“Pressure’s what you make of it.” — Daniel Ricciardo
Years ago, a colleague told me, “If the pressure is too high, it’s going to find a way to relieve. If you don’t plan for pressure relief, then you don’t get to pick how and where it relieves. And that can be disastrous.”
For every piece of equipment, there is a pressure that is too high. We relieve pressure by releasing enough material to keep the pressure from being too high. Pressure relief systems control the place from which enough material releases and the place to which it releases.
The challenge, then, is to know not only how high the pressure must be to be too high, but how much material must be released. A pressure relief system that doesn’t begin releasing material until the pressure is already too high will not protect us. Likewise, a pressure relief system that doesn’t release enough material to keep the pressure from becoming too high will not protect us.
This is true for every piece of equipment, although some equipment has no way to experience high pressure. So, while some equipment does not need pressure relief, because there is no way for it to experience pressure that is too high, any equipment that has the potential to experience pressure that is too high is equipment that needs a pressure relief system.
That includes the equipment in a distillery.
Causes of High Pressure
The equipment most commonly in need of pressure relief is a vessel. The includes pressure vessels (with a pressure rating of more than 15 psig), low-pressure vessels (with a pressure rating between more than 0.5 psig up to 15 psig), and atmospheric vessels (with a pressure rating between 0 psig and 0.5 psig).
The most widely recognized cause of overpressure is an external fire. The scenario begins when equipment containing a liquid 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.
API 2000, the standard developed by the American Petroleum Institute for venting atmospheric and low-pressure tanks (regardless of the industry) identifies 19 types of causes for overpressure other than external fire. In distilleries, the most relevant are probably liquid inflow and overflow and abnormal heat transfer.
The gold standard for pressure vessels is the Boiler and Pressure Vessel Code (BPVC), put out by the American Society of Mechanical Engineers. When it comes to causes of high pressure, the BPVC refers to API 521, the code for pressure vessels. API 521 lists 16 types of causes for overpressure, including external fire. In distilleries, the most relevant are probably blocked outlets, abnormal process heat or vapor input, cooling failure, or failure of automatic controls.
A Tragic Case
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, designed to relieve 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.
Distillery Equipment Likely to Need Pressure Relief
Vessels and pumps are the equipment most likely to require pressure relief. Even then, not all vessels and pumps.
When it comes to pumps, centrifugal pumps do not typically require pressure relief, although they may expose receiving vessels to their deadhead pressure. That would mean that the vessel may require protection from the pump-in scenario.
Positive displacement pumps are more likely to require pressure relief, specifically for the case where the pump discharge is blocked in. Gear pumps, in particular, have the ability to generate a discharge pressure that exceeds the rating of the pump casing and piping, and require pressure relief. It has become normal to provide gear pumps with an external pressure relief valve, in addition to the internal pressure relief valve that most gear pumps come equipped with, because there is no way to test the internal pressure valve.
Another type of positive displacement pump that is often used is the air-driven double diaphragm pump. Unlike gear pumps, the pressure that an air-driven double diaphragm pump can generate is limited to the pressure of the air driving it. When the discharge from an air-driven double diaphragm pump is blocked, the pump just stops pumping. There is no catastrophic pump casing or discharge piping failure.
As for vessels, a vessel that can be blocked in can experience overpressure, if only because of the external fire case. In distilleries, this could include cookers, fermenters, slurry tanks, still pots, the shell on shell-and-tube heat exchangers, filter housings, collection tanks, proofing tanks, blending tanks, surge tanks, and day tanks. Dust collection bag houses also require pressure relief, not because of external fires, but because of the potential for internal dust explosions.
Importantly, while pressure relief may consist of pressure relief valves or rupture disks, it may also consist of being open to atmosphere.
Open to Atmosphere
Fermenters have a pressure that is too high, but in distilleries, fermenters often do not require pressure relief systems. This is because the fermenters in distilleries, unlike those in breweries, are often open top vessels. An open top invariably provides enough venting capacity. Vessels with a closed top may also be open to atmosphere, through a vent line.
Vent lines may provide enough venting capacity if they are big enough. By “big enough”, we mean big enough for the emergency venting situation, not just big enough for normal operation. It’s easy to imagine that a 4” vent line from a 4000-gallon vessel is big enough, and calculations would most likely support this. On the other hand, a ¼” vent line from a 4000-gallon vessel is undoubtedly too small. So, somewhere between ¼” and 4” is the minimum size that is adequate for vent line to provide pressure relief in an emergency. Most likely, the sizing case will be the external fire.
For equipment that cannot be left open to atmosphere, however, some kind of relief device will be necessary.
Sizing Pressure Relief Devices
Pressure relief devices must be sized and specified. The specification must include a set pressure, which cannot be higher than the rated pressure of the equipment the device is protecting. The specification must also include the relieving capacity of the device, at the set pressure of the device. The higher the set pressure, the greater the capacity, so how a particular device performs can vary from one installation to the next.
Before specifying the relieving capacity of the device, it is important to determine the demand for all of the relieving cases. Usually, the highest demand will be the external fire case, but not always. Especially with equipment like still pots, where heat is deliberately and necessarily applied.
Document the Pressure Relief Design and Design Basis
The still pot that experienced an overpressure failure at the Silver Trail Distillery was manufactured by Revenoor, Inc. a company no longer operating. At the time of the tragedy, the owner, Terry Wilhelm was quoted in the incident investigation report as saying, “I just put those on there to keep the lawyers off me.”
Sizing pressure relief devices is not a legal exercise. Neither is it a trivial technical exercise, but it is not complicated. It needs to be done, and whether we do it ourselves, rely on the manufacturer, or hire a third party, we must insist that the design and design basis be documented. What is the emergency demand the system should be capable of handling? What is the set pressure that the system must have? And what is the capacity of our system at that set pressure (and is it greater than the demand)?
Otherwise, we are putting our operation and workers at risk.
