1. Introduction

     Mechanical seal is device to seal machines between rotating part (shaft), and stationary part (pump housing) and for sealing purpose a film is formed between the rotating portion and the stationary portion of mechanical seal

 The absence of this fluid film shall be avoided, since it will result in frictional heat and the destruction of the mechanical seal. This condition occurs, for example, on dry run of the pump.

Based on API 682, all mechanical seal shall be cartridge design.

 Codification/specification of seal based on API 682

 C2 A1 C 11 62

 C2: Category 2

A1: Arrangement 1

C  : Type C

11: Flush Plan 11

62: Quench Plan 62

Seal category, type, and arrangement based on API 682

 Seal categories:

1) Category 1 seals: ANSI/ASME B73.1, ANSI/ASME B73.2 and ISO 3069 Type C seal chamber (non-ISO 13709 seal chambers). Seal chamber temperatures from – 40 deg C to 260 deg C and absolute pressures up to 22 bar.

2) Category 2 seals: ISO 13709, API 610 seal chamber. Seal chamber temperatures from – 40 deg C to 400 deg C and absolute pressures up to 42 bar.

3) Category 3: ISO 13709, API 610 seal chamber. Seal chamber temperatures from – 40 deg C to 400 deg C and absolute pressures up to 42 bar. More detail test and documentation refer to API 682. 

 Operating temperatures and pressures beyond the API 682 standard may require “engineered seal”, designed for specific requirement

 Seal types:

1) Type A seal: rotating flexible element, balanced, inside-mounted, cartridge design, pusher seal, multiple springs, temperature up to 176 deg C (stationary optional),

2) Type B seal: rotating flexible element, balanced, inside-mounted, cartridge design, non-pusher (bellows) seal, temperature up to 176 deg C (stationary optional),

3) Type C seal: stationary flexible element, balanced, inside-mounted, cartridge design, non-pusher (metal bellows) seal, temperature up to 400 deg C (rotary optional)

 Seal arrangements:

1) Arrangement 1: Single Seal

2) Arrangement 2: Dual un-pressurized seal (use buffer fluid)

3) Arrangement 3: Dual pressurized seal (use barrier fluid)

 Seal orientation (Arrangement 2 and Arrangement 3):

 1) Face-to-back (tandem): one mating ring is between the two flexible elements and one flexible element is between the two mating seal rings

2) Back-to-back: both of the flexible elements are between the mating seal rings

3) Face-to-face: both of the mating seal rings are between the flexible elements.

 Another definition of sealing methods:

1) Contacting wet seals CW: mating faces are not designed to intentionally create aerodynamic or hydrodynamic forces to sustain a specific separation gap

2) Non-contacting seals (whether wet or dry) NC: mating faces are designed to intentionally create aerodynamic or hydrodynamic separating forces to sustain a specific separation gap

3) Containment seals (whether contacting or non-contacting) CS: Seal design with one flexible element, seal ring and mating ring mounted in the containment seal chamber.

 On dual pressurized seal, the lubricating film is generated by the barrier fluid which should be 1.5 up to 2.0 bar above the product pressure in the seal area. In case of a leakage, the barrier liquid penetrates the product.

 Dual mechanical seal are usually used for application as follows:

• If a fluid product leakage needs to be avoided,

• When aggressive media are used or at high pressures and temperatures,

• For many polymerizing, sticky media and media which tend to sedimentation ion,

• For vacuum applications.

 

Figure 1. Mechanical seal cross-sectional view

Figure 2. Mechanical seal (cartridge type)

Figure 4. Pusher type and non-pusher type

Figure 5. Below type

2. Mechanical seal code based on API 610, 8 edition

On the latest edition, this seal code does not specify anymore in API 610 and seal arrangement refers to API 682.

But some vendor and end-user sometimes still use this designation.

1st letter

Balanced (B), Unbalanced (U)

2nd letter

Single (S), dual un-pressurized (T), Dual Pressurized (D)

3rd letter
T – Seal gland type (P = plain, no throttle bushing; T = throttle bushing with quench, leakage and/or drain connections; A = auxiliary sealing device, type to be specified)

4th letter

Table H-4 Fourth Letter

Letter Stationary Seal Ring Gasket Seal Ring to Sleeve Gasket
E FKM FKM
F FKM FKM
G PTFE PTFE
H Nitrile Nitrile
I FFKM FFKM elastomer
R Graphite Foil Graphite Foil
X As specified As specified
Z Spiral wound Graphite foil

 5th Letter
Table H-5 Fifth Letter

Letter Sealing Ring Face Material
Ring 1 Ring 2
L Carbon Tungsten carbide 1
M Carbon Tungsten carbide 2
N Carbon Silicon carbide
O Tungsten carbide 2  Silicon carbide
P Silicon carbide       Silicon carbide
X As specified          As specified         

 

3. Seal Selection

Mechanical seal selection shall consider the following item:

  • Liquid: the metal parts must be corrosion resistant, usually steel, bronze, stainless steel, or Hastelloy. The mating faces must also resist corrosion and wear. Carbon, ceramic, silicon carbide or tungsten carbide may be considered. Stationary sealing members of Buna, EPR, FKM (Viton) and PTFE (Teflon) are common. Material shall also be checked to find the performance against the fluid. For example FKM (Viton) is not recommended for fluid contain H2S
  • Pressure: balanced or unbalanced, is based on the pressure on the seal and on the seal size.
  • Temperature: materials must be selected to handle liquid temperature.
  • Characteristics of Liquid: abrasive liquids (like slurry) create excessive wear and short seal life. Flushing from an external source allows the use of mechanical seals. On light hydrocarbons balanced seals are often used for longer seal life even though pressures are low.
  • Reliability and Emission Concerns: seal type and arrangement selected must meet the desired reliability and emission standards where the pump will be applied. Dual seals (pressurized or un-pressurized) with buffer/barrier fluid could be a choice.

Detail selection is provided by API 682 as international standard for mechanical seal but purchaser shall also consult to seal vendor specialist to get proper seal selection.

4. Seal Flush Plan

Seal flush plan selection mainly based on operating condition of pump itself. The simple flush plan is plan 11 that circulate form discharge pump to mechanical seal. Plan 23 is usually used on boiler feed water pump with single seal. For toxic fluid that leakage to atmosphere is not allowed, double mechanical seal used, for example plan 11 + 52 (with buffer fluid) or 11 + 53A (with barrier fluid). Seal plan selection will be responsibility of purchaser and vendor to decide the proper seal.

Below is seal flush plan according to API 682:

Seal Flush Plan 01: Internal seal chamber flush from pump discharge

Seal Flush Plan 02: Dead-ended seal chamber with no flush

Seal Flush Plan 11: Seal flush from pump discharge through orifice

Seal Flush Plan 12: Recirculation from pump discharge through a strainer and flow control orifice to the seal. Not recommended as per API 682 explanation

Seal Flush Plan 13: Recirculation from seal chamber to pump suction through orifice. Standard flush plan on vertical pump.

Seal Flush Plan 14: Seal flush from pump discharge and recirculation to pump suction with orifice. Combination of plan 11 and plan 13.

 Seal Flush Plan 21: Seal flush plan from pump discharge through orifice and cooler.

Seal Flush Plan 22: Recirculation from pump discharge through a strainer, a flow control orifice, and a cooler and into to the seal chamber. Not recommended as per API 682 explanation

Seal Flush Plan 23: Seal flush plan from internal pumping device through cooler. Standard flush plan in hot water services

Seal Flush Plan 31: Seal flush from pump discharge through cyclone separator. Centrifuge solids are returned to pump suction.

Seal Flush Plan 32: Seal flush plan from external clean source

Seal Flush Plan 41: Seal flush from pump discharge through cyclone separator and cooler. Combination of plan 21 and 31.

Seal Flush Plan 51: External reservoir providing a dead-ended blanket for fluid to quench connection of the gland.

 Seal Flush Plan 52: Un-pressurized buffer fluid circulation through reservoir. Fluid is circulated

 Seal Flush Plan 53A: Pressurized barrier fluid circulation through reservoir. Fluid is circulated by pumping ring in the dual seal assembly.

 Seal Flush Plan 53B: Pressurized barrier fluid circulation with bladder accumulator. Fluid is circulated by pumping ring in the dual seal assembly.

 Seal Flush Plan 53C: Pressurized barrier fluid circulation with piston accumulator. Fluid is circulated by a pumping ring in the dual assembly.

 Seal Flush Plan 54: Pressurized barrier fluid circulation by external system.

 Seal Flush Plan 61: Tapped and plugged connections for the purchaser’s use. Typically this plan is used when the purchaser is to provide fluid (such as steam, gas, or water) to an external sealing device.

 Seal Flush Plan 62: External quench on atmospheric side of seal. Quench fluids typically steam, nitrogen, or water.

 Seal Flush Plan 71: Tapped connections for purchaser’s use. Typically this plan is used when the purchaser may use buffer gas in the future.

 Seal Flush Plan 72: Un-pressurized buffer gas control system. Containment seal support typically with nitrogen buffer gas.

 Seal Flush Plan 74: Pressurized barrier gas control system. Gas seal support typically with nitrogen barrier gas.

 Seal Flush Plan 75: Drain from containment seal cavity to liquid collector and vapor recovery

 Seal Flush Plan 76: Vent form containment seal cavity to vapor recovery

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