Making The Right Choice
By: Lisa Davis
Engineer

Type "C" rotary joints with early failure rates.

Upon asking the relevant questions, I discovered that the customers were using ferrous trim Type C unions in water applications. The better choice for these applications is the E-75 RS.

When determining whether to use the Type C or the E75-RS, first determine if the media is steam, low pressure steam/hot water (condensate) mixture, or water (more on this later…). If the media is steam only or steam/hot water combination, then the Type C is the appropriate choice. If the media is water, then use the E-75 RS.

E-75 RS

Simple, right? Now, to complicate the situation even further, we offer the Type C in two different trims, bronze and ferrous. If the media is steam only, the ferrous trim Type C is the right choice. This joint has a long dependable history in steam service. However, in some steam systems operating at lower pressure you will encounter condensate mixed with the steam. The lower the steam pressure, the higher the likelihood of finding condensate in the media. This condensate (water) will rapidly rust the ferrous trim Type C. This is where the bronze trim Type C really shines. The bronze, having better corrosion resistance properties in water will outperform the ferrous trim Type C in steam/condensate media mixtures, making it the right choice for low-pressure steam applications.

Just to make things even more interesting, sometimes it can be difficult to know with certainty what exactly the media is, within certain limitations. Take a hot water system. Sometimes a customer thinks they are running hot water, when in fact, they are running steam, or a steam/hot water mixture. How does this happen? The temperature at which water transitions to steam at a given pressure is well defined and is called the saturation temperature. You might also know it as the boiling point. The boiling point decreases as pressure decreases and increases as pressure increases. If your actual pressure at some point in a system is lower than you think it is (or lower than it is at the measurement point), then your corresponding saturation temperature, or boiling point, is lower than you think it is. Frequently, system pressure is measured at or near the pump. Pressure drops in the system due to plumbing, insufficient flow, and elevation changes reduce the actual pressure that the unions see. Older systems have losses due to buildup. On average a 5 year old pipe has 1.4 times the loss that a new pipe does. If we have a 4” line supplying 850 gpm, the pressure drop at the end of 100 feet of new straight pipe would be around 20 psi, 5 year old pipe, 28 psi, 10 year old pipe, 44 psi.

The point here is that it is possible to have water at the pump and steam at the union!

In a nutshell:
Steam = ferrous trim Type C
Steam/Condensate (low pressure steam) = bronze Trim Type C
Water Only = E-75 RS

Type "C" Bronze Trim	Type "C" Ferrous Trim

Of course, you can always contact the Engineering Department at Barco if you want to discuss an application. We will help you sort out the optimum choice for your situation!

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