Is a pump seal a primary or a secondary grade of release?
Abstract
It depends on the seal. A mechanical seal not expected to release in normal operation is normally a secondary source (Zone 2). A packed gland, which must leak to stay lubricated, is primary. Many seals are assessed for both: a small normal weep and a larger failure.
It depends on the type of seal and how it is expected to behave. A mechanical seal that is not expected to release in normal operation is a secondary source, which in the open air usually gives Zone 2 around the pump. A seal that releases as part of normal running, above all a packed gland that has to leak to stay lubricated, is a primary source and adds a Zone 1 close to the seal. Many pumps are assessed for both cases.
Short answer
- Grade is about expected frequency, not size. IEC 60079-10-1 makes a seal secondary when release is not expected in normal operation, and primary when it is.
- A good-quality mechanical seal on a pump that is maintained is normally graded secondary. Industry codes such as EI 15 give hazard radii for pumps as secondary releases.
- A packed gland (compression packing) is designed to leak a little all the time for cooling and lubrication. With a flammable liquid above its flash point, that is a primary release.
- Seal failure is a separate, larger, secondary release. Its hole size and release rate decide how far the Zone 2 reaches.
- Seal-less pumps and dual seals with a barrier or buffer system and leak detection reduce the release or remove it.
What the standard says, in plain words
The International Electrotechnical Vocabulary, in its notes on normal operation, makes two points that settle most pump discussions. First, a breakdown of a pump seal that needs repair or shutdown is not normal operation: such a release is secondary. Second, minor releases can be part of normal operation, and it gives seals that rely on wetting by the pumped fluid as the example. The same notes appeared in the definitions of IEC 60079-10-1:2008.
So the question for each pump is: does this seal release flammable material during normal running, and how much? If the answer is "practically nothing that can form an explosive atmosphere", the seal is assessed as secondary. If it is "yes, a steady weep of volatile liquid or vapour", there is a primary release, normally small, which may give a small Zone 1 or, with high dilution and good availability of ventilation, a zone of negligible extent (Zone 1 NE).
Seal types compared
| Seal arrangement | Normal operation | Usual grade |
|---|---|---|
| Packed gland (compression packing) | a properly adjusted gland leaks steadily, because the leakage cools and lubricates the packing | primary for the normal leak, plus secondary for packing failure |
| Single mechanical seal | thin fluid film between the faces; very small emission | secondary, unless the service or seal history shows regular leakage |
| Dual mechanical seal with barrier or buffer fluid and leak detection | process fluid contained by a second seal; a failure is detected | secondary, with a smaller or shorter release |
| Seal-less pump (canned motor or magnetic drive) | no shaft seal to atmosphere | casing flanges and connections remain secondary sources |
The exact leakage rate of a gland is a maintenance setting, not a design value. What matters for classification is that leakage is part of normal running and that the liquid evaporates or flashes. A heavy fuel oil well below its flash point leaking from a gland may not form an explosive atmosphere at all; light naphtha or a liquefied gas will.
How the grade turns into zones
The secondary release on seal failure usually sets the outer Zone 2, because it is the larger release. The primary weep, where one exists, gives a small Zone 1 close to the seal. Both then pass through the ventilation step of IEC 60079-10-1:
- In open air with medium dilution, the seal failure gives Zone 2.
- In an enclosed pump room with low dilution, the same secondary release can give Zone 1, so the room ventilation and its availability become the deciding factor.
- Heavy vapours from the leak spread at grade, so the Zone 2 is usually extended at low level beyond the pump, and pits and trenches nearby are treated more severely.
For scale, the US OSHA table for bulk plants (29 CFR 1910.106, Table H-18) puts an outdoor pump in Division 2 within 0.9 m (3 ft) of any edge, extending up to 0.45 m (18 in) above grade within 3 m (10 ft) horizontally. These are Division boundaries for that code's assumptions, not zone extents.
Example: A transfer pump handles toluene (flash point 4 °C) outdoors. The original packed gland leaks steadily, so the classifier draws a small Zone 1 around the gland inside the Zone 2 set by a packing failure. After a retrofit to a mechanical seal that is not expected to leak in normal operation and is inspected on routine rounds, only the secondary release remains: Zone 2 around the pump with a low-level extension for the heavy vapour.
Common mistakes
- Treating every seal as secondary by default. Packed glands and seals with a known leakage history release in normal operation.
- Assessing only the normal weep. The larger failure case usually decides the extent of Zone 2.
- Ignoring the seal support system. Seal pot vents, quench drains and leak-off lines discharge somewhere. If they vent locally, they are sources of their own.
- Changing the seal without reviewing the classification. A seal upgrade can remove a Zone 1, and a downgrade can create one.
Related on excodex
- Grades of release: continuous, primary and secondary
- Typical hazardous area examples: tank vents, pumps, compressors and loading points
- Are flanges and valves always secondary grade sources of release?
- What hole size should be assumed for a secondary grade release?
- Standards spaces: IEC 60079-10-1, EI 15
Standards and further reading
- IEC 60079-10-1:2020 (edition 3.0), classification of areas, explosive gas atmospheres: preview; the 2008 edition preview shows the notes on minor releases and seal failures in its definition of normal operation: preview
- International Electrotechnical Vocabulary 426-28-07, normal operation (non-electrical equipment), with the notes on seals that rely on wetting and on seal failures: electropedia.org
- Energy Institute, Model Code of Safe Practice Part 15 (EI 15), area classification for installations handling flammable fluids, 5th edition (October 2024), with hazard radii for secondary releases from pumps, compressors, flanges and valves: energyinst.org
- US OSHA, 29 CFR 1910.106, Table H-18: osha.gov
This article is general technical information prepared by the excodex editorial team. It does not replace the applicable standards, the equipment certificates and instructions, national regulations or the assessment of a competent person.