Balance in Mechanical Seals & Choosing Pumps for Condition Monitoring

Q. What does the term “balance” mean when referring to a mechanical seal? A. A balanced seal is a mechanical seal configuration in which the fluid closing forces on the seal faces have been modified through seal design. Seal balance, or balance ratio of a mechanical seal, is simply the ratio of two geometric areas. These areas are called the closing area (Ac) and the opening area (Ao). The closing area is different when pressure is on the outer diameter of the seal than when the pressure is on the inner diameter. When the pressure is on the outer diameter, the closing area is from the seal face outer diameter down to the lowest point, where the secondary seal rests against the shaft or sleeve. When the pressure is on the inner diameter, the closing area is from the highest point, where the secondary seal rests against the primary ring counter bore, down to the sleeve diameter.

The opening force is always the area of the sealing faces. The balance ratio is then Ac/Ao. A seal with a balance ratio less than 100 percent is called a balanced seal. A seal with a balance ratio greater than 100 percent is called an unbalanced seal. Most balanced seals have a balance ratio between 60 and 90 percent. Most unbalanced seals have a balance ratio between 110 and 160 percent. Pusher seals normally require a step in the shaft/sleeve or internal hardware to achieve a balanced design. Metal bellows seals do not require this step. The balance diameter, or mean effective diameter (MED), of metal bellows seals is located near the middle of the convolution. When pressure is applied to the outer diameter of the seal, the MED shifts downward, lowering seal balance. The opposite is true when the seal is subject to internal pressure. The rate of change in the balance depends on the face width and the bellows leaflet design. Pusher seals can be designed to withstand pressure from either direction. This is accomplished by trapping the O-ring between two diameters as shown in Figure 4.1. The cavity must be long enough to allow the O-ring to move, allowing pressure to act on the primary ring. These designs allow the seal to withstand system upsets. For more information on the design of mechanical seals, refer to HI’s guidebook Mechanical Seals for Pumps: Application Guidelines at www.pumps.org. Q. I believe monitoring the condition of our pumps will benefit the plant. How do I decide which pumps to monitor and how do I justify the cost of monitoring them? A. Various strategies for asset reliability management are shown in table 9.6.5.1.3. Beginning with noncritical machines, reactive maintenance is simply allowing the machine to fail before repairing it. These are usually easily replaceable machines where failure is unlikely. The next stage is preventative maintenance applied to machines considered essential to a plant’s operation. Here maintenance is periodic (via route-based monitoring) or nearly continuous (wired or wireless online monitoring). Predictive maintenance focuses mainly on expensive machines critical to the plant’s operation. Table 9.6.5.1.3 summarizes these strategies, reactive, preventive, and predictive from the U.S. Department of Energy (DOE) office of Energy Efficiency & Renewable Energy Operation & Maintenance Best Practices Guide. To the extent that condition monitoring technologies support improved reliability of equipment, the criticality of the asset governs the strategy. To justify the decision to monitor machinery with diagnostics, you can look at the typical annual maintenance cost for the machinery and compare this to the cost of applying effective diagnostic monitoring. In some industries, the typical practice is based on the assumption that monitoring gives a significant financial advantage to the organization by reducing the scope of maintenance versus run-to-failure. In cases where incremental monitoring costs are lower than 50 percent of annual maintenance costs, monitoring is justified and should be implemented. Annual maintenance costs include:

• all parts and labor costs
• all clean-up and decontamination costs
• all preventive maintenance activities (inspections, lubrications, etc.)
• all lost revenue from production downtime during maintenance (only if sold out)
• all rework costs associated repairs done incorrectly

For more information condition monitoring of pumps, refer to ANSI/HI 9.6.5 Rotodynamic Pumps – Guideline for Condition Monitoring at www.pumps.org.