What is a Double Mechanical Seal and Why do I need a Flush?

Throughout this blog, we’ve spent a lot of time talking about mechanical seals for Waukesha Positive Displacement and Centrifugal pumps. For both pumps, we have single and double seal options. As you probably know, when you run a double seal (either o-ring or mechanical), you need to use a flush. But why is that? Let’s tackle that topic in today’s post.

A mechanical seal is a relatively simple device. Most commonly, it features a stationary seal and a rotating seal seat. In Waukesha Universal pumps, the standard seal configuration is a carbon stationary seal and a ceramic rotating seal seat. In a C114 with Type D seal, on the other hand, the stationary face is actually the stainless steel backplate and the carbon seal is the rotating piece.

The mechanical seal works by drawing a very small amount of fluid between the seal faces, preventing them from touching, which would destroy the seal. The product lubricates the seal faces, but can leak vapor from the high pressure side (product side) to the low pressure side (atmosphere).

Many times, for high pressure, high temperature, hazardous, or applications with sticky media, we’ll opt for a double mechanical seal. Functionality in a double mechanical seal is similar to a single mechanical seal. The inboard or primary seal keeps the product contained in the pump housing while the outboard or secondary seal prevents flush liquid from leaking to atmosphere. The flush is needed to extend seal life by both cleaning the seal faces and cooling the seal.

In regard to cooling, this is important, because as mentioned above, we are typically running a double seal in a high pressure or high temperature application. When two surfaces rub, they create friction and heat. Typically the product would act as a lubricant, but in the higher temperature applications, this isn’t enough to dissipate the heat, resulting in wear and eventual seal failure.

For sticky products, we need the seal flush to help clean the seal faces. Sticky, abrasive, and products that tend to plate can build up on the seals and cause wear, resulting in an uneven sealing surface. And what do you do when something gets sticky? You rinse with water. With abrasive products, if the rotary and stationary seal faces are not perfectly flat, we run the risk of a leak. We could just tackle this with a single mechanical seal with a flush, but typically we’ll see customers opt for the double mechanical option. With the double mechanical seal, the inner seal acts as the primary seal between the product on the pump side and atmosphere, while the secondary outer seal acts to prevent the flush liquid from leaking into the atmosphere.

Total containment is the final reason we’ll touch on that people will opt for a double mechanical seal, especially with hazardous fluids. In a single mechanical seal, we’re basically creating a low pressure zone between the two seal faces that draws product in and flashes it off to atmosphere. This isn’t good for a hazardous product. So again what we’ll do is opt for a double mechanical seal where the inner seal will at as the primary product seal and then the flush, typically run at a higher pressure will act as a barrier in the secondary seal, allowing complete containment of the hazardous product.

To close this post, it’s important to remember that for high pressure, high temperature applications or applications where we are running sticky, abrasive products, as well as hazardous applications, we’ll want to run a double mechanical seal with flush for maximum seal life. The double mechanical seal with flush allows for us to lubricate the seal faces, reducing heat and product accumulation, and thus maximizing seal life. The flush also creates a high pressure barrier that prevents hazardous products from leaking to atmosphere. If you have any questions about which seal you should choose for your Waukesha Sanitary Positive Displacement pump, contact a Triplex Sales Engineer today!

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