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Mechanical seals


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Brief overview about Mechanical seals

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Mechanical seals

  1. 1. Seals are devices used to prevent or eliminate leakage of fluids What is Seal?
  2. 2. Seal Classification Seal Static seals Gasket Sealant Dynamic seals Rotating shaft seal Face seal Axial seal Mechanical Seals Radial seal Lip ring seal Packing Circumferential split seal Interstitial seal Labyrinth seal Bush seal Reciprocating shaft seal Packings Piston ring
  3. 3. 1. Set of primary seal faces: one rotary and one stationary. 2. Set of secondary seals such as, O- ring and shaft packing. 3. Mechanical seal hardware including; gland ring, collar, compression ring, spring and bellows. The Basic Mechanical Seal
  4. 4. The primary seal is achieved by two very flat, lapped faces which create a difficult leakage path perpendicular to the shaft. Rubbing contact between these two flat mating surfaces minimizes leakage. As in all seals, one face is held stationary in housing and the other face is fixed to, and rotates with the shaft. One of the faces is usually a non-galling material such as carbon-graphite. The other is usually a relatively hard material like silicon- carbide. Dissimilar materials are usually used for the stationary insert and the rotating seal ring face in order to prevent adhesion of the two faces. Mechanical Seal Mechanism (How it’s work?)
  5. 5. 1. Between stationary element and casing. 2. Between rotating element and shaft. 3. Between mating surfaces of stationary and rotating seal faces. Main Sealing Points
  6. 6. Classified by arrangement Mechanical seal Single Inside Outside Double Dual pressurized Dual Pressurized Gas (Non-contacting) Tandem (unpressurized) Cartridge Classified by design Mechanical Seal Classification Mechanical seal Balanced Unbalanced
  7. 7. Single Mechanical Seal Sin gle I n s id e Sin gle Ou ts id e Description It is a mechanical seal with one set of primary seal (one dynamic seal face). They called inside seals because the seal parts are located inside the seal housing. It is the most common type of mechanical seal for pumps. It is a mechanical seal with one set of primary seal (one dynamic seal face). They called outside seals because the entire rotation portion of the seal is outside the seal chamber. Advantage • Can be easily modified to accommodate seal flush plan. • Can be balanced to withstand high seal environment pressures. • Hardware items of the seal don’t come in contact with liquid. Disadvantage • Hardware items of the seal come in contact with liquid which may corrosive. • Pressure of sealed liquid pushes the two seal faces apart. • Limited to sealing pressure 25 to 30 psig unbalanced and 60 psig balanced. • Exposed to dust and other environmental contaminants. Photo/Drawing
  8. 8. Double Mechanical Seal D u a l P r e s s u r i ze d D u a l P r e s s u r i ze d G a s ( N o n - c o n t a c t i n g ) Ta n d e m ( D u a l u n p re s s u r i ze d ) Description Dual mechanical seal is a combination of two seals mounted back-to-back with a non-hazardous barrier liquid injected between the two seals at pressure higher than pumped liquid pressure. Dual pressurized gas mechanical seal is a combination of two seals mounted back-to-back with a barrier gas (normally N2) injected between the two seals at pressure 25 to 30 psi higher than pumped liquid pressure. When shaft begins to rotate, pressure is built up within spiral grooves that machined in mating rings which separates the seal faces. This effect result in non-contacting faces at both dynamic seal faces. Tandem mechanical seal is a combination of two seals mounted back-to-front with a non-hazardous buffer liquid injected between the two seals at pressure less than pumped liquid pressure. Applic ation Corrosive liquids, toxic liquids and liquids containing abrasive solids. Abrasive, flashing, flammable and toxic liquids. Corrosive liquids, toxic liquids and liquids containing abrasive solids. Feature Pumped liquid can’t leak into the environment. Provide reduction in power consumed in seal (Less mechanical losses). If primary seal fails, end user can plan to repair/replace seal in advance without any product escaping to the environment if secondary seal still alive. Photo/Drawing
  9. 9. Cartridge Mechanical Seal Cartrid ge Mech an ical Seal Description Mechanical seal Pre-mounted on a sleeve and fit directly to shaft or shaft sleeve ( available single, double and tandem) . Advantages • No requirement for seal setting measurement for installation. • Eliminate seal setting errors. • Low maintenance cost. Photo/Drawing
  10. 10. Balanced & Unbalanced Mechanical Seal B alan ced Un b alan ced BalanceRatio The balance ratio of a mechanical seal is an area ratio and is related to the seal face load. Balance ratio is defined as the ratio of the closing area to the opening area. Balance Ratio < 1 Balance Ratio > 1 Advantages • Less hydraulic force acting on seal faces. • Generate less heat. • Higher pressure limit. • Can handle liquids with poor lubricity and high vapor pressure. • Inexpensive. • Less leak. • Stable when subjected to vibration, misalignment and cavitation. Disadvanta ges • Relatively higher cost. • Very minor leak is expected. • Higher Hydraulic force acting on seal faces. • Low pressure limit. Photo/Drawing
  11. 11. Piping Plan API Mechanical Seal Piping Plan Why flushing plan is required? 1. To lubricate and cool mechanical seal. 2. To remove foreign particles. 3. To remove carbon deposition and engrossed materials on mating surfaces. 4. Prevent seal from dry running. 5. To maintain the operational parameters of mechanical seal. Mechanical Seal Type API Seal Plan Single seal 01, 02, 11, 13, 14, 21, 23, 31, 32 and 41 Double seal 52, 53A, 53B, 53C and 54 Double seal (Gas Barrier) 72, 74, 75 and 76
  12. 12. Buffer and Barrier Fluid Buffer Fluid Barrier Fluid Characteristics of Buffer/Barrier Fluid • Safe to use, handle and store. • Compatible with seal materials. • Not a volatile organic compound, volatile hazardous air pollutant or other regulated compound. • Good flow qualities at operational temperatures (including very low temperature service). • Nonflammable. • A stable liquid at ambient temperatures. • Good lubricity. • Non-foaming when pressurized. • Good heat transfer properties. • Low gas solubility. • Compatible with process fluid. • Inexpensive.
  13. 13. How to Install Mechanical Seal?
  14. 14. • Zero or minimal leakage. • Elimination of shaft and sleeve wear. • Low maintenance cost. • Ability to seal higher pressure and more corrosive liquids. • Lower mechanical losses compared to conventional packing. Advantages of Mechanical Seal Over Conventional Packing
  15. 15. • Liquid type. • Pressure. • Temperature. • Characteristics of liquid. • Reliability and emission concern. Mechanical Seal Selection Criteria
  16. 16. Mechanical properties 1. High modulus of elasticity. 2. High tensile strength. 3. Low coefficient of friction. 4. Excellent wear resistance. Chemical properties 1. Corrosion resistance. Mechanical Seal Material Properties Thermal properties 1. Low coefficient of expansion. 2. High thermal conductivity. 3. Resistance to thermal shock. 4. Thermal stability.
  17. 17. Mating faces 1. Carbon-Graphite. 2. Tungsten Carbide(TC). 3. Silicon Carbide (SiC). 4. Ceramic. Elastomers 1. Teflon. 2. Viton. 3. Buna N. 4. Silicone rubber. 5. Neoprene. 6. Butyl. Mechanical Seal Material Metals Parts 1. Stainless steel. 2. Bronze. 3. Hastelloy (super alloys).
  18. 18. Operational error 1. Dry running. 2. Suction chocking. 3. Foreign material. 4. Material incompatibility. 5. Abnormal process parameters. 6. Flushing plan off. Maintenance error 1. Improper installation. 2. Shaft misalignment. 3. Shaft run out. 4. Failed bearing. 5. Unavailability of required flushing plan. How Mechanical Seal Can Fail ?