Considerations for Seal Flush Systems for an Industrial Fluid Oil Buffer

As more focus has been placed on safety, reliability, and the environmental impact of pumping equipment, dual mechanical seals have become more prevalent in industries such as chemical, petrochemical, and midstream oil and gas. Dual mechanical seals provide superior leak prevention and consist of an inboard (primary) seal and an outboard (secondary) seal. Inboard seals keep process fluid within the pump housing and outboard seals prevent buffer/barrier fluid leakage to the atmosphere. Dual mechanical seals use buffer or barrier fluid in addition to process fluid to provide lubrication and heat removal. Unpressurized dual seals use a buffer fluid to accomplish this.

For some applications, water and glycol solutions, alcohols, and kerosene and diesel fuels may not be suitable. In such cases, lube oils and synthetic lube oils are often used in seal flush systems for dual mechanical seals. Below, we will detail general considerations for oil buffer fluids as well as considerations for applicable seal flush plans for an industrial fluid oil buffer.

Industrial Fluid Oil Buffer API Seal Flush Plans

Inadequate or incorrect seal flush systems often account for a significant portion of mechanical seal failures. Dual mechanical seals require seal flush systems to provide the proper lubrication and cooling to function properly and extend seal life. For operations involving harsh process conditions, seal flush plans may require additional components, such as filters, coolers, or cyclone separators, and/or instrumentation to ensure reliable dual mechanical seal functionality. Fluid systems experts should be consulted to determine the appropriate selection of additional components for specific applications and process conditions to optimize seal flush performance.

Let’s take a look at the API plans that are suitable for oil buffer fluids.

API Plan 52: Buffer Fluid Seal Pot

API Plan 52 utilizes a reservoir and circulates unpressurized buffer fluid between the inboard and outboard seals using a pumping ring to provide positive circulation through the flush system and into the seal. Process fluid and buffer fluid must be compatible. Frictional losses in the buffer fluid inlet and outlet lines can be minimized through proper tubing size selection, use of large radii and/or 45º bends, and reduction of tube length.

API Plan 52 provides additional heat removal from the inboard seal when required and acts as a secondary seal to prevent process fluid from leaking into the atmosphere.

API Plan 52 considerations:

• Restriction orifice plates may be used in the vent line to maintain backpressure in the seal pot.
• Piping or tubing from the seal to the seal pot should be self-venting.
• Buffer fluid should be regularly assessed and replaced when necessary.
• Temperatures at the seal inlet and outlet line should be measured to indicate proper flow.
• Seal pot pressure and fluid level should be monitored:
  • An increase in seal pot pressure or fluid level may be indicative of inboard seal leakage. 
  • A decrease in seal pot pressure or fluid level may be indicative of outboard seal leakage.
• Seal pot pressure should be maintained at a lower pressure than the seal chamber (and less than 40 psi).
• API Plan 52 should not be used for contaminated and polymerizing process fluid.

API Plan 55: Buffer Fluid Circulated by an External System

API Plan 55 is a custom-engineered system that circulates unpressurized buffer fluid to and from the seal chamber via a pump located on an external reservoir. This plan can contain additional components, such as coolers and filters, depending on application parameters and process conditions. 

API Plan 55 provides both cooling and lubrication to the outboard seal independent of the operation of the pump.

API Plan 55 considerations:

• Temperatures at the seal inlet and outlet line should be measured to indicate proper flow.
• Instrumentation, such as flowmeters, should be used to ensure proper flow. An increase in flow rate may be indicative of outboard seal leakage.
• Seal pot pressure should be maintained at a lower pressure than the seal chamber (and less than 40 psi).
• API Plan 55 should not be used for contaminated and polymerizing process fluid.
• Additional engineering and/or components may be required depending on the application.

Considerations for Selecting an Oil Buffer Fluid

Petroleum-based hydraulic and lubricating oils—and synthetic lubricating oils specifically formulated for buffer or barrier use—provide exceptional lubrication to mechanical seal faces, offering the potential for prolonged seal operating life. In general, oils have fewer compatibility issues with seal materials with the exception of carbon seal faces. The selection of such fluids should be determined with a mechanical seal manufacturer. 

Generally speaking, an ideal buffer fluid should have the following characteristics:

• Compatibility with sealing materials and process fluid
• Safe to use, handle, and store
• Non-flammable
• Good lubrication and heat transfer characteristics
• Flows well at low temperatures
• Non-foaming or gas absorbing
• Stable at ambient temperatures

The Best Method for Seal Flush Plan Optimization

When utilizing an industrial fluid oil buffer, choosing the right seal flush plan components and instrumentation for your application is critical for mechanical seal reliability. Optimization of seal flush plans for specific applications can prevent mechanical seal failure and unnecessary rotating equipment downtime.

If you are experiencing seal flush lubrication or cooling inefficiencies or are looking to improve mechanical seal reliability, Field Advisors at Edmonton Valve & Fitting can perform a detailed on-site consultation to understand your process and recommend custom solutions, such as additional components and instrumentation, to ensure reliable and efficient rotating equipment operations.