| VACUUM
GENERATION |
 |
Vacuum
Systems are necessary for a variety of applications in Petrochemicals, Refineries,
Process, Chemical, Plastics, Pharmaceuticals, Synthetics Fibre, Pulp &
Paper, Power, Sugar, Desalination, Evaporators, Edible Oil, Refining and
Steel plants for distillation, drying, flash cooling stripping, evaporation,
deodorization, degassing etc that frequently take place at less than atmospheric
pressure. |
| There are
various types of vacuum-producing devices but the simplest and probably
most widely used vacuum producer is the Ejector. It has established the
superiority and versatility of its use as compared to the mechanical vacuum
pump. |
| All
Ejectors offer a range of attractions |
Simple design, with no moving parts and practically no wear. |
|
Can be mounted in any orientation |
|
Can be fabricated of virtually any metal, as well as various types of plastics.
The latter are usually fiber-reinforced grades. |
|
Lowest capital cost among vacuum-producing devices |
|
Offers the largest throughput capacity of any vacuum-producing device -
can handle more than 1,000,000 ft3/min of process fluid as they
can convey them at very high velocities. |
|
No special start-up or shutdown procedures required |
|
Can handle condensable loads |
|
Simple repair and maintenance
|
| Ejectors
are especially attractive when the process load contains condensable or
corrosive vapors, very low absolute pressures are needed, or the vacuum
- producing capacity required is very large. However, these devices are
not confined to such applications; they should also be evaluated, along
with other options in other process situations. |
| Ejectors
also work well as boosters upstream of liquid-ring pumps (below). This combination
can minimize capital and utility costs with no sacrifice in performance. |
A Steam
Jet Ejector is simplified type of vacuum pump or compressor, consisting
of three basic
parts : 1) Nozzle 2) Mixing Chamber 3) Diffuser
|
| The Nozzle
discharges a high velocity Steam Ejector across a suction chamber that is
connected to the equipment to be evacuated. The process vapours are entrained
by this Steam Jet and carry into a Venturi shape diffuser, which convert
the velocity energy of the steam into pressure energy enabling discharge
ultimately to atmosphere |
| The Ejectors
are generally categorized into one of four types |
| a) Single
stage. |
| b) Multi-stage
condensing. |
| c) Multi-stage
non-condensing |
| d) Multi-stage
with both condensing and non-condensing stages. |
| Lower suction
pressure is obtained by staging of the Ejectors. The number of stages of
Ejectors is depended on the suction pressure required. Nominal range of
suction pressure for number of Ejector is stated below.
|
| Due to high cost of
power available from most of the Stage Electricity Boards, many of the large
Industries are installing their own captive power plants. Many a times,
low-pressure steam is available as exhaust from the back pressure turbine
of their captive power plant. Instead of condensing this waste low-pressure
steam and using it as boiler feed water, the same can be utilized to motivate
Ejectors for their process applications. As the cost of this low-pressure
waste steam is very less, the running cost of the plant reduces drastically
as the Booster/Ejector Vacuum System might utilize majority of the steam
in a plant. |
| Boosters and Ejectors
have been supplied for as low as 0.5 Kg/cm2 (g) motive steam pressure and
as high as 40 Kg/cm2 (g). Cooling water temperature also plays a very major
role for the efficiency of the Ejector System. Roughly, for every 20 C,
increase in the cooling water inlet temperature of the inter-condenser from
320 C will increase the steam consumption by about 10% and vice versa. Therefore,
cooling tower should always be properly maintained. |
| Also, nowadays considering
high cost of energy, mainly steam, most of the installations have incorporated
Multiple Nozzle Boosters which consume about15 - 20% lesser steam compared
to conventional single nozzle Booster designed for the same motive and suction
pressure and capacity. |
| Boosters and Ejectors
always need to be performance tested to ensure foolproof performance at
site with the correct utilities. Most of the Ejector manufactures in India
do not have such a performance test center at their works because of which
many a times, modification need to be carried out at site. However, even
after the modification, it is not necessary that the Ejector Vacuum System
will function with the right specifications and might have to be operated
at off design conditions. Boosters and Ejectors are thoroughly tested for
performance with simulated site condition. This ensures trouble free commissioning
of the system and performance with the right utility figures.
|
| Unsatisfactorily performance
of an Ejector System can be caused by external or internal causes. Unsatisfactory
performance can also be classified as sudden or gradual. The gradual loss
of vacuum will normally suggest internal erosion or corrosion, whereas a
sudden loss of vacuum will normally suggest external causes. Since it is
easier to check external causes of trouble, all possible external causes
should be checked first.
|
| External
Causes of Trouble |
| 1) Low steam pressure. |
| 2) Wet steam |
| 3) High water temperature
or insufficient water flow. |
| 4) Entrained air in
condenser water. |
| 5) High discharge pressure
|
| 6) Fluctuating water
pressure |
| 7) Change in load -
excessive air leakage. |
| Let us assume that
a multi-stage system has lost vacuum. The possible external causes are quickly
checked and are found trouble free. We must now look for internal causes
of trouble. |
| Internal
Causes of Trouble |
| 1) Eroded or corroded
parts, particularly nozzles and diffusers |
| 2) Clogged nozzles,
diffusers and strainers. |
| 3) Leaks in steam chests. |
| 4) Clogged or fouled
water supply |
| 5) Clogged water discharge |
| 6) Excessive leakage
- cracked or worn parts. |
| 7) Inter condenser
water nozzle eroded. |