MECHANICAL SEALS : INTRODUCTION | | What is a Mechanical Seal?

INTRODUCTION:

          In many industries 'Mechanical shaft seals' failures are the major cause of pump's downtime. So we decided to give some information towards mechanical seal basics.

          "Mechanical sealing is a process of preventing leakage and containing fluid within the vessel" (like pumps, Agitators) where the rotating part of the shaft passes through the stationary part of the pump housing or some cases, the housing rotates around the shaft.  

          Years ago, pump shafts are sealed by using Gland packing rope. Gland packing is braided material, rope like material. This type of seal required a skilled worker and these require a small amount of leakage just to lubricate the packing and keep it cool. without this arrangement the packing gland becomes hard. While the shaft rotates friction produces between rope and shaft this causes wear of packing. Packing need to contact the shaft it will wear the shaft too. 

          After facing these kind of problems "Mechanical Seal" development came, which fulfills the task of controlling the leakage produced around the pump shaft with two very flat surfaces (one rotating and one stationary or constant). These mechanical seal faces also require some (very small) leaks across the faces, to form a hydrodynamic film, this leakage is usually evaporating and not identifiable. This fluid comes from processed fluid or from an external source. To achieve a precise gap for this fluid film is a design challenge.

          Now a days most of the pump shafts are sealed by using mechanical seals. However, due to the sensitive parts used in this new sealing system, mechanical seal failure are the biggest cause of pump down time.

Types of mechanical seals

TYPES OF MECHANICAL SEALS

          Mechanical Seals are the devices which arrests leakage of fluids when joined the parts or mechanisms together. Mechanical seals are available in different designs. We have to understand first how they function, it will help us to select the suitable for our application.

Different types of mechanical seals are mentioned below they are:

1. Conventional

2. Pusher

3. Non-Pusher

4. Blanced

5. Unbalanced

6. Cartridge 

7. Cortage

1. Pusher type:

GEAR PUMPS TYPES AND ITS WORKING

          The most common pumping system used in the process of chemicals having high viscocity is the Gear pump. The gear pump was invented around 1600 by Johannes Kepler.
          The Gear Pump is a "Positive displacement pump", which helps us to move fluid with the help of inbuilt gears. This type of pump has two (or) more gears that create vacuum force to drive the fluid within the pump. The pump is built with various components such as shaft, rotors, and casing.   

Design:

          A basic gear pump consists of a rotary housing or casing or stator containing two or more inter-meshing gears (Helical and Herringbone gears) or lobed cams. Tight tolerances are required between the casing & gears and between gears (The fluid being pumped will lubricate this small clearance and help prevent friction and therefore wear of the rotors and casing). Atypical housing will have an inlet and outlet, for suction and discharge respectively. There are two main types of gear pumps: 1) External gear pumps (Exterior bearing type) which use two external gears (Figure 1) and 2) Internal gear pumps (Internal bearing type) which use internal and external gears (Figure 2). The term positive displacement for gear pumps describes, they compel or force a fixed amount of fluid they move for each revolution.

Working:

Ø  In this type of pump, only one of the rotor is driven. The inter-meshing gears rotate the other rotor. As the rotors rotate, the liquid or gas, (this type of mechanism can also be used in compressor), enters from the suction line and fills the space between the teeth of the gears and becomes trapped forming small 'Slugs' of fluid between teeth.

Ø  The slugs are then carried round by the rotation of the teeth to the discharge side of the pump.

Ø At this point, the gears mesh together and, as they do so, the fluid is displaced from each cavity by the inter-meshing teeth.

Ø  Since the fluid cannot pass the points of near contact of the inter-meshed teeth nor between the teeth and casing, it can only pass into the discharge line via the putlet.

Ø  As the rotation continues, the teeth at the suction end are opened up again and the same amount of the fluid will fill the spaces and he process repeated. The liquid at the discharge end is constantly being displaced.

General viscosity range : 2 to 4,00,000 cst (EPW, 2012)

Pump formulas:

· Flow rate in US gal/min = Pump Capacity X rpm

· Power in hp = US gal/min X (lbf/in³)/1714

Types of Gear Pumps:

Based on design these pumps are basically classified into two types they are:
     . External Gear Pump
     . Internal Gear Pump

1) External Gear Pump:

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          In external gear pump two identical external gears (Figure 1) that displace non-lubricating fluids (gears are oil lubricated). The mechanism is usually driven by one of the toothed gears, the intermeshing which inturn dives the other (Driven or Idle gear). The both gears are supported by bush bearing or ball bearing it is based on the application of load. The drive gear driven by motor directly through coupling. Drive shaft leak is controlled by gland rope or by mechanical seal. There are three factors in the regulation of flow; Volume of cavity between the teeth, gears speed, and the amount of fluid that slips back to the inlet (tolerance dependent) via the mechanism. There are three main types of external gears: Spur, Helical and Herringbone gears. Helical and Herringbone gears deliver more flow at higher pressure while also being quieter, but many require a great inlet pressure than spur gear.

2) Internal Gear Pump:

          An internal gear pump uses internal and external gears (Figure 2). The gears themselves are lubricated by the fluid, which is of a lubricating nature. The internal design is reliable, easy to operate and maintain- due to the presence of only two moving parts. Only one drive gear is required for the mechanism to function but it is possible to use two. The pump will usually contain at least one bushing. The design can also be modified to include a cresent shaped portion that improves performance when pumping high viscosity fluids. Internal gear pumps have relatively low speed and inlet pressure requirements. 

Considerations:

          Gear pumps are generally operated at high speed and thus give a fairly pulse-free discharge flow and pressure. Where these pumps are operated at slower speeds, as in pumping viscous liquids, the output tends to pulse due to the meshing of the teeth.

          Any air gap drawn into the pump with the liquid, will be carries through with the liquid and will not cause cavitation. This action of the pump means that it is a 'self priming' pump. The discharge pressure may however, fluctuate.

The output from this type of pump is directly proportional to the speed of operation. If the speed is doubled, the output will be doubled and the pressure will have very little effect. (Due to higher pressures, due to the fine clearences between the teeth and between the casing and the rotors, a small leakage back to the suction side will occur resulting in a very small drop in actual flow rate. The higher the discharge pressure, the more likely that internal leakage will occur).

          Rotory pumps are widely used for viscous liquids and are self-lubricating by the fluid being pumped.

          This means that the external source of lubrication can't be used as it would contaminate the fluid being pumped. Gear pumps are capable of moving small suspended solids but due to the meshing of gears they can be damaged by pumping large solids. However, if a rotory pump is used for dirty liquids or slurries, solid particles may act as abrasives, can get between the small clearences and cause wear of the teeth and casing. This will result in loss of efficiency and expensive repair or replacement of the pump.

          In terms of construction materials, gear pumps can be made form a wide variety of materials, ranging from bronze, iron and stainless steel to cast iron, depending on the application and fluid properties.

OTHER TYPES OF ROTARY PUMPS

The following types of pumps have same characteristics to the gear pump, they are 

a) Lobe pump

b) Sliding vane pump

c) screw pump

Advantages of Rotary Pumps

· They can deliver liquid to high pressures.

· Self - priming.

· Relatively smooth output, (especially at high speed).

· Positive Acting.

· They can pump viscous liquids.

Disadvantages of Rotary Pumps

· Compared to centrifugal pumps these are more expensive.

· Should not be used for fluids containing suspended solids.

· Wear is more if not pumping viscous material.

· Must NEVER be used with the discharge closed.

Areas:

Marine, Chemical, Petrochemical, Food and General industries.

Applications:

Transfer, lubrication, processing and hydraulic.

Fluids/Materials:

High viscosity fluids, fuel oils, lube oils, various chemicals, resins, paints, pulp, acids, etc.,

MONOBLOCK PUMP TYPES AND APPLICATIONS

          A Monoblock Pump is a mechanical device. We all know that 'Mono' means single. The prime mover of the pump (i.e., Motor) and pump are built together in same housing, that type of pumps is called as "Monoblock Pumps".

          In which a common shaft contains all the rotating parts. Because of this, during the transmission from motor to shaft, loss of energy is minimal. Motor assembly and pump assembly is the main components of monoblock pumps. The device's impellor and volute chamber are designed in a manner that it offers best hydraulic efficiency and characteristic related to suction lift. Most advanced machinery, dynamic balanced impellor, seals and ball bearings are used for the manufacturing process. Due to the dynamically balanced rotor, there is less noise and vibration. The shaft is made up of high-quality steel.

Types of Monoblock Pumps:

          Different types of monoblock pumps are available in the market, mainly include

              1) Centrifugal Monoblock pumps

              2) Submersible pumps

              3) Booster pumps 

Features/Advantages: –

a) It's unique design that helps in the prevention of overloading and the burning of the motor even during prolonged use.

b) They are simple and compact. They need less space for installation.

c) They are cost effective.

d) Weared parts can be easily replaceable.

e) These pumps bears fatter efficiency curve which helps in efficient functioning.

f) With dynamically balanced rotating parts, there is less noise and vibration while working.

g) The pump has less complexity in installation and maintenance is also easy.

h) They are self supporting, these pumps do not need additional base plates and coupling for installation.

i) Automatic release of air while priming.

j) These pumps has a sturdy design as they can endure three phase voltage fluctuations.  

PROBLEMS ON AVERAGE - 1

Examples 1: what will be the average of 11, 12, 13, 14, 15, 16, 17 series?

Solution: The common difference is same i.e., 1, so the series is in A.P. Average is the middle term when the number of terms is odd, So the middle term is 14 which is our average of the series.

Example 2: What will be the average of 11, 12, 13, 14, 15, 16, 17, 18?

Solution: Since the common difference is same for each of the term we can say that the series is in A.P. So we have discussed that when the number of terms are even then the average will be the average of two middle terms.

Now the two middle terms are 14 and 15, and the average is (14+15)/2 = 14.5

Example 3: The average of five numbers is 29. If one number is excluded the average becomes 27. What is the excluded number ?

Solution: let the excluded number is

= (29 x 5) – ( 27 x 4 )

= 145 – 108

= 37 .

Example 4: Find the average of first 20 natural numbers?

Solution: 

Sum of first n natural numbers = n ( n + 1 ) /2

So, we can find easily average of first 20 natural numbers 20 x 21 / 2 = 210 

So, then Required average is = 210 / 20 = 10.5.

Example 5 : Find the average of first 20 multiplies of 5 .

Solution: 

Required average = 5 ( 1 + 2 + 3 +……………….. + 20) /20

= ( 5 x 20 x 21 / 20 x 2) = 2100 / 40 = 52.5 .

So the Required average is 52.5.