Screw Pump Mechanical Seal Options - API 682 Compliance Guide

Mechanical Seal Basics

How Mechanical Seals Work

┌─────────────────────────────────────────────────────┐
│                                                     │
│     Rotating Face ────────► ◄──────── Stationary   │
│     (on shaft)      Thin film        Face (in      │
│                     of fluid          housing)     │
│                                                     │
│     Spring pressure maintains contact               │
│     Fluid film provides lubrication                 │
│                                                     │
└─────────────────────────────────────────────────────┘

Key Components

Seal Arrangements

Single Seal (Arrangement 1)

Process ─────┤ SEAL ├───── Atmosphere
             ╔═════╗
             ║     ║
         ═══════════════
             Shaft

Applications:

• Non-hazardous fluids
• Standard process service
• Most common arrangement
• Lower cost

Tandem Seals (Arrangement 2)

Process ─┤ SEAL 1 ├─ Buffer ─┤ SEAL 2 ├─ Atm
         ╔═══════╗           ╔═══════╗
         ║       ║   Fluid   ║       ║
     ════════════════════════════════════
                  Shaft

Applications:

• Hazardous fluids requiring backup
• When leak detection needed
• Extended seal life
• Not zero-emission

Double Seals (Arrangement 3)

Process ─┤ SEAL 1 ├─ Barrier ─┤ SEAL 2 ├─ Atm
         ╔═══════╗   (Higher  ╔═══════╗
         ║       ║   pressure)║       ║
     ════════════════════════════════════
                  Shaft

Applications:

• Zero emission required
• Highly hazardous fluids
• Carcinogenic fluids
• Regulatory requirements

API 682 Overview

Standard Scope

API 682 defines standard piping configurations (flush plans) for mechanical seals on:

• Centrifugal pumps (API 610)
• Rotary pumps (API 676)
• Includes screw pumps

Seal Categories

API 682 Flush Plans for Screw Pumps

Single Seal Plans

Tandem/Double Seal Plans

Plan Selection for Screw Pumps

Standard Service: Plan 11

         ┌─────┐
Discharge│ ○   │← Orifice (restricts flow)
    ─────┤     │
         │ SEAL│
         │     │
    ─────┤     │
Seal     └─────┘
Chamber

Flow: Discharge → Orifice → Seal → Drain back to process

When to Use:

• Clean fluids
• Viscosity < 200 cSt
• Temperature < 80°C
• Most standard applications

Advantages:

• Simple installation
• No external piping
• Low cost
• Reliable

High Viscosity Service: Plan 12

         ┌─────┐
Discharge│ ▣   │← Y-Strainer (removes particles)
    ─────┤ ○   │← Orifice
         │ SEAL│
         │     │
    ─────┤     │
         └─────┘

Flow: Discharge → Strainer → Orifice → Seal → Drain

When to Use:

• Viscosity > 200 cSt
• Fluids with minor particles
• Extended seal life needed
• Self-venting required (horizontal pumps)

Advantages:

• Removes particles before seal
• Self-venting capability
• Better seal life in viscous service
• Standard strainer maintenance

Hot Service: Plan 13 or Plan 23

Plan 13: Reverse circulation (seal → suction)

• Uses cooler suction as flush source
• Simple, no heat exchanger

Plan 23: Includes heat exchanger

• External cooling of flush fluid
• For very hot service (>150°C)
• Maintains seal face temperature

Zero Emission: Plan 53A/B/C

         ┌─────────────┐
         │ Accumulator │
         │ ▲           │
         │ │ Barrier   │
    ─────┤ │ Fluid     ├─────
         │             │
    ═════╡ SEAL 1│SEAL 2╞═════
         └─────────────┘

Barrier pressure > Process pressure
Zero process fluid emission

When to Use:

• Carcinogenic fluids
• Regulatory requirements
• Zero emission mandate
• Highly toxic service

Seal Face Materials

Common Combinations

Material Selection by Fluid

Elastomer Selection

Common Elastomers

Selection Guide

Specification Format

Seal Specification Template

=== MECHANICAL SEAL SPECIFICATION ===

Service:
Tag number: _______________
Pump model: _______________
Shaft size: ___ mm

Process Conditions:
Fluid: _______________
Temperature: ___ °C (normal), ___ °C (max)
Pressure at seal: ___ barg
Viscosity: ___ cSt

Seal Requirements:
Arrangement: □ Single □ Tandem □ Double
API 682 Category: □ 1 □ 2 □ 3
Flush Plan: Plan ___

Materials:
Rotating face: _______________
Stationary face: _______________
Elastomers: _______________
Metal parts: _______________

Flush System:
Plan: API 682 Plan ___
Flush fluid: _______________
Flush rate: ___ L/min
Inlet temperature: ___ °C

Special Requirements:
□ API 682 compliance required
□ Cartridge seal required
□ Monitoring provisions (pressure, temperature)
□ Other: _______________

Maintenance Considerations

Seal Life Expectations

Maintenance Activities

Summary: Plan Selection Guide