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Dual Mechanical Seals with Liquid Buffer/Barrier Fluids

FSA Knowledge Series

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Abstract

This Fluid Sealing Association Knowledge Series training presentation introduces liquid buffer and barrier systems together with an overview of their desirable properties. A description is provided on:

▪ Dual seal arrangements

▪ Unpressurized and pressurized piping plans

▪ Liquid buffer and barrier fluid characteristics

▪ Liquid buffer and barrier fluid types and properties

What is a Dual Seal?

As operators of pumping equipment become more focused on the safety, reliability and environmental impact resulting from shaft seal leakage, dual mechanical seals have become more prevalent in industry.

A dual mechanical seal offers a second (outer) seal to achieve the containment of the pumped fluid by creating a cavity, or chamber, between the inner and outer seal that can be filled with a fluid.

When this fluid is unpressurized (at a pressure less than the seal chamber pressure) it forms a buffer between the pumped fluid and atmosphere and is commonly referred to as a buffer fluid.

When pressurized (at a pressure greater than the seal chamber pressure) it forms a barrier between the pumped fluid and atmosphere and is known as a barrier fluid.

Liquid Buffer and Barrier Fluids

In addition to separating the pumped fluid from the atmosphere, liquid buffer and barrier fluids also provide:

▪ Lubrication to the mechanical seal faces

▪ Transport of heat energy from the mechanical seal to a heat exchanger. (This heat energy includes both frictional heat generated by the seal and absorbed heat from the surrounding pump casing and pumped fluid)

Typical Dual Seal

Pumped Fluid

Supply and return connections to the Buffer/Barrier fluid system

Buffer/Barrier Fluid Cavity

Inner Mechanical Seal

Outer Mechanical Seal

Typical Dual Mechanical Seal cross section

Buffer and Barrier Fluid Sources

There are several methods of storing, monitoring and delivering the buffer or barrier fluid to the dual mechanical seal.

Each method is identified by a piping plan number that is used to describe the minimum requirements of each system.

The most commonly referenced piping plan designations originate from the American Petroleum Institute’s standard API-682.

▪ API Plan 52 - Vessel unpressurized buffer fluid

▪ API Plan 53A - Vessel pressurized barrier fluid

▪ API Plan 53B - Bladder accumulator pressurized barrier fluid

▪ API Plan 53C - Piston amplified pressurized barrier fluid

▪ API Plan 54 - External system pressurized barrier fluid

▪ API Plan 55 - External system unpressurized buffer fluid

Unpressurized Buffer Fluid Systems

Unpressurized systems using buffer fluids

An API Plan 52 provides a reservoir that stores the buffer fluid.

Supply and return lines are connected to the mechanical seal and circulation of the buffer fluid is achieved by an internal circulating device (pumping ring) within the mechanical seal.

The vapor space above the buffer fluid in the reservoir is vented to atmospheric pressure typically via a flare or vapor recovery system.

Unpressurized Buffer Fluid Systems

Unpressurized systems using buffer fluids (continued)

The reservoir can be instrumented to measure the liquid level and pressure in the reservoir. Ports are fitted to the reservoir to facilitate maintenance activities such as inspection and cleaning or refilling and draining the buffer fluid. Cooling is provided in the form of an internal heat exchanger.

Simplified schematic of an API Plan 52 system

Pressurized Barrier Fluid Systems

Pressurized systems using barrier fluids

Pressurized dual seal systems contain the same essential components as an unpressurized system. They contain:

▪ A stored supply of barrier fluid,

▪ A means to dissipate heat absorbed by the barrier fluids,

▪ Instrumentation to monitor the performance of the system,

▪ Inter-connecting piping to transport the barrier fluid from the mechanical seal to the heat exchanger, and finally

▪ A means to pressurize the barrier fluid.

There are 4 methods employed to pressurize the barrier fluid:

API Plan 53A

A pressurized gas blanket in the reservoir pressurizes the fluid. Nitrogen is normally used, and the pressure is controlled via a pressure regulator. The barrier fluid is in direct contact with the pressurized gas.

Simplified schematic of an API Plan 53A system

API Plan 53B

Compression of a gas filled bladder. Pressure is generated as a nitrogen filled bladder is compressed as barrier fluid is added into the bladder accumulator. The bladder prevents direct contact of the pressurized gas with the barrier fluid

Simplified schematic of an API Plan 53B system

API Plan 53C

A pressure amplifying piston uses pressure from within the pump (typically the seal chamber) to amplify the barrier pressure by the ratio of the area on each side of the piston. The barrier fluid is not exposed to any pressurized gas in this arrangement

Simplified schematic of an API Plan 53C system

API Plan 54

An external system is used to pressurize the barrier fluid and circulate the barrier system.

An API plan 54 system can be broadly classified into two groups: closed loop systems where barrier fluid is stored in a large reservoir and pumps are used to pressurize and circulate the barrier fluid and open loop systems where a compatible process stream is used as the barrier fluid and is circulated through the mechanical seal and then returned to another point downstream in the process.

Simplified schematic of a closed loop API Plan 54 system

API Plan 55

An external system used to provide circulation of unpressurized buffer fluid (a pressure less than the seal chamber pressure). An API plan 55 system can be broadly classified into two groups: closed loop systems where buffer fluid is stored in a large reservoir and pumps are used to circulate the buffer fluid and open loop systems where a compatible process stream is used as the buffer fluid and is circulated through the mechanical seal and then returned to another point downstream in the process.

Simplified schematic of a closed loop API Plan 55 system

Buffer/Barrier Fluid Characteristics

Important characteristics of a buffer/barrier fluid

There are several critical properties of a buffer or barrier fluid that need consideration when making a selection. An ideal buffer or barrier fluid will have the following properties:

▪ Safe to use, handle, and store

▪ Not a VOC, VHAP, or other regulated compound

▪ Nonflammable

▪ Good lubricity

▪ Good heat transfer properties

▪ Compatible with process fluid

▪ Compatible with seal materials

▪ Good flow qualities at operational temperatures

▪ Remain a stable liquid at ambient temperatures

▪ Non-foaming when pressurized

▪ Low solubility of gas

▪ Inexpensive

Buffer/Barrier Fluid Families

Families of buffer/barrier fluid

Fluids that have characteristics suitable for use as a buffer and barrier fluids can be broadly into the following groups:

▪ Water and glycol solutions

▪ Alcohols

▪ Kerosene and diesel fuels

▪ Petroleum based hydraulic and lubricating oils

▪ Synthetic hydraulic oils

▪ Heat transfer fluids

Typical Buffer/Barrier Fluid Properties

Typical buffer and barrier fluid properties

Summary

The buffer or barrier fluid needs to move heat energy from the mechanical seal to the heat exchanger without degradation of the fluid. It needs to have good compatibility with the process fluid and the seal components while providing good lubrication to the mechanical seal faces.

The correct selection of a buffer or barrier fluid will help achieve these goals and maximize the reliability of the dual seal sealing system.