MECHANICAL SEALS FOR CENTRIFUGAL & ROTARY PUMPS (AS PER API 682)

We know that rotary/centrifugal pumps are used to develop head and discharge liquid at a specified flow rate. The medium to be pumped will be enclosed in a casing. The rotor (impeller, vanes, screw, lobes etc.) will impart the required dynamic motion to the liquid.

The liquid inside casing should be contained in order to prevent leakages, which might reduce the efficiency, cause environmental problems etc. The gaps between two static parts can be sealed by using gaskets of appropriate size. But the gap between shaft and casing cannot be sealed in this way. For this purpose, we use mechanical seals.

Here we are discussing the types of mechanical seals as per API 682 and some common terms related to seals. API 682 is a standard developed for the mechanical seals used in centrifugal/rotary pumps.

TERMS

1. Barrier Fluid: It is a pressurized fluid used in dual mechanical seals or double seals to isolate the process fluid from the environment. It also acts as a cooling and lubrication medium for seal faces. The pressure of barrier fluid is higher than that of the process fluid.
2. Buffer Fluid: It is a fluid used in the dual mechanical seal which is supplied at a pressure lower than the process fluid pressure. The main purpose is for lubrication.
3. Flush: This is the fluid introduced at process fluid side for cooling and lubrication of seal faces. This is found in single seals. It is generally provided very near to the seal faces.
4. Quench: It is a neutral fluid introduced at the atmospheric side of the seal, used to prevent the solid formation that might damage the seal faces. Generally, water or steam is used for this process.
5. Throat bushing: It is a restrictive close clearance provided around the sleeve/shaft between the seal and impeller. A Teflon or graphite bush provided on the back plate serves this purpose.
6. Throttle bushing: It is a restrictive close clearance provided around the sleeve/shaft at the outboard/atmospheric end of mechanical seal gland. A Teflon or graphite bush provided on the back plate serves this purpose.
7. Gland: It is the seal housing with threaded holes to provide a quench/flush connection.
8. Stationary face: The face of the seal which does not rotate along with the shaft. These are generally mounted on the gland and/or backplate, depending on the type of seals. Cartridge type seals come with an integral assembly of both stationary and rotary faces.
9. Rotary face: The face of a seal which rotates along with the shaft. These are mounted on shafts.
10. Pusher type seal: The secondary the seal is mechanically pushed along the shaft or sleeve to compensate for the face wear.
11. Non-pusher type seal: The secondary seal is fixed to the shaft.
12. Internal circulating device/ Pumping ring: It is located in seal chamber, mounted on the shaft, used to circulate barrier fluid through cooler or buffer fluid reservoir.

Required Minimum Flush Flow rate and Pressure in Seal

Minimum required seal chamber pressure for inner unpressurized dual seals and single seals is 3.5 bar (50 psi) or 10% above maximum fluid vapour pressure at seal chamber fluid temperature.

Minimum seal flush flow rate is 8 L/min (2 gpm) per seal.

API 682 coverage based on temperature and pressure range

Temperature range: -40⁰C to 260⁰C (-40⁰F to 500⁰F)

Pressure range: 0 bar to 34.5 bar (0 psia to 515 psia)

PARTS OF MECHANICAL SEAL

• Seal faces: As explained above there will be rotary and stationary faces. Ideally, the faces will not come in to contact with each other, as the process fluid or barrier/buffer fluid acts as a lubricant to separate them. The material of construction will be Teflon, graphite/carbon, Silicon Carbide (SiC), Tungsten Carbide(TiC). These are used in different combinations based on the service.
• The seal face mating surfaces are lapped to a high degree of precision to maintain the flatness.
• Retainers: These are metallic parts used to hold the rotary seal faces. These are provided with ant-rotation slots/pins to prevent the seal faces from rotating.
• Fluoroelastomer/ perfluoroelastomer: These are the O-ring materials used for different service temperature. These are provided to prevent leakage through shaft/casing and seal face joints. It is like a gasket for the seal faces. Fluoroelastomers are used at normal operating temperature. Viton material belongs to this category. Perfluoro elastomers are used in high temperature and chemical services. Kalrez and Chemraz materials belong under this category.
• Springs: These are found in pusher type seals used to mechanically hold the seal faces in position and maintain the uniform gap between seal faces irrespective of wear and minor pressure fluctuations.

SEAL PLANS

Single Seals

Plan 01

Plan 02

Plan 03

Plan 11

Plan 12

Plan 13

Plan 14

Plan 21

Plan 22

Plan 23

Plan 31

Plan 32

Plan 41

Plan 51

Dual/Double Seals

Plan 52

Plan 53A

Plan 53B

Plan 53C

Plan 54

Plan 55

Quench Seals

Plan 61

Plan 62

Single Seals with Leakage collection/detection

Plan 65A

Plan 65B

Plan 66A

Plan 66B

Secondary Containment Seals

Plan 71

Plan 72

Plan 75

Plan 76

Dual Gas Seals

Plan 74

Engineered Piping Plan not defined by any of the above mentioned

Plan 99