A replacement mechanical seal matches the shaft diameter and the pump can be assembled, yet it leaks or heats during operation. The face material may not be the problem. The installed working length determines where the seal operates within its spring and compensation travel. More compression does not automatically mean better sealing.
Verify the exact seal design, installation datums and complete axial stack before approving the replacement.
Free height is not installed working length
The height measured out of the package differs from the operating length in the pump. Depending on the construction, the seat, rotating element, shaft shoulder, impeller, spacers and retention parts determine the final position. Similar overall free heights can hide different face positions or spring travel.
Insufficient compression can compromise closure and following capability. Excess compression can increase friction, reduce compensation travel or approach a mechanical limit. However, face-closing force also depends on pressure acting over relevant areas and the seal’s balance and geometry. Spring force alone is not the complete seal load.
Establish the operating position from a dimensional stack
Choose a common datum and calculate the installed length using approved dimensions. Include relevant tolerances and axial movement. Where endplay or thermal growth is permitted, confirm the seal remains within its usable range throughout it.
Adding a spacer to eliminate impeller contact may also change spring compression. A correction to one interface therefore needs an associated seal check.
For illustration, assume a spring can be approximated as linear over the relevant travel, with stiffness 12 N/mm. An extra 0.4 mm compression changes its force by about 4.8 N. This demonstrates sensitivity to dimensions; it is not a seal specification and cannot predict leakage or temperature. Use the actual spring characteristic and complete closing-force assessment for design decisions.
| Item | Confirm | Inadequate substitute |
|---|---|---|
| Installation datum | Working face relative to locating surfaces | Overall free height alone |
| Working length | Nominal value, tolerances and travel | “It fits in the space” |
| Axial changes | Endplay, growth and revised parts | Cold stationary position alone |
| Face condition | Flatness, cleanliness, liquid film and medium | Spring compression alone |
Assembly condition can change the result
Contaminated faces, a tilted seat, a twisted elastomer, lead-in damage or incomplete seating can coexist with a length error. Follow the specific seal and pump procedure, including compatible assembly materials and appropriate tools. Do not repeatedly touch working faces or add sealant to compensate for an unexplained dimensional mismatch.
Isolate power and release pressure before work. Competent repair personnel should record original positions, contact marks, seal revision and spacer arrangement before dismantling. A removed spring returning to its free state cannot by itself reveal the installed compression it previously experienced.
Static tightness does not complete running verification
A static test provides useful information, but running introduces face following, temperature and liquid-film conditions. Verify leakage and related behaviour at the approved duty, retaining temperature state and time sequence. Do not dry-run the faces as a diagnostic demonstration, and do not ask ordinary users to dismantle energised or submerged equipment.
A change in seal supplier, shaft, impeller or end cover needs review of the associated working length. Spare orders should identify the full seal designation, installed dimensions and applicable revision. “Mechanical seal for this shaft diameter” leaves important compatibility unresolved.
If leakage persists, investigate face damage, shaft condition, medium and assembly as well as the working position. Increasing compression without locating the mechanism can increase friction while leaving the original leak path intact.
Length symbols and datums can differ between suppliers. Confirm their definitions before transferring dimensions between drawings. An overall length on one sheet must not be entered as working length on another without verification. An assembly section showing the seat, rotating element and locating faces helps purchasing and service compare the same dimension.
Frequently asked questions
Can shortening the spring reduce heat?
Do not modify it informally. That changes force, travel and reliability. The designer must establish a suitable replacement or correction.
Are seals with identical free height interchangeable?
Not necessarily. Working length, face position, materials, elastomers and construction can differ.
Will one extra washer cure a small leak?
Not reliably. Diagnose the leak and dimensional stack first. A washer may also alter impeller position and bearing loading.