Above roughly 20 bar (290 psi) stuffing-box pressure, specify balanced. Below that, unbalanced is often the correct and cheaper engineering choice.
The decision turns on one number: the balance ratio b = Ah/Ac.
Competitors quote ranges like 60-90 percent without showing the formula or running a calculation. This article runs the numbers and anchors the pressure thresholds so you can write a defensible rationale on the datasheet.
What “Balanced” Actually Means on a Seal Face
“Balanced seal” is a geometric label, not a statement of force equilibrium. It means the seal achieves lower net face pressure through a stepped sleeve or stepped face geometry. A balanced seal is deliberately overbalanced — closing force slightly greater than opening force — to keep faces closed under vibration, pressure pulses, and thermal transients.
In industry vocabulary, “balanced” means b is less than 1.0 — the closing area exposed to stuffing-box pressure has been reduced below the face area, not that forces are equal.
One disambiguation before the math. Searches for “balance shaft seal” refer to engine balance-shaft oil seals — a different component in reciprocating engines that has nothing to do with pump seal hydraulic balance. For the underlying theory of why face geometry matters, the deeper treatment of hydraulic balance in a mechanical seal walks through the force equilibrium equations in full.
Balance Ratio Formula and Worked Example
The balance ratio b = Ah/Ac is the fraction of face area loaded by stuffing-box pressure, where Ah is the hydraulically loaded closing area and Ac is the total face contact area. Two variants exist depending on whether pressure acts on the outside or inside of the face.
For outside-diameter pressurization (the common centrifugal pump case):
b = (D1² − D3²) / (D1² − D2²)
where D1 is the face outer diameter, D2 is the face inner diameter, and D3 is the balance diameter (the stepped shoulder). For inside-diameter pressurization, the formula inverts to D3² − D2² over D1² − D2². Which variant applies depends on which side of the faces holds the pressurized fluid.
Running the Force Numbers
Take a 2 in² face area with 100 psi stuffing-box pressure. An unbalanced design exposes the full 2 in² to pressure: closing force is 100 psi × 2 in² = 200 lbs.
Opening force from the pressure-drop film across the face is roughly half that, about 100 lbs. Net closing load is 100 lbs and the face-contact pressure doubles what the hydrodynamic film can sustain.
Now step the sleeve so only 1 in² is hydraulically loaded. Closing becomes 100 psi × 1 in² = 100 lbs, which matches opening. That gives a balance ratio of 50 percent on paper, but most real designs overbalance deliberately to b = 0.70 — keeping a small net closing margin so the seal stays shut under transients.
The 70/30 overbalance is why the textbook range lands at 60 to 90 percent for balanced seals and 110 to 160 percent for unbalanced. These numbers are the compromise between “not enough closing to survive a cavitation pulse” and “too much closing to survive the heat.”
Spring load matters here and most engineers forget it. A new spring contributes 20 to 40 psi of closing pressure, decaying to roughly 10 psi at end of life. That seemingly small number dominates the closing-force budget when stuffing-box pressure is under 5 bar — a fact that inverts the usual intuition in low-pressure service.
Pressure and PV Thresholds Where Balanced Becomes Mandatory
Unbalanced seals are limited to about 10 barg (145 psi). Above that, face-contact pressure outruns the hydrodynamic film and the seal overheats. Balanced standard seals extend the envelope up to roughly 10 MPa (100 bar, 1450 psi) before the next regime change.
A commonly cited industry heuristic puts the practical transition at approximately 250 psid (roughly 17 to 20 bar differential). The number is not a code citation — it reflects where face PV exceeds what a 110-160 percent balance ratio can tolerate with carbon-versus-ceramic or carbon-versus-silicon-carbide face pairs.
Above 10 MPa or where PV exceeds roughly 100 MPa·m/s, even contact-style balanced seals are insufficient. Controlled-leakage hydrodynamic seals (pumping-groove designs, gas seals) take over.
High-pressure boiler feed, high-speed compressor applications, and multi-stage pipeline pumps run into this upper boundary regularly. The high-pressure pump seal selection workflow walks through the full envelope including API 682 category mapping.
One thermal rule: maintain at least 20 °C margin between the fluid boiling point at sealed pressure and the operating temperature. Specialized low-friction pairs (silicon carbide against antimony-impregnated carbon) tolerate a 5 °C margin, but standard pairs need the full cushion to avoid film vaporization.
Pros and Cons by Configuration
Balanced and unbalanced configurations are trade-offs, not a ranking.
| Dimension | Balanced (b = 0.60–0.90) | Unbalanced (b = 1.10–1.60) |
|---|---|---|
| Pressure ceiling | Up to ~10 MPa (100 bar) | ~10 barg (~145 psi) |
| Heat generation | Baseline | ~30–40 percent higher at same P and speed |
| Face wear rate | Lower | Higher, shorter MTBF above 10 barg |
| Stability under vibration | Lower (smaller closing margin) | Higher (1.1–1.5 pusher safety factor) |
| Shaft/sleeve prep | Stepped sleeve required | Plain sleeve, lower cost |
| Cost | Higher (geometry + sleeve machining) | Lower |
| Best fit | High-pressure, flashing hydrocarbons, API services | Low-P water, cavitating or vibrating services |
Replacement seals in both configurations — including Burgmann MG1-compatible balanced variants — are available off the shelf, so the specification decision is not constrained by supply lead time.
The heat number is where most unbalanced misapplications get caught. An unbalanced seal above its pressure limit generates enough face heat to thermally check carbon or crack silicon carbide within weeks — not the 2-3 year design life of a correctly specified balanced seal. I have pulled failed unbalanced seals from 15-bar services where the face was glazed from thermal distortion before the first rainy season ended.
When Unbalanced Is the Correct Engineering Choice
Unbalanced seals win in three specific regimes, where defaulting to balanced is over-specifying.
First, low-pressure water or high-lubricity fluids below 10 bar: the spring load dominates the closing force budget, and the higher closing-to-opening safety factor (1.1 to 1.5 for pusher seals, 1.1 to 1.3 for non-pusher) gives better face contact under cavitation and vibration.
Second, services with frequent pressure reversals, flashing upstream of the seal, or high vibration. The balanced seal’s smaller closing margin means faces can lift during a transient, pulling in fluid or vapor. An unbalanced seal’s 1.25 to 1.35 ratio keeps the faces pressed together through the disturbance.
Third, simple commodity services. For a cooling-water booster pump running at 5 bar in a stable mechanical room, the unbalanced design is correct — not a compromise.
The spring-load floor is the clearest argument. At 3 bar stuffing-box pressure with a 20-psi spring, the hydraulic contribution to closing force is smaller than the spring contribution — a balanced seal can be under-loaded in this regime, losing face contact during a mild pressure dip.
The pusher versus non-pusher architecture decision is the other axis here; balance ratio is independent of whether secondary sealing is a pusher or a bellows.
API 682 and the Selection Decision Framework
API 682 does mandate “balanced” explicitly: all three seal types must be balanced cartridge designs. Its Category 2 and Category 3 pressure, temperature, and hydrocarbon service envelopes also fall almost entirely within the regime where balanced is required for viable face loading.
For every service, Category 1 included, the standard requires balanced seals through its seal-type definitions — in explicit language. OEMs build their API 682-compliant lines (EagleBurgmann, John Crane, AESSEAL) predominantly as balanced designs because the standard’s service conditions demand it.
The defensible specification framework runs in this order. Check stuffing-box pressure first. If it exceeds 20 bar or 250 psid, specify balanced — stop here.
If it is below 10 bar, check the fluid. For water and non-flashing hydrocarbons, a balance ratio of 0.75 is the standard design point; for flashing hydrocarbons, 0.80 to 0.85 gives extra closing margin. Unbalanced at 1.25 to 1.35 is appropriate for low-pressure water where stability under disturbance outranks heat reduction.
PV is the final check. If face velocity times pressure exceeds roughly 100 MPa·m/s, no contact seal survives — move to a hydrodynamic or gas seal. The broader context on mechanical seal types for centrifugal pumps shows where the balanced/unbalanced axis sits among the Type 1, Type 21, cartridge, and component-seal decisions.
The Bottom Line
Compute the balance ratio before you specify. If stuffing-box pressure exceeds 20 bar or 250 psid, write balanced on the datasheet with a target b of 0.75 for water and 0.80 to 0.85 for flashing hydrocarbons.
Below 10 bar in stable water service, unbalanced at b = 1.25 to 1.35 is not a compromise — it is the correct choice, and the higher safety factor buys real margin against vibration and cavitation.
Above 10 MPa or PV greater than 100 MPa·m/s, neither configuration holds and the conversation shifts to hydrodynamic and gas seals. The balance ratio formula exists so you can decide with a number, not a default.