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The Fluid Systems Engineering and Management Blog

by Swagelok Northern California

Understanding the Difference Between Seal Plan 53A and 53B to Guide System Selection and Design

by Malik Durojaiye, on 2/24/21 8:45 AM


The API Plan 53 with variations A, B, and C are some of the most commonly used support systems for dual pressurized mechanical seals. They deliver a clean external barrier fluid at a pressure greater than the product pressure on the inner seal. And even when a mechanical seal fails, these plans virtually guarantee zero emissions to the atmosphere. The plans are well suited to pumping applications for toxic or hazardous fluids. In this blog, I’ll focus on two similar variations—53A and 53B—describing their designs and capabilities and contrasting the two plans to give you a better understanding of where and why you would deploy each of the plans. But, first, a very brief description of each plan.

API Plan 53A



This plan uses a pumping ring to circulate pressurized barrier fluid from a reservoir to the dual seal. Cooling coils in the reservoir manage barrier fluid temperature. Plant nitrogen pressurizes the fluid to at least 1.3 bar (20 psig) above the maximum seal chamber pressure. API Plan 53A supports a single pump.

API Plan 53B



API Plan 53B also uses a pumping ring to circulate pressurized barrier fluid to a dual mechanical seal. A bladder accumulator pressurizes the barrier fluid with nitrogen charging the accumulator. This plan can operate at higher pressures 10.34 bar (150 psig) to 51.71 bar (750 psig) because it does not allow pressurizing gas to be absorbed into barrier fluid. A heat exchanger manages barrier fluid temperature. API Plan 53B can simultaneously support multiple pumps.


In both of these API plans, barrier fluid must be chemically compatible with process fluid as the barrier fluid will leak across the inner seal face into the process fluid. As barrier fluid leaks across the seal faces, the reservoir or bladder accumulator needs to be replenished with fluid. Level switches or visual level indicators are used to monitor barrier fluid levels.

Similarities & Differences Between Seal Plan 53A and 53B

Let’s take a more focused look at the key similarities and differences between the two API plans. In the table below I’ve attempted to summarize the key characteristics to help a process engineer choose between plans.


API Plan 53A

API Plan 53B

Pressure range

Maximum of 10.34 bar (150 psig)

10.34 bar (150 psig) to 51.71 bar (750 psig)

Barrier Fluid Reservoir

Capacity is between 1 to 5 gallons of barrier fluid. Larger capacity reservoirs help extend fluid life and require fewer refills of barrier fluid.

Accumulator design limits the volume of barrier fluid; the majority of accumulator volume is gas. In some designs, barrier fluid may be as little as one-half gallon. As a result, fluid is thermally cycled more frequently, thereby reducing fluid life.

Pressure source

Plant nitrogen supply

Bladder accumulator pressurized by plant nitrogen supply

Gas absorption 

Barrier fluid can absorb nitrogen gas resulting in reduced barrier fluid pressure that does not effectively lubricate the seal faces and maintain seal integrity. 

Bladder accumulator design prevents barrier fluid from absorbing nitrogen gas and providers higher barrier fluid pressure.


Cooling coils are located in the reservoir with water flow rates varying between 1 and 3 gpm.

Air or water-cooled heat exchanger


Wear particles heavier than the barrier fluid settle to the reservoir bottom and do not recirculate to the seal chamber.

Wear particles have no place to settle and are recirculated through the seal chamber, potentially damaging seal faces.

Number of seal systems supported.

Supports only a single, dual seal pump.  

Can be configured to support one or more dual seal pumps, with equal or unequal process pressure conditions 


Comparatively less expensive because of its relatively simple design 

Higher cost because of components needed to maintain pressure and temperature


Key Differences Between Seal Plan 53A and 53B Guide Selection

The main deciding factor in choosing between the two plans is the process fluid pressure that needs to be maintained. Pressures above 10.34 bar (150 psig) favor 53B. Also note that 53B can be configured so that a single system supports multiple pumps, which from an operational perspective may be ideal, even if high-pressure is not a deciding factor. Plan 53B is also the better choice (heat exchanger) for maintaining barrier fluid temperatures in high-temperature hydrocarbon pumping environments. From the perspective of design simplicity and implementation, 53A comes out on top.

There are several other factors to consider in determining the design of the seal support system. Let’s consider barrier fluids. Water or water/glycol fluids will dissipate heat more efficiently than petroleum-based fluids and determine the cooling capacity of the reservoir or heat exchanger. You’ll have to ensure that appropriate barrier fluid levels are maintained and operations personnel notified when fluids should be refilled. Your mechanical seal vendor should be able to provide metrics for fluid loss across the inner seal to help in the planning.  

Location and infrastructure also play an important role in the system design. If plant nitrogen is not available (such as in remote locations) Plan 53B is the best option, because bottle nitrogen can be used to pressurize the bladder accumulator. Significant ambient temperature variations can affect the pressure in bladder accumulators and jeopardize reliability. Capacity and bladder pressurization are critical design considerations.

Let a Seal Support Professional Guide Your Plan Selection and Design

You’ll achieve the best outcomes in determining which of the API Plan 53 variations best fit your needs when you consult with an experienced mechanical seal support partner. They’ll not only help you evaluate the pros and cons of 53A versus 53B, but they’ll also help design the system for optimum reliability and performance, providing definitive answers to design questions such as:

  • What capacity cooling components will be needed?
  • How will an API Plan 53A fit into your existing infrastructure?
  • Which instrumentation (temperature, pressure, flow, and level gauges) is needed to monitor performance and identify developing problems?
  • What’s the best configuration for API Plan 53B supporting multiple pumps with varying process pressures?

At Swagelok, we’ve been guiding industries worldwide in determining the API 682 plans that best meet their seal support requirements for more than 50 years. Whether our Field Engineers consult on-site to gain an in-depth understanding of your specific process conditions or consult virtually, they bring a team of worldwide experience and expertise. As a result, you’ll receive the best technical guidance, optimal seal support system design, and product quality that will ensure smooth implementation and years of reliable service.

To learn how Swagelok Northern California can help you determine which API Plan 53 configurations best meets your seal support system requirements by relying on our expert on-site or virtual consultation, system design, and ISO 9001 fabrication processes, contact our team today by calling 510-933-6200.

image8 (1)-1About Malik Durojaiye | Custom Solutions Engineer, Assembly Services

Malik Durojaiye began his Swagelok career in 2019 as a Custom Solutions Engineer in our Assembly Services group. Prior to Swagelok, Malik developed as a design engineer as well as a manufacturing engineer for 6 years serving Kentucky and California with Altec Industries; a leading provider of products and services to the electric utility, telecommunications, tree care, lights and signs, and contractor markets.

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