Air Ejectors (2 of 3)

Air Ejectors (2 of 3)

-An air ejector or steam ejector is a device which uses the motion of moving fluid (Motive Fluid) to transport another fluid (Suction fluid). It is has a wide range of application in steam ejector in boiler condenser, fresh water generator and in priming the centrifugal pump.

-It works on the principle of convergent /divergent nozzle as it provides the venturi effect at the point of diffusion as the tube gets narrows at the throat the velocity of the fluid increases and because of the venturi affect it pressure decreases, vacuum will occur in the diffuser throat where the suction line will be provided.

-The operating medium of an air ejector can be either high-pressure gas or liquid. This is passed through a nozzle and the pressure energy is converted into velocity energy. The high-velocity fluid aspirates the air and the non-condensable gases and the mixture is projected into a diffuser which reconverts the velocity energy into pressure energy.

-Steam is suitable operating medium and is used in the steam-jet air ejector. The steam consumption is controlled by the compression ratio of the air and this factor influences the decision to adopt either single or multiple stage ejectors for a particular condition.

-To meet the requirements when raising vacuum a starting ejector is provided. This is high capacity, high steam consumption ejector, of single stage and without an after condenser.

-A main air ejector with standby unit is usually provided for normal operation. The heat in the operating steam is partially recovered in the condensate which flows through the inter and after coolers.

 

-If an installation is to operate with a direct cooling system, as is provided by the sea or a river, vacuum as high as 29.2 in.Hg would be expected during the winter months.

-A cooling tower installation will operate at a slightly poorer vacuum than a direct cooled station and this vacuum can usually be handled by a two-stage ejector.

CONDENSER – AIR EXTRACTION EQUIPMENT ( 1 Of 3 )

CONDENSER – AIR EXTRACTION EQUIPMENT

- Turbines are designed for a particular operating conditions like steam inlet pressure, steam inlet temperature and turbine exhaust pressure/ exhaust vacuum, which affects the performance of the turbines in a significant way. Variations in these parameters affect the steam consumption in the turbines and also the turbine efficiency. Theoretical turbine efficiency is calculated as work done by the turbine to the heat supplied to generate the steam.

- Higher exhaust pressure/ lower vacuum, increases the steam consumption in the turbine, keeping all other operating parameters constant. Exhaust pressure lower than the specified will reduce the steam consumption and improves the turbine efficiency. Similarly exhaust vacuum lower than the specified, will lower the turbine efficiency and reduces the steam consumption.

- The air extraction equipment must be capable of meeting two conditions; one met during normal operation, the other when raising vacuum on the turbo-generator unit.

- When raising vacuum the air extraction equipment must deal rapidly with a large quantity of air and sufficient capacity must be installed to reduce pressure quickly in the condenser to a level which allows the turbine to be started. The last row blades of a 500 MW turbine will overheat if they run at speed and at low load in a poor vacuum. Thus, a vacuum of 20 in.Hg must be obtained before steam is admitted to the turbine and a vacuum of 26 in.Hg for full speed. It is important that the time taken to bring a turbo-generator on load shall not be increased by sufficient air extraction capacity.

- Under normal operating conditions the quantity of air to be exhausted is lower. It consists of air leakage into the condenser via flanges and glands and also of incondensable gases that are present in the steam exhausting from the turbine. Air and incondensable must be removed from the condenser as their presence in any quantity impairs the heat transfer capability of the condenser and hence its performance. Conversely, excessive air extraction capacity should not be run, as this involves unnecessary running costs and also results in the extraction of an unwanted quantity of water vapor from the condenser steam space.

“We will talk about the equipment for the extraction in the 2 upcoming articles”

Plant Safety: Learn from the Mistakes of Others

 “THIS ARTICLE GIVE US LESSONS FROM AN ACCIDENT HAPPENED AT U.S. WITH HEAT EXCHANGERS EXPLOSION,”

-On January 27, the U.S. Chemical Safety Board (CSB) released a case study examining the causes of a heat exchanger rupture and ammonia release at the Goodyear Tire and Rubber Plant in Houston in 2008. Use these lessons learned to check your employee tracking system and to determine whether your plant has installed valves that violate the American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code.

-The CSB event summary begins by noting that the accident occurred on June 11, 2008, when an overpressure in a heat exchanger led to a violent rupture of the exchanger that hurled debris, which struck and killed a Goodyear employee walking through the area. The heat exchanger contained pressurized anhydrous ammonia, a colorless, toxic chemical used as a coolant in the production of synthetic rubber; five workers were exposed to ammonia released by the rupture.

-On the day prior to the accident, maintenance work required closing several valves on the heat exchanger. CSB investigators found that workers closed a valve that isolated the exchanger from a relief valve, to replace a burst rupture disk located below the relief valve.

-The next day, at about 7:30 a.m., an operator closed another valve—this one blocking a second, automatic pressure control valve—to begin cleaning the process line with steam. The operator was unaware that the isolation valve was also closed, thus leaving no means of relieving excess pressure in the exchanger. As a result, pressure continued to increase until the heat exchanger exploded violently. 

-The report further notes that maintenance work activity was not properly communicated between maintenance and operations personnel, resulting in a subsequent shift not being notified of the isolation of the pressure relief line.

Plant Design Mistakes

-The CSB’s final report outlines several lessons learned, including the need to adhere to the existing ASME Boiler and Pressure Vessel Code.

-CSB Investigations Supervisor Robert Hall said, “We found the accident likely would not have happened had operators followed the ASME code. It’s crucial that workers continuously monitor an isolated pressure relief system throughout the course of a repair and reopen blocked valves immediately after the work is completed.”

-The CSB’s report notes that the ASME code states that “Overpressure protections shall be continually provided... whenever there is a possibility that the vessel can be over-pressurized by a pressure source.”

‘This article for, Dr. Robert Peltier , PE “

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BOILER (MASTER FUEL TRIP)

Steam Generator ( boiler ) is very important equipment at thermal power units , Boiler is the steam supply for the unit to rotate the turbine's rotor , hence produce electricity at Generator's stator.

Boiler usually equipped with solar and Main gas firing equipments , and you can imagine that huge boiler disappear in seconds unless operator take his precaution from operation hazards., hence Boiler always equipped with a protection relays to trip the boiler out of service if hazards conditions has happened , these relays are enclosed at huge protection system is called ( Master Fuel Trip ).

From its name ( Master Fuel Trip ) , it produces trip signal for fuel systems from operation , then trip the boiler out of service away from danger. this relay needs many conditions to operate , these conditions will be found at the following figure.

 


MASTER FUEL TRIP , CONDITIONS :-


1-REHEATER PROTECTION
2-DELAYED LIGHTING
3-LOSS OF ALL FUEL INPUT
4-DRUM LEVEL HI HI
5-DRUM LEVEL LOW LOW
6-ALL FD FAN STOP
7-FURNACE PRESSURE HI HI
8-CCS WATCHDOG ABNORMAL
9-COMBUSTION AIR FLOW LOW LOW
10-CONDENSER PROTECTION
11-PARTIAL LOSS OF FLAME
12-BOILER EMERGENCY TRIP
13-LOSS OF ALL FLAME
14-HP/LP BYPASS VALVE ABNORMAL CLOSE
15-ALL FUEL TRIP DELAY TRIP

LOAD SWING TEST

YESTERDAY ,  We made a test at our power station ( 2*350 MW - Gas/Oil Fired ) , this test is LOAD SWING TEST.

This test is implies the load of the unit at 350MW , for a while . and collect our data from the unit especially the boiler and Turbine system. , Then Decrease the sharply to the lowest load at 250 MW .

Hold time at this minimum load  and collect data , and identify the performance of BOILER MASTER and GOVERNOR behaviors, and judge on the performance for these equipment.

Repeat this swing two and three times about this range , and collect the data for the system.

What Happened with our unit ??

It is noted that when decreasing the load , the water heaters ( Medium Pressure Heaters ) are going out of service because of hi level , this is very dangerous for the behavior of steam unit.

because when water heaters , the cascaded drains causes to trip all the heaters from service , and suddenly the load may increase sharply , especially the high pressure heaters. 

Pressure Measurement

Pressure Measurement

-A major concern in process control applications is the measurement of fluid pressure. The term fluid means a substance that can flow; hence, the term applies to both liquids and gases. Both will occupy the container in which they are placed.

-The pressure is equal to the force applied to the walls divided by the area that is perpendicular to the force. For a liquid at rest, the pressure exerted by the fluid at any point is perpendicular to the boundary of the liquid. Pressure is defined as a force applied to, or distributed over, a surface area as:-

P=F/A

Gauge and Absolute Pressure

-Absolute pressure is the pressure measured above total vacuum or zero absolute, where zero absolute represents a total lack of pressure.

-Gauge pressure is the pressure measured above atmospheric or barometric pressure. It represents the positive difference between measured pressure and existing atmospheric pressure.

 

Pressure Gauges

-Pressure gauges are used for local indication and are the most common type of pressure-measurement instrument used in process industries. Pressure gauges consist of a dial or indicator and a pressure element. A pressure element converts pressure into a mechanical motion.

-Most mechanical pressure elements rely on the pressure that acts on a surface area inside the element to produce a force that causes a mechanical deflection. The common elements used are Bourdon tubes diaphragms and bellows elements.

-The following figure shows one of the most common and least expensive pressure gauges used in the process industries. This pressure gauge uses a “C” type Bourdon tube. In this device, a section of tubing that is closed at one end is partially flattened and coiled. When a pressure is applied to the open end, the tube uncoils. This movement provides a displacement that is proportional to the applied pressure. The tube is mechanically linked to a pointer on a pressure dial to give a calibrated reading.

- A Diaphragm is another device that is commonly used to convert pressure into a physical movement. A diaphragm is a flexible membrane. When two are fastened together they form a container called a capsule. In pressure- measuring instruments, the diaphragms are normally metallic. Pressure applied inside the diaphragm capsule causes it to expand and produce motion along its axis.

- A diaphragm acts like a spring and will extend or contract until a force is developed that balances the pressure difference force. The amount of movement depends on how much spring there is in the type of metal used.

- To amplify the motion that a diaphragm capsule produces, several capsules are connected end to end, as shown in the following figure. You can use diaphragm- type pressure gauges to measure gauge, absolute, or differential pressure.

- The Bellows pressure element is very similar to a diaphragm-type gauge in that it converts a pressure into a physical displacement. The difference is that typically the movement in a bellows is much more of a straight-line expansion. A typical bellows-type pressure gauge is manufactured by forming many accordion-like pleats into a cylindrical tube

-You can use the motion that the pressure input signal produces to position a pointer, recorder pen, or the wiper of a potentiometer.

BOILER STEAM PURITY

BOILER STEAM PURITY

- It is important that the steam leaving the boiler drum should be free from impurities which could be deposited in the superheater or the turbine.  

- Such impurities may arise from two distinct causes, one being the property which steam at high temperature and pressure has of dissolving significant amounts of certain substances, notably SILICA & CAUSTIC SODA.

- the other cause is more familiar, namely priming or carry-over, where there is incomplete separation of the two phases in the drum and droplets of boiler water are entrained in the steam leaving the drum.

- Impurities  caused by steam solubility can only be controlled by careful regulation of the boiler water analysis, particularly with regard to alkalinity and silica, and it is accepted that if the silica in steam doesn’t exceed 0.02ppm , it will cause no trouble by deposition in the superheater or the turbine.

- The concentration of silica in the boiler water which is in equilibrium with 0.02ppm in the saturated steam varies with the operating pressure as shown in the following figure.