api plan for double mechanical seal in stock

Pumping processes involving toxic or hazardous fluids that can’t risk leakage because of stringent environmental regulations require a double mechanical seal. Compared to a single mechanical seal, a double seal gives you significantly greater protection against leaks. With a double mechanical seal, you have an arrangement of two mechanical seals (a primary or inboard seal and a secondary or outboard seal) in series—back-to-back, tandem, or face-to-face. Each seal has a rotating (R) surface and a stationary (S) seal surface. These seals can be arranged in one of three patterns.

In a back-to-back arrangement, the stationary seal faces are positioned back-to-back with the rotating seal faces on the outside. The back-to-back arrangement is easy to install and used for many general pumping applications.

The tandem arrangement has the two pairs of seals mounted with the same orientation. This arrangement is preferred for toxic or hazardous applications because the outboard seal provides full pressure back-up, allowing the outboard seal to back up in the event of an inboard seal failure.

In the face-to-face arrangement, the rotating seal faces share a common stationary seal face. This arrangement is useful when equipment space is too constrained to permit back-to-back or tandem seal arrangements.

The American Petroleum Institute (API) Standard 682 classifies double mechanical seals into two configurations—pressurized and unpressurized. The pressurized arrangement has a barrier fluid delivered to the double mechanical seal by a seal support system. The barrier fluid is delivered at a higher pressure than the process fluid and must be chemically compatible with the process fluid as it will lubricate the inboard seal faces and mix with the process fluid. The unpressurized arrangement has a buffer fluid delivered to the double mechanical seal by a seal support system. The buffer fluid is delivered at a lower pressure than the process fluid.

The barrier and buffer fluids you use can be liquid or gas. They provide lubrication and help maintain the required operating temperature of the seal faces. The typical choices are water and water/glycol mixtures, low-viscosity petroleum or synthetic oils, kerosene, diesel, and nitrogen.

To gain a better understanding of the differences between the uses of barrier and buffer fluids, let’s look at two common API plans for double mechanical seals—API Plan 52 Buffer Fluid Seal Pot and API Plan 53A Barrier Fluid Seal Pot Pressurized by Nitrogen.

API Plan 52 takes buffer (unpressurized) fluid from a reservoir (seal pot), delivers it to the seal chamber, circulates it between the inboard and outboard seals using a pumping ring located driven by shaft rotation, then returns the fluid to the reservoir. In the event of an inboard seal failure, process fluid leaks into the seal chamber. When that occurs an increase in buffer fluid pressure and/or level alerts operators to the problem. The outboard seal, however, contains leakage until maintenance can replace the damaged seal.

This plan can include cooling coils in the reservoir to maintain the required buffer fluid temperature, visual or mechanical fluid level indicators, pressure and level transmitters, and connection to a collection system and buffer fluid replenishment source.

The overall design of this API plan for a double mechanical seal is relatively simple in comparison to other plans. Design decisions involving tubing size, length, geometry, type (carbon vs stainless steel), buffer fluid type, and volume of the buffer fluid reservoir are critical in maintaining the proper operating environment for the double seal. If you don’t have this expertise in-house, work with an experienced, local seal support system vendor to ensure the API Plan 52 is designed to meet your specific pumping requirements.

API Plan 53A is conceptually similar to API Plan 52 with the difference that the fluid being circulated between the double mechanical seals is under pressure. A pumping ring is used to circulate the fluid. The reservoir that contains the barrier fluid is pressurized by plant nitrogen. Reservoir pressure should be set a minimum of 20 to 25 psi (1.4 to 1.73 bar) above the maximum seal chamber pressure, allowing the barrier fluid to leak (and lubricate) across the inboard seal faces into the process fluid. For this reason, the barrier fluid must be chemically compatible with the process fluid.

Because barrier fluid is depleted as it moves across the inboard seal faces, it needs to be replenished. This can be done manually or automatically by way of a system that serves multiple pumps. API Plan 53A design options include reservoir type and volume, cooling coils, fluid level and pressure indicators, and transmitters to alert to level or pressure changes that indicate seal failure.

When you choose an API plan for a double mechanical seal, your primary decision is between a buffer or barrier plan. I’ve highlighted two of the API plans for double mechanical seals above to show the basic differences. There are multiple API plans for double mechanical seals to choose from—pressurization from bladder or piston accumulators, plant nitrogen delivered directly to the seal chamber, and custom-engineered external systems. Your choice will be determined by the process fluid and pumping conditions and the type of double mechanical seal your vendor recommends.

With this information in hand, it’s best to work with an experienced local seal support system vendor. They’ll be able to meet with you on-site to review the specifications for the pumping process, the pump, and the double mechanical seal. They’ll evaluate your existing infrastructure and its influence on seal support system design. Based on this information, they’ll then design the seal support system to meet the specific pumping requirements.

If you work with a global vendor like Swagelok, based on the design, we can quickly assemble and thoroughly test the API plan at our local facilities prior to delivery. We’re also conveniently available for follow-up consultations, on-site, remotely, or by way of a quick phone call.

For well over 50 years, Swagelok has worked closely with Northern California process industries to confidently choose the right API plans for pumping needs. Our locally based Field Engineers and certified technicians provide field verification of your seal support requirements, designs based on best practices gained from global experience.

To find out more about howSwagelok Northern California can help you choose the right API plan for double mechanical seals, as well as process and atmospheric side seals,contact our team today by calling

Morgan holds a B.S. in Mechanical Engineering from the University of California at Santa Barbara. He is certified in Section IX, Grab Sample Panel Configuration, and Mechanical Efficiency Program Specification (API 682). He is also well-versed in B31.3 Process Piping Code. Before joining Swagelok Northern California, he was a Manufacturing Engineer at Sierra Instruments, primarily focused on capillary thermal meters for the semiconductor industry (ASML).

In Part 3 of our series on Double/Dual Mechanical Seals we take a look at the best piping plans/support systems to put in place to increase seal/equipment reliability and reduce energy/water costs.

More importantly, to extend the life of a double/dual seal, you want to control the fluid film that comes into contact with the primary seal faces to establish the ideal lubrication, temperature and pressure within the seal. Across all types of mechanical seal failures, inadequate/incorrect seal support systemsare the second highest cause of seal failures (Figure 1).

Standard environmental control plans (also called piping plans) have been developed for double/dual seals and choosing the right environmental control plan is critical to your seal performance and reliability.

This plan is used when working with hazardous process fluids in which no leakage into the product can be tolerated. It provides a backup seal in case of inboard seal failure.

If an inner seal leak is not detected early enough, the higher pressure process fluid will displace the buffer fluid. This can result in the process fluid completely filling the barrier fluid chamber between the inboard and outboard primary seals. Should the outer seal leak, product will be released into the atmosphere.

This plan is used in services where no product leakage into the atmosphere can be tolerated.  Plan 53’s biggest advantage over Plan 54DM, which does not use a tank, is its limited fluid volume.  The amount of fluid that can enter the product system is limited to the volume of the barrier fluid tank itself (usually 2-3 gallons or 8-12 liters).

Uses a pressurized external reservoir or barrier fluid tank to provide a clean, pressurized barrier fluid to both the inboard and outboard primary seals of a pressurized double/dual seal.

Leakage of barrier fluid into the product will always occur to some extent.  Normally this leakage will be small and can be monitored by the barrier tank fluid level.  Therefore, the chosen barrier fluid must always be compatible with the process fluid.

The tank pressure must be maintained at the proper level.  If the barrier fluid tank pressure drops, the system will begin to operate like a Plan 52, or unpressurized dual seal, which does not provide the same level of sealing integrity.  Specifically, the inner seal leakage direction will be reversed and the barrier fluid will, over time, become contaminated with the process fluid.

Directs a clean compatible fluid into and out of the dual mechanical seal barrier/buffer fluid ports. The purpose of this fluid is to prevent the pumped fluid from damaging the inboard seal faces, to remove heat from the seal, and to lubricate the outboard seal faces.

Careful consideration should be given to the reliability of the barrier fluid source. When the source is interrupted or contaminated, the resulting seal failures can be very costly.

The higher the differential pressure, the more transfer/migration into the product. The control should never be used where the barrier fluid pressure is likely to fall below seal chamber pressure. If this were to happen, the failure of one inboard seal from any mechanical seal in the system could contaminate the entire barrier fluid system with product and cause additional seal failures.

Once the inboard seal wears out or fails, the amount of barrier fluid that entered the product system is virtually unlimited unless it is shut down quickly.  Alerting instruments should be put in place to avoid this.

Using appropriate monitoring devices, alarms, switches, etc. will provide a means to monitor the health of your double/dual seal to identify potential issues in advance avoiding costly unplanned outages. A basic Plan 52 buffer system will have a sight glass to monitor the buffer fluid level, a pressure gauge and connections to and from the mechanical seal, as well as a vent and drain valve. Cooling coils and level switches are typically available as optional items.

A Plan 53 would have the same features as the Plan 52 with the addition of a pressure regulator, safety valve, and an optional pressure switch. These systems can be customized to meet customer specifications should they desire to have additional features incorporated into the system.

For assistance with using the best seal support system for your specific challenge, ask your local Chesterton office or contact our Ask the Expert service.

Plan 53B systems have been viewed as an “engineering challenge” around the world, often with long lead times. The innovative modular concept permits 12 modular options to be applied to create an API 53B System for any application. This modular process facilitates efficient stock control which in turn provides AESSEAL® API 53B Systems with rapid delivery times. Modularity eases the production of documentation for each Plan 53B product and also makes it easier to determine the correct solution for the application.

Pressurised barrier fluid circulation in outboard seal of dual seal configuration. Circulation is maintained by using pumping ring in running condition and with thermosyphon effect in stand still condition. The pressure is maintained in the seal circuit by a bladder accumulator.

Mechanical seal failure due to unfavorable operating conditions is an issue in every industry. Double mechanical seals especially require proper sealing accessories to create suitable operating environments which are key to increasing MTBF. Reservoir systems are one of the most common and effective options to supply cooling fluid crucial to successful seal operation.

A double -or dual – cartridge seal is defined as an arrangement of two mechanical seals in a series. These seals may be configured in various orientations within the cartridge. The seals themselves are referred to as the “primary” (or inboard) seal and the “secondary” (or outboard) seal. A double seal arrangement is the superior option to a single cartridge when it is imperative the product being pumped does not leak into the atmosphere. The API (American Petroleum Institute) Standard 682 classifies dual seals into two configurations. These configurations also apply to ASME (American Society of Engineers) B73.1 and ASME B73.2 pump designs.

Arrangement 2 (Unpressurized) Designs: the buffer fluid is the operating environment for the secondary seal and forms a “buffer” between the process fluid and the atmosphere.

Arrangement 3 (Pressurized) Designs: the barrier fluid is the operating environment for both the inboard and outboard seal, preventing process leakage to the atmosphere.

Buffer and barrier fluids may be either liquid or gas. These fluids lubricate seal faces during operation as well as regulate operating temperatures by moving heat—both generated and absorbed—away from the faces.

Seal support systems are necessary for the smooth operation of a dual mechanical seal. Here are two of the most common piping plans for these systems.

This is an unpressurized system consisting of a reservoir, supply and return lines, and an internal circulation device within the outer seal (commonly referred to as a pumping ring). The buffer fluid circulation rate is dependent on how this circulation device functions during seal operation.

Reservoirs may be fitted with a variety of measurement devices such as a liquid level indicator and pressure gauges as well as valves and switches to aid in various operation and maintenance functions. For instance, a typical support system configuration for natural gas liquids (NGL) would issue an alarm (visual, audible, or both) when the inner seal fails. In addition, the outer seal would take over the primary seal function until maintenance is performed.

This system forces gas from an external pressurized source into the reservoir to pressurize the barrier fluid. This means the reservoir pressure will be above seal chamber pressure; a guideline is a minimum of 20 to 25 psig (1.4 to 1.73 bar) above the maximum process pressure.

The Plan 53A is also used to maintain a specific operating temperature range to ensure optimum lubrication at the seal faces. The reservoir houses a cooling coil which actively cools the barrier fluid as necessary.

As with the Plan 52, a circulation device is used to move the barrier fluid. Replenishing a Plan 53A system with fresh barrier fluid can be as simple as a using a hand pump or a more complex arrangement which satisfies multiple seals.

& 5. Fuels such as diesel or kerosene, and alcohol solutions may also we used, but it is imperative that these fuels be evaluated for safety and compatibility before put into service.

Liquid-lubricated dual mechanical seals require an external source of clean, cool lubricating fluid. The following fluid reservoir systems create this enhanced sealing environment, enabling longer operational life for dual seals.

In the oil and gas industry, reliable seal operation is critical to running efficient, safe processes. In conjunction with API 682 Piping Plans 52 and 53A, seal support systems aid in smooth seal operation.

If you require an engineered seal support system or are interested in additional options to Flexaseal’s ANSI PLUS and ANSI LITE support systems, please contact our applications engineering team.

Plan 52 uses an external reservoir for providing buffer fluid for the outer seal of an unpressurised dual seal arrangement. Cooling coils in the reservoir are available for removing heat from the buffer fluid.

For Plan 53A, reservoir pressure is produced by a gas, usually nitrogen. A pumping ring maintains circulation during operation. Thermosiphon action is in effect during standstill. Reservoir size can be optimised in accordance with the flow rate. Any particles tend to settle at the bottom of the reservoir and don"t get recirculated. Similar to Plan 52, cooling coils are also used. Proper safeguards against the backflow of barrier fluid into the external supply of nitrogen also need to be considered.

Plan 53B makes use of an accumulator for isolating the pressurizing gas from the buffer fluid. A heat exchanger is also included in the loop in order to cool the fluid. It can either be water cooled, finned tubed, or air-cooled, depending on the system heat load.

Plan 54 employs an external source for providing a clean pressurised fluid. Plan 54 systems can be custom-made so as suit various application requirements and provide pressurised flow to various seal installations, thus keeping costs down.

Mechanical seals are devices that seal machines between rotating parts (shafts) and stationary parts (pump housing) and are an integral part to the pump. Their main job is to prevent the pumped product from leaking into the environment and are manufactured as single or double seals. What"s the difference between the two?

A single mechanical seal consists of two very flat surfaces that are pressed together by a spring and slide against each other. Between these two surfaces is a fluid film generated by the pumped product. This fluid film prevents the mechanical seal from touching the stationary ring. An absence of this fluid film (dry running of the pump) results in frictional heat and ultimate destruction of the mechanical seal.

Mechanical seals tend to leak a vapor from the high pressure side to the low pressure side. This fluid lubricates the seal faces and absorbs the heat generated from the associated friction, which crosses the seal faces as a liquid and vaporizes into the atmosphere. So, it"s common practice to use a single mechanical seal if the pumped product poses little to no risk to the environment.

A double mechanical seal consists of two seals arranged in a series. The inboard, or “primary seal” keeps the product contained within the pump housing. The outboard, or “secondary seal” prevents the flush liquid from leaking into the atmosphere.

Two rotating seal rings are arranged facing away from each other. The lubricating film is generated by the barrier fluid. This arrangement is commonly found in the chemical industry. In case of leakage, the barrier liquid penetrates the product.

The spring loaded rotary seal faces are arranged face to face and slide from the opposite direction to one or two stationary seal parts. This is a popular choice for the food industry, particularly for products which tend to stick. In case of leakage, the barrier liquid penetrates the product. If the product is considered “hot”, the barrier liquid acts as a cooling agent for the mechanical seal.

Are you still using packing for your pumps? Read about the differences between packing and mechanical seals to see if switching to mechanical seals makes sense for your plant. A qualified engineer will help you decide which type of mechanical seal is best for your application.

The API plans presented in this section are developed in accordance with the API 682, 3 revision / API 610, 10 revision standard. This is the standard scheme of the drilling pipes, which are widely used in industry. It is possible to customize these plans to meet the needs of customers.

The flushing of the seal from the outlet to the seal chamber via the aperture and flushing the seals from the seal chamber to the inlet through the diaphragm

Diagram of the system for ensuring the operation of a single seal with an impeller that creates fluid circulation through the stuffing box along an Autonomous circuit.

If the pressure in the oil seal chamber of the pump is less than the design pressure of the tank (4mpa), the installation of a safety valve on the tank pipelines is not required.

"Tandem" type mechanical seals can be used both with a refrigerator at the pump"s working medium temperature up to 400 °C, and without it at the working medium temperature up to 150 °C.

Diagram of the system for ensuring the operability of a double seal with a tank. The system operates at constant maintenance of the pressure of the shut-off fluid (pressure in the tank) within:

At pump working medium temperatures up to 150°C seals are used without a refrigerator, at the temperature of the pumped medium 150...400°C-with a refrigerator.

For servicing seals of a group of pumps that perform the same task and are located close to each other, it is possible to use the system diagram shown below.

The most commonly used scheme is a system with the supply of shut-off fluid from a separate pipeline with an overpressure m through the seal of the threads.

At pump working medium temperatures up to 150°C seals are used without a refrigerator, at the temperature of the pumped medium 150...400°C-with a refrigerator.

For condensate pumps, where dry operation of the mechanical seals is not excluded, the guaranteed supply of the shut-off fluid can be carried out according to the following scheme.

At pump working medium temperatures up to 150°C seals are used without a refrigerator, at the temperature of the pumped medium 150...400°C-with a refrigerator.

API plan 65 allows you to determine the volume of leaks through the mechanical seal. If the friction pair breaks through, the external strapping tank is equipped with an upper-level alarm that will trigger as soon as the liquid level in the tank increases.

An API Plan 53A type system designed specifically to minimize water usage of double mechanical seal supported by a conventional quench and drain system. The system is completely self-managing after installation and maintains fluid levels and pressure. In the event of a fault condition within the Mechanical Seal or Pump a flow indicator provides a visual alert to the operator to investigate the fault condition. The savings in water consumption can be significant and Chesterton provides all of the services required to audit and calculate the potential returns.

An API Plan 52 system designed to provide important cooling, lubrication and containment for Double Mechanical seals operating in conditions where barrier fluid is not allowed to enter the finished product and/or the finished product must be contained in the event of fault condition. The system can be configured to the users requirements with various options and instrumentation, including ATEX equipment.

An API Plan 53A system designed to provide support for Double Mechanical seals operating in conditions where the process fluid does not meet the ideal conditions for support of the Mechanical Seal, and a complimentary barrier fluid must be utilized that is compatible with the process media. The system can be configured to the users requirements with various options and instrumentation, including ATEX equipment.

In the past there was only one Plan 53, but with the 2nd Edition of API 682 and the 1st Edition of ISO 21049 other variations of Plan 53"s were created.

Plan 53A is the former Plan 53. Plan 53B is what had been in the past denoted as Plan 53 Modified; this is especially popular in European and other countries in the Middle East. Plan 53C is a variation of this that has also been used in the past and is now formally recognized.

The major difference in the plans is that Plan 53A uses an external reservoir, while Plans 53B and 53C run within a closed loop system with a make-up system piped to it for replenishment of the barrier fluid.

In dual pressurized sealing arrangements the inner process seal can have its own flush plan; in such applications the complete flush plan system designation should include both plans. For example, Plan 11/53A means that the inner seal has its own flush plan, Plan 11. The API/ISO default is for no separate flush plan when using any of the Plan 53"s, but this can vary with the application conditions.

With the older traditional back-to-back seal arrangement the inboard seal usually does not require a separate flush. In applications such a hydrofluoric acid, where it is both extremely hazardous and corrosive, a Plan 32 can be used in conjunction with a Plan 53. The dual pressurized face-to-back seal arrangement eliminates some of the potential problems associated with the back-to-back design. This face-to-back seal arrangement sometimes incorporates a reverse pressure capability that is not a default with the back-to-back design.

Also, face-to-back arrangements do not have a dead zone underneath the inboard seal that can become clogged by dirty process fluid and lead to seal hang-up. However, the face-to-back arrangement is not a cure-all. With the product on the seal O.D. and with it being used on API pumps that still incorporate throat bushings, it is advantageous to provide a flush for the inboard seal on a number of applications.

Abrasives can accumulate in the more closed API type seal chambers compared to the newer generation chemical duty pumps with large cylindrical bore or tapered bore chambers. The use of a Plan 11 or similar bypass type flush for the inner seal has advantages. It can help keep the seal chamber clean. It also has an improved overall heat transfer setup versus just using a Plan 53 system alone.

In comparison to a Plan 54, Plans 53A/B/C are usually less complex and less expensive. With Plans 53A/B/C, both the inner and the outer seals are lubricated by the barrier fluid, which can be selected for optimum seal performance. Plans 53A/B/C are usually selected for dirty, abrasive, or polymerizing process services which might be difficult to seal directly with single seals or with dual unpressurized seals using a Plan 52. There will always be some leakage of the barrier fluid into the process with any pressurized system.

With some of the Plan 53 systems the volume of barrier fluid is limited, especially compared to a Plan 54 system. Venting of the seal chamber is essential for all Plan 53"s where vapor locking can if vapor bubbles collect near the pumping ring or in the piping.

Plan 53A uses an external reservoir to provide barrier fluid for a pressurized dual seal arrangement. Reservoir pressure is produced by a gas, usually nitrogen, at a pressure greater than the maximum process pressure being sealed. The gas pressure is regulated by a system that is outside the schematic of the piping plan. Circulation of the barrier fluid is maintained by an internal pumping ring.

Like Plan 52 reservoirs, cooling is accomplished internal coil of tubing to remove the heat. Also like Plan 52 reservoirs, the volume of barrier liquid can vary from two gallons to 5+ gallons, where API and ISO standards specify 3-gal and 5-gal, depending upon the shaft diameter.

For non-API specifications, smaller reservoirs - typically 2-gal - are often used, especially at ambient pumping temperatures. Pressure alarms, pressure gages and level switches are typically standard equipment and are required by API 682/ISO 21049.

The usual guideline for Plan 53 barrier pressures is that they be a minimum of 20-psi to 50-psi above the maximum process pressure seen by the seal. Barrier pressure is normally supplied by a plant wide distribution system. Nitrogen bottles should not be used as they require a lot of attention and maintenance.

API 682/ISO 21049 recommends that the system be limited to 150-psig due to gas entrainment into the barrier fluid. Field experience has shown that with the proper barrier fluid, Plan 53A systems can be used up to 300-psig if the temperature is controlled to less than 250-deg F. A variation to this would be to use an accumulator to eliminate gas entrainment.

The volume of barrier fluid is dependent upon the size of the reservoir. Larger flow rates should use larger reservoir sizes so that retention time in the reservoir is maximized for longer fluid life.

Disadvantages (vs. other Plan 53 systems)The barrier fluid in Plan 53A is subject to gas entrainment due to direct exposure to the pressurizing gas. Different barrier fluids have varying levels of gas entrainment.

Installation should be limited to a single seal installation even on between bearing pumps. Therefore for a large number of installations, Plan 53A can be more expensive than Plan 53B or 53C.

nlike a Plan 53A that incorporates a pressurized reservoir within the circulation loop, Plan 53B incorporates a bladder type accumulator along with the piping and an air or water cooled heat exchanger to provide for barrier fluid capacity.

Some installations use finned tubing as the heat exchanger, but these should be used with caution as the heat removal depends upon a positive air flow across the tubing to be effective. Gas entrainment is not a problem with this plan since it incorporates bladder accumulator to maintain the barrier pressure within the closed loop circuit.

The accumulator should be pre-pressurized to between 80 percent and 90 percent of the barrier pressure. This creates a problem in that it limits the volume of fluid within the Plan 53B circuit. The majority of the accumulator volume is gas. The basic setup is comprised of two parts; the closed loop circulating system made up of the piping and heat exchanger and the make up system.

Flow in the circulating system is usually induced by an internal pumping device. The make up system can be configured a number of ways based upon the customer"s preference, ranging from a simple hand pump to an elaborate pumping system feeding multiple pumps/seals.

Like Plan 53A, the flow rate of the Plan 53B circuit is controlled by the pumping ring design, peripheral speed, barrier fluid viscosity, and resistance of the piping circuit; the piping circuit of 53B includes a heat exchanger. The sizing of the heat exchanger depends upon the heat load of the system. The heat exchanger should be designed to contribute minimum resistance.

API 682, 3rd edition does not provide guidelines for sizing the accumulator of Plan 53B, but the total fluid volume of the system should be about the same as the volume of a 53A system.

Disadvantages (vs. other Plan 53 systems)The volume of fluid within the closed loop circuit is very limited, as little as one-half gallon in some instances.

With the limited fluid volume the barrier fluid gets thermally cycled on a much more frequent basis than a Plan 53A, so the service life of the fluid is reduced.

The finite volume of the accumulator requires a designed pressure operating range between refills (in excess of that required for a Plan 53A) and this must be built into the pressure rating of the seals.

The separate heat exchanger introduces additional flow resistance to the piping system and will have a lower flow rate than an otherwise identical Plan 53A.

Plan 53C is a variation of Plan 53B that uses a piston accumulator to track the pressure of the seal chamber. In Plan 53C, the piston accumulator has a reference line from the seal chamber to the bottom of the accumulator. There are differences in diameter of the internal piston so that a higher pressure is generated on the top half, which in turn is piped to the circuit loop into and out of the seal chamber.

Similar to Plan 53B, there is no gas pressurizing the barrier fluid so there is no chance of gas entrainment. Also, like Plan 53B flow is generated by a pumping ring through a heat exchanger. The heat exchanger can be water cooled, air cooled or can be finned tubing if the heat load is small enough. This system should be used with caution, as the reference line to the accumulator is subject to the process fluid. The process fluid may be corrosive, abrasive, or a slurry that could potentially clog the pressure reference line threatening the tracking ability of the system.

The advantages and disadvantages are the same as the Plan 53B system. Additionally, the disadvantage of this system is that pressure spikes or pressure drops in the process pressure will vary the pressure on the outer seal that may create a temporary leakage condition. Also, tracking pressures can always be subject to delays that can cause a temporary loss of positive pressure differential across the inboard seal.

PHOTO : https://www.eagleburgmann.com/media/literature-competences-products-solutions/division-mechanical-seals/competences/brochure-barrier-buffer-media-for-mechanical-seals

Double mechanical cartridge seals have a barrier fluid between both pair of seal faces. One pair is located at the product side, the other pair is located on the atmospherically side. In-between is the streamed barrier fluid to cool the seal faces and to ensure that no contamination of the environment will appear. Several plans and methods are used in industry to seal different fluids, while realizing a long run time. These plans are advised by the American petroleum institute, called API. Most plans reconsider extra systems which are built and manufactured by STB.

try to play around with the hydrostatic pressure. slowly reduce the pressure by adjusting the nitrogen purge pressure and at the same time consistently try to hand rotate the pump. if the jamming is loosening as you gradually reduce the pressure, then it is indeed the jamming is due to this 5 bar. Hence this 5 bar is literally pushing the rotating face onto statinonary face until you are not able to rotate it manually. good that you try to rotate it before starting it. If you start the pump with this 5 bar, you could have end up with significant wear on the seal faces.

i do not know whether this behavior is anticipated or not. as long as the seal pot level is not going down at stop, i know the seal is fit for operation. if it require reduction (not total release) in the hydrostatic pressure prior starting up, so be it. you can always increase the pressure if the pot level is increasing after that. if it is holding, you can decide to maintain the reduced pressure