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Why does vacuum in steam condenser reduce or drop??

  1-High exhaust temperature: Vacuum drops or maintains at lower side due to high exhaust steam temperature flow into steam condenser. This high exhaust temperature is mainly due to 1-Operation of Turbine at lower loads 2-More clearance in labyrinth seals 3-Not operating exhaust hood sprays 4-More load on condenser 5-Breaking of ejector U loop 2-Low circulating cooling water flow Vacuum in condenser reduces due to inadequate cooling water flow through steam condenser. This is mainly due to; 1-Problems associated with pumps 2-Air pockets in pipe line 3-Leakages in cooling water line 4-Stuck of discharge valve of pump 3-High cooling water temperature at condenser inlet Higher cooling water temperature at condenser inlet results into reduction of vacuum due to poor heat transfer from steam to water 4-Poor heat transfer in condenser Very less or poor heat transfer in steam condenser reduces vacuum to very low level resulting into high exhaust temperature &am

40 important question & answers on belt conveyor for power plant engineers & operators

1-What are the equipments used in coal handling plant (CHP)?
Starting from coal shed
  • Coal elevator
  • Coal grizzly hopper
  • Coal vibro feeder
  • Coal conveyors (belt conveyor)
  • Coal vibro screen
  • Coal crusher
  • Coal dust extraction system
  • Shuttle conveyor
2-Name the various parts of the belt conveyors?
  • Belt
  • Carrying idlers
  • Return idlers
  • Impact idlers
  • Transoms
  • Pulleys
  • Vertical gravity take up system
  • Power transmission system (Motor, Gear box, couplings)
  • Main structure & trestles
Tools & tackles used in power plant maintenance


3-How do you specify the conveyor belt?
Belt is specified based on
  • Its material of composition
  • Number of ply
  • Belt rating
  • Belt width

4-What are the various materials used for manufacturing of belts
Nylon, Polyester, Aramide, Polyester-Nylon, Steel fabric
5-What are the various pulleys used in belt conveyors?
  • Head pulley
  • Tail pulley
  • Snub pulley
  • Bend pulley
  • Take up pulley
6-Which is the largest and smallest pulley among above all pulley?
  • Largest pulley : Head pulley
  • Smallest pulley : Snub pulley
7-What are the different troughing angles used for transoms?
Depending on application and load carrying capacity, different troughing angles transoms are used namely
  • 180 degree: For flat belts and low loads
  • 45 degree: Higher capacity
  • 35 degree: Medium capacity
  • 20 degree: Low capacity
  • 10 and 15 degrees.
8-What are the different protection systems provided for belt conveyors?
  • Pull Cord Switch (PCS): Used for emergency stopping of the belt.
  • Belt Sway Switch (BSS): Protects belt from swaying or restricts over side movement
  • Zero Speed Switches (ZSS): This arrangement is fitted to tail pulley or snub pulley to protect the belt from further damage during belt cut. 

9-What  do you mean by the transoms in belt conveyors?
Transoms: Transoms are the brackets on which idlers are fitted to locate them properly.
Following types of transoms are noticed in general belt conveyor system.
  • Carrying idler transom
  • Return idler transom
  • Self aligning carrying transom
  • Self aligning return transom
  • Garland idler transom (refer below image)
10-What are the functions of carrying idler, return idler, impact idler & side guide idler
  • Carrying idlers: Fitted on carrying side of the belt (top side of the belt)
  • Return idler: Fitted on return side of the conveyor (bottom side). It is the largest idler among all the idlers used in conveyors
  • Impact idlers: It is same as carrying idler, it is fitted on carrying side of the conveyor below the discharge chute of preceding conveyor
  • Side guide idler: Fitted to the self aligning transoms to avoid the belt swaying at one side

10a-What are the various grades of belts, explain their applications
  • M Grade: Heavy duty resistant to abrasion, sharp and rugged materials. Work in normal atmosphere temperature. Generally used for coal, bagasse and other biomass fuel applications.
  • N Grade: Used for coal, bagasse and biomass, low resistant to abrasion, sharp and rugged materials.
  • B (G) Grade: Work in normal temperature used only for low duty.
  • HRS: Used for high temperature applications up to 100 °C surface temperature and 200 °C material temperature.
  • HRE Grade: Used for high temperature applications up to 150 °C surface temperature and 400 °C material temperature.
11-List down the various problems that you face in belt conveyors.
  • Belt running off at head pulley due to improper alignment of troughing idlers near head pulley.
  • Belt running off at tail pulley due to build up of material on return idlers.
  • Excessive wear on bottom side of belt.
  • Excessive wear on carrying and return side of belt.
  • Overstretch of belt due to excessive tension.
  • Belt slipping on tail pulley.
  • Pulley hub key way damage.
  • Belt moving on one side only.
  • Fuel leakage and spillage due to improper arrangement of scrapers and skirting.
  • Conveyor Drive Problem: Vibrations in gear box and motors, abnormal sound in gear box, coupling bolts loose.
12-What are the reasons for operating ZSS?
Zero speed switches operate on following conditions:
  • Belt cut
  • Belt slip on head pulley
13-What are the reasons for excessive wear out of belt on carrying side?
  • Excessive wear out on carrying side of belt is due to:
  • Dirt, frozen or misaligned return idlers
  • Excessive sag between troughing idlers causing load to move and shift on belt as it passes over idlers
  • Abrasive skirt boards
  • Poor loading
14-What is the reason for excessive wear on bottom side of belt?
  • Slippage Between Belt and Drive Pulley or Pulleys: Check the wear out of pulley rubber lagging.
  • Sticking Troughing Idlers: Check the operation of belt cleaners (scrapers).
  • Material Ground Between Pulley and Belt: Arrest fuel leakage and remove the collected fuel regularly.
15-What are the possible reasons for belt running off at head pulley?
  • Build up of material on return idlers.
  • Unequal loading.
  • Improper alignment of head pulley.
  • Not working of self aligning transums situated near head pulley.
16-What is the reason for a conveyor belt runs to one side for some distance along conveyor length?
It is due to Improper loading of belt and failure of self aligning transoms.
 17-What actions will you take if you found conveyor carrying side wearout occurs continuously?
Following actions should be taken to avoid wear out:
  • Install belt-cleaning plows at head end and tail end.
  • Clean, repair and align return idlers.
  • Increase belt tension if too low.
  • Reduce idler spacing.
  • Repair and inspect skirt board rubbers regularly.
  • Feed load on belt in same direction at same speed.
18-What actions will you take if belt has excessive tension?
If belt has excessive tension, then following actions shall be taken:
  • Decrease the belt tension by lagging on drive pulleys.
  • Increase arc of contact of drive pulley.
  • Increase speed, if possible, keeping shift tonnage the same.
  • Reduce tonnage at slower speed.
  • Tighten screw take-up just enough to keep belt from slipping.
  • On a gravity take-up reduce counter weight to minimum amount sufficient to keep belt from slipping.
19-What is the preferred speed of coal belt conveyors in power plants?
For medium capacity power plants 10 to 50 MW : 0.9 to 1 m/sec
For Higher thermal power plants = 0.9 to 1.5 m/sec
20-What is meant by transition distance?
Transition distance is traditionally defined as the distance from the center line of the first fully troughed idler roll to the centre line of either the head or tail pulley
21-Tell briefly about belt cleaners/scrapers.
Belt cleaning techniques are aimed at removing the greatest possible percentage of carryover at the source i.e. at the head/discharge pulley. Belt cleaners must not however cause damage to the belt in the course of operation.
Types of Belt Cleaners:
Contact Type:
Generally primary, secondary and tertiary belt cleaners are used.
Primary belt scraper is installed inside the head chute at a point below the natural material trajectory where the belt is in contact with the pulley. Primary scrapers are usually contact scrapers.
Secondary belt scraper is usually installed within the head chute however this scraper is positioned to contact the belt once it leaves the head pulley. Secondary scrapers are usually contact scrapers and are either blade-type scrapers or rotating brushes although the latter is not common.
Tertiary scrapers are sometimes installed although this is not common practice. In this case the scraper is outside the head chute and scrapings must be returned to the head chute by a mechanical chain conveyor or similar. Tertiary scrapers are usually contact scrapers.

Belt washing systems are also used in cases where the material carried over is extremely difficult to remove from the belt and/or where the disposal or handling of the washing water plus spillage can be done easily or conveniently. Washing systems are usually considered to be non-contact cleaners in as much as water is used to clean the belt and squeeze rollers provide a drying function.
 22-What is the function of Take up unit (TU)?
The role of the take-up device is to create and maintain adequate pre-tension to allow the conveyor drive pulley to drive the belt under all running conditions (empty or loaded).
23-What are the main two types of Take up unit (TU)?
1-Fixed type : screw take up unit (STU)
2-Vertical gravity take up (VGTU)
24-What is the percentage of belt tension adjustment in normal running condition?
It is 0.8 to 1.2% of total length
25-What is the minimum length of the take up system ?
Minimum length of take up is  the 1.5% of Conveyor length (Center to Center)  
26-A belt conveyor of length 100 meter need to install VGTU system calculates the minimum length of take up.
Minimum length of take up = 1.5% X 100 = 1.5 X 100/100 =1.5 meters
27-A conveyor length is 200 meters and speed is 1 meter/sec, then calculate the time taken for this conveyor for one complete rotation
Total appx.length of the belt = 2 X conveyor length + Conveyor length X 1.5%

                                              =2 X 200 + 200 X 1.5% = 403 meters


Time taken for one complete rotation of the belt 
= 403 m X 1 sec =403 seconds =403/60 = 6.71 minutes.

28-What is meant by transition distance?


Transition distance is defined as the distance from the centre line of the first fully troughed idler roll to the centre line of either the head or tail pulley.

29-Explain VGTU system in belt conveyors.


The role of the take-up device is to create and maintain adequate pre-tension to allow the conveyor drive pulley to drive the belt under all running conditions (empty or loaded). Generally there are two main types of take-up. The fixed type normally referred to as a screw take-up STU and the automatic or Gravity take-up (GTU). The screw take-up is normally used on short conveyors up to a length of 50 meters. Over 50 meter an automatic take-up system should be used to ensure constant pre-tension. The length of the take-up depends on the both the center distance of the conveyor and the type of belt. The screw take-up on shorter belts is normally located at the tail end. Tracking of the belt with the screw take-up should only be performed as a last resort as it may cause permanent belt stretch.

The GTU performs best located close to the drive pulley. The GTU bend pulleys will have a minimum diameter based on the belt specification. Movement of the GTU on both the vertical and horizontal axis should be limited to avoid excessive movement and associated tracking problems. The GTU should be protected with a return belt plough and must be adequately guarded.

30-What is the significance of counter weight?



The weight of the GTU should maintain a constant belt sag between the idlers (normally H/L 5 1–2% of length of take up) hence calculating the correct weight is important. A GTU underweight will lead to excessive belt sag thus increasing belt bend resistance and reducing overall belt and idler life. A GTU over weight will lead to excessive belt tension resulting in belt and joint stretch.
31-What is the significance of covers in belt?
Covers are used in conveyor belt constructions in order to protect the base conveyor belt carcass and to extend its service life. In addition, covers do provide the finished belt with a wide variety of desirable properties, including the following:
32-A belt conveyor of length 100 meter need to install VGTU system calculate the minimum length of take up.

Minimum length of take up = Conveyor length (centre to Centre) X 1.5%
                                              = 100 X 1.5/100 = 1.5 meters

33-A 200 metre length conveyor belt has a drive pulley size OD 510 mm 3 Length 1800 mm, this pulley is directly coupled to planetary reduction gear box having reduction ratio 40:1 and 1475 rpm motor is used to drive this gear box. Then calculate the time required for conveyor for one complete rotation.

We have,
Conveyor length: 200 meters
Pulley OD: 510 mm = 0.51 m
Motor output speed = 1475 rpm
Gear box reduction ratio = 40:1
Gear box output speed N = 1475/40 = 36.87 rpm
Head pulley speed = Gear box output speed (As head pulley is directly coupled to gear box)
Pulley speed in m/sec. = V = Pi X D X N/60
                                          = (3.142 X 0.51 X 36.87/60) = 0.98 m/sec

Belt speed is 0.98 m/sec, given that conveyor total length is 200 meters.
Then the length of the belt = 2 X conveyor C/C length + Conveyor C/C length X 2 X 1.5% (VGTU) + 0.5 X No. of pulleys (Pulley and belt contact radius length is considered 0.5 meter for each pulleys)
= 2 X 200 + 200 X 2 X (1.5/100) + 0.5 X 6 (6 Nos of pulleys)
= 409 meters
Time required for one complete revolution = Belt length/Speed
                                                                     = 409/(0.98 X 60) = 6.95 minutes
34-A conveyor of 7.5 KW having belt width 800 mm and speed 0.95 m/sec., calculate the maximum tension at the start of the belt.

Belt rated power P = 7.5 KW
Belt speed V = 0.95 m/sec.
Belt width W = 800 mm = 0.8 meters
Maximum tension at the start of the belt = (3.2 X P)/(V X W)
                                                                 = (3.2 X 7.5)/(0.95 3 0.8)
                                                                 = 31.5 KN/M

35-A 1400 mm width inclined belt conveyor has its head pulley at the elevation of 22 meter from ground, is used to convey 45 TPH coal. The head pulley is coupled to motor of RPM 1450 through planetary gear box of efficiency 80%. Then calculate the motor rated power to drive the conveyor. Consider motor efficiency 90%.

Belt width W =1400 mm
Vertical lift = L = 22 meter
Conveyor capacity Q = 45 TPH
Gear box efficiency = 80%
Motor efficiency = 90%
We have power required to drive the inclined belt P = 4.7 X L X Q/1000
                                                                                   = 4.7 X 22 X 45/1000
                                                                                   = 4.65 KW
Motor input power = 4.65/(Gear box eff. 3 Motor eff.)
                               = 4.65/(0.8 X 0.9) = 6.45 KW

Motor rated power = Motor input power X 1.15 = 6.45 X 1.15 = 7.42 KW (Take 15% margin)
Select 7.5 KW motor

36-Calculate the length of a 3 ply 1600 mm width conveyor belt, which is coiled on a 100 mm pipe. The thickness of the belt is 10 mm and belt is coiled around 65 turns.

We have,

Diameter of centre hole of coiled belt, d = 100 mm X 0.1 m

Thickness of the belt T = 10 mm = 0.01 m

No. of turns N = 65

Outer diameter of belt, D= 2 X 65 X 0.01 = 1.9 m

Length of belt = (d + (D - d)/2) X π X N

                        = (0.1 + (1.9 - 0.1)/2) X 3.142 X 65

                        = 204.23 meters

37-Write a short note on operation checks of conveyor belt?
Belts:
Watch out for spillage of material onto the return strand, where it can get between the belt and pulleys.
See that the belt is properly aligned and does not contact steel structure. This is especially important on the return strand that is generally not as easy to see.
Mark breaks, damaged and worn spots and report them for early repair.
Keep large lumps (coal lumps, stones, other foreign materials) of material off belts. If they do get on, remove them carefully.
Idlers and Pulleys:
Note and mark “frozen” or damaged idlers for repair or replacement.
Check troughing and return training/self aligning idlers for proper operation.
Check scrapers and plows for proper operation.
Watch for material to build up on decking or floor under idlers or pulleys. This is especially important at training/self aligning idlers.
 Chutes and Hoppers:
Note and correct clogged or “bridged” material.
Prevent large lumps from dropping onto unprotected belt.

Check to see that the material is centred on the belt and that the belt is not overloaded.
Replace all damaged impact idlers; do not replace impact idlers with regular idler.
38-What is the significance of pulley lagging?
Lagging is done to improve Pulley traction, either bolt or vulcanize the lagging to the pulley face. The lagging will prevent slipping at the head pulley and also cushion the wear between the belt and the pulley surface. In wet conditions, grooved lagging works best.

39-What is the function of hold back device in conveyor belts?
Hold back is a device used to restrict the reverse/back movement of belt during sudden stop or trip of conveyor.

40-What is belt splicing? What are the different types of splicing?
Splicing is the process of joining up of two trimmed ends of a conveyor belt utilizing a defined chemical or mechanical procedure.

Vulcanized Splicing (cold and hot) and Mechanical Splicing are generally used for belt jointing.

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