Chemical Engineering September 2021 - 51

TABLE 2. TYPICAL OPERATING STEPS FOR AUTOMATIC SAND FILTERS
Step
Service
Drain down
Air scour
Refill
Backwash
Duration Valves open Valves closed Backwash
pump
24 h
1,200 s
900 s
900 s
600 s
1,2
3,4
4,5
4,6
7,8
the refinery's maintenance department
that both backwash pumps
were still in good condition. Everything
from actual blade size to failure
record was thoroughly reviewed
and accepted. Therefore, the pump
characteristic curve illustrated in Figure
10 should provide a good flowrate
estimation.
Before commencing the backwash
operation, the positioning bolt of the
backwash inlet valve (valve no. 7 in
Figure 4) was pre-adjusted at approximately
50% of the full insertion
length as a preventive measure to
prevent excessive filter media loss.
Regarding initial field pressure data,
3,4,5,6,7,8
1,2,5,6,7,8
1,2,3,6,7,8
1,2,3,5,7,8
1,2,3,4,5,6
Off
Off
Off
Off
On
Air scour
compressor
Off
Off
On
Off
Off
he found that Pdischarge - Psuction was
2.50 bars (approximately 150 m3/h
of backwash flow), while the target is
1.95 bars. Consequently, an instrument
technician adjusted the bolt
length so the differential pressure
became as targeted (approximately
1.95 bars).
More than that, a visual check
of backwash water also confirmed
that the backwash flowrate was
too low to expand the filter media,
because the water appeared quite
clear. Once the differential pressure
approached 1.95 bars, a significant
amount of sludge and silt had
been observed in the backwash
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water, with a negligible amount of
anthracite. The dirty appearance
of backwash water implied an efficient
backwash operation. It was
also proof that the differential pressure
target of 1.95 bars worked,
although a full backwash performance
test would still be required.
Ensure robustness against fluctuations
in the water level. On the
day this engineer adjusted the backwash
flowrate, he asked the board
operator to reduce the water level in
the storage tank to 85%, the minimum
level.
To ensure the system's robustness,
he also monitored the backwash
operation when the water level
was at 95%, the maximum level.
As per the field monitoring, he
found minimal anthracite in the
backwash water, with the estimated
backwash flowrate of 310 m3/h (or
3% above design) using the same
technique described earlier. Ideally,
the maximum and minimum water
level should be properly controlled to
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Chemical Engineering September 2021

Table of Contents for the Digital Edition of Chemical Engineering September 2021
