Considerations for Double Mechanical Seal Applications

One of the more common calls we get on a day in and day out basis questions about pump seals. Whether it’s seal type selection, material selection, or troubleshooting a leaky seal, at Triplex we field seal questions all the time. A lot of the toughest questions we get are on double mechanical seals- what are they, when do we use them, and what should we consider when installing the pump. In the post, we’ll run through what a double mechanical seal is, why you would use it, and also touch on materials and piping considerations. So let’s get into it.

DM Seal- Inner NF Tungsten vs. Carbon Outer Seal

First things first- what is a double mechanical seal? A double mechanical is just what the name would imply- two seals! With the Waukesha series of Universal pumps, we accomplish this by adding an additional stationary outer seal. This outer or secondary seal rides on the same seal seat as the stationary inner seal, but unlike the inner seal, ideally never sees product. Instead, it seals the flush or barrier fluid between the outer seal and atmosphere. This enables us to precisely and effectively use a flush fluid to lubricate, cool, and support the inner seal. This flush fluid is perfect for sticky or abrasive applications.

As we just alluded to, we use a double mechanical seal in applications where a single primary seal is unable to handle the duty. Think hot, sticky, or abrasive applications, as well as applications where the product tends to plate or coat. By using the secondary or “double” seal, we’re able to use a compatible barrier fluid to flush the primary seal and keep sealing zones clean and cool.

So clearly, the double seal is a lot about the flush. But what is the flush? The flush fluid is typically water, but can be any compatible fluid. In chocolate applications (a product that is not compatible with water), for instance, we may use propylene glycol or coca butter as a flush fluid. We’ll even see the product used as the flush fluid in very specific applications. At the end of the day, as long as it’s clean, lubricating, and relatively cool, it’ll work to support the inner seal.

Which brings us to the next consideration when specifying a double mechanical seal- what seal material should you use? Even though double seals are typically associated with the most abrasive and challenging applications, because all we’re really sealing is the flush fluid, usually a carbon outer seal is more than sufficient. There are other outer seal materials available, but unless you’re using a slightly abrasive product as a flush, a premium seal material probably isn’t worth it.

A typical double mechanical flush set up

From a piping standpoint, there are a few things we want to consider. First is direction. On the Universal 1, we can connect to either connection on the upper or lower seal. On Universal 2 applications, we only have two flush connections and we’ll want to ensure we have the flush entering the bottom flush connection and discharging through the upper flush connection. Typically, there will be an on/off valve that we suggest is installed on the inlet side. On the discharge side of the flush piping, we suggest a pressure gauge and an optional restrictor valve to help ensure flow and maintain backpressure on the flush.

And because we most commonly see double mechanical seals fail upon system restart (after the flush has been turned off for a period of time), we’ll want to run discharge piping up and away from the pump in order to maintain the flush as a low point sump to help wet the seal during downtime.

 And what flow rate flush do you need? We recommend ¼ GPM.

So just to recap, we use double mechanical seals in the most aggressive applications that require a flush to help lubricate and keep seal faces clean. We recommend a low-cost carbon material for this outer seal on both Universal 1 and Universal 2 applications. We want to bring the flush in from a low point and discharge from a high point if possible. And we always try to keep flushing fluid in the seal, maintaining flush rates of about ¼ GPM. Keep these tips in mind for your next double mechanical seal application and you’ll seal the impossible! And as always, if you have any questions about your double mechanical seal or any of your Waukesha Universal pump needs, please contact a Triplex Sales Engineer today!

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