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BOP REPORT (EJECTOR SYSTEM)

NAME : K.NANDA KISHOREDATE : 26/10/2007

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HOGGER EJECTOR:

A "HOGGER" is the large air ejector used when the condenser is being lit off

(started up) to pull an initial vacuum, usually to 10-15" Hg. At this point the service

air ejectors are lit off and continue pulling vacuum down to normal operating

range.

EJECTOR

Ejector consists of a motive fluid inlet nozzle and a converging-diverging outlet

nozzle. In this case of a steam ejector, the motive fluid is high-pressure steam.

The Venturi effect, a particular case of Bernoulli's principle, applies to the

operation of this device. The high-pressure steam is converted into a high-velocity

jet at the throat of the convergent-divergent nozzle which creates a low pressure at

that point. The low pressure draws the suction fluid (a vapor or gas in this case)

into the convergent-divergent nozzle where it mixes with the high-pressure steam.

In essence, the pressure energy of the inlet steam is converted to kinetic energy in

the form of velocity head at the throat of the convergent-divergent nozzle. As the

mixed fluid then expands in the divergent diffuser, the kinetic energy is converted

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back to pressure energy at the diffuser outlet in accordance with Bernoulli's

principle.

The compression ratio of the steam jet ejector, P2/P1, is defined as ratio of the

ejector's outlet pressure, P2, to the inlet pressure of the suction vapour or gas, P1.

The entrainment ratio of the steam jet ejector, Ws/Wv, is defined as the amount of

motive steam, Ws (in kg/hr), required to entrain and compress a given amount, Wv

(in kg/hr), of suction vapour or gas.

SYSTEM PROCESS:

Steam Ejectors are often used to pull vacuum on surface condensers,

evaporators, etc. A high pressure, motive, fluid (usually steam) enters the ejector

chest through a nozzle and then expands. This converts its pressure energy to

velocity. The increased velocity causes reduced pressure, which sucks in and

entrains gas from the suction. The diffuser section then recompresses the mixed

steam/gas stream to some intermediate pressure. The exhaust is then sent to a

condenser which quickly condenses the steam at a low pressure and temperature

so that the volume quickly decreases.

EJECTOR STAGES

Ejectors can be classified as single-stage or multistage. Multistage ejectors may be

further divided into condensing or non condensing types.

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The single-stage ejector, the simplest and most common type, is generally

recommended for pressures ranging from atmospheric pressure (30 in. Hg

absolute) to 3 in. Hg abs. Discharge is typically at or near atmospheric pressure.

Multistage non condensing ejectors (MNEs) are used to produce suction pressures

lower than 3 or 4 in. Hg abs. Steam consumption in an MNE is relatively high. Each

successive stage is required to handle the load plus the motive steam from the

previous stage. They are used for intermittent service or when condensing water is

not available. MNEs are usually two-stage.

Multistage condensing ejectors (MCEs) are available in two through six stages.

Inter condensers (surface or direct-contact) between stages condense steam from

the preceding stage, reducing the load to be compressed in the succeeding stage. A

multistage system is shown in Figure 1.

Four-, five- and six-stage ejectors are used to achieve suction pressures as low as 5

µm Hg abs. Under such vacuum conditions, pressure between the preliminary

stages is too low to permit condensation of ejector steam, and only the final two

stages are fitted with condensers. MCEs remove condensable vapor ahead of a

given ejector stage. They also permit use of a smaller ejector, and a reduction in

the amount of steam required. Condenser nomenclature is determined by the

corresponding operating conditions and functions.

Pre condensers are used when the absolute pressure of the process is sufficiently

high to allow condensation at the temperature of the available water supply. Non

condensable are removed from the pre condenser by one or more ejector stages.

Condensers or inter-condensers liquefy process vapor and motive steam from one

or more preceding booster ejectors. After condensers condense steam discharging

from the last-stage ejector, generally at atmospheric pressures.

There are two basic types of condensers –

1. Direct-contact.

2. Surface-contact.

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In direct-contact (countercurrent, barometric design) condensers, cooling

water is mixed directly with the vapor to be condensed, and then discharged

to atmosphere through a barometric leg or tailpipe of sufficient length to

overcome the atmospheric pressure. A means of cleaning up or otherwise

disposing the water that has become contaminated by process material is

often required.

The surface-contact condenser permits main-condenser cooling water to be

used as cooling water through inter- and aftercondensers, for energy and

process-water conservation.

Figure-1

This illustration represents a four-stage ejector system with two surface-contact intercondensers. A simple single-

stage system would comprise only the section in the outlined box. In either case, motive force is provided by steam

jets, which draw vapor from a vessel and through the system. The condensers act to reduce the load on the next

ejector.

Ejector systems have no moving parts; thus, they are designed for optimum

performance at a single set of conditions.

A key performance measure is the compression ratio: the ratio of the discharge

pressure to the suction pressure (note that the pressure of the motive steam is not

included). A single ejector stage can achieve compression ratios up to 8:1, although

values in the 3:1 to 5:1 range are more typical. The discharge pressure is set by the

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condenser pressure -- minimum pressure is the condensing pressure of steam at

the vapor outlet temperature.

Compression ratios can be increased by using several stages. In this arrangement, vacuum is pulled on

each condenser by a second ejector. This results in a lower vacuum on the process.

Number of Stages Suction Pressure (lowest)1 75 mmHg2 12 mmHg3 1 mmHg4 0.2 mmHg5 0.02 mmHg6 .002 mmHg

The average compression ratio for a system is best approximated as the overall

compression ratio to the 1/NS power (NS is the number of stages).

STEAM TRAP STRAINER

Steam traps, pass steam and condensate through a strainer before entering the

trap.  A circular baffle keeps the entering steam and condensate from impinging on

the cylinder or on the disk. The impulse type of steam trap is dependent on the

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principle that hot water under pressure tends to flash into steam when the

pressure is reduced.

The only moving part in the steam trap is the disk.   A flange near the top of the

disk acts as a piston. The working surface above the flange is larger than the

working surface below the flange.

INLET

OUTLET

STRAINER