The use of gas lubricated mechanical seals in pumps has increased for a variety of reasons. This two part article will address the principles of gas seals, some basic application questions, what to expect from seal performance and the environmental factors that may affect performance.
What Are Gas Lubricated Mechanical Seals?
Gas seals run in a clean, controlled fluid environment and are designed to remain non-contacting and non-wearing for the expected operating conditions. Seal faces operate on a barrier fluid film, typically in pressurized dual seal arrangements like in Figure 1.
Typical barrier fluids used in gas seals include nitrogen, steam, purified air or other inert gases. Each of the sealing end faces have some macro-topographic design pattern, are wider than plain face seals and are loaded with lighter spring force. These features enable generation of the hydrodynamic pressures that prevent contact of the seal faces.
Since they are non-contacting and do not wear, gas lubricated mechanical seals produce consistent seal performance, even in a widely varying duty cycle, and reduce power consumption at the seal faces. The seal generates little heat, and any gas leakage absorbs heat away through gas expansion. It is imperative that the seal faces are separated with a gas barrier film. If the faces contact, the seal will not last long running dry at typical pump speeds.
Figure 1. Typical dual gas seal for pumps
Since gas has poor lubricity, why rely on it to "lubricate" the seal faces? The reason is that in some pump applications, the process fluid to be sealed can be volatile, non-lubricating and sensitive to heat. It can also polymerize to foul the seal face, corrode the seal parts or be hazardous to the environment.
Dual seal arrangements with liquid barrier fluid are commonly used to overcome the above problems. In situations where the liquid barrier fluid can contaminate the process, or it is cost prohibitive to add the circulating cooled barrier system, the gas barrier fluid can become a viable choice.
A seal support system (example in Figure 2) is usually installed, including a gas control panel with coalescing filter to minimize liquid contamination, ensure cleanness of the gas and monitor leakage flow rate.
Figure 2. Typical gas seal support system
What Keeps the Seal Faces Separated?
There are many methods for generating the gas barrier film. The underlying principle uses a shallow step height change on the face to squeeze the film and generate fluid pressure (see Figure 3). Design variations like those shown in Figure 4 include Rayleigh pad, spiral groove, wavy face and others. All demonstrate various capabilities to control the gas flow and generate the face separating pressure.
If the gas pressure generated depends on the relative sliding of the seal faces, it is called hydrodynamic pressure. If it depends only on the pressure differential and is effective even at standstill, it is called hydrostatic pressure. Depending on the operational requirements, typical gas lubricated mechanical seals use varying degrees of combinations of both hydrodynamic and hydrostatic effects to maximize protection for the seal faces. They use carbon-graphite as the primary ring material to run against a silicon carbide or tungsten carbide mating ring, which helps the seal survive short durations of face contact in transient or emergency conditions. For higher pressure applications, carbide primary rings are used to take advantage of higher Young's Modulus and strength.
Figure 3. Hydrodynamic pressure generation in a step bearing
Figure 4. Typical gas seal face patterns
What Are Acceptable Leak Rates?
Gas leak rates are typically small, in the order of SCFH (standard cubic feet per hour). Predicting the actual leak rate from the theoretical rate is difficult since the leakage is sensitive to manufacturing variations and deviates from the idealized theoretical model. It is better to use the initial leak rate for any specific new seal installation as the baseline for future reference. If the operating leak rate exceeds 10 times the initial, contact the seal manufacturer to see if the seal needs to be repaired.
Barrier gas pressure is typically set at about 30 psi higher than the process pressure. As a rule of thumb, only about one-third of the total barrier gas flow leaks through the inboard seal into the process. Gas ingestion into the process is typically not an issue. The process fluid carries it through the system and vents it out at some stage. However, for a closed loop system, the user and seal manufacturer need to determine if the gas will accumulate and cause operational problem such as cavitation or vapor (gas) lock, which may starve the pump.