After the seal arrangement is matched to the required containment duty, the choice becomes clear: use a single seal for acceptable controlled leakage, a tandem arrangement for backup containment, or a pressurized dual seal when pumped hydrocarbon must not reach atmosphere. A pump discharge pressure of 13 barg does not decide the arrangement by itself.
Where does the hydrocarbon leakage path run?
Start at the process side and follow the fluid path through each restriction. The pressure that loads the inboard seal faces is the seal-chamber pressure, not automatically the pump discharge pressure. Suction pressure, internal pump geometry, balance features, recirculation connections, vapor pressure, temperature, and operating point can all change the pressure and phase condition at the seal.
With a single mechanical seal, the inboard seal is also the atmospheric containment boundary. A small film passes across the faces as part of normal seal operation. Depending on the fluid and face temperature, that leakage can remain liquid, flash, or vaporize. The design question is therefore not whether the seal leaks at a microscopic level, but where that leakage goes and whether the resulting liquid or vapor release is acceptable.
A tandem arrangement places an outboard seal behind the process seal. The cavity between them contains buffer fluid or receives process leakage from the inboard seal. The outboard seal can contain leakage if the inboard seal deteriorates, so this architecture provides backup containment.
A pressurized dual arrangement maintains barrier fluid between the two seals at a pressure above the process-side seal pressure. The pressure gradient drives clean barrier fluid toward the process rather than allowing hydrocarbon to migrate outward. The outer seal normally contains barrier fluid, not pumped product. Its primary function is zero outward process leakage during normal operation, not standby replacement of the inner seal.
Which seal architectures address this service?
| Approach | Normal leakage path | Containment function | Support requirement | Best fit |
|---|---|---|---|---|
| Single mechanical seal | Process fluid crosses one seal toward atmosphere or a closed collection connection | No second seal boundary | May require venting, drainage, flushing, or cooling according to the fluid and seal design | Leakage is acceptable and can be routed safely |
| Tandem, unpressurized dual seal | Process leakage enters the cavity between seals; the outboard seal limits release | Backup containment after inboard leakage | An unpressurized buffer system such as Plan 52, with monitoring and any required cooling |
Reduced emissions and secondary containment are required, but barrier-fluid ingress into the process is undesirable |
| Pressurized dual seal | Barrier fluid flows toward the process across the inboard faces | Prevents pumped product from migrating outward during normal operation | A pressurized barrier system such as Plan 53B, plus monitoring and heat removal where required |
Atmospheric release of the pumped product is unacceptable |
The word “dual” is not precise enough for procurement. State whether the arrangement is tandem and unpressurized or pressurized with barrier fluid. Otherwise, technically different proposals may appear equivalent even though they address different risks.
What symptoms point to each design requirement?
| Observed condition or requirement | Mechanism to examine | Design response |
|---|---|---|
| Visible drips or hydrocarbon odor | Leakage is reaching atmosphere as liquid or vapor | Check the seal faces, collection path, cavity pressure, and vent destination; consider tandem containment |
| No pumped product may reach atmosphere | The pressure gradient must oppose outward hydrocarbon migration | Use a pressurized dual arrangement and verify barrier pressure against the maximum seal-chamber pressure |
| Remote or lightly attended pump area | A degrading inboard seal may remain undetected | Add secondary containment and instruments that create an actionable alarm |
| Barrier or buffer temperature rises | Seal-face heat and fluid circulation exceed passive heat rejection | Review circulation, reservoir or accumulator condition, and cooling duty |
| High-viscosity process fluid enters a buffer cavity | Contaminated buffer fluid circulates poorly and carries more heat | Detect inboard leakage early and avoid operating indefinitely with contaminated fluid |
| Barrier-fluid level or volume declines | Fluid is leaking toward the process, atmosphere, or an external connection | Trend the loss rate, inspect both seals and piping, and correct the leak rather than repeatedly topping up |
Tank farms are not automatically low-risk installations. Stored inventory, loading operations, valves, flanges, sampling points, ignition sources, and intermittent staffing can make even a low-pressure release significant. Apply the site VOC rules, hazardous-area assessment, environmental limits, and risk analysis to the actual release path.
What criteria decide between single, tandem, and pressurized dual seals?
Classify the required outcome before comparing utility consumption. The first decision is whether any normal hydrocarbon release is permissible. If the answer is no, a single seal is not equivalent to a pressurized dual seal, even if both can operate mechanically at the stated pressure.
If controlled leakage is permissible, determine whether it can be routed to a closed vapor space. A connection to the tank vapor section can prevent local atmospheric discharge, but only when the receiving system can accept the vapor and liquid under every operating state. Check backpressure, drainage, vapor recovery capacity, isolation philosophy, ignition control, and the consequences of a blocked line.
An unpressurized tandem seal becomes the stronger choice when the main requirement is backup containment. One proposed method is to connect the interseal cavity toward pump suction through an orifice so the cavity operates closer to suction pressure than seal-chamber pressure. Treat that as an engineered system, not a universal detail: calculate flow through the restriction, verify cavity pressure across startup and shutdown, prevent reverse flow, and confirm that the routing does not defeat leak detection or send vapor into an unsuitable destination.
Select a pressurized dual seal when the release criterion requires the process fluid to remain behind an inward barrier-fluid pressure gradient. Confirm that limited barrier-fluid entry into the pumped stream is compatible with product quality and downstream processing. Discharge pressure of 13 barg is only an input to the hydraulic review; obtain the maximum transient seal-chamber pressure for the final barrier-pressure setting.
Which support plan works without a continuous liquid utility?
| Support plan | Pressure source | Site dependency | Primary monitoring need |
|---|---|---|---|
Plan 52 |
Unpressurized buffer system | Buffer-fluid inventory and any required cooling | Level, pressure, temperature, and evidence of process leakage into the buffer |
Plan 53A |
Nitrogen header pressurizes a seal reservoir | Reliable nitrogen supply | Reservoir pressure, level, and temperature |
Plan 53B |
Accumulator maintains barrier-fluid pressure | Stored pressure rather than a continuous nitrogen header connection | Barrier pressure or volume, temperature, refill demand, and accumulator bladder condition |
Plan 53C |
Piston pot follows a reference pressure from the discharge or seal chamber | Correct reference-pressure connection and piston operation | Barrier pressure, piston position or available volume, and temperature |
Plan 53B can suit a site without a continuous liquid utility because the accumulator supplies the pressure function. It does not eliminate maintenance or instrumentation. Operations must receive an alarm before usable barrier-fluid volume is exhausted, and maintenance must have a method to refill the fluid and restore the accumulator bladder charge.
Heat removal is a separate duty from pressurization. If cooling water or another cooling fluid is unavailable, an air-cooled heat exchanger can reject barrier-fluid heat. Size it from seal heat generation, circulation rate, ambient temperature, solar exposure, and allowable barrier-fluid temperature. Salty air and high humidity affect material selection and exchanger fouling or corrosion control.
How should the recommended arrangement be specified?
For aromatic feed or naphtha service, recommend the arrangement from the release criterion. Use Plan 53B with a pressurized dual seal when no pumped product may escape during normal operation and the process tolerates barrier-fluid ingress. Use a tandem arrangement with Plan 52 when the goal is secondary containment and an unpressurized buffer is preferred. Retain a single seal only after the site accepts the calculated and routed leakage consequences.
- Record the fluid composition, vapor behavior at seal conditions, temperature range, viscosity, solids or fouling tendency, and compatibility with candidate buffer or barrier fluids.
- Obtain seal-chamber pressure at minimum and maximum flow, startup, shutdown, blocked or restricted discharge, and any other credible transient. Do not substitute the 13 barg discharge value without checking the chamber hydraulics.
- Define the containment statement in testable language: controlled leakage allowed, backup containment required, or no outward process leakage allowed.
- Choose single, tandem unpressurized, or pressurized dual architecture from that statement. State the architecture explicitly on the datasheet and purchase specification.
- Select the support plan. For
Plan 53B, define the required barrier-pressure relationship, usable accumulator volume, refill method, bladder service method, instruments, alarms, and isolation points from the seal supplier’s calculations. - Calculate heat rejection for continuous operation and the worst ambient condition. Specify air cooling where no liquid cooling utility exists, with materials suited to humidity or salt exposure.
- Place pressure, level or volume, and temperature indications where a loss of containment, loss of pressure margin, or loss of cooling creates an alarm before the seal loses its protective function.
- Document alarm response: reduce load or stop the pump, isolate it when required, identify which seal is leaking, restore the support system, and inspect contamination before restart.
How is the installation verified before service?
Test the complete pressure and fluid path, not just the seal cartridge. Confirm line routing against the approved diagram, verify that vents rise without liquid traps, check drains for a safe destination, prove the correct orifice orientation where one is used, and verify that valves cannot leave the seal operating with a blocked circulation path.
For a tandem system, establish the normal buffer pressure, level, and temperature before startup. Trend them as pump pressure increases. A rising buffer pressure, changing level, or hydrocarbon contamination can identify inboard-seal leakage; leakage at the atmospheric side points toward the outboard seal or its connections.
For Plan 53B, verify the accumulator condition, barrier-fluid inventory, and pressure indication before starting the pump. During startup, confirm that barrier pressure remains above the actual seal-chamber pressure through the full operating transition. Then prove every low-pressure, low-volume or level, and high-temperature alarm by simulation or controlled test according to the site test procedure.
Run long enough to reach stable temperatures. Record seal-chamber pressure, support-system pressure, fluid level or available volume, supply and return temperatures, cooler ambient conditions, and refill rate. Acceptance requires stable thermal behavior, the specified pressure relationship, no process-fluid release at the atmospheric boundary, and alarms that reach the responsible operator.
Frequently Asked Questions
What happens if a single seal is used for naphtha service?
Normal face leakage has only one containment boundary and may reach the collection system or atmosphere as liquid or vapor. Use it only when the site accepts that release path and the vent or drain system can handle every operating condition.
What happens if Plan 53B barrier pressure falls below seal-chamber pressure?
The pressure gradient can reverse, allowing process fluid into the barrier cavity and removing the arrangement’s no-outward-product function. Alarm on loss of pressure margin and apply the defined shutdown or isolation response.
What happens if hydrocarbon enters a Plan 52 buffer system?
It indicates leakage across the inboard seal and can alter viscosity, temperature, lubrication, and vapor behavior in the cavity. Use pressure, level, temperature, and fluid-condition checks to identify the leak before the outboard seal becomes the only containment boundary.
What happens if no cooling-water utility is available?
An air-cooled heat exchanger can remove buffer- or barrier-fluid heat when sized for the seal duty and worst ambient condition. Account for salty air or high humidity in the exchanger material and maintenance plan.
What happens if the pump discharge is 13 barg?
Do not set the seal-support pressure from discharge pressure alone; measure or calculate the maximum seal-chamber pressure through steady and transient operation. Final verification is a recorded operating test showing stable temperature, the specified barrier-pressure margin, no atmospheric process leakage, and successful alarm actuation.