How to Pick a Centrifugal Pump Part 7: Mechanical Seal Components and Materials

By Chad Edmondson

Part 6 of the “How to Pick a Centrifugal Pump” explained the value of internally flushed seals. One question that our customer service team fields every week concerns the material of construction of seals for various applications. This week we’ll look at the seal material you should specify.

Mechanical seals are simple devices. There are metal parts including a spring, a stationary seat, and a rotating seat (also called the primary ring) and there are the elastomers parts.

Water is the most common heat transfer fluid used in HVAC pump applications; glycol-based fluids are the second most common. In either case, the fluid in these applications contain very little particulate and are typically well below 225°F. Given these “easy service” conditions, the majority of seals used in HVAC applications are what is known as “carbon-ceramic.”

For typical HVAC applications, carbon-ceramic have stainless steel metals, BUNA elastomers, a 99.5% pure aluminum oxide ceramic stationary seal face, and a carbon rotating face. These seals work well with the temperatures mentioned above and a pH neutral range of 7.0-9.0. They can handle up to 400 ppm of dissolved solids and 20 ppm of undissolved solids which satisfied most system requirements.

In most applications carbon-ceramic seals work fine but there are certain circumstances when a different type of material for the mechanical seal should be specified. These include:  

Systems with high pH levels. Most HVAC applications maintain a pH from 7.0 to 9.0. Once in a while the pH is too high for the carbon-ceramic seal material. The main issue may be located in the chemical treatment portion of your specification. There are specifications that call for the pH to be maintained at levels in the 9.0-11.0 range. If your specification calls for this range, the pump seal material specification should be changed to EPR/Carbon/Tungsten Carbide (TC) or EPR/Silicon Carbide (SiC) /Silicon Carbide (SiC). We recommend the EPR/SiC/SiC material since that seal can handle pH up to 12.5 which gives some “wiggle room.”

Higher solids levels. Solids, otherwise known as dirt, are another area of concern for mechanical seals. If the system is dirty or has silica in the water, you may find, once again, that you need the EPR/SiC/SiC seal. The standard Buna/Carbon/Ceramic seal in HVAC systems cannot handle any silica and the solids handling capability was mentioned earlier in this article. The silicon carbide seal can handle 60 times the dissolved solids content and double the undissolved solids content with 20 ppm silica content thrown in for good measure.

So, why not just always specify EPR/SiC/SiC seals? There are two reasons: cost and lead-time. This seal will cost three times as much as the standard seal.  In addition, since the carbon-ceramic seal is standard, there may be additional lead-time to get a pump with a special seal.

Improperly mixed glycols. The term glycol is a bit of a misnomer when it comes to heat transfer solutions in HVAC systems. Glycol is used in many applications from shaving lotions to whipping cream to automotive antifreeze. In HVAC systems you want a properly mixed glycol-based heat transfer fluid that has the correct inhibitors for the application. If you use automotive antifreeze in HVAC systems, the silica based inhibitors will create a gel in the coils that blocks heat transfer and flow. That is, if the seals don’t leak first!

We commonly see Dowtherm® SR-1 and Dowfrost™ HD heat transfer fluids used for these type of applications. These products are made for use in our industry but should be mixed according to the manufacturer’s instructions. Dow recommends they be pre-mixed with deionized water before filling the system. If you simply mix these product with city or well water the calcium and magnesium in water will mix with inhibitors and cause a particulate that exceeds the ppm of normal seals.

Occasionally contractors will put the glycol based fluid in the system, filled it with water and then turn on the pump to “mix” the solution in the piping system. This subjects the pump seals to shots of up to 100% ethylene or propylene glycol which is well beyond the maximum recommended amounts for even silicon carbide seals.

Our experience is that the standard carbon ceramic seal work fine with properly mixed glycol products designed for our industry. However, brands vary in their quality and make-up so we cannot say that carbon-ceramic seals are appropriate for all glycol mixtures. These fluids do carry a higher pH than water, so we recommend specifying a glycol seal if there is any question.