard (29 CFR 1910.106). In general, though, most will turn to API Standard 2000, Venting Atmospheric and Low-Pressure Storage Tanks, 7^th Edition (2014).

Demand

Anyone designing a pressure relief system must first understand what the demand on the system will be. All relief systems, whether they are called “pressure relief” or “thermal relief”, relieve pressure by releasing material. “Demand” is the rate at which material must be released.

The most widely recognized demand case is that of an external fire. The scenario begins when equipment is engulfed in flames. Heat transfers through the equipment walls to the liquid contained within, increasing the temperature of the liquid, in turn increasing the vapor pressure of the liquid. Eventually the vapor pressure exceeds the equipment’s ability to contain the pressure and something breaks.

While almost universal, the external fire case is not the only possible cause for a demand on a pressure relief system. API 2000 identifies nineteen other causes:

• Liquid inflow
• Flashing of inflowing liquid
• Vacuum from outflow
• Changes in outdoor temperatures
• Pressure transfer through vapor breakthrough
• Inert pads and purges
• Abnormal heat transfer
• Internal failure of heat-transfer devices
• Vent treatment system
• Utility failure
• Changes in temperature of input stream to tank
• Chemical reactions
• Liquid overfill
• Atmospheric pressure changes
• Control valve failure
• Steam condensation
• Uninsulated heated tanks
• Internal explosion/deflagration
• Mixing products of different composition

There is probably no low-pressure or atmospheric tank anywhere to which all twenty causes apply, but OSHA expects equipment designers at least to consider each, if only to determine that they don’t apply.

Relief Devices and Capacity

The purpose of pressure relief design is to choose the place where high pressure breaks free. Successful pressure relief systems release material at a rate high enough to keep the pressure below what the rest of the equipment can withstand. It is not sufficient to specify the set pressure for the relieving device. The designer must also specify the capacity of the relieving device, so that when the pressure reaches the set pressure, the system will release enough material to relieve the excess pressure.

The capacity of a specific relief device is dependent on the set pressure. The higher the set pressure, the harder the stuff inside the tank pushes to get out. So, while a high-pressure vessel can get away with a relatively small relief device, low-pressure vessels need big relief devices to provide enough relief area. Rupture discs and pressure relief valves, typical devices in high-pressure relief systems, are not generally available with the set pressures required for low-pressure or atmospheric equipment.

A common relief device for atmospheric vessels is an atmospheric vent, a pipe that opens directly to the atmosphere. Often, someone will insist, “That tank can’t overpressure; it’s got an atmospheric vent.” Atmospheric vents are reliable if they’re big enough, but they are not perfect. They have a potential for plugging. The plugging might be caused by birds or wasps building nests, by monomer vapor polymerizing in the vent, or by solids carried through the vent which then deposit on the vent walls. Solids like the dissolved sodium hydroxide in 50% caustic or the spent grain in whiskey mash.

Other relief devices for low-pressure tanks include end-of-line conservation vents and pipe-away conservation vents for use with vent gas treatment systems. When a lot of relieving area is required, a relieving manway can provide as much as a 36” diameter relief if there is a large enough nozzle on the tank to install it.

Low-pressure Tanks Need Relief

The fact that atmospheric and low-pressure tanks operate at a pressure that is not considered “high pressure” does not mean that they cannot experience a pressure that is “too high.”

Thomas Carlyle, the nineteenth-century Scottish philosopher, is usually credited with saying, “No pressure, no diamonds.” What he didn’t address is that not all pressure produces diamonds. Far more often, excess pressure simply produces catastrophe. For instance, a failed tank. An adequate pressure relief system requires knowledge of the potential causes of excess pressure and then designing for those causes.

Ensure Relief Is Adequate

While a catastrophic failure of a pressure vessel is typically much worse than the catastrophic failure of a low-pressure tank, low-pressure vessel failures are not without consequences. Ask the folks who survived the incident at Silver Trail Distillery. Designing pressure relief for low-pressure or atmospheric tanks requires different approaches than designing pressure relief for pressure vessels.

Make sure that your low-pressure and atmospheric tanks have appropriate pressure relief, not just for ordinary operating conditions, but for abnormal conditions as well.