DesaIkaion, 25 (1978) 227-23 1 @ Elsetier Scientific Publishing Company, Amsterdam - Printed in The Netherlands

SEAWATER REVERSE OSMOSIS SYSTEM PROJECTS, A MANUFAC-

TURER’S VIEWPOINT*

BILL ANDREW?I FoI_vmet&s, Inc., &ma Clara, CA 9XHI (U.S.A.1

(Received November 5,1977)

SUMMARY

A reverse osmosis desalination system will provide excellent service if care is taken in the design, selection, installation and operation of the system. Precautions should be taken not to overly complicate the design and operation. The environ- ment dictates that the highest quality components should be used. Satisfactory operation is highly dependent upon the experience of the manufacturer and the preparations taken prior to installation and commissioning of the system.

INTRODUCTION

Since March 1975, POLYMETRICS has built over 27 seawater reverse osmosis systems for operation in 16 different locations. The total capacity exceeds 1,700 mskday (440,000 gpd).

The seawater desaiting market has been weft developed by the manufacturers of thermal desalting equipment. To-enter this existing market with a new technolo- gy, the reverse osmosis (RO) manufacturer must have certain specific objectives. One objective is to produce sufficient income to justify the large development costs. Another major objective is to insure that the process is successfully applied. This paper will discuss some major factors that are instrumental in determining the success or failure of a reverse osmosis desalting application.

SSTF, CONDMONS

A desalting plant is usually required in focations that have no economic afternate supply for the support of fife or industry. Usually, the customer requires a reliable system instal!ed in the minimum amount of time at the least possible

* Presented at the Intematiooal Congress on Desalination and Water Re-Use, N~v. 27-Dec. 3, 1977, Tokyo.

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cost. As a consequence, it should be initiahy assumed that some of the following conditions will apply:

- Poor communications - Poor transportation - Adverse environment - Construction problems - Lack of reliable power source - Language difficulties - Customs/Visa problems - inexperienced operators

The existence of the above conditions can adversely affect the installation time and operation. It is essential that accurate data be obtained on site conditions at the earliest possible opportunity. The best approach is to arrange for a site visit by a project engineer from the factory.

The design of the intake structure has a considerable impact on the perform- ance of the reverse osmosis system. The silt and contamination levels in the feed can greatly affect fiItration and membrane replacement costs. Cooperation between the customer and system manufacturer will ensure optimum design of the intake.

In some cases, it may prove more economical to build a new structure, rather than using an existing intake.

DESIGN

Equipmen? quality

The desalting environment is possibly the most severe application for mechanical and electrical equipment. For land-based systems it often encompasses the extremes of both marine and land environments_ The equipment must be

designed to perform in an environment with the following hazards:

- High ambient temperature - High relative humidity - Salt spray in air - Atmospheric dust

The above environmental hazards act in conjunction with the presence of seawater, to provide an extremely difhcult operating environment for mechanical and eIectricaI equipment.

Much literature has been written on the internal corrosion of metallic elements containing seawater. It is important that resistance to external environ- mental attack is given equal consideration. The instruments, electrical controls,

coatings, and moving mechanical equipment shouId be given special attention. The environment dictates that the highest quality components and materials should be used throughout desatting plants.

SEAWATER RO SYSTEhf PROJECTS 229

The designer of a thermal desalting plant has a large number of variables to consider before he obtains the optimum design of a desalting plant. The two items that have greatest impact on capital cost are the materials of construction and the thermal efficiency. The result of this is that thermal plant designs are usually tailored to meet the customer’s required operating/capital cost ratio. As a conse- quence, the customer often has a plant that has never been built before with the same thermal characteristics. Small vapor-compression plants are one of the few

examples of thermal designs that can really be called “standard)‘. The designer of a reverse osmosis desalting pfant is somewhat more fortunate,

in that he has a lesser number of process variables to consider. Once the membrane to be employed is determined, the operating variabies to be monitored are well defined, so the instrumentation package is known. The greatest variable is the type of pretreatment to be employed. As a consequence, the only significant variables that the designer has are the extent of the pretreatment and the quality of the equipment. These variables both have a significant impact upon the capital cost of the system.

Assuming that the designer has elected to use the highest quality equipment, his only real variable is the extent of the pretreatment The pretreatment equipment

can be defined by careful analysis of the feedwater. If the different pretreatment processes are designed with a modular concept, then the necessary modules can be included in the system.

By selecting the described design philosophy, the manufacturer can offer a line of standard systems with different output capacities. Once the feedwater quality and required capacity are known, the installation can be easily defined. In this manner the customer obtains a system that has been refined and proven.

One of the disadvantages of the standard system concept is that the design is determined by the manufacturer. The customer is unable to impose his prefer- ences on the design. The improved reliability obtained by using a proven design far outweighs this disadvantage of unfamiliarity.

The role of the consulting engineer is somewhat different when standard systems are employed. Specifications need only state the water requirements and detail the site conditions. The necessity for a detailed process specification is eliminated. The expertise of the consultant is required for tech&al evaluation of bids, based upon performance of plants of the same design, which are operating under similar conditions. If the plant selection is made on this basis, then the probability of successful application is high.

Mechanical design

Reverse osmosis is a once-through system and thus does not employ re- circulation of fluids. This greatly simplifies understanding of the process flow and the layout of components. The advantage of simplicity can easily be lost if consid-

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eration is not taken to avoid unnecessary complexities in the mechanical design. The first step in the mechanical design is to arrange the floor plan of the

major components. It is not uncominon to have a number of individual units operating on the same site. For this reason, the floor plan of a single unit often has the shape of a long rectangle. Several units can then be placed adjacent with access aisles running the Iength of the long axis. The aisles need only be wide enough for handling of the largest component that will require removal. It would seem logical that the components should be placed in a long line with the same order as the process-flow drawing shows. In the case of POLYMETRICS’ sea- water design, this would result in the high-pressure pump being halfway along the aisles, and the membranes at one end. However, the pump is much heavier and larger than the membrane elements. By reversing the order to place the membranes in the middle and the pump at one end, the aisle width is reduced. Such a design consideration illustrates that it is important to optimize the layout for improved acct;ss and reduced floor area.

As installation can be a significant part of the total cost for a desalting plant, every effort should be made to reduce installation effort by careful mechanical design. This may dictate that subunits should be skid-mounted. Overhead and under fioor piping should be earefuily avoided. Equally, field welding should be eliminated. Errors in reconnection of electrical wiring can be a major cause of commissioning problems. Reconnection should be minimized.

On large systems a customer may request a central control room remote from the reverse osmosis units. Even the largest RO unit shou!d not require more than two variables to be controlled. These do not require automatic control loops, but only occasional manual adjustment. For reliability of operation, these controls should be located on the unit. The central control room is then reduced to a central monitoring room, with start/stop switches for each unit. If this philosophy is employed, the units can be commissioned as soon as they are installed. The central monitoring room can be constructed after start-up and the repeater instru- ments installed.

TESTING

The design concept of modular skid-mounted RO units lends itself readily for in-plant testing. Wherever possible, the entire RO unit, including pretreatment, should be assembled at the factory. The operation and mechanical integrity can then be fully tested prior to shipment. This is an excellent time to familiarize the operations manager with the system. The operating personnel for the installation should be present for the manufacturer’s in-plant testing.

INSTALLATION AND COMhiISSIOMNG

The construction of the intake and brine outfall should be completed first. The intake pump can then be operated while system installation is being done.

SEAWATER RO SYSTEM PROJECTS 231

This allows the intake structure to stabilize and eliminate any mechanical probtems. The manufacturer’s project engineer should be on site for the final stages of

installation. He should be responsible for supervising the commissioning and acceptance of the unit. Acceptance is usually based upon a minimum of 5 days continuous operation.

OPERATION

Unless the operator has previous experience with similar equipment, it is essential that a routine service contract be arranged for at Ieast one year. If there are frequent operatiag personnel changes, it may be necessary to have continuing routine service- The manufacturer’s field service engineer provides training, equipment service and advice to the customer.

Accurate daily logs of all operating parameters must be kept. If this is not done, prob!ems cannot be analysed properly, thus creating a difficult situation for both the plant owner and the system’s manufacturer. The customer should stock a comprehensive inventory of consumables and spare parts. This is preferable to the installation of standby pumps and other equipment.