Application of Dry Gas Seals in the Pumping of Liquid Hydrocarbons

Related Vendors

EagleBurgmann Germany GmbH & Co. KG

DÜPERTHAL Sicherheitstechnik GmbH & Co. KG

Maag Pump Systems AG

Evaluation of the different phases illustrates the factors that affect a mechanical seal’s reliability:

• Start-up: The pump is primed. Under suction pressure conditions, the liquid ethane penetrates between the seal faces and vaporizes as a result of the pressure drop at the inner diameter of the seal face and stationary seat. Once the pump has started, building up the normal operating pressure (which exceeds the vapor pressure of the medium) in the seal compartment sometimes takes too long. Even a low heat generated between the seal faces can increase the vapor pressure sufficiently to allow the liquid medium in the sealing gap to evaporate. This results in damage to the seal faces and the damage patterns are startlingly clear.

• Slow-roll: The problem here is the same as for start-up, only compounded. The discharge pressure cannot be generated until the rotation reaches a threshold speed. The pressure in the seal compartment rises too slowly to guarantee the necessary ethane vapor margin. More and more heat is generated between the seal faces, increasing the probability of insufficient lubrication and thus seal damage.

• Standby: The conditions are similar. The seals are also often left sitting idle for months without flushing. It has been shown that deposits collecting on the seal faces and around the seal during standby in turn have negative effects on the seal compartment.

• Inefficient operation: Running the pump outside the optimal range and with the wrong operating point parameters results in increased demand for drive power and a reduced delivery rate. Both of these negatively impact the vapor margin in the seal area, which can result in dry running.

Other events that need to be considered when dimensioning the seal supply are temperature fluctuations in the medium, frequent start/stop cycles and, not least, operator error.

In our case, all these factors conspired to give the Texan operator’s ethane pump a MTBF (Mean Time Between Failures) of a little over three weeks. It was clear that something had to be done to counteract the seal failure, resulting product losses and greatly impaired plant availability. The causes were analyzed in collaboration with the customer, then a team of Eagle Burgmann application and design engineers concentrated on finding a reliable and durable sealing solution.

Taking up the Challenge

Applications with low vapor margins, such as ethane in our case, have one thing in common: the liquid tends to transition to the gaseous state. A “liquid-lubricated gas seal” would therefore appear to be a perfect solution. Various seal concepts were investigated.

The Eagle Burgmann product portfolio already includes gas-lubricated seals for pumps — Please continue reading on the next page ...

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