Single vs Double Mechanical Seals: How Do You Choose?

Single vs Double Mechanical Seals: How Do You Choose?

What is a single mechanical seal?

A single seal has one pair of flat faces, one stationary and one turning with the shaft. The pumped fluid forms the thin lubricating film between those faces, so a small amount of it works through the faces and escapes to atmosphere as vapour. On most water-like duties that is normal behaviour, not a fault.

What is a double mechanical seal?

A double seal puts two sets of faces in one unit with a separate barrier or buffer fluid between them. That middle fluid, not the process fluid, lubricates the faces. In a pressurised arrangement the barrier fluid is held above the seal chamber pressure, so anything crossing the faces moves into the process rather than the process leaking out. In an unpressurised arrangement the outer fluid acts as a back-up that catches leakage if the inner seal lets by.

When is a single seal enough?

When the fluid lubricates reasonably well, carries no solids that would hang the faces open, and a small vapour loss to atmosphere is acceptable on safety and environmental grounds. Clean water, mild chemicals and many general plant duties fall here. One seal, no support equipment, least to go wrong.

When should you move to a double seal?

When any of those three conditions fail. Fluids that are toxic, flammable or otherwise cannot be allowed to reach atmosphere. Fluids that crystallise, set, or carry solids that would jam the faces. Poor lubricants, or fluids so close to boiling that the film between the faces flashes to vapour. A double seal with the right barrier fluid covers all three, because the faces only ever see the clean barrier fluid.

What does a double seal need from you?

A barrier or buffer system: a reservoir of the fluid, a way to hold it at pressure, and usually cooling to carry heat away. That system needs routine checking, because a double seal run without barrier pressure can fail faster than the single seal it replaced. The seal is only half of the installation; the support system is the other half.

Is a double seal always better?

No. It costs more, needs more looking after, and adds its own failure points. Where a single seal suits the duty, it is often the more reliable choice simply because there is less of it. The question is not which seal is best, it is which seal the duty needs.

How do we choose for your pump?

Send us the fluid, its temperature and pressure at the seal, any solids or crystallising tendency, and whether any leakage to atmosphere is acceptable. We select and supply mechanical seals for pumps and rotating equipment, and we will tell you plainly which arrangement the duty calls for and why.