expert engg - plan 53 c · api 682 discourages this plan above 150 psi barrier fluid pressure....

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Manufacturer & ExporterMechanical Seals and other Engineering Products

Expert Engineering PLAN 53 C We do not want that a toxic, volatile or flammable liquid should leak to atmosphere. In such case normally double seals are recommended for pumps. API (American Petroleum Institute) has recommended various plans (API Plan 52, 53A, 53B and 53C) in standard API 682 for double seals (In earlier editions only API Plan 52 and 53 were there. Now API Plan 53 B and 53C have been added and old plan 53 is renamed as API Plan 53A.) All three API Plan 53 versions are intended to isolate the pumped product from the atmosphere and create a favourable environment for the mechanical seal. All the above plans are explained in short below. Plan 52 The buffer liquid is provided in a reservoir for the outer seal of a double seal. During normal operation, an internal pumping ring maintains circulation. The reservoir is usually continuously vented to a vapour recovery system and is maintained at a pressure less than the pressure in the seal chamber. The system is not good for volatile fluids. It allows vapour to go to the flare. Actual loss of product is not known. For vapour pressure more than 5kg/cm2 (abs) , API Plan 52 is not recommended

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Plan 53 A Plan 53A is the simplest plan of the three; it has no moving parts and is easy to operate. The barrier fluid is maintained in a reservoir. The barrier fluid is pressurised by an external source (nitrogen cylinder or a regulated supply of nitrogen). API 682 discourages this plan above 150 psi barrier fluid pressure. Normally a pumping ring is provided in between the two seals. It circulates the barrier fluid through the loop. In the reservoir normally a cooling coil is provided which removes the heat from the barrier fluid. The reservoir as well provides makeup fluid for normal seal leakage. Excessive seal leakage is detected by a change in fluid level in the reservoir The primary disadvantage of this plan is that there is an interface between the pressurisation gas and the barrier fluid. At higher pressures, this can lead to significant gas absorption into the barrier fluid resulting in poor seal performance. Other disadvantage is that the barrier fluid pressure is maintained higher than the maximum stuffing box press. Outboard seal is subjected to maximum pressure all the time (even when pump is not running). Also inboard seal faces variable differential pressure. Instrumentation requirement is more.

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Plan 53B In Plan53B, the pressurizing gas does not come into direct contact with the barrier fluid. Plan 53B is a dual pressurised system that eliminates direct gas contact with the barrier liquid. The Plan 53B uses a bladder accumulator, which acts as a barrier between the gas and the barrier fluid. The bladder is pressurized with gas prior to filling the system with barrier liquid. As the system is filled, the bladder is compressed thus provides a positive pressure on the barrier liquid. A pumping ring circulates the barrier fluid through a loop that includes a seal cooler and other instrumentation.

Excessive seal leakage is detected by drop in the pressure in the seal loop. The disadvantage is that the barrier fluid pressure is maintained higher than the maximum stuffing box press. Outboard seal faces maximum pressure all the time. Also inboard seal faces variable differential pressure. Only small volume of liquid is in circulation, therefore chances of thermal degradation of barrier liquid is more. Also the circulation of barrier fluid quantity is reduced (in comparison with API Plan 53A) due to additional cooler. Also instrumentation requirement is more. This plan is normally more expensive than Plan 53A.

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PLAN 53 C Plan 53C is a dual pressurized system that eliminates the use of gas for pressurization through the use of a piston accumulator. Barrier liquid is stored on the topside of the piston. The seal chamber is connected to the bottom of the piston accumulator. Piston design is such that slightly higher pressure (about 1 to 2 kg/cm2) is generated at the top of the accumulator. The top portion of accumulator is connected into the seal loop.. A pumping ring circulates the barrier fluid through a loop that includes a seal cooler and other instrumentation. The leakage is detected by change in level. Also the circulation of barrier fluid quantity is reduced (in comparison with API Plan 53A) due to additional cooler. This system is ideal for applications that have fluctuating pressures. Since the piston accumulator pressurizes the barrier fluid based on the stuffing box pressure, the barrier pressure automatically tracks actual operating conditions including system upset. In Plan 53 C barrier fluid pressure is maintained slightly higher than stuffing box pressure. Therefore the outboard seal becomes more comfortable and life expectancy in comparison with other plans becomes more. The inboard seal faces constant differential pressure so this also becomes comfortable in comparison with API Plan 53 A and Plan 53 B.

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The Plan is suitable for tandem seal where inboard seal is not reversed balanced ( Plan 53 A and Plan 53 B are not suitable as seal chamber pressure is maintained at

about 2 kg/cm2 more than the maximum stuffing box pressure. Plan 53 C is superior to Plan 53 A and Plan 53 B. In this plan no Nitrogen pressurisation is required. Also minimum instrumentation is required. Only low liquid alarm and differential pressure alarm is required. Disadvantages: There is a chance of sluggish movement of piston and in the cases of small leakage the response may be poor. Also only small volume of liquid is in circulation, therefore chances of thermal degradation of barrier liquid is more. This plan is comparatively more expensive than Plan 53 B.

For volatile liquids API Plan 53 C will be superior to other API Plans

Advantages of API PLAN 53C

For clear liquids, among all above plans API Plan 53 C is the best as it always provides just adequate barrier fluid pressure. Following are the advantages of API Plan 53C

1. Less instrumentation in comparison with other plans.

2. No external gas or bladder required.

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3. Barrier fluid pressure always is maintained nearer to actual stuffing box pressure.

4. Pressure alarm is not required.

5. ‘O’ Rings of any material suitable for process liquid can be provided.

6. For gas/ vapour sealing for top mounted agitator seals.

Expert Engineering Plan 53 C Expert Engineering has replaced piston with metal bellow. Operation Principle: The Metal Bellow Tracker provides a simple means of pressurizing double pressurised, mechanical seals. It automatically adjusts to maintain seal pressure during normal barrier fluid leakage. Therefore, barrier fluid pressure is constantly referenced to the sealed pressure and maintains differential pressure across inner primary seal face in the range of 15 to 30 PSI (1 to 2 Bar). This is regardless of any suction pressure changes and provides ideal operating conditions for the mechanical seal. The Metal Bellow Tracker achieves this without need of external nitrogen gas supplies, external pumps, power supplies, or bladder accumulators. Since no ‘O’ ring is being used for separating the process side fluid and barrier fluid, it eliminates the sluggish motion / poor response of piston. The bellow provides immediate response of the pressure change on process side pressure to the barrier fluid pressure.

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Bellow tracker gives advantages of all conventional Plan 53 A, Plan 53 B and Plan 53 C as shown in bellow summery table.

SUMMERY Comparison of Plan 52 and Plan 53 C Design Features Benefits API Plan 52 API Plan 53C

Barrier Fluid is at higher pressure than stuffing box pressure

Zero Fugitive emission Yes No

Primary Seal Interface fluid is barrier liquid ( Not the Peroduct)

Increased Mechanical seal Life 9specially with light Hydrocarbon Products)

No Yes

Barrier fluid remains uncontaminated by the product throughout its life

Increased Mechanical Seal Life

No Yes

Barrier fluid pressure automatically changes with varying stuffing box pressure

No No Yes

Need for the seal system connected to flare line or to vapour recovery system

Reduced installation and maintenance cost

Yes No

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Comparison of Plan 53A, Plan 53B, Plan 53 C and Expert EngineeringPlan 53 C Design Features API Plan

53A API Plan 53B

API Plan 53C

Expert EngineeringPlan 53C

Need to evaluate stuffing box pressure Yes Yes No No Barrier fluid pressure Maximum

stuffing box pressure + 2 kg/cm2

Maximum stuffing box pressure + 2 kg/cm2

stuffing box pressure + 1 - 2 kg/cm2

stuffing box pressure + 1 - 2 kg/cm2

Differential pressure between barrier fluid and stuffing box fixed

No No Partial Partial

Barrier fluid pressure automatically changes with varying stuffing box pressure

No No Yes Yes

Suitable for use with low suction pressure Yes Yes No Yes Nitrogen Gas required Yes Yes No No Nitrogen absorption into barrier fluid Yes No No No Instrumental requirement More More less less Suitability for low variation in stuffing box pressure

Yes Yes No Yes

Large amount of useable seal barrier fluid Yes No No Yes Thermal degradation of working fluid Slow Fast Fast Slow Magnetic level indicator No No Yes Yes Flow restriction of barrier liquid due to extra cooler

No Yes Yes Less

Power consumed by Mechanical Seals More More Less Less Can work when inboard seal is not reversed balanced

No No Yes Yes

Effect of Solar Radiation Yes Yes No No Sluggish Movement of Piston No No Yes No Considering all above points Expert Engineering has decided to standardise on Plan 53 C.

expert engg - plan 53 c · api 682 discourages this plan above 150 psi barrier fluid pressure....

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