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This Fluid Sealing Association Knowledge Series training presentation introduces API Piping Plan 72. A description is provided on:
▪ What is an API Plan 72?
▪ How an API Plan 72 Works
▪ What does an API Plan 72 do?
▪ What an API Plan 72 cannot do
▪ Optional Features for an API Plan 72
▪ Cost to Operate with an API Plan 72
▪ How to Size an API Plan 72
▪ How to Install an API Plan 72
▪ General API Plan 72 Commissioning Guidelines
▪ How to Operate API Plan 72
▪ General Troubleshooting of API Plan 72
▪ Alternatives to API Plan 72
What Are Piping Plans?
▪ Piping plans collectively are different piping arrangements of fluid used to improve the conditions the mechanical seal operates in with the objective of improving the mechanical seal’s life.
▪ The American Petroleum Institute adopted numbers and created definitions for each piping plan configuration, thereby allowing a common language across the industry to simply describe a particular configuration.
▪ The American Petroleum Institute standard API-682 is where the definition of each piping plan can be found and where they may periodically be updated.
What
is an API Plan 72?
API Plan 72 delivers a filtered low-pressure buffer gas between the inner primary seal and an outer containment seal. This inert buffer gas, commonly nitrogen, is typically vented to a vapor recovery system (API Plan 76). An API Plan 72 is panel mounted and features instrumentation to monitor buffer gas pressure and flow rate.
Flow transmitter with local readout
Isolation valve
Gas source
Coalescing filter
Filter drain valve
Pressure transmitter with local readout
How an API Plan 72 Works
Dual unpressurized pump seals may feature an inner primary seal lubricated with the process fluid and an outer dry containment seal. In this configuration, an external source of low-pressure (buffer) gas may be introduced between the inner and outer seal face pairs to direct leakage. Leakage can be either liquid or vapor (or a combination of both). The outer containment seal operates on the low-pressure gas, but in the event of a primary seal failure, it is capable of operating as a processlubricated seal.
The continuous flow of buffer gas between the seal faces provides a clean environment for the containment seal while directing most vaporized process leakage to the vapor recovery system (API Plan 76). A flow restricting device, typically a carbon bushing, guides the incoming buffer gas over the outer containment seal and directs any condensing process leakage to collection (API Plan 75) before reaching the outer containment seal. This creates near-zero process emissions to atmosphere.
How an API Plan 72 Works
An API Plan 72 typically features a panel-mounted group of instruments and valves that filter the buffer gas, regulate the outlet pressure, and measures gas pressure and flow being delivered to the mechanical seal. An API Plan 72 should be combined with an API Plan 75 for liquid collection and/or an API Plan 76 for vapor recovery.
Nitrogen is commonly used as the buffer gas since it is readily available and, as an inert gas, it does not create any adverse chemical reactions with process fluids. Less readily available inert gases, such as argon, can also be used.
To condition and monitor the buffer gas, the API Plan 72 panel typically includes:
Isolation valve – Isolates the buffer gas source from the mechanical seal to allow for maintenance or filter replacement.
Coalescing filter – A 2 to 3 micrometer (μm) coalescing filter traps liquids and particles that can potentially damage the seal faces.
How an API Plan 72 Works
Pressure regulator – Regulates the gas pressure being delivered to the mechanical seal.
Pressure transmitter – Local pressure readout is used when adjusting the pressure regulator. High pressure alarms are set in the data acquisition system to monitor for excessive primary seal leakage. Low pressure alarms are set for loss of gas supply pressure.
Orifice – Narrow plate with a small flow-through hole that restricts flow. These are commonly constructed from piping components and can be welded or threaded.
Flow transmitter – A proper flow is required to maintain the clean environment for the containment seal and minimize environmental emissions. Too low of a flow will increase emissions, while too high of a flow could create elevated buffer gas pressure to the containment seal, potentially reducing seal life.
Check valve – Prevents backflow of process fluid and/or vapor into the buffer gas supply line in the event of loss of buffer gas pressure or an abnormal process upset.
What does an API Plan 72 do?
An API Plan 72, used with a dual unpressurized mechanical seal with an outer dry containment seal, delivers low-pressure buffer gas to dilute primary seal leakage before it escapes to the atmosphere. When combined with leakage collection systems—API Plan 76 for vaporizing leakage or API Plan 75 for condensing leakage— most emissions are captured and diverted to recovery systems. This configuration minimizes fluid escape, achieving near-zero process emissions.
Instrumentation in API Plan 72 regulates and monitors gas pressure and flow. Changes in buffer gas pressure provide early warning of seal degradation.
What an API Plan 72 cannot do
During normal operation of a dual unpressurized mechanical seal, process leakage is diluted with the buffer gas, with most leakage typically being directed towards a leakage collection system. There are scenarios where an API Plan 72 is not suitable, such as:
▪ Applications where zero process leakage to the atmosphere is required.
▪ Locations where no buffer gas source is available.
▪ Applications where there is no access to a leakage recovery system.
▪ Applications where process fluid leakage crystallizes at buffer gas system pressure.
▪ Installations in confined spaces where atmospheric leakage can create an oxygen deficient environment.
Optional Features for an API Plan 72
Optional features for an API Plan 72 include:
▪ Instrumentation - Pressure: Analog versus digital instruments: buffer gas pressure measurement and alarms can be achieved with analog instruments (e.g., pressure gauges and pressure switches). These achieve a simple and low-cost option but lack the ability to trend measurements over time. A pressure transmitter with local readout provides a digital feedback to a data acquisition system that can trend measurements over time, allow high and low-pressure alarms, enabling seal performance monitoring.
▪ Orifice – May be replaced with needle valve.
Optional Features for an API Plan 72
▪ Instrumentation - Flow: Analog versus digital instruments: buffer gas flow measurement and alarms can be achieved with an analog flow meter, typically a low range variable area flow meter with a flow switch. A flow transmitter with local readout provides a digital feedback to a data acquisition system that can trend measurements over time, allow high and low-flow alarms, enabling seal performance monitoring.
Optional Features for an API Plan 72
▪ Filter differential pressure – Adding a differential pressure switch to the filter can be used to provide an indicator for changing the filter element.
▪ Duplex filter – Enables changing the filter element without the need to depressurize the system.
▪ Dual outlet for between bearing pumps: For pumps with a seal at each end of the shaft, a second outlet can be added to the API Plan 72 providing instrumentation that can monitor the performance of each seal independently.
Optional
Features for an API Plan 72
▪ Check valve – Check valves are available with a variety of materials for their internal seals. The seal material should be selected for compatibility with the process fluid at the process temperature. Non-elastomeric check valves are available for high temperature fluids. A check valve with a low cracking pressure should be selected.
▪ Backup buffer gas bottle rack – A bottle rack with multiple gas bottles can be incorporated into an API Plan 72 design to provide an alternate temporary source of buffer gas should the primary source fail. Using bottled gas as the primary gas source is not recommended.
Optional Features for an API Plan 72
▪ Weather enclosures – An API Plan 72 can be mounted in a weatherproof enclosure protecting the instruments from the extremes of cold, dust, salt air, and humidity. Heaters can be added to the enclosure for cold environments. Sunshades can be added to the instruments, or the entire API Plan 72, for hot environments.
Cost to Operate with an API Plan 72
Based on the dual unpressurized seal’s design, the power consumption can vary but is less than a dual pressurized wet seal. Additionally, the buffer gas consumption for a dual unpressurized seal with an API Plan 72 is typically less than that of a dual pressurized gas seal utilizing an API Plan 74.
Refer to the Fluid Sealing Association’s Lifecycle Cost Calculator (LCC) for a more detailed analysis.
How to Size an API Plan 72
Sizing an API Plan 72 is determined by selecting components specified to the pressure rating of the pumping system and selecting instruments that can measure the pressure and flow rates at the operating conditions and at the high-pressure alarm conditions.
Buffer gas pressure
Dual unpressurized mechanical seals must operate with the buffer gas pressure in the seal chamber lower than the process pressure, but they should be designed with a buffer gas pressure that is slightly greater than any vapor recovery system’s pressure.
▪ Minimum buffer gas pressure = Normal vapor recovery system pressure + minimum differential pressure (> Check valve cracking pressure).
▪ Maximum buffer gas pressure = As established by the seal manufacturer, but note API 682 states buffer gas pressure should be no greater than 0.7 bar (10 psi).
How to Size an API Plan 72
Buffer gas consumption rate
The gas consumption rate is based primarily on the flow restriction device in the vapor recovery system and influenced by the mechanical seal design. When an optional needle valve is used to control buffer gas flow, rates are typically set between 85 and 170 NLPH (3-6 SCFH).
Instrument set points and alarms
▪ Pressure set point – The recommended set pressure for the pressure regulator is at or above the minimum required buffer gas pressure.
▪ High pressure alarm point – An alarm should be triggered if the buffer gas pressure in the containment seal chamber is above the maximum buffer gas pressure, indicating primary seal failure.
▪ Pressure gauge and pressure switch, or pressure transmitter, can be sized based off the buffer gas pressure set and alarm points.
How to Install an API Plan 72
The API Plan 72 valves and instruments are panel mounted. This panel should be mounted on a stand at a height to comfortably operate the valves and read the instruments.
The stand can be mounted on the pump baseplate or adjacent to the pump in a location that does not interfere with inspection and maintenance activities for the pump. Unlike liquid buffer systems, an API Plan 72 support system does not need to be immediately adjacent to the mechanical seal.
Avoid creating trip hazards with the pipe/tube that connects the API Plan 72 panel to the mechanical seal.
How to Install an API Plan 72
▪ Install ground wires to all electrical instruments, the API Plan 72 panel, and to the stand.
▪ Caution should be used where the distance between the API Plan 72 panel and mechanical seal is large. A pressure drop between the API Plan 72 panel and seal can occur in the small-bore tubing that can reduce the buffer flow being delivered to the seal. Increasing the tube size can eliminate this concern.
To commission an API Plan 72:
1) Connect the API Plan 72 panel to the buffer gas source and to the mechanical seal and ensure any leakage collection systems (API Plan 75 and/or API Plan 76) are connected and valved open.
2) Purge air out of the API Plan 72 panel and interconnecting piping using the buffer gas. A fitting near the mechanical seal may need to be cracked open or disconnected to allow purging.
3) Pressurize the API Plan 72 system and check for leaks. Take care not to over pressurize the buffer system, which can lead to primary seal failure from reverse pressurization. Consult seal manufacturer for limits.
4) Purge the filter drain connection.
General API Plan 72 Commissioning Guidelines Cont.
To commission an API Plan 72:
5) Adjust the pressure regulator to the correct pressure to start buffer gas flow.
6) If an optional needle valve is being used to control buffer gas flow, adjust needle valve to reach desired buffer gas flow.
7) Flow should be delivered to the seal before introducing fluid into the pump casing. Complete the pump venting and commissioning procedures.
8) The seal and API Plan 72 are ready for startup.
How to Operate API Plan 72
With the pump in operation, the only maintenance item is the filter. After one month of operation, the filter element should be inspected, and if no contamination is detected, inspection should occur every six months. The filter element replacement interval will be dictated by the cleanliness of the supply gas but should be no more than three years.
The buffer gas consumption rate should be checked and recorded during routine inspections.
Small pulsations in the buffer gas flow rate are normal as they are the result of the cycle of the check valve cracking open, allowing gas to flow, and then re-seating.
General Troubleshooting of API Plan 72
Symptom Potential causes
▪ Low/zero flow
Buffer gas isolation valve closed
Buffer gas pressure below seal chamber pressure
Filter clogged
Failure of inner seal
▪ Increasing/high flow
Vapor recovery system pressure too high
Flow indicator malfunction
Containment seal face performance deteriorating
Hang-up of containment seal flexible element
Buffer pressure set too high
Leaks in interconnecting tubing
Vapor recovery system pressure too low
General Troubleshooting of API Plan 72
Symptom Potential causes
▪ Low pressure
Buffer gas isolation valve closed
Buffer gas supply pressure too low
Pressure regulator set incorrectly
Excessive containment seal leakage
Leaks in interconnecting tubing
▪ High pressure
Pressure regulator set incorrectly
Heat exposure/solar radiation
Primary seal failure
Leakage collection flow blockage
Alternatives to API Plan 72
Alternative piping plans that provide a liquid buffer system:
Plan 52
Dual seal with liquid buffer reservoir
Plan 55
Dual seal with liquid buffer from an external system
Alternatives to API Plan 72
Alternative piping plans that provide a pressurized barrier system:
Plan 74
Dual non-contacting seal with external barrier gas supply
API Plan 53A
Dual seal with gas pressurized liquid barrier reservoir
API Plan 72 Summary
The combination of a dual mechanical seal together with the API Plan 72 support system offers operators a shaft sealing solution that provides:
▪ Near-zero emissions to atmosphere
▪ Lower capital investment (compared to seals with a liquid buffer system)
▪ Lower life cycle cost (compared to seals with a liquid buffer system)
▪ A small footprint for the support system
▪ Low maintenance requirements
When installed and operated with best practices this sealing solution can deliver long term reliability and improved rotating machinery availability.
