How it works — the five-step seal chain
Every contactive mechanical seal can be sized with the same five links: face loading, face speed, PV value, friction torque and power, and the flush flow needed to carry the heat away. Each step feeds the next — change one input and the whole chain re-runs.
1 · Face total pressure \(P_f\)
\(\Delta P\) is the pressure drop across the faces, \(b\) the balance ratio (hydraulic closing area over face area), \(k\) the pressure-gradient factor of the fluid film, and \(P_{sp}\) the spring pressure. The factor \(k\) says how much of \(\Delta P\) the film itself carries: a parallel liquid film ≈ 0.5, a converging (flashing) film 0.70–0.75, a gas film ≈ 0.66.
Balance ratio from seal dimensions (OD-pressurized seal): $$b=\frac{OD^{2}-BD^{2}}{OD^{2}-ID^{2}}$$ with \(BD\) the balance diameter. \(b<1\) = balanced seal (normal duty 0.65–0.85); \(b\ge1\) = unbalanced — keep \(\Delta P\) below ~10 bar (145 psi).
2 · PV value
\(V_m\) is the mean face speed at the average face diameter \(D_m\). PV is the single number used to screen face-material capability. Mind the definition: PV values from different sources are computed with different load definitions and must never be compared across definitions. This tool uses the closing-load PV above as a screening figure — confirm the final selection against the seal vendor's own PV table.
3 · Friction torque and power
\(b_f\) is the face width and \(f\) the effective face friction coefficient — roughly 0.05 for low-viscosity light hydrocarbons, 0.07 for medium-viscosity water-based services, 0.10 for lubricating oils (carbon vs SiC/WC faces). At start-up friction runs 3–5× the running value — size the motor for breakout torque when start frequency is high.
4 · Seal-chamber temperature rise
All face friction power \(P\) is assumed to be carried away by the flush flow \(Q\). Keep ΔT < 5 °C (10 °F) — above that, increase flush flow (Plan 11/32) or add a cooler (Plan 21/23) before the film flashes.
5 · Pressure-gradient factor k and friction factor f
| Fluid state | k | Service (face pair carbon vs SiC/WC) | f |
|---|---|---|---|
| Non-flashing liquid | 0.50 | Low-viscosity (light hydrocarbons) | 0.05 |
| Flashing liquid | 0.70–0.75 | Medium-viscosity (water-based) | 0.07 |
| All vapor / gas | 0.66 | High-viscosity (lubricating oils) | 0.10 |
Leakage criterion for stuffing-box packing (min-leak rule)
| Box gauge pressure | Minimum leakage (drops/min) | ≈ cc/min |
|---|---|---|
| ≤ 60 psi (≤ 4.0 bar) | 60 | 4 |
| 61–100 psi (4.1–6.8 bar) | 190 | ~13 |
| 101–250 psi (6.9–17 bar) | 470 | ~31 |
Compression packing is intentionally leaky: the through-leak lubricates and cools the rubbing faces. Measured leakage below the table value means the packing runs dry — expect glazing, burning and sleeve scoring. This rule does not apply to mechanical seals, which are held to far lower emission limits.
PV screening verdict (🟢🟡🔴)
- 🟢 PV ≤ 100 bar·m/s (≈ 285,000 psi·ft/min) — routine balanced-seal duty.
- 🟡 PV 100–500 bar·m/s — high-load service: verify heat balance and confirm the face-pair PV limit with the vendor.
- 🔴 PV > 500 bar·m/s — beyond routine wet-contact capability: move to a pressurized double seal (Plan 53A/54) or a gas seal.
- 🟡 Balance ratio outside 0.65–0.85 — off the standard window; \(b\ge1.0\) unbalanced seals are limited to ΔP ≲ 10 bar (145 psi) 🔴.
- 🟡 Spring pressure outside 100–240 kPa (15–35 psi) — off the contactive-seal window (dry-running seals use 14–40 kPa).
- 🔴 Chamber temperature rise ΔT > 5 °C — increase flush flow or add a cooler.
Worked example (anchor case)
ΔP = 10 bar, b = 0.75, water (k = 0.50), Psp = 200 kPa, Dm = 60 mm, bf = 10 mm, N = 2950 rpm, f = 0.07. Then:
| Step | Value |
|---|---|
| \(P_f=1000(0.75-0.5)+200\) | 450 kPa (65.3 psi) |
| \(V_m=\pi\cdot0.06\cdot2950/60\) | 9.27 m/s |
| \(PV\) | 41.7 bar·m/s (≈ 119,000 psi·ft/min) 🟢 |
| \(T=450\,000\cdot\pi\cdot0.06\cdot0.01\cdot0.07\cdot0.03\) | 1.78 N·m (15.7 lb·in) |
| \(P=1.78\times2950/9550\) | 0.550 kW (0.738 hp) |
| \(\Delta T\) at 6 L/min flush, SG = 1.0, \(C_p\)=1.0 | 1.32 °C (2.4 °F) 🟢 |
API 682 flush-plan selector
Tick the service conditions below the calculator and the selector scores twelve common piping plans (01/02/11/13/21/23/31/32/52/53A/62/76) against their strengths and weaknesses, returning the top-2 match. Quick reading of the winners:
- Clean, moderate temperature → Plan 11 (discharge through orifice), the default of the industry.
- Solids → Plan 31 (hydrocyclone, clean stream to the seal) or Plan 32 (external clean flush).
- Hot service → Plan 21 (cooled discharge flush) or Plan 23 (closed thermosyphon — minimal cooler load, no cold injection into the pump).
- Toxic / hazardous → pressurized double seal Plan 53A (barrier above seal-chamber pressure) or unconstrained double Plan 52 with vent to vapour recovery.
- Crystallizing → Plan 32 external flush with a compatible fluid, or Plan 62 external quench to keep the atmospheric side clear.
Symbol table
| Symbol | Meaning | Units |
|---|---|---|
| \(\Delta P\) | pressure drop across the faces | kPa / psi |
| \(b\) | balance ratio (closing area / face area) | — |
| \(k\) | pressure-gradient factor of the film | — |
| \(P_{sp}\) | spring pressure | kPa / psi |
| \(P_f\) | total face pressure | kPa / psi |
| \(D_m,\,b_f\) | mean face diameter, face width | mm / in |
| \(V_m\) | mean face speed | m/s / ft/min |
| \(PV\) | face pressure × mean speed | bar·m/s / psi·ft/min |
| \(A,\,R_m\) | face area, mean face radius | m², m / in², in |
| \(f\) | effective face friction coefficient | — |
| \(T,\,P\) | friction torque, friction power | N·m, kW / lb·in, hp |
| \(SG,\,C_p,\,Q\) | specific gravity, specific heat, flush flow | —, kcal/kg·°C, L/min |
| \(\Delta T\) | seal-chamber temperature rise | °C / °F |
Engineering criteria applied
- Balance-ratio window 0.65–0.85 for balanced contactive seals; unbalanced (\(b\ge1\)) only below ΔP ≈ 10 bar (145 psi).
- Spring-pressure window 100–240 kPa (15–35 psi) for contactive liquid-film seals; dry-running designs sit at 14–40 kPa.
- ΔT < 5 °C (10 °F) across the seal chamber — the flash-prevention rule.
- PV screening as above, in the closing-load definition of this tool; vendor tables use their own definitions — never cross-compare.
- Start-up torque — breakout friction is 3–5× running friction; check the driver for frequent starts.