A mechanical seal is a device that prevents fluid leakage between rotating and stationary parts in pumps, mixers, and other equipment. The choice between single and double seal arrangements depends on your specific application needs.
Single seals work well for most standard applications, while double seals provide extra protection for hazardous or expensive fluids. Each arrangement type offers unique advantages for different operating conditions.
Single Mechanical Seals (Arrangement 1)
Single mechanical seals use one set of sealing faces to contain the process fluid. They’re the simplest and most cost-effective option.
These seals work by pressing two extremely flat surfaces together – one rotates with the shaft, the other stays fixed to the housing. A thin film of liquid between these faces provides lubrication and cooling.
Single seals handle most water pumps, oil systems, and chemical processes perfectly well. They take up less space than double seals and cost less to install.
The main limitation appears when dealing with dangerous chemicals or fluids that crystallize easily. If a single seal fails, the process fluid leaks directly to the atmosphere. This makes them unsuitable for toxic substances or expensive products where any leakage creates serious problems.
Double Mechanical Seals
Double mechanical seals use two sets of sealing faces for extra protection against leakage. They create a barrier between the process fluid and the outside environment.
The space between the two seals contains a barrier or buffer fluid. This fluid serves multiple purposes – it cools the seals, provides lubrication, and acts as a safety barrier if the inner seal fails.
Different double seal configurations exist to match various operating requirements. We’ll explore three main categories: unpressurized and pressurized dual seals based on API standards, followed by specific mounting arrangements like tandem, back-to-back, and face-to-face designs.
Unpressurized Dual Seals (API Arrangement 2)
Unpressurized dual seals maintain their barrier fluid at atmospheric pressure or slightly above. The barrier fluid pressure stays lower than the process fluid pressure.
If the inner seal leaks, the process fluid enters the barrier fluid chamber rather than escaping to the atmosphere.
The system needs a reservoir to hold the barrier fluid and allow for thermal expansion. Regular monitoring of the barrier fluid level helps detect inner seal problems early.
These seals suit applications where complete containment matters but the process fluid isn’t extremely hazardous. They’re common in food processing and pharmaceutical equipment.
Pressurized Dual Seals (API Arrangement 3)
Pressurized dual seals keep their barrier fluid at a pressure higher than the process fluid – typically 15-25 PSI above. This pressure difference ensures any leakage flows from the barrier fluid into the process, not the other way around.
Think of it like inflating a balloon inside a jar. The higher pressure inside the balloon prevents anything from the jar entering the balloon.
This arrangement provides maximum safety for toxic or hazardous fluids. Nuclear plants, chlorine compressors, and other critical applications rely on pressurized dual seals.
The downside involves more complex support systems. You need pressure sources, control valves, and monitoring equipment to maintain proper barrier fluid pressure.
Tandem Seals
Tandem seals mount both seal sets facing the same direction, one behind the other. The inner seal handles the full pressure while the outer seal acts as a backup.
During normal operation, the outer seal experiences almost no pressure differential. It springs into action only if the inner seal fails.
This arrangement works well when shaft space is limited. Many older pumps designed for packing can be retrofitted with tandem seals without major modifications.
The barrier fluid between the seals usually connects to a reservoir at atmospheric pressure. Any increase in barrier fluid level signals inner seal leakage.
Back-to-Back Seals
Back-to-back seals face away from each other, with the barrier fluid chamber between them. Both seals share the pressure load.
This design handles pressure from either direction effectively. If process pressure fluctuates or reverses, both seals continue working properly.
The arrangement excels in mixer applications where vacuum conditions might occur. The pressurized barrier fluid prevents air from entering the process during vacuum operation.
Each seal faces relatively low pressure differentials, which extends seal life. However, the design requires more axial space than tandem arrangements.
Face-to-Face Seals
Face-to-face seals mount with their sealing faces pointing toward each other. The barrier fluid chamber sits outside the seal faces.
This configuration creates the most compact double seal arrangement. When shaft space is extremely limited, face-to-face seals provide double seal benefits in minimal space.
The design places unique demands on the barrier fluid system. Proper circulation becomes critical since the fluid must flow through narrow passages.
These seals often appear in high-speed equipment where minimizing seal chamber length helps reduce shaft deflection. The compact design also suits retrofit applications with space constraints.
