Mechanical Seal Configurations and Selection

Mechanical Seal Configurations and Selection FSA Knowledge Series

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Abstract This Fluid Sealing Association Knowledge Series training presentation introduces the various configurations of mechanical seals together with an overview of their selection process. A description is provided on:

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Seal arrangements – single and dual seals

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Dual seal configurations

▪

Piping plans

▪

Seal selection

Mechanical Seal Components ▪

All mechanical seal consist of the same basic elements. These elements may vary in form with different seal types, however the function they provide is the same Seal Plate (Gland) Gasket

Seal Plate (Gland) Stationary Face Secondary Seal

Flexible Element (Spring or bellows/diaphragm)

PROCESS FLUID

ATMOSPHERE

Dynamic Secondary Seal Rotating Seal Face

Stationary Seal Face Seal Face Pair

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Seal Arrangements

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Seal Arrangements Mechanical seals can be grouped into 3 distinct arrangements: Arrangement 1) Single mechanical seal A mechanical seal assembly with one pair of seal faces

Arrangement 2) Dual unpressurized mechanical seal A mechanical seal assembly with two pairs of seal faces. The cavity formed between the two seals is unpressurised and can be filled with a liquid or gas Arrangement 3) Dual pressurized mechanical seal A mechanical seal assembly with two pairs of seal faces. The cavity formed between the two seals is pressurized above the process fluid pressure and can be filled with a liquid or gas

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Seal Arrangements Typical arrangement 1 single mechanical seal Single mechanical seals can incorporate various features to allow the environment on either the process fluid or atmospheric side of the seal to be altered. These include ports (connections) in the seal plate (gland), close clearance bushing or lip seals on the atmospheric side of the seal.

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Seal Arrangements Arrangement 2 and 3 dual mechanical seal Dual mechanical seals incorporate two mechanical seals in the seal assembly.

These two seal can be arranged in 3 different Configurations (positions of the inner and outer seal relative to each other).

In addition, the cavity that is formed between the two seal can be filled with a liquid or gas that is connected to a seal support system (that controls and monitors the fluid).

This fluid cavity can remain unpressurised (Arrangement 2) or pressurized (Arrangement 3)

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Arrangement 2 and 3 Configurations Face-to-Back Configuration In this configuration, the face of the inboard seal is adjacent to the back of the outboard seal. This configuration can be operated as an Arrangement 2 or 3 seal.

Traditionally referred to as a Tandem seal, however this description often implies an Arrangement 2 seal only.

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Dual Seal Configurations Back-to-Back Configuration In this configuration, the back of the inboard seal is adjacent to the back of the outboard seal. This configuration is commonly used in high pressure Arrangement 3 seals. Traditionally referred to as a Double seal, however this description often implies an arrangement 3 seal only.

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Dual Seal Configurations Face-to-Face Configuration In this configuration, the face of the inboard seal is adjacent to the face of the outboard seal. This configuration is commonly used in high pressure applications or equipment with high shaft speeds as an Arrangement 3 seal.

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Dual Seal Configurations Face-to-Face Configuration An alternative Face-to-Face configuration, with the mechanical seal flexible element rotating with the shaft, is commonly used in abrasive chemical applications or pumped fluids with suspended solids.

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Seal Environment

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Seal Environment Reliable performance of a mechanical seal is principally governed by the environment that the seal is operating in.

The selection of a mechanical seal type, arrangement and configuration are dictated by the fluid properties and operating conditions.

Piping plans can be selected to alter the environment around the mechanical seal to enhance reliability and safety.

These piping plans are defined in a standard published by the American Petroleum Institute, API-682.

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Piping Plans Basic Types of Piping Plans Piping plans can be grouped into 3 types. These types relate to which part of the mechanical seal environment the piping plan effects Piping Plans can be connected to:

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Process fluid side of the seal

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Atmospheric side of the seal

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Barrier or buffer cavity of a dual seal

Process Fluid Flush Plans Process Fluid Piping Plans change the seal chamber conditions by: ▪

Changing temperature

▪

Changing Pressure

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Creating flow to remove debris

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Removing trapped vapor Process Fluid Piping Plans Can be used on all seal Arrangements

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Process Fluid Flush Plans Common Process Fluid Piping Plans :

API Plan 02 API Plan 11

API Plan 13 API Plan 14 API Plan 21 API Plan 23 API Plan 32 17

Provisions for heating/cooling jacket around the seal chamber and/or seal plate (Gland plate) Recirculation of fluid from the pump discharge to the seal chamber Recirculation of fluid from the seal chamber to the pump suction Plan 11 and 13 together Plan 11 with an inline heat exchanger Seal chamber fluid circulated through a heat exchanger External flush fluid injected into the seal chamber

Atmospheric Piping Plans Atmospheric Piping Plans change the environment around the mechanical seal by: ▪

Changing temperature

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Cleaning to remove debris

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Displacing atmosphere (oxygen or humidity)

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Monitoring leakage Atmospheric Piping Plans Can be used on all seal Arrangements

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Atmospheric Piping Plans Common Atmospheric Piping Plans :

API Plan 51 API Plan 62

API Plan 65A/B API Plan 66A/B

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Dead-ended vessel providing an atmospheric quench liquid.

Quench fluid (liquid, gas or steam) injection at the atmospheric side of the seal Leakage detection (detected by leakage flow) Leakage detection (detected by leakage pressure)

Dual Seal Piping Plans Dual Seal Piping Plans Barrier/buffer cavity Piping Plans change the environment between the inner and outer mechanical seal by: ▪

Changing temperature

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Providing a safety buffer or barrier

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Providing an alternate fluid to lubricate the seal faces Dual Seal Piping Plans Can only be used on Arrangement 2 and 3 seals

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Dual Seal Piping Plans Dual Seal Piping Plans : Arrangement 2 (Unpressurized dual seals) API Plan 52

API Plan 55 API Plan 72 API Plan 75 API Plan 76

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Liquid buffer stored in a reservoir and delivered to the mechanical seal Liquid buffer delivered to the mechanical seal from an external system Gas buffer delivered to the mechanical seal from a gas control panel Condensing leakage detection used with Plan 72 Non-condensing leakage detection used with Plan 72

Dual Seal Piping Plans Dual Seal Piping Plans : Arrangement 3 (Pressurized dual seals) API Plan 53A

API Plan 53B API Plan 53C API Plan 54 API Plan 74

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Liquid barrier stored in a reservoir and pressurized by a gas blanket Liquid barrier pressurized by a gas bladder accumulator Liquid barrier pressurized by a piston accumulator

Liquid barrier delivered to the mechanical seal from an external system Gas Barrier delivered to the mechanical seal from a gas control panel

Seal Selection

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Seal Selection The process of selecting a seal involves: ▪

Reviewing the operating conditions in the seal chamber

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The process fluid physical and chemical properties

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Any hazards the process fluid may present to personnel and the environment

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Reviewing industry or user specifications and requirements

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Reviewing federal or local mandated specifications

* Contact your seal vendor for more detailed guidance in the seal selection process 24

Seal Type Selection The selection of seal type is determined by the pumping conditions and pumped fluid properties. This can be generalized as: ▪

If the pumped fluid has a low specific gravity (density) or has a low vapor pressure at the pumping temperature, dual seals should be selected (arrangement 2 or 3)

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If the pumping temperature is outside the limits of elastomers (O-rings), then a metal bellows with non-elastomeric secondary seal should be selected

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If the pumped fluid has a moderately low specific gravity, then a pusher seal should be selected

* Contact your seal vendor for more detailed guidance in the seal selection process 25

Seal Type Selection Selecting the seal Type

Temperature (°F)

-200

0

200

400

600

800

Specific Gravity

1.2

0.8

0.6

High Temperature Bellows Seals

Pusher or Bellows Seals

1.0

Low Temp Bellows Seals

Pusher Seals

0.4

Dual Seals 0.2

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* Contact your seal vendor for more detailed guidance in the seal selection process

-100

0

100

200

Temperature (°C)

300

400

Seal Arrangement Selection Selecting the seal Arrangement Is the process fluid:

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Very hazardous or a poor lubricant or abrasive

Arrangement 3

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Hazardous

Arrangement 2

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Non-hazardous

Arrangement 1

* Contact your seal vendor for more detailed guidance in the seal selection process 27

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