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For experts, this one is going to be the most difficult. Because in the long years that we become experts, we almost forgot the perspective of simplicity. We forgot the questions that were in our mind when you were young, novice, like a newbie. Somewhere, someone, something, cleared those dreadful questions that normal people don't ask experts. If you remember those questions and who, how, what, when, which book, which movie helped in making you an expert, write about it here. Be very worry some of the language because the terms that you use may not mean anything to domain outsiders.

Generator Bus Duct Erection Featured

Now What?

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There are times when life puts a wall in front of us, when no medicines work, when all available tools are of no use, when all that we know is irrelevant, when none of our experience comes in handy. At moments like these, all that goes on in our mind translates in to one simple question - Now what? This simple but profound question stirs our emotions, baffles our intelligence, makes us helpless, forces us to reboot and relearn and leaves us with memories for a life time. Times when we came to face this question are the turning points in our lives. They shape our lives, build our characters and make us who we are.

But those are also the moments when innovation triggers in some corner of the human brain. New concepts, new instruments, and new methods evolve. The initial spark of imagination in the mind of one passionate and fearless individual appears absurd at first but slowly gets refined and gains acceptance. Over the years it spreads to impact everything - companies, industries, communities, societies and even countries. Thus the old appears irrelevant and often incorrect and gives way to new.

The life cycle of large infrastructure projects is no different. They also go through this inevitable process of evolution. When major components of large projects cannot be interfaced correctly to work properly because wrong components have been delivered at project site, this question comes back to hunt everyone - Now what?

Whenever items delivered to a project are incompatible, site Field and Commissioning engineers take on the challenge. If possible and acceptable, they try to modify the interface in such way that the item can be used without extensive rework and modification to avoid costly reorder. But once in a while the sheer physical size of a component is so large that it is just not possible to carry out any physical modification whatsoever in any manner because suitable interface changes are just not possible. This is exactly what happened at one NTPC power station, where during the commissioning of the generator transformer it was found out that the generator bus ducts were of wrong design. This story describes what the impacts were and what was done at site to resolve the problem to go ahead with the project.

Large utility generators like those of 500 MW capacity, have three bus ducts, one for each phase that connect the generator to the generator transformer. These are aluminum tubes of about a meter and half in diameter, more than a few centimeters thick with an aluminum conductor centrally supported by insulators. These tubes come in pre-fabricated transportable spools and are welded together at site. When the spools are welded together as designed, the bus duct takes the shape it is intended to and connects the generator, generator transformer and the unit auxiliary transformers in a pre designed manner.

Assembly Of Bus Duct 1200

The main GT at the project comprised of three single-phase transformer units (connected externally through the generator bus duct) to act as a three phase transformer. On the generator side, the low voltage windings of the three single-phase units are connected as a "delta". For forming a delta on the LV winding, each bus duct coming out of the generator is split into two smaller ducts. R&Y go to the first transformer, Y&B go to the second and B&R to the third transformer. On the grid side, the high voltage windings of the three single-phase units connect as "star". This particular connection creates a phase angle difference of 30 degrees between the HV and LV of the GT. Keeping this phase shift in view, the unit auxiliary transformers are so designed that this phase shift is compensated.

 

Main Bus Duct Graphics 1200

 

Why Generator transformer, unit auxiliary transformer and the startup or station transformer must be so chosen that the two voltage sources are in-phase? This is necessary to ensure that uninterrupted auxiliary power can be available to the power station through a station transformer when the generator transformer trips.
Read on for some more explanation.
 
To generate electricity in a power plant, a number of equipment have to run and all of them consume electricity. Typically, 5 to 10 percent of power generated by a power plant gets consumed in the very process of generating that power. This power is called - auxiliary power. Even when a power plant is under shut down (not running), it consumes some auxiliary power for equipment safety and maintenance.
 
When a power plant generates power, it takes its auxiliary power requirement from its own generation through unit auxiliary transformers. When it stops or is not running it takes this auxiliary power requirement from a station transformer that is always connected to the grid. This switching over of power from one source to the other is called "change-over". Change-over of auxiliary power to plant has to happen without interruption, otherwise, all the plant equipment will trip during the interruption. This means, there is a small time interval during which both the unit auxiliary transformer and the station transformer have to remain connected together to the auxiliary loads, after which the source not required can be switched off.
 
For two power sources to be connected together, their voltages have to be in same phase angle. For this to happen, the phase shift due to the GT must be compensated by the unit aux. transformer. This requires that the vector group of the GT and the unit aux. transformers must complement each other in such a way that the unit transformers should bring back the phase angle of the voltage by the same angle that is shifted by the GT.
 
When the bus duct was and assembled at site, we observed that Y phase of generator was going to the R phase of the transformer and R phase of the generator was going to the Y phase of the transformer. Similarly, B&Y (in place of Y&B) and R&B (in place of B&R) was going in to the second and third transformer. This was applying a voltage with 180-degree phase shift to each transformer. This meant the phase angle between the high voltage and low voltage of the transformer would now be (180+30) or 210 degrees in stead of 30 degrees as designed. The phase shift a transformer introduces is conventionally represented on a clock face - e.g 30 degrees represents 1 O clock, 150 representing 5 O clock etc. By that convention, the GT was connected as a YdN7 and not as YdN1 as intended.
 
 
Phase Angle 1200
 
 
 
The massiveness of the mistake was such that virtually nothing was possible to rectify the problem. As far as the GT and the bus duct were concerned, there was no possibility of any modification at site. Period.
 
But so what if the phase angle is 210 degrees instead of 30 degrees? 
Will there be any problem in synchronization to the grid? The answer is - No. The generator is a free running source and once synchronized, will keep generating happily without any problem. But after synchronization, the output voltage of the unit auxiliary transformers will be out of phase with the station auxiliary bus voltage coming from the station transformer. Because of this, the two sources cannot be connected in parallel and switching from one source to the other source will not be possible without interruption of power to the plant. This will pose serious problems in reliable and safe operation of the plant. There was nothing that could be done with bus duct. There was nothing that could be done with the generator transformer. Therefore, on everybody's face there was that question we began with - Now What?
 
 
 
Vector Group Matching1 1200
 
 
 
The only way to match these two voltages was by changing the vector group of the unit auxiliary transformers in such way that the new connection will shift the voltage by another 150 degrees so that the total shift becomes 360 degrees which is equivalent to 0 degrees. 150 degree phase shift is possible with a DyN5 connection. Thus the only option was to reconnect the unit auxiliary transformers as DyN5 from their existing configuration of DyN11. This could be only possible by changing the connections of the winding inside the transformers.
 
We did not know if this was possible and practical. We contacted transformer manufacturer and discussed the problem. The designers of the manufacturer agreed to carry out connection changes inside the transformer at the site to change the vector group from Dyn11 to Dyn5.
 
The design practice for generator transformers of this service are generally of YdN1 type. However, with the wrongly manufactured bus duct, this was not possible and the change described above was the only solution possible. Nevertheless, using GT of YdN7 vector group has not resulted in any problems that can be attributed to this rather unconventional choice of the vector group. 
 
The generating unit and the transformer are running satisfactorily since over 25 years.
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Codes & Standards

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At a project site in US, there was a container-office in the maintenance bay used as a conference room for odd meetings. What was written on the entrance of this container was interesting. On a A4 size computer printout, the following was written.

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An old Television Antennae Featured

Signals from the Moon !

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1984, LA Olympics.
Those were the days when satellite and DTH TV had still not come to India and television broadcast was limited to major Indian cities and "larger" second tire cities. Raipur was two hundred KM from Korba and had a regular TV broadcasting center.

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24 wires !

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Many times in life, a journey becomes more enjoyable and interesting than the destination. Often we forgot tasks we completed and the achievements that came along, but we always remember the the manner in which it was done, the people who were with us, and the unique situation that we were in.

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The Sixth Sense

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Ingenuity and creativity comes from the need to perform tasks that others would consider impossible.  In order to perform these feats of magic, you need to develop a 6th or a 7th sense. That gives you the ability to think outside the box.

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The Story of Pumps

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Imagine, you are taking a hot shower in a five star hotel in a hill resort and suddenly shower goes dry! Or working in a power plant and the power goes off with boiler tripping due to low water level in the drum!!
Think of city of Chennai or Mumbai; millions of people are dependent on supply of drinking water, the lifeline, from a very distant source to the tune of 50-100 km.

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The Story of Pumps - 2

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In our first story, we have already looked at the history of the development of the Pumps and realized that the whole endeavor was to invent a mechanism(s) which can reverse the natural phenomenon of water flow by gravity.

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THE STORY OF PUMPS - 3

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In our first story, we have seen the evolution of pumps. In the second, we have learnt how centrifugal pump infuses energy into the fluid through the impeller. Besides, there are positive displacement pumps, which also impart energy into the fluid through physical movement of the liquid mass. So how do we decide which type of pump is best suited for a hydraulic system design? Let’s take a glimpse at the classification of pumps and understand the basic working philosophy of each type.

Broad classification of pumps based on working principle can be seen in Figure 1 below –

TSOP 3 F 1
Figure 1 - Broad Classification of Pumps

A.    Rotodynamic
Leading by the term, rotodynamic pumps work on the philosophy of continuously adding kinetic energy to the working fluid through a rotating device i.e., the impeller. The category has been further divided into three (3) sub-categories based on impeller construction vis-à-vis the direction of fluid flow exiting the impeller. Figure 2 below provides typical sectional view and characteristics of these categories of impellers.

TSOP 3 F 2Figure 2 – Typical impeller construction & characteristics in Rotodynamic Pumps

•    The Radial Vane or Centrifugal type
Impellers under this category will force the fluid to exit the impeller in the radial direction by the influence of centrifugal force while the fluid enters the impeller eye in the axial direction. The discharge fluid gets collected at the pump volute casing and guided to the discharge. Please refer to Figure 3a. Technically, this type of pumps has low Specific Speed and is useful for developing high discharge head and can handle the relatively lower quantity of fluid. Impellers can be open / semi-open or shrouded type depending on the process fluid characteristics. Refer to typical sectional details of the pumps in Figure 3b, 3c & 3d below for clarity.

TSOP 3 F 3a1  TSOP 3 F 3a2 

Fig 3a – Working Principle of a centrifugal pump

 TSOP 3 F 3b1  TSOP 3 F 3b2

Fig 3b – Cut away view of a centrifugal pump

 TSOP 3 F 3c

TSOP 3 F 3d

Fig 3c - Sectional view of a double suction pump
Fig 3d - Typical view of multistage centrifugal pump

•    Mixed Flow type
Impellers under this category and Francis type force the fluid to exit the impeller in an angular direction as can be seen in Figure 4a. Pumps with mixed flow type impellers have medium Specific Speed as can be seen in Figure 2 and is useful for developing medium discharge head. It can handle comparatively higher flow than a centrifugal unit. Typical view of open and closed type mixed flow impellers and sectional view of single stage mixed flow pumps are shown in Figures 4b & 4c below.

TSOP 3 F 4a
Figure 4aTypical Mixed Flow Pump working principle

TSOP 3 F 4b1  TSOP 3 F 4b2 

Figure 4b – Typical Mixed Flow Impeller construction

TSOP 3 F 4c1   TSOP 3 F 4c2

Figure 4c – Typical Mixed Flow pump construction

•    Axial Flow type
Impellers under this category are sometimes denoted as turbine type and are designed to push the fluid in the axial direction as can be seen in Figure 5a. Pumps with axial flow type impellers can handle large fluid volume as compared to other two categories and develop low discharge head. The impeller is characterized by the highest band of specific speed as can be seen in Figure 2. Typical impeller construction and sectional view of axial flow pumps are indicated in Figure 5b.

TSOP 3 F 5aFigure 5a – Typical Axial Flow Impeller & pump construction

TSOP 3 F 5b1   TSOP 3 F 5b2 TSOP 3 F 5b3 

Figure 5b – Typical view of Axial Flow Impeller & pump cross-section

Rotodynamic impellers have been further developed through researches by the pioneers in the pump industry to satisfy various process needs, handle dirty water & slurry as well as for pumping fluids other than water. For handling slurry or waste water containing solids, semi-open or open type non-clogging impellers are used. Typical view of these impellers can be seen in Figure 6.

 TSOP 3 F 6 1 TSOP 3 F 6 2  TSOP 3 F 6 3  TSOP 3 F 6 4 

Figure 6 – Typical view of closed, semi-open and open type Impellers

B.    Positive Displacement Type
As the name implies, this category of pumps physically pushes the fluid from suction to discharge. Positive Displacement Pump has an expanding cavity at the suction side and a decreasing cavity on the discharge side. Liquid flows into the pumps due to fall in pressure as the cavity on the suction side expands and the flows out of the discharge as the cavity collapses. Volume flow through the pump is constant in each cycle of operation.
Positive displacement pumps are classified into two main categories based on working principle.
•    Rotary
A rotary pump traps fluid between a rotating element and the closed casing at the suction side and transports the fluid along with the rotary element until it gets discharged at the outlet due to space constraint. It is normally a fixed volume machine and maintains a constant & uniform flow across all discharge pressures. Single or multiple rotor elements can be used in a rotary pump design, as can be seen in the following section. Major distinctive type rotary pump designs are -

  • Gear Pumps
 TSOP 3 F 7 1 TSOP 3 F 7 2 

Figure 7 – Typical working view of Gear Pump and construction

Pumping activity is achieved by teeth of two rotating gears meshing each     other fitted in an enclosed casing. Liquid trapped between the casing and the gear slots at the suction is carried by the teeth and discharges at the pump outlet. The meshing of teeth of two gears at center prohibits liquid backflow from the suction to discharge side. Both the gears are driven by a common driver.

  • Twin Lobe Pumps
    The arrangement is similar to gear pump with the difference that liquid is trapped in the rotating lobes as can be seen in the diagram below. Faces of the lobes continuously touch at the center thereby creating a seal between suction and discharge. Lobes are synchronized and driven through a common drive unit and timing gear.
TSOP 3 F 8 1   TSOP 3 F 8 2 TSOP 3 F 8 3 

Figure 8 – Typical view of Twin Lobe pump & cross-section

  • Vane Pumps
    Vane pump is a single rotor design in which the rotor is placed eccentrically in the pump     housing as shown in Figure 9a so that the rotor keeps a small clearance with the casing on one side. As the rotor rotates, sliding vanes eject out of the rotor due to centrifugal  force and trap the liquid between vane and casing. Trapped liquid rotates with the vane     and gets discharged at the outlet due to a reduction in the trapped volume.
  •  TSOP 3 F 9a1  TSOP 3 F 9a2

Figure 9a – Typical view of sliding vane pump & cross-section

TSOP 3 F 9b1  TSOP 3 F 9b2 

Figure 9b – Typical view of Flexible vane pump

In the case of flexible vane design, vanes collapse to reduce the trapped volume as they     approach closer to casing near discharge point to force the liquid to come out of the pump. Please see Figure 9b.

  • Screw Pumps
    The concept of screw pump was invented by Archimedes around 200 BC, which says that the angular motion of the screw when rotated in a static body (Figure 10a), can lift water from lower to a higher level. The concept is still used in the transportation of granular material. The design has been modified further to have double and triple screw pumps which are extensively used in oil and viscous fluid pumping.

  • Let’s look at the sectional view of a twin screw pump in Figure 10b.  The liquid trapped within screw threads and casing moves along the rotating screws till it reaches at the end. Both the screws are driven by a single prime mover using timing gears.

TSOP 3 F 10aFigure 10a – Typical Archimedean single screw pump

 TSOP 3 F 10b1 TSOP 3 F 10b2 

Figure 10b – Typical view of Twin Screw Pump & cross-section

  • Progressive Cavity Pumps
    Progress Cavity Pump uses a single screw rotor installed in a flexible casing. As the rotor rotates within the flexible stator, it pushes the trapped liquid in the direction of the screw. These pumps are used for slurry and high viscous fluids transportation. Typical sectional view of a progressive cavity pump is shown in Figure 11.
 TSOP 3 F 11 1 TSOP 3 F 11 2 

Figure 11 – Typical cross-sectional view of a Progressive Cavity Pump

•    Reciprocating
As the name implies, to and fro motion of a moving element is used for pumping the fluid. In a reciprocating pump, the liquid is drawn into the cylinder through the suction valve as the piston or plunger moves away from suction point creating a vacuum in the cylinder. Reaching the other end, piston reverses its motion due to crankshaft arrangement and the trapped liquid is discharged through the outlet valve under positive pressure due to a reduction in trapped volume while the suction valve remains closed. See Figure 12. The discharge from a reciprocating pump is therefore pulsating and the discharge volume is fairly constant for a specific pump size and drive speed irrespective of the discharge pressure. The operation of the pump is also independent of the rotational direction of the drive when compared to a rotary or rotodynamic unit.

TSOP 3 F 12 1  TSOP 3 F 12 2 

Figure 12 – Working Principle of a Reciprocating Piston Type Pump

Reciprocating Type Pumps have been further categorized based on their reciprocating mechanism as can be seen below -

  • Plunger or Piston Type
    Piston or plunger type pumps utilize piston movement as pumping principle, as already explained in Figure 12. Let’s also look at the Figure 13a below. The reciprocating motion of the piston is obtained using a rotating device and cam arrangement. As the piston moves away from piston head, the vacuum created in the cylinder pulls up the ball at inlet check valve and liquid rushes inside. Ball at outlet check valve remains seated preventing backflow from discharge side. With the reversal of piston motion, ball at suction valve goes back to sitting position due to gravity and pressure developed inside the cylinder pushes the liquid through the discharge valve.
 TSOP 3 F 13a1 TSOP 3 F 13a2 

Figure 13a – Working Principle of a Reciprocating Piston Type Pump

 TSOP 3 F 13b1  TSOP 3 F 13b2
Single Acting  Double Acting

Figure 13b – Working Principle of single and double acting Reciprocating Piston Type Pump

The piston or plunger type pumps can be single or double acting type as can be seen in Figure 13b. In a double acting system, two sets of suction and discharge valves are used, one set at each end of the cylinder so that while piston moves from left to right, the left side of the cylinder is in suction mode and right side goes in discharge mode. The situation reverses as the piston moves from right to left.

  • Diaphragm Type
    In a diaphragm pump, movement of a flexible diaphragm using a cam and rotating device creates the changes in the volume of the pumping chamber in a cyclic manner and thereby produce pumping action with the help of suction and discharge valve. See Figure 14a. The capacity of the pumping system is generally low as it is dependent on the size and flexibility of the diaphragm and the rotational speed of the cam. The advantage of this pump is that the wetting of components by the handling fluid is limited to the pumping chamber and the process flow line and therefore useful in handling highly toxic and/or corrosive fluid.
    The design has been further modified to ensure uniform movement of the diaphragm using the plunger and hydraulic mechanism. Reciprocating movement of the plunger by cam arrangement in a confined hydraulic system causes the diaphragm to follow the plunger due to pressure fluctuation in hydraulic fluid and thereby produces pumping action in the process fluid. See Figure 14b.

TSOP 3 F 14aFigure 14a – Working Principle of a Diaphragm Pump

TSOP 3 F 14b1   TSOP 3 F 14b2

Figure 14b – Working of a Diaphragm Pump using hydraulic device

Metering Pumps or the Controlled Volume Pumps are basically a modified version of the diaphragm pump, wherein the stroke or the frequency of the plunger movement is regulated using a stroke control mechanism or a variable frequency drive. Figure 15a below shows how a micrometer adjuster is used to regulate the stroke length of the plunger and thereby discharge volume. Figure 15b shows a cut-away view of the metering pump.

TSOP 3 F 15a
Figure 15a – Working of a Metering Pump using hydraulic

TSOP 3 F 15b
Figure 15b – Cut away view of a Metering Pump

 

I believe it has been an enjoying session for a beginner to understand pump types and their working principle. Further details may be studied in various literature published on the subject.

In our next story, we will talk about performance characteristics of pumps and their behavior in a hydraulic system. Stay Tuned !

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