Global Business Environment
for desalination and water treatment projects
Plant availability
There is a widespread confusion between the plant availability
and the plant reliability. The latter relates to the procured
equipment failure rate and is the negotiation point of the project package
whereas the first is not as it depends on the repair capabilities of the
plant personnel and the adopted preventive maintenance plan (PMP). It
necessarily includes such time-consuming procedures as CIP cleaning,
the membranes reshuffling and replacement and the seawater intake pipe pigging.
Quantitatively availability is always less than reliability. Depending
on the plant design the latter varies within 97.5 – 99.5%. Availability
below 95% is not rare.
So only the plant reliability defined as (1 – repair_time*capacity_drop/8760)*100%
is subject to the performance guarantee.
When a plant is reliable, maintenance costs decrease, unplanned outages
and incidents are few and far between, the cost of production goes down,
and profits go up.
The plant reliability design requires comprehensive system approach and
is a compound result of the following vectors.
Requirements for more reliable design and construction are set out in the
equipment specification. In practice the equipment is considered reliable
if it has enough similar application references, good operating records
and proven sufficient service life.
The pump will be more reliable if it complies with the following criteria
For optimally selected and maintained pumps MTBF (mean time between failures)
is far above 5 years, average values being 2.5 – 4 years and pump
repair costs being $10,000 - $25,000 (the author estimate). This does not
include lost opportunity costs.
The pump motors bigger than 1000 kW shall be ordered with reduced vibration
levels and shall be designed at least for 300 starts per year. The motor
life may be increased substantially if ‘soft’ start is used.
Interconnecting piping is rarely a source of trouble if the following
points are taken into account.
Valves last longer if their sizing and specification are done with the following
points in mind.
The membrane manufactures guarantee mechanical integrity of the reverse
osmosis membranes if and only if the pressure feed variation is kept
below 1 Bar/sec during the plant startup and stop.
At present practically no seawater desalination plant
(except
the Palmahim SWRO plant, Israel) complies with this requirement –
start/stop procedure is not controlled by interlock on excessive pressure
variation. For new plants with the 16-inch membranes, planned to
stop daily this limitation on the pressure feed variation may turn into
a bottleneck problem hard to crack.
Spinning reserve capacity is the difference between the required design
capacity and the rated one. Centrifugal pumps working at BEP always have
rotating reserve capacity over 10%. Drives - AC motors and variable speed
drives - are sized by 10-15% over the maximum continuous load. At low
ambient temperatures motor may be overloaded by 5 – 10%. Dosing
pumps are always selected with high redundant capacity sometimes surpassing
100%.
If the pumping station is equipped with 5 pumps each having 20% spinning
reserve capacity and driven by VSD, than the capacity drop caused by one
pump failure may be recovered to 80% by overloading the rest of the pumps.
So there is no need in backup capacity. No tender addresses this issue.
Backup capacity is an effective means of providing high reliability for
equipment modules connected in parallel. For instance, at the normal operation
conditions all dual-media filters are in operation. In case one filter
is shut down, its load is re-distributed between the balance of the filters
without impairing the product quality. For seawater desalination plants
working at 65 - 75 Barg, the pump backup capacities are very expensive.
To reduce capital costs and given the axially split pumps high reliability,
instead of the installed pump only it's rotor with impellers, bearings
and mechanical seals may be kept in the warehouse. Another point to consider
is that in warm surface seawater (above 28oC) even superduplex steels
are subject to pitting and crevice corrosion (but at substantially less
rates than the SS316 steel).
Structural reliability analysis (SRA) tries to assess the whole system
reliability if the reliability data for the system each equipment piece
is known. This analysis is an important part of the plant conceptual design
starting from comparison of the plant different process schemes and the
equipment unit sizes and quantities. Such comparison objective is to minimize
the life cycle costs.
To the best of my knowledge SRA is never done by the book primarily due
to a lack of the reliability database, software tools, and special training.
Lightweight version of SRA treats the desalination plant as a chain of
processes – seawater pumping, pretreatment, feeding to desalination
units, SWRO membrane arrays, energy recovery and , finally, post-treatment.
The plant is generally admitted reliable if any of the above-mentioned
chain pieces has the same level of reliability.
The reference point for SRA is the pressure center design – the
term coined by the designers for the Ashkelon desalination plant (Israel, 2005).
(Power engineering uses the cross-linked design term. One cannot
help noticing that the desalination industry exactly follows the steps
of the power engineering development of seventies – transition from
the cross-linked design to the packaged one.)
The pressure center offers highest structural reliability and energy efficiency.
On the other hand it has the following disadvantages (in the order of
impact - lessons learnt from the power engineering history).
Due to the above-mentioned points the pressure center design is not a preferred
choice for the majority of the modern desalination plants.
The reliability of the desalinated water production shall be treated differently
from the reliability of the product chemistry maintenance: the product consumers
are always tolerable to the temporary offsets from the product time-averaged
quality criteria.
For example, if the seawater desalination plant produces technical-quality
water for the nearby iron ore refining factory, the only meaningful requirement
would be the water non-corrosivity corresponding to the LSI index above
-1. Non-treated water (at LSI below -1.5) may trigger corrosion of the construction
steels but at the rates substantially lower than those observed in seawater
for SS316 and the duplex steels - main construction materials of the desalination
plants. Under the circumstances, the post-treatment system with the ‘quality’
reliability of 90% would be an acceptable solution. In the case of the lime
milk dosing system this figure may mean 30 – 50% (!) decrease in the
installation costs as compared to the system with the 98% reliability.
Structural reliability requirements shall be relaxed for the equipment
or systems with the average annual load substantially less than the installed
capacity.
Let us consider the seawater desalination plant with Boron removal system
(BRS) treating only part of the permeate quantity produced in the first
pass (split RO scheme). Due to strong effect of the seawater temperature
on the permeate quality, its flow rate to BRS varies from zero at winter
time to 35-40% in summer. Such operation reduces the BRS installed capacity
annual utilization to approximately 4500 hours. For such units the design
criteria shall foster the design ruggedness and inexpensiveness even at
the expense of power efficiency and shall allow for the temporary offsets
in the product quality.
By the same token, auxiliary systems like CIP and chlorination (for intake
and posttreatment systems) with the installed capacity usage less than
100 hours annually shall not contain any reserved capacities.
Excerpt from the pump reliability database
1
Operation
continuous
2
Power, kW
1000
3
Application
high pressure booster
4
Equipment class
axially split centrifugal pump
5
Manufacturer model
Sulzer SMN 402-620
6
qty
8
7
Cumulative operation time, hours*
140000
8
Down time, hours
104
9
Unavailability
0.000742857
10
MTTR, hours
8
11
MTBF, hours
21875
12
SM downtime, hours
98
13
SM unavailability
0.0007
14
MTTM, hours
6.5
15
MTBSM, hours
9285.7
Where MTTR - mean time to restore, SM - scheduled maintenance, MTTM - mean time to maintain,
MTBSM - mean time between scheduled maintenance instances.