It’s no secret that a leaking vent or thief hatch can soon become a frozen vent, and a frozen vent is bad news for everyone when it chokes off a tank’s ability to breathe. Not only can a frozen vent lead to costly tank damage, but the results dangerous to your operators, the public and is not great for the environment. Keeping your tank vents from freezing keeps your tanks safe.

image result for tank explosion

Plugged vents can lead to exploded tanks, like these.  Frozen hatches are a common cause of losing venting capacity.


When a vent relieves frequently, continuously or leaks slowly in a cold atmosphere, water vapor in the relived gas can condense and freeze on the outlet of the vent. Two modes of failure can result: the valve seal can freeze to the seat preventing the valve from opening, or the frost can build up in the throat of the valve where it chokes off the flow path and can eventually block it entirely.

image result for frozen thief hatch, image result for frozen vents

This competitor thief hatch is no longer working because it is covered in ice as a result of continuous venting. If this vent was not left open, the tank would have a real risk of catastrophic damage.

Common Causes for Leaking Vents

The following are the main causes of leaking vents:

  1. Lack of vent maintenance;
  2. Set pressure too close to tank operating pressure;
  3. Inappropriate application; and,
  4. Improper reseating and poor manufacturing tolerances leading to sealing issues.

Lack of vent maintenance

The more active a vent is, the sooner it needs to be maintained. If the vent begins to leak suddenly without any change to the operating conditions, most likely it needs to be maintained. Sometimes cleaning up the sealing surface and reapplying lubricant to the gasket provided by Hawkeye will solve the issue. Other times a simple gasket replacement will do the trick. A regular maintenance program should be in place by Producers to prevent these issues.

Set pressure too close to operating pressure

The above is for a vent that suddenly begins to leak after being in service for a long period of time. No brand-new vent should leak from the start. If it does, a common issue is having the set pressure of the vent is very close to the operating pressure of the tank.

The closer the vent set pressure is to the operating pressure, the less downward force is continuously applied to help seal the vent. Venting standards limit leakage up to 75% of the set pressure, so it is best practice to operate below 75% of the set pressure. Let’s consider a tank that normally operates at 8 oz/in2. If a vent is set at 10 oz/in2 and does leak at 75% of its set pressure (7.5 oz/in2), the risk of freezing the vent is prevalent.

Inappropriate Vent

Using an inappropriate vent for a specific application does occur less frequently but can cause tremendous negative effects when it does occur. It is the duty of the Producer to ensure any material being exposed to the contents of the tank are suitable and chemically compatible. A common mismatch in the oil and gas industry is using a Viton gasket where methanol is present. Methanol will cause the Viton to swell and result in a leaking vent.

Inadequate Manufacturing Tolerances

This last cause is a result of the type of vent purchased and not due to operation errors. After a pallet applies pressure to a gasket continuously, the gasket will experience memory effects. This will create an engravement by the circumference of the pallet. The pallet reseating on top of the engraved circumference is crucial to avoid a leak. Decreasing the tolerance in the pallet assembly will allow it to reseat correctly. This issue is found in vents that do not have a guided pallet, such as a standard thief hatches.

The Solution: Series 4000 Marsh Hawk

The series 4000 Marsh Hawk is a tank relief vent (TRV) and not just a thief hatch. No product that is designed to do two things does either well, but a Marsh Hawk is a vent first and gives tank access second.

The Marsh Hawk is a deep-set thief hatch where the gasket sits inside of the tank. This protects the gasket from the environment. The Marsh Hawk is the most adopted vent by end-users when they realize they have leaking issues. Outfitting the Marsh Hawk with a de-icing system adds an extra line of defense for cold environments. This is unique to the Marsh Hawk and can not be found on any other vent in the industry. Solutions the Marsh Hawk offers to the four problems mentioned previously are as follows:

  1. The Marsh Hawk utilizes one gasket for both pressure and vacuum relief. This drives maintenance costs down tremendously. It can also be maintained without being unbolted from the top of the tank; minimal downtime to your operations. Contact your Hawkeye representative for a better understanding of how this can be done.
  2. Adjusting the set pressure of a Marsh Hawk is very simple. If a user wants to increase the set pressure after realizing it is close to the operating pressure, it can be done very quickly, on-site and with minimal efforts.
  3. The Marsh Hawk is the most modifiable vent available. Different gaskets and coatings are available for the Marsh Hawk.
  4. The Marsh Hawk is deep-set into the tank. Not only does this keep the gasket warm, but the barrel guides the pallet and gasket assembly to always reseal in the same position.

Marsh Hawk deep-set tank gaugeMarsh Hawk and internal assembly

Read more about the Series 4000 Marsh Hawk TRV on our product page.


A standard Thief Hatch does the job in many cases. In Canada, we are faced with a challenging environment and meeting regulations is important to avoid unnecessary costs. Being local, Hawkeye understands the issues Oil and Gas companies face in Canada and therefore, has designed vents to solve these issues.


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