Posted: 30 Dec 2010 02:35 PM PST

FEED WATER PUMP HYDRAULIC COUPLING

Eighty Percentage 80% of thermal power plants has boiler feed pump , that takes the water from the feedwater system ( from the DA) and provide this water to the boiler system , to generate steam which is responsible for rolling the Turbine,Therefore Generate Electricity.

Normally Feed water pumped to the boiler is pumped to the Boiler's Drum where at the top point of the boiler, so we have to provide big pump that can handle big pressure with great flow to the boiler.

That is happened by this huge pump ( Boiler Feed Pump ) , BFP is usually rotate with 5000 rpm ,150 barg and can provide about 300 T/H.

This big pump needs a prime mover with high electrical rating - Power Plants usually use Electrical Induction motor - to rotate this pump, the rating of this prime mover is usually about ( 5 MWatts , 6.6 KVolts , 1600 rpm ). It is very huge rating if you imagin , 5 MWatts can provide Electricity to a small town.

so , as you see in the picture ( left hand is the prime mover_motor) , (Right hand is the pump) and what is it at the middle of the picture??

Hydraulic Coupling

Hydraulic Coupling is used to transmit Power in a wear-free manner from a prime mover (driving machine) to a power consumer(driven machine-pump). the power is transmitted in the following way:-

* by means of a connecting coupling between the driving machine and geared variable speed coupling.

* by means of a step-up gear unit between the input shaft and primary shaft.

* hydro-dynamically by means of the working oil between the primary wheel and the secondary wheel.

* by means of connecting coupling between the driving machine and geared variable speed coupling.

the control at the hydraulic coupling is done by the scoop tube control, it provide infinitely variable adjustment of the driven machine's speed. the power from the driving machine is transmitted to the primary wheel to the working oil, the working oil is accelerated in the primary wheel, and the mechanical energy is converted into the energy of fluid flow. the secondary wheel picks up the flow energy and converts it into mechanical energy. this energy is transmitted to the driven machine.

Speed Control

the speed of the driven machine is infinitely variable. this is accomplished by varying the amount of oil in the coupling during operation with the aid of the adjustable scoope tube.

* Scoop tube advanced as far as possible into the scoop champer of the coupling (0% position):minimum oil ring , minimum speed.

* Scoop tube retracted as far as possible out of the scoop champer of the coupling (100% position): maximum oil ring, maximum output speed.
THESE FILES ARE SO GOOD FOR THIS TOPIC.

Quote:

hydr_couplg.exe
speed control.pdf
Vorecon_applications.pdf

Can Your Boiler Feed Pump Handle a Deaerator Pressure Transient?

Posted: 30 Dec 2010 06:37 AM PST

Can Your Boiler Feed Pump Handle a Deaerator Pressure Transient?

"This is part of an article talking about the importance of DA and BFP positions at power plant "

In a typical steam power plant, the boiler feedwater (BFW) pump takes suction from the deaerator (DA) and discharges high-pressure water to the boiler through the feedwater heaters. During normal operation, the DA is supplied with steam turbine extraction steam to mix with and heat the feedwater. Other purposes for the DA are to provide the required net positive suction head (NPSH) for the BFW pump and to serve as a storage tank to ensure a continuous supply of feedwater during rapid changes in BFW demand.
The available net positive suction head provided to a boiler feedwater pump can drop enough during a pressure excursion to cause cavitation and damage to the pump's internal parts. A careful analysis of various operating profiles can ensure that the pump operates safely during the pressure fluctuations that occur after a steam turbine trip or large load change.
How does the plant designer or operator determine the adequacy of the BFW pump selection or the DA and feedwater system design? It's not uncommon to find that the BFW pump was originally specified based on steady-state conditions and did not consider the DA pressure transients that occur during a steam turbine trip (with the boiler remaining in service) or a sudden steam turbine load reduction. If the NPSH available to the BFW pump during the pressure transient drops below that required by the pump for only a short period of time, cavitation and damage to the pump internals often result.
An NPSH deficit in an existing system or a new system under development can be avoided by using some very simple analytic tools.

Find the NPSH margin

The deaerator is installed at some elevation above the BFW pump to provide the NPSH required by the pump. By definition, the NPSHr is the total suction head over and above the vapor pressure of the liquid pumped.
The DA elevation minus the dynamic losses in the BFW suction piping between the DA and the BFW pump equals the NPSH available (NPSHa) to the pump. The difference between the value of the NPSHa and that required (NPSHr) by the pump gives the NPSH margin.
The NPSH margin or the NPSH margin ratio (NPSHa/NPSHr) is an important factor in ensuring adequate service life of the pump and minimizing noise, vibration, cavitation, and seal damage. The NPSH margin requirement increases as the suction energy level (for example, high suction specific speed, high peripheral velocity of impeller, and the like) of the pump increases. In the case of the BFW pump, this ratio could be in the range of 1.8 to 2.5. These margins are typically based on steady-state operation.
In addition, the NPSH margin improves the ability of the BFW pump to handle a DA pressure transient. Once a design is determined to have an adequate NPSH margin, the next step is to determine if the NPSH margin is adequate during a pressure transient.

Expect Deaerator Pressure Decay

Immediately after a steam turbine generator trip, turbine extraction steam is no longer available to the deaerator, resulting in decay of the DA pressure. Also during a sudden steam turbine generator load reduction, the extraction steam pressure decreases until the extraction stage supplying the DA can no longer maintain DA pressure. This also results in DA pressure decay as the lower-temperature condensate continues to enter the DA, cooling the stored feedwater
The decrease in DA pressure causes some of the water in the DA storage tank to flash to steam until saturation pressure is reached at the new DA pressure. The water in the BFW pump suction line has a static head exerted on it by the level in the DA storage tank, preventing it from flashing immediately. Therefore, the water in the suction line can be considered as a slug of hot fluid that must be moved through the pump in some finite amount of time. In other words, the pump will not perceive a decrease in vapor pressure (or a decrease in water temperature) until the entire slug of hot water has passed through the pump.
During the passage of the hot-water slug, the combination of high vapor pressure at the pump suction along with a decrease in pump suction pressure (due to DA pressure decay), results in a "critical point" at which the suction pressure may drop below the minimum required pressure (that is, the vapor pressure of the hot-water slug plus the pressure equivalent of the NPSHr). This low suction pressure could result in cavitation damage to the pump internals due to insufficient net positive suction head

Short Residence Time

The time required for passage of the hot-water slug through the pump suction line is the "residence time." Residence time can be expressed as the suction line volume divided by the volumetric flow rate (or, alternatively, as the mass of liquid in the suction line divided by the mass flow rate)
Note that because the vapor pressure at pump suction is modeled to decay only after the residence time has elapsed, the critical point occurs at the end of the residence time interval. The challenge is to determine the DA pressure at this critical point and thereby the system NPSH margin.

Options for Adding NPSH to the System

The main BFW pumps are generally large, high-energy pumps needing large amounts of NPSHr. One solution would be to raise the DA to a higher elevation to increase the NPSHa. This solution is normally not practical or cost-effective. Another approach is to install a low-speed, low-NPSH booster pump upstream of the BFW pump. The booster pump discharge pressure then provides the added NPSH required by the BFW pump. In addition, the same NPSH analysis must be made on the booster pump. The only difference is that in the case of the booster pump arrangement, the critical point and the critical point margin need to be evaluated at the booster pump suction as well as the BFW pump suction.

Additional Transient Condition

An additional transient condition that the system designer must consider occurs during a "hot start." In this situation, steam flash (water-steam mixture) can occur at the pump suction and cause cavitation damage to the pump internals. However, the mechanism causing steam flash is slightly different than what was discussed earlier.
On a plant trip, the DA pressure drops and the water temperature inside the DA drops. However, the pump and suction piping near the pump remain at a higher temperature due to the mass of the metal. As a result, when the pump is operated on a hot restart of the plant, steam flash and cavitation are likely to occur at the pump suction

"This article for, Magdy Mahmoud is manager of engineering for PGESCo., Egypt."

Open Feed Water Heater (Deaerator)

Posted: 30 Dec 2010 06:30 AM PST

Open Feed Water Heater (Deaerator)

An open feadwater heater , also called direct-contact and deaerating (DA) heater , is one that heats the feedwater by directly mixing it with bled steam from the turbine. Usually only one DA is used at Power Plant.
Because the pressure in such a heater can't exceed the turbine pressure at the point of extraction, a pump (Main Boiler Feed Water Pump) must follow the heater. The confluence of steam and water flows makes possible the efficient removal of noncondensables as well as the heating of the feedwater.

The DA heater is usually positioned in the feedwater line at a pressure to prevent air inleakage and at a temperature at which Oxygen retention is least likely. Most DA heaters are designed for Oxygen concentration in the outlet feedwater below 0.005Cm3/L
The DA outlet feedwater is at or near saturation. Pumping saturated water results in cavitation because of the pressure drop below saturated pressure, thus causing flashing on the back side of pump vanes. The DA heater is therefore usually positioned in the powerplant steam-generator house high above its pump by perhaps 60 ft. This provides sufficient pump inlet pressure to render the saturated water compressed (or subcooled) and prevents cavitations.

There are three types of DA heaters for industrial and utility use.
1) Spray-Type deaerators .
In this type the feedwater enters the heater through nozzles that spray it into the extraction-steam-filled heater space. The water is heated and scrubbed to release the noncondensables gases. A second agitation of the now-heated feedwater by another steam flow is provided by an internal baffling system.

2) Tray-Type deaerators.
Here the feedwater is directed onto a series of cascading horizontal trays. It falls in sheets or tubes from tray to tray and comes into contact with rising extraction steam admitted from the bottom of the tray system. As scrubbing occurs and noncondensables gases and some steam rise, they come into contact with colder water, resulting in a reduced volume of high concentration of noncondensables to vent into the atmosphere.

3) Combination spray-tray deaerator
In this type, the feedwater is first sprayed into a steam-filled space, then made to cascade down trays. This combination type with horizontal stainless steel trays is currently preferred by utilities.

You noticed that just below the heater, is a relatively large feedwater tank (Storage Tank) which allows sufficient water for rapid load variations.

Crazy Engineers .CoM

Thursday, December 30, 2010

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