Positive Displacement Pumps

Positive Displacement Pumps — Sealing Overview

Positive displacement (PD) pumps move a fixed volume per revolution. Progressive cavity, rotary lobe, gear and screw pumps handle what centrifugal pumps cannot: high viscosity, shear-sensitive food products, sludges and slurries, and products that must be dosed at a steady rate. API 676 covers rotary PD pumps for the process industries.

The seal duty differs from a centrifugal pump in three ways. Speeds are low, so the faces generate little heat from speed, but viscous products shear in the seal chamber and can overheat it. On progressive cavity pumps and most gear and screw pumps the seal sits on the suction side and sees suction pressure. And many PD pumps run in both directions, so the seal must not rely on a direction-sensitive pumping feature.

Key Sealing Challenges

• Viscosity: thick products drag on the rotating parts and can stop springs from following the faces. Balanced seals with springs outside the product cope better.
• Products that harden: sugar syrups, resins, bitumen and polymers set on the atmospheric side as they cool or dry. A Plan 62 water or steam quench keeps the faces free.
• Reversing rotation: lobe pumps are often reversed for cleaning or emptying lines. Seals must be bi-directional.
• Hygiene: food and pharmaceutical duties need cleanable seal designs, FDA-compliant elastomers and resin-impregnated (not antimony) carbon.

Fault Diagnosis Example

Symptom: Seal on a molasses gear pump leaks after every weekend shutdown.
  ├── Incorrect Response: "Increase spring force."
  ├── Diagnostic Evidence Required:
  │     ├── Is there hardened product on the atmospheric side of the faces?
  │     ├── Is the seal housing heat-traced, and was the quench left on during shutdown?
  │     └── Are the faces chipped from starting against set product?
  └── Engineered Solution:
        ├── Fit a Plan 62 steam or hot-water quench and keep it on during shutdown.
        ├── Heat the pump and seal housing before start-up.
        └── Use SiC vs SiC faces, which resist chipping.

Frequently Asked Questions

What pressure does the seal on a progressive cavity pump see?

In the standard arrangement the seal is in the suction housing, so it sees suction pressure, not the discharge pressure the pump develops. Confirm the arrangement, because some designs are reversed.

Why do lobe pumps need bi-directional seals?

Lobe pumps are often run backwards to empty lines or to clean. A seal with a direction-sensitive pumping feature can run dry or unload its faces in reverse.

Can a gear pump running on packing be converted to a mechanical seal?

Usually yes, if the stuffing box is deep enough for a cartridge or component seal. Send the pump make, model and stuffing-box dimensions.

Related: Beverage & Brewery · Polymer & Rubber · Molasses-Based Distillery · Slurry Pumps · All process pump solutions.

Request a Seal Selection

Send your pump make and model, product, viscosity, temperature, pressure, shaft size and speed to info@gipfelengg.com or use our enquiry form. Gipfel Engineering (ISO 9001:2015, NSIC-registered MSME, Mumbai) designs and manufactures mechanical seals, seal support systems and process pumps.