“Science explorers are like an ideal gas. They can expand to fill any volume, but they can only do work under pressure…and the pressure’s on.” — Robert Ballard
Every time that someone at a process facility tells me that they use compressed gas to transfer liquids from one tank to another, I wince. It’s not because it cannot be done without incident—obviously, it is done without incident at facilities around the world every day. It’s because there are hazards associated with this means of liquid transfer that they don’t seem to be aware of.
What are these hazards that make me wince? Why will I, as a process safety professional, always prefer to see liquid transfer by means of a pump?
Source Tank Overpressure
It is quite common for the vessel from which liquid is being transferred to be a low-pressure vessel, 15 psig or less. On the other hand, it is quite common for the compressed gas that transfers the liquid to come from a plant utility—plant air or plant nitrogen—operated at around 90 psig. The normal design uses a regulator on the drop down from the plant utility to the tank.
Regulators aren’t perfect, however. They can be expected to fail to the open position about once per decade. When they do, the tank to which they supply compressed gas experiences full line pressure, enough to cause a tank rupture. So, the normal design also includes a relief device sized to handle the regulator failure case. This relief device is not required when a pump is transferring the liquid. Of course, since they are critical to the safe operation of the vessel, the regulator and relief device must be subjected to periodic testing and inspection, something more for the maintenance department to add to their already long list of things to do (not that pumps can go without maintenance).
Vapor Breakthrough
Because liquids are incompressible fluids, they don’t need much head space at all to decompress from a high pressure to a low pressure. So, it is possible to transfer liquid at 90 psig through a pipe into an atmospheric tank without exposing the tank to 90 psig. When the receiving tank is liquid full, however, the tank will experience the full line pressure and resulting damage. But it won’t be catastrophic damage; a release at the rate the pump is pushing liquid in will be enough to decompress the liquid.
On the other hand, when compressed gas pushes the liquid, the liquid transfer eventually ends, and the compressed gas reaches the receiving tank. So, instead of an incompressible fluid that requires little expansion to reduce pressure, this “breakthrough” results in a gas that must expand in proportion to the required pressure drop. When there is not enough room to expand, it poses a significant relief case, something that doesn’t occur with pumped liquid.
No Vapor Conservation
An increasingly common feature in the design of liquid transfer lines is an accompanying vapor return line. When a pump pulls liquid out of a source tank and pushes it to a receiving tank, the head space in the source tank increases, putting the source tank under vacuum, The head space in the receiving tank decreases by exactly the same volume, putting the receiving tank under pressure. When a vapor return line connects the two tanks, the vapor being displaced from the receiving tank fills the increasing head space in the source tank, maintaining a perfect equilibrium.
When pressurized gas pushes the liquid from the source tank, it addresses the problem of vacuum in the source tank. But the vapor in the shrinking headspace of the receiving tank must go somewhere. And it cannot go back to the source tank. As a result, it is lost to the process and becomes a yield loss, albeit small, built into the design.
Venting Vapor-Laden Gas to Atmosphere
When pressurized gas transfers liquid from a source tank to a receiving tank, the vapor in the shrinking headspace of the receiving tank must go out a vent. When the vapor is flammable or toxic, the vent typically will be equipped with a treatment system. No treatment system has a removal efficiency of 100%, meaning that at least some of the flammable or toxic vapor is released to the atmosphere. And, believe it or not, some tank vents are not equipped with a treatment system at all.
When liquid transfer is by means of a pump, it allows for a vapor return line as part of the design which offers at least the potential of 100% vapor containment.
Inability to Stop in Emergency
The four previous reasons for using a pump instead of pressurized gas for liquid transfer are all design issues, issues that can be addressed during design. The fifth reason for using a pump instead of compressed gas to transfer liquids is an inescapable problem of physics: liquids are incompressible and gases are not. When a pump shuts off, the pressure instantly disappears. On the other hand, when the supply of compressed gas shuts off, the system is still pressurized. And the system remains pressurized , even as the compressed gas vents.
From a process safety perspective, the ability to instantly take the pressure away from a system in an emergency will always be desirable. Compressed gas as a means of liquid transfer does not allow for that, while pumping does.
The “Hazard” That I Don’t Worry About
Occasionally when I begin to point out the hazards of using pressurized gas as the motive force for transferring liquids, I’ll be interrupted. “Oh, don’t worry. At least we don’t use air. We know that increases the flammability in the head space, so we use nitrogen.” Except that leaking nitrogen can be an asphyxiation hazard, there is nothing wrong with using nitrogen instead of air. If the liquid is flammable with a flash point less than the ambient temperature, putting nitrogen in the head space of the vessel instead of air is certainly helpful.
When the flammable liquid has a flash point higher than the ambient temperature, putting nitrogen in the head space doesn’t hurt, but increasing the pressure in the head space with compressed air is not going to make the headspace more flammable.
What?!
Consider ethanol. Its flash point is 55°F (13 C). Ethanol’s lower explosive (LEL) is 3.3 vol% in air at atmospheric pressure, which is same as saying that the mole fraction of ethanol vapor in air at atmospheric pressure is 0.033. In other words, the partial pressure of ethanol in the vapor space of a tank containing pure ethanol at 55°F (13 C) is 0.033 atm (0.485 psia). The partial pressure of the ethanol won’t change as long as the temperature doesn’t change. When compressed air increases the pressure of the headspace from 1 atmabs to 3.04 atmabs (44.696 psia, 30 psig), the mole fraction of ethanol drops from 0.033 to 0.011. The head space becomes considerably less flammable.
In fact, a study shows that the flash point of many common flammable liquids increases significantly as the pressure increases — about 18°F (10 C) when the pressure goes from ambient to 15 psig. So, a flammable vapor becomes even less flammable as a non-flammable gas—even air—pressurizes the head space.
Take Another Look
The primary reason for using compressed gas to transfer liquids, especially if the plant air or plant nitrogen system already exists and has plenty of capacity, is the lower initial capital costs. Pumps cost money. And once installed, they have their own maintenance costs. But from a process safety perspective, they have the potential to be much safer in an emergency.
If you are using compressed gas to transfer liquids, would installing a transfer pump make for an safer process? It’s worth a look.
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