What is a Hydrophobic Sterile Tank Vent and Why Does it Need to be Heated?

January 26, 2015

In the high purity fluid handling industry, there is no such thing as a “typical” plant. Layouts and installations vary from plant to plant, company to company. That being said, certain pieces of equipment, such as pumps, sanitary diaphragm valves, gaskets, and flexible hose assemblies are ubiquitous throughout all facilities. We’ve talked extensively about those on this page before. One piece of commonplace equipment we haven’t spent as much time emphasizing are tanks. This post will take a closer look at pharmaceutical storage tanks, specifically tank vents and common considerations that go in to selecting the right one for your application.
To begin, why do we need a tank vent filter? Well, let’s start by thinking about why we’re using a tank in the first place. Tanks are expensive, so it makes sense that they are commonly used to store valuable product or utilities- such as water for injection or buffer media. And the whole point of having a sanitary, clean system is protecting this valuable product form microbial or particulate contamination. Seems easy enough, right? Let’s just seal the tank, don’t let anything in or out, and call it a day- no problem. Well not so fast.

When liquid is added or removed from a tank, air must move in or out of the tank to fill or adjust to the changing airspace above the liquid. Unlike solids and liquids, gases can expand and compress easily. This means that as we pump into a tank and fluid takes up a larger part of the tank, the gas will compress, increasing the pressure of the tank.  And when we pump out of the tank, we will create a vacuum, assuming there is no airflow in or out of the tank. Remembering the ideal gas laws, we also know that temperature swings can affect the volume of gas in a tank. During an SIP cycle, for example, cool down following 140 C steam can result in a huge temperature swing and the rapid collapse of the volume of gas/fluid within the tank. This can lead to problems ranging from a rupture disc blowing, to a catastrophic collapse of the tank if the vacuum created exceeds the vacuum rating of the tank.

So if sealing the tank is not going to work, we need to figure out how to allow air to flow in or out of the tank without contaminating the contents. So how do we accomplish this? Usually it’s done with the use of a 0.2 micron hydrophobic sterilizing grade filter cartridge. As the name implies, hydrophobic filters don’t like water.  Hydrophobic filters will not “wet out” like hydrophilic filters that are used for liquid filtration. Because of this hydrophobic filters will readily let gas pass through the membrane.  Usually made of Teflon, these hydrophobic filter membranes eliminate significant microbial contamination risk.

When sizing a vent filter, there’s usually several unknowns, but there are a few pieces of information to focus in on and that we can use to a make educated guess at the correct size. The first is to identify the air inflow rate. If this is not available, it can be determined by using the liquid flow rate (gpm), multiplied by the conversion factor of 7.81. This will give you CFM, or cubic feet per minute of air flow, a standard measurement used in the sizing of tank vent filters. Next, we want to identify the maximum vacuum rating of the tank. All ASME pressure vessels will have this stamped on the side of the tank. Other bulk storage tanks won’t carry the ASME stamp, indicating it is not a rated vessel.

Finally, we want to identify the maximum operating pressure of the tank. In tanks that contain fluid at an elevated temperature, the fluid has a higher vapor pressure, resulting in water “carryover” with air at the top of the tank during fill. Why is this important? Well if the temperature of the vent filter is not maintained, this water vapor can condense on the membrane of the filter. This results in blinding, decreasing the available membrane filter area and the possibility creating of a vacuum during tank drawdown. For this reason, it is common to see sterile vent filter housings equipped with steam jackets or electrically heat traced to not only keep the temperature of the housing elevated, but also maintain and control the temperature at the filter, extending membrane life.

So there you have it- why tank vent filters are so important and why it is important to heat them. For your next sanitary vessel application, keep in mind that when liquid is added or removed, we need to allow air to move in and out of the vessel to compensate. If you have any additional questions about your next tank venting application, contact a Holland Sales Engineer today.