DESALINATION

Any functional design shall be reliable over entire lifecycle. Tenders’ request for the desalination plant reliability analysis (RA) becomes a commonplace. Generally this analysis shall demonstrate that the offered plant design is reliable enough and convince the client that the selected technical subsystems (chemical dosing, post-treatment reactors, sludge treatment, etc.) are optimized to the availability criteria.
Currently most engineering companies have neither reliability expert on staff nor design-for-reliability culture. Reliability is addressed neither by international standards nor industry practices. From the author experience, contrary to the process engineer intuition considering RA a fancy stuff, even at the bidding stage RA effectively identifies the subsystems with low level of availability and adds substantially to systems engineering perspective. Later at the engineering stage RA helps compare design alternatives, and track reliability improvement.
The plant design for reliability requires comprehensive system approach and is a compound result of the following vectors.

  1. Equipment design reliability,
  2. Spinning reserve capacities,
  3. Backup capacities,
  4. Plant structural reliability.

The above-mentioned is not covered by the traditional Reliability Engineering coming into play after the plant construction.

CP implementation of Design For Reliability is based on the following fundamental axioms.

  1. Reliability is a third in importance criterion of the plant design excellence after the product water quality and energy consumption.
  2. Reliability is controllable: it may and must be designed into sub-systems and processes.
  3. Reliability practices must begin early in the design process and be well integrated into the project engineering and management cycle.

CP reliability framework includes the following.

  1. CP records the designer previous experience in engineering the same type systems
  2. CP assesses whether the module or item failure is remotely detectable
  3. CP audits control loop redundancy
  4. CP enforces new substantially more rigid standards for safety interlocks
  5. CP auto-generates comprehensive body of safety interlocks covering 10 times more off-design situations
  6. CP auto-generates Reliability Block Diagram based on PFD/CFD and identifies the hotspots – equipment modules (EM) with inadequate reliability (highest likelihood of failure occurrence).
  7. CP auto-generates the reliability analysis report for the plant operation range
  8. CP auto-generates the spare parts an items stock matching requested reliability
  9. CP maintains the MTTF and MTTB database for desalination and water treatment equipment

The goodness of reliability prediction critically depend upon the historical reliability data collection. Requested by the tenders “proven experience in the plant design and O&M (operation and maintenance)” is actually disguised request to demonstrate the reliability data collection framework to track and improve the reliability design based on feedback from site. Without this framework any EPC contractor may be easily disqualified. Below is an excerpt from the pump reliability database.

Table 1: 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 is mean time to restore, MTBF – mean time between failures, SM - scheduled maintenance, MTTM - mean time to maintain, MTBSM - mean time between scheduled maintenance instances.
Such a framework is part of Maintenance Management Software (MMS). Its procurement and activation (the asset database compilation) is usually not in the scope of the plant designer. This fact drastically devaluates the designer proven experience and blocks transition to the long-term partnership with the client.
The EPC contractors do not include MMS in the plant scope by the following reasons.

  1. MMS activation is a time-consuming as Basis of Design Book - main document of the project - does not contain the maintenance data
  2. To be meaningful, feedback from MMS should be compared and analyzed across many projects and be rooted into the design-for-reliability logic. No commercial MMS meets this requirement.
  3. The designers avoid linking the operating plant availability with Preventive Maintenance Plan (PMP) and O&M due to lack of historical data. 

CP changes everything.
Auto-generated spare part/item stock is tied down to P&ID items and groups and may be analyzed across all the projects executed by the company. CP auto-generates Preventive Maintenance Plan (PMP) that includes frequency of service, the time required to complete the task, annual work schedule, issuing work orders and the budget. 
For RO desalination plants, most of O&M work-hours are allocated to the RO membranes history tracking, sampling, testing, replacement and re-shuffling inside the pressure vessels. These tasks become daunting in the megaprojects dealing with thousands of membrane pieces. CP offers full software package for performing these tasks; it develops the annual schedule, the work scope, and sequences for membrane testing and replacement.
CP prepares the database (with all of the P&ID information downloaded) for the valve-exercising program. It includes the following.

  1. Valve service list in the order of importance
  2. Valve exercising procedures schedule (similar to start/stop sequences)
  3. Annual schedule
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