Sealing Sense
API 682 Piping Plan Series
A guide to the API Plan 21.
Brandon Eckardt | Fluid Sealing Association member, Flowserve
| IMAGE 1: Fouled plan 21 cooler (Image courtesy of Flowserve)
Mechanical seals are very important for pump reliability, as their performance often determines whether a plant runs smoothly or suffers costly interruptions. To improve seal life and operating conditions, support systems are often used to create a favorable environment around the mechanical seal. Among them, an American Petroleum Institute (API) Plan 21 is a practical solution for cooling process fluid before it enters the seal chamber, offering operators a way to manage temperature, lubricity and vapor pressure margin with a relatively simple arrangement.
An API Plan 21 is best understood as a cooled version of an API Plan 11. In an API Plan 11, fluid is taken directly from the pump discharge and routed into the seal chamber through a flow control device (typically an orifice). An API Plan 21 adds a heat exchanger to this arrangement, cooling the fluid before it enters the seal chamber. The system relies on the differential pressure between the pump discharge and the seal chamber to generate flow. As soon as the pump generates discharge pressure, fluid circulates through the piping, passes through the exchanger and enters the seal chamber to cool and lubricate the seal faces. Given that the flush fluid is taken directly from the process stream, an API Plan 21 does not introduce an external fluid to the process stream, but rather conditions the process fluid itself to create a more favorable environment for seal operation.
Benefits & Limitations
By cooling the flush fluid, an API Plan 21 lowers the temperature at the seal interface, reducing thermal stress and extending seal life. Cooling also increases fluid viscosity, which improves the lubricating film between the rotating and stationary seal faces. Lowering the fluid temperature increases the vapor pressure margin, reducing the risk of flashing or vaporization as the fluid crosses the faces. When combined with a close clearance seal chamber throat bushing, an API Plan 21 can raise the seal chamber pressure, further improving vapor pressure margin. Continuous flow also helps prevent solids from accumulating in and around the seal, which could lead to hang-up and wear. In practice, these advantages translate into longer seal life, reduced maintenance and improved reliability in high-temperature applications.
An API Plan 21 does have some limitations, though. It cannot introduce a clean external fluid to isolate the seal chamber from the process, as the flush fluid is always the process fluid in the pump. If there are solids in the process stream, it cannot completely remove them from the seal chamber. It is also unsuitable for fluids that freeze, solidify or thicken within the system when cooled. Additionally, since the process fluid only passes through the heat exchanger once, the amount of temperature reduction of the flush fluid is limited, and a very high heat load is put through the heat exchanger at the same time. This can result in a water-cooled heat exchanger becoming easily fouled, requiring more maintenance or necessitating the switch to an air-cooled design. Understanding these limitations is essential when evaluating whether an API Plan 21 is the right choice for a given application, and in some cases, alternatives such as an API Plan 23 may provide better performance or lower operating costs.
API Plan 21 Options & Costs
Operators have the ability to customize API Plan 21 systems with optional features that improve performance and monitoring. Flow control can be achieved through orifice plates, choke tubes or compression fittings with built-in restrictions, and multiple orifices may be installed in high differential pressure applications. A throat bushing can be added to the bottom of the seal chamber to tighten clearances and build back pressure in the seal chamber. Instrumentation such as pressure gauges upstream and downstream of the orifice, temperature gauges on both process and cooling sides of the heat exchanger and flow indicators for cooling water provide valuable operating data. Heat exchangers themselves can be designed as shell-and-tube, tube-in-tube, natural convection or forced convection units, depending on cooling requirements and available utilities. These options allow operators to adapt an API Plan 21 to a wide range of operating environments and improve monitoring for reliability.
While the initial investment for an API Plan 21 system is relatively low, operating costs can be substantial. Cooling duty requirements are often high compared to other plans, because hot process fluid continuously enters the heat exchanger. Utility consumption, whether in the form of cooling water for a water-cooled heat exchanger or electricity for a forced convection heat exchanger, is a major component of the overall cost. Heat removed from the process must also be replaced, increasing process energy demand. Routine monitoring, cleaning of heat exchangers and inspection of flow control devices adds labor costs. Higher energy consumption also translates into a larger carbon footprint. A life cycle cost analysis is recommended to fully understand the long-term impact of an API Plan 21 operation. While the system may be inexpensive to install, the ongoing costs of utilities, maintenance and energy balance can be significant.
| IMAGE 2: Typical API Plan 21 setup (Image courtesy of the Fluid Sealing Association)
Sizing & Installation
Proper sizing of an API Plan 21 system is essential for reliable performance. A target seal chamber temperature must be established to ensure adequate margin above vapor pressure and acceptable viscosity. Heat load is then determined by combining seal face heat generation with heat soak from the surrounding pump components, and the heat exchanger is selected to remove this load. Best practices limit temperature rise through the chamber to 15 F (8 C) for water and low-volatility hydrocarbons, 30 F (16 C) for lubricating fluids and 5 F (3 C) for volatile hydrocarbons.
Installation practices are equally important. Orifice diameters should be no smaller than 3.2 millimeters (mm) (0.125 inches [in.]) to prevent clogging. API 682 recommends DN15 (½ in.) pipe for shafts up to 60 mm and DN20 (¾ in.) for larger shafts. Connections should be made on the side of the pump volute to avoid solids being centrifuged into the flush.
To achieve self-venting, the flush should enter at the highest point of the seal chamber or seal gland plate, with piping sloped upward to the heat exchanger. The exchanger is typically mounted adjacent to the pump, elevated above the shaft centerline and arranged for counter-flow to maximize efficiency. Attention to these details during installation ensures reliable operation and minimizes startup issues.
Commissioning & Operation
Commissioning an API Plan 21 system is simple, but it involves some critical steps to ensure proper function.
- Verify the flush piping is connected to the correct port on the pump or seal chamber.
- Check that the proper orifice size has been installed in the flush line.
- With the pump casing filled, vented and pressurized, check for leaks on all the API Plan 21 joints.
- Vent the seal chamber and API Plan 21 system via a high point vent.
- Start the supply of cooling water to the heat exchanger (or electrical motor on a forced convection heat exchanger).
- The pump is now ready to start.
During operation, an API Plan 21 system requires routine monitoring and maintenance. Operators should periodically check for leaks at joints, adjust the cooling water flow rate as needed and measure inlet and outlet temperatures of both process and cooling water to monitor exchanger efficiency.
During seal maintenance, flow control devices should be inspected for wear or clogging, and heat exchanger surfaces should be cleaned to remove fouling. Because an API Plan 21 flow is driven by pump discharge pressure, operation is inherently tied to pump performance, and any changes in pump conditions will directly affect flush flow.
Commissioning & Operation
Commissioning an API Plan 21 system is simple, but it involves some critical steps to ensure proper function.
- Verify the flush piping is connected to the correct port on the pump or seal chamber.
- Check that the proper orifice size has been installed in the flush line.
- With the pump casing filled, vented and pressurized, check for leaks on all the API Plan 21 joints.
- Vent the seal chamber and API Plan 21 system via a high point vent.
- Start the supply of cooling water to the heat exchanger (or electrical motor on a forced convection heat exchanger).
- The pump is now ready to start.
During operation, an API Plan 21 system requires routine monitoring and maintenance. Operators should periodically check for leaks at joints, adjust the cooling water flow rate as needed and measure inlet and outlet temperatures of both process and cooling water to monitor exchanger efficiency.
During seal maintenance, flow control devices should be inspected for wear or clogging, and heat exchanger surfaces should be cleaned to remove fouling. Because an API Plan 21 flow is driven by pump discharge pressure, operation is inherently tied to pump performance, and any changes in pump conditions will directly affect flush flow.
| IMAGE 3: Counter-flow heat exchanger (Image courtesy of Flowserve)
Troubleshooting & Alternatives
Monitoring the system via instrumentation can provide clues to its health. An increased outlet temperature may result from higher pump temperature, reduced cooling water flow, fouling of the heat transfer surfaces or throat bushing wear. A decreased outlet temperature could be due to lower pump temperature, increased cooling water flow or clogging of flow control devices.
Systematic troubleshooting should consider both process and utility conditions to identify root causes and restore reliable operation. While an API Plan 21 is a versatile and effective option, it is not always the best choice. In some applications, an API Plan 11 may suffice, offering a simpler arrangement without cooling. In others, an API Plan 23 may be more efficient, recirculating fluid from the seal chamber through a heat exchanger and reducing cooling duty compared to an API Plan 21. Selection should be based on process temperature, fluid properties and life cycle cost considerations, with input from seal vendors to ensure the best match for operating conditions.
An API Plan 21 offers a practical solution for cooling the process fluid before it enters the seal chamber, improving mechanical seal reliability in high-temperature applications. While relatively simple in design, its operating costs can be significant due to high cooling duty requirements.
By carefully sizing the system, selecting appropriate optional features, following best installation practices and maintaining diligent operation and monitoring, an API Plan 21 can deliver years of reliable service. Operators should also evaluate alternatives such as an API Plan 23 when cooling requirements are particularly high. Ultimately, the choice of piping plan should be made in collaboration with seal vendors and guided by a thorough understanding of process conditions, utility availability and life cycle costs.
We invite your suggestions for article topics as well as questions on sealing issues so we can better respond to the needs of the industry. Please direct your suggestions and questions to sealingsensequestions@fluidsealing.com.
Brandon Eckardt is an engineering specialist at Flowserve, working as a technical expert on mechanical seals and their application. For more information, visit flowserve.com.
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