Document text
TDA Progress Report 42-85
N86-28291 t
January- March 1986
I
Reverse Osmosis Water Purification System
H. G. Ahlstrom, P. S. Hames, and F. J. Menninger
Ground Antenna and Facilities Engineering Section
! A new reverse osmosis water purification system , which uses a programmable con-
troller (PC) as the control system , was designed and built to maintain the cleanliness and
level of water for various systems of a 64-m antenna . The installation operates with other
equipment of the antenna at the Goldstone Deep Space Communication Complex. The
reverse osmosis system was designed to be fully automatic; with the PC many complex
sequential and timed logic networks were easily implemented and are modifiable. The PC
monitors water levels , pressures , flows , control panel requests ; and set points on analog
I meters ; with this information various processes are initiated , monitored , modified , halted ,
I or eliminated as required by the equipment being supplied pure water.
J
I. Introduction
A reverse osmosis water purification system was designed
and installed for the Goldstone Deep Space Communication
Complex, California. This article describes the water treatment
requirements, control philosophy, and hardware and software
specifications.
The moving part of the 64-m antenna shown in Fig. 1, a
complex structure weighing over 3.2 XI 0 6 kg (7 X 10 6 lb),
rotates horizontally on a film of oil 0.2 mm (0.008 in.) thick
beneath three large pads. The risk of losing a spacecraft track
is high if the hydrostatic bearing fails due to insufficient
cooling.
There are three requirements for pure water for systems
that support the antenna operation. The first is cooling water
for the hydraulic system that maintains the critical film of
oil beneath each pad. The second requirement is for pure
coolant water for the high-power transmitter. The third pure
water use is for make-up water for the boilers and chilled-
water loop of the air conditioning system. Water supplied to
the station is pumped from nearby underground wells. The
chemical composition and quality are not adequate for direct
use; therefore, a purification system is necessary to meet the
specifications.
II. Functional Requirements
The performance and functional requirements for the pure
water system can be summarized as follows:
The two principal design criteria for the water purification
system are reliability and flexibility - reliability is derived
from the mission requirements and flexibility from the need
for versatility in monitoring and control, to permit unattended
operation. This versatility must meet constantly changing
process requirements, which include such events as placing an
operation on “hold” to perform another process, and sensing
the proper conditions for returning to the original operation.
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Without automatic controls, such variations would require
labor-intensive attention to the equipment.
Several water purification systems were considered, includ-
ing gas/oil fired water distillation, solar stills, reverse osmosis,
and purchasing purified water from a commercial water sup-
plier. The stringent cleanliness standards for the product of
this system, as well as the reliability required, led to the selec-
tion of reverse osmosis as the type of purification preferred.
The need for reliability and flexibility resulted also in the
selection of a programmable controller to manage the opera-
tion. The reverse osmosis system is shown in Fig. 2.
It was required that the plant be nearly maintenance free.
The new design should have process error checking and fully
automated operation, should automatically perform routine
“housekeeping” functions, and should be capable of remotely
controlled operation.
III. Reverse Osmosis Principles of Operation
Osmosis is the spontaneous passage of a liquid from a dilute
to a more concentrated solution through a semipermeable
membrane, which allows passage of the liquid but not of
dissolved solids. Reverse osmosis (R.O.) is the forced reversal
of this natural phenomenon, usually accomplished by apply-
ing enough pressure to the concentrated solution to overcome
the natural pressure (osmotic pressure) of the less concen-
trated solution.
This property of some membrane materials makes them
ideal for water purification. In general application, the mem-
brane unit is made up of either a bundle of fibers approxi-
mately the diameter of a human hair, or a spiral-wound
membranous material, within an epoxy tube. In either case,
the membrane surrounds a central core, within which the
clean, or “product” water flows after permeating through the
membrane to the core.
The reject water, or “concentrate,” emerges from the out-
side membrane material on the same side where the raw water
entered. No chemical reactions or phase transitions are involved.
One of the principal advantages of semipermeable membranes
over other methods of separating water from its contaminants
is that, with proper care, the membranes can be expected to
last from three to five years.
This article focuses on the application of a programmable
controller (PC) to control the R.O. process. It is therefore
necessary to be aware of the constraints on the operation of
the system and the process variations to take advantage of the
flexibility the programmable controller provides.
IV. The Reverse Osmosis Process
The water purification process at the Goldstone antenna is
divided into three stages: pretreatment, reverse osmosis, and
post-treatment. The process is controlled by a programmable
controller. The controller’s actions are determined by the
status of switches on the control panel and various sensors on
the process equipment. An instrumentation panel monitors
various analog signals and reports out-of-bounds conditions to
the PC.
Figure 3 is a simplified flow diagram of the R.O. system.
Many essential controls are omitted for simplification; the
diagram is not intended as a complete representation but only
as an aid to the discussion.
A. Pretreatment Stage
The reverse osmosis unit processes raw water from either
the local water supply (demand tank fill) or the cooling tower
sump water (tower recycle). The water cannot be run directly
into the R.O. membranes; it must first go through preliminary
processing so as not to damage the membranes. This pretreat-
ment involves flowing the water through a sand filter (to
remove large dirt particles) and a carbon filter (to remove
chlorine, which would oxidize the membranes), injecting
acid to control pH level to a slightly acidic condition (excess
alkalinity can foul membranes), and flowing the water through
a 10-micron prefilter and then past sensors that check for
pH level and the presence of chlorine. (Raw water typically
has a pH of 8.5 to 11.5; the R.O. membranes require the pH to
be approximately 6.0.) Now the water contains very few
nondissolved solids, but is still high in dissolved solids. It is
now ready for the reverse osmosis treatment.
B. Reverse Osmosis Treatment Stage
The reverse osmosis treatment section consists of pressure
switches, flow controls, conductivity ratio sensors, flow
meters, R.O. membranes, pressure gauges, and a main pump.
Each of these is critical to the production of purified water.
The inlet pressure to the pump must remain above 10 psi
to avoid cavitation in the pump. The inlet pressure to the
membranes must stay below 400 psi; otherwise the membranes
will be damaged. Pressure switches monitor these critical
items.
The flow controls are adjusted to maintain a ratio of 50
percent product water and 50 percent reject water, with a
production flow of 15 gallons per minute. The reject water
contains all the impurities removed from the product water
and drains into the sewer.
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The conductivity ratio sensors are part of the membrane
efficiency meter, shown in Fig. 4, measuring the amount of
dissolved solids in the water. The total dissolved solids in
feedwater are typically 500 to 600 parts per million (ppm).
Product water typically contains less than 45 ppm. The meter
determines what percentage of dissolved solids is being rejected
by the membranes; when it drops below 90 percent, the
PRODUCT WATER LOW REJECT RATIO error signal is
generated.
C. Post-Treatment Stage
Product water flows through an ultraviolet sterilizer, and
then to either the demand tank by way of a degassifier, or
to the cooling tower for sump recycle. The ultraviolet steri-
lizer kills bacteria in the water. The degassifier is a tall cham-
ber above the demand tank, filled with saddle packing; water
flows down over these saddles as air is simultaneously blown
up over them removing dissolved gasses.
From the degassifier, product water flows into the demand
tank until it reaches a preset level, monitored by a level switch.
The demand tank can then drain to either the shutdown flush
pump, or the demand pump. The shutdown flush pump
supplies product water to the main pump for a cleaning flush
of the membranes after every operation. The demand pump
supplies water to the additional PC-controlled functions.
V. Additional Control Functions of the PC
In addition to processing water through the R.O. unit, the
PC manages the water level of the cooling tower, a transmitter
coolant storage tank, and an air conditioning chilled water
storage tank. In addition, it manages the cleanliness of the
cooling tower. The demand pump pumps water to three
places, as selected by solenoid valves: the heating and air
conditioning system (make-up water), the cooling tower sump,
or the transmitter coolant system storage tank through a
deionizer.
A. Cooling Tower Functions
A front panel switch, as shown in Fig. 5, tells the PC
whether to refill the sump with raw water, purified water from
the R.O., or nothing. The cleanliness of the tower water is
monitored by a conductivity meter on the instrument panel.
When a preset high limit in the conductivity meter is reached,
the PC starts a cleanup process as determined by another
front panel switch, which tells the PC whether to recycle the
sump water through the R.O. or to pump it to the drain
(blow down). Blow down is the operation of removing water
from the sump and replacing it with fresh water. The con-
troller blows down the sump by pumping water to drain and
replacing the lost water with either raw or R.O. water, as
selected. (If neither recycle nor blow down is selected, an
alarm is generated.) When conductivity reaches the low limit,
the PC ends the cleanup process.
In recycle mode, the R.O. returns only 50 percent of the
water it removes from the tower. If the refill switch is on
R.O., then the other 50 percent is furnished from the demand
tank. Since the R.O. must be flushed after each water produc-
ing cycle, there must be enough water in the demand tank to
flush the R.O. membranes. Therefore, when the demand tank
is below this middle level switch the PC will first fill the demand
tank before proceeding with the recycle request.
B. Transmitter Tank Fill Function
The front panel switch selects between MANUAL-OFF-
AUTOMATIC refill of the transmitter coolant tank. In auto-
matic, a level switch indicates to the PC the need for water.
(In manual, the level switch is overridden.) If the demand tank
is not empty and the tower is not being refilled, the demand
pump is turned on and a valve opens to send water to the
transmitter coolant tank through a deionizer. The deionizer
further purifies the water to a resistivity of at least 2 megohms/
centimeter. (With a fresh deionizer, the output can be as high
as 16 megohms/centimeter.) The water is monitored by a
resistivity meter that has a preset low-level trip point, at which
the XMTR WATER LOW RESISTIVITY error signal is gener-
ated when the water is top conductive. To prevent excess
water from being pumped to the transmitter, this function can
be active for only ten hours. Then the system must be reset
before more water will be pumped to the transmitter.
C. Air Conditioning System Fill Function
In normal operation, a manual valve is opened and water is
allowed to flow down to a chilled-water storage tank at the
control building for the air conditioning system and to the
humidifiers. (There is an elevation difference of approximately
30 feet.) If a large quantity of water is required, the demand
pump may be turned on and the line to the storage tank pres-
surized (G-86 PRESSURIZE front panel indicator on). This
function has the lowest priority, and is interruptible by the
tower-refill-with-R.O.-water process or the transmitter-tank -fill
process. To prevent excess water from being pumped to the
storage tank, this function is active for only 10 hours. Then
the system must be reset before more water may be pumped.
VI. Housekeeping Functions
At the end of each water-producing operation, the system
automatically goes into a “shutdown flush” cycle. The flush
pump pumps product water from the demand tank to the
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main pump ahd then to the reverse osmosis unit. The product
water from this operation returns to the demand tank or
sump, depending on the production cycle that was just fin*
ished. The purpose of this flush is to feed very pure water
into the membranes so as to flush out any contaminants.
A high-flow flush is performed once each day. (This is
different from a shutdown flush and is performed even if no
water production cycles have taken place that day.) A sole-
noid valve is opened to bypass the reject flow control valve;
the reduced backpressure causes 90 percent of the water to
go out the reject ports, thus flushing any solids from the sur-
face of the membranes. The high-flow flush of the membranes
lasts for fifteen minutes. In addition, once a week the sand
filter and the carbon filter are backflushed, one at a time.
Each of these filters is also backflushed when a high pressure
drop exists across the filter.
All of these operations are part of programmed mainte-
nance activities, independent of any other actions, and are
initiated by the programmable controller’s software clock.
They are, however, subject to the same monitoring and alarm
controls that accompany all other operations. The timing is
part of the PC program; no additional hardware is required.
The sand filter and carbon filter backflush and the high-
flow flush may be activated manually by using the power of
the PC. This capability was accomplished without the use of
an additional front panel switch. When all process selector
switches are turned to OFF, and the ALARM ACKNOWLEDGE
button is pushed twice, all three flushes are enabled. The sand
filter is flushed first, followed by the carbon filter, and then
the high-flow flush. At any time, each flush can be canceled by
pushing the ALARM ACKNOWLEDGE button twice. This
action establishes a form of reset, in which the first push
holds everything, preventing the machine from running; the
second push resets all sequences.
VII. Process Alarms
If any of eleven parameters is out of limits, an alarm
sequence is initiated. Once a process is started, each instru-
ment can generate an error signal, after a time delay has been
satisfied to allow for transient water conditions. Once an error
signal is generated, it will create an error routine that shuts off
the applicable process. The eight processes of the R.O. system
for purposes of alarm shutdown are demand tank fill, tower
refill, tower recycle, tower blowdown, transmitter tank refill,
air conditioning system pressurize, high-flow flush, and shut-
down flush. After a five-minute delay, the controller will
attempt to restart the interrupted process. This allows for the
air bubbles that can get into the water supply. The delay
can be manually overridden by pushing the ALARM ACK-
NOWLEDGE button twice. If, after two restart attempts,
the error still exists, the interrupted process will be shut off
until it is manually reset by operator action (pressing the
ALARM ACKNOWLEDGE button twice). The error is now a
fatal error. When a fatal error occurs, an alarm is sent by the
controller to a console at the communications center of the
complex.
When an operator acknowledges an alarm, the system is
placed in a “hold” condition with a 30-minute allowance for
maintenance. If someone should forget to reactivate the sys-
tem, the hold condition is canceled; if no reset has been input
by this time, the system goes into SYSTEM SHUTDOWN
mode and transmits an alarm.
VIII. Process History
The PC has been programmed to restart the system and
continue, if an error does not reappear. This feature makes it
difficult for maintenance personnel to determine whether
there have been any errors since the last time the system was
checked. Maintenance personnel may not check on the system
for up to a week or two at a time. Repetition of a certain
error can be a telltale signal that a certain section is becoming
marginal and should be checked closely before it shuts the
system down. As a maintenance feature, each error increments
its own counter inside the PC when it shuts the system down.
There are also counters assigned to each process. Some
count the number of times a process has occurred, and others
count the number of five -minute increments each is on.
These are useful in analyzing how the machinery has been
used since the last time it was checked. The number of five-
minute increments a process has been on can be multiplied
by a constant to give an accounting of how much water was
sent for each use; this number is within ten percent of the
actual number and does not require costly totalizing flow
meters to report back to the PC. There are two totalizing flow
meters, one for the product water and one for the reject water,
but they do not report back to the PC.
IX. The Programmable Controller Rationale
The large number of control and monitoring actions required
in a system such as this makes the programmable controller an
ideal choice for control methods. The necessity to effect the
control actions, make the control change decisions, and pro-
vide the requisite isolation of sensor inputs from control
outputs could suggest a completely computer-controlled
system. But the sophistication of expensive computer hard-
ware would constitute control “overkill” in this case, and the
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cost would be compounded by the environmental protection
necessary for most computer installations. The programmable
controller modules are rugged and provide the ideal compro-
mise, with cost savings on both ends.
For example, during pre-treatment, the water is monitored
for excessive pressure drop at three stages, as well as for high
or low pH or the presence of chlorine. If any sensor detects
an out-of-limits condition, an error routine is generated and
the condition is displayed on the appropriate analog meter.
At the same time, corrective action is started by the PC. If
another process was in progress at the time, it might continue,
or be placed on “hold,” depending on the relative importance
of the two actions. Note that the relative importance can
change; the first priority of the system is to keep the cooling
tower sump filled; if the sump is not low, and another process,
for instance the production of pure water, is ongoing and a
high pressure drop occurs, the process would be suspended
until the excessive pressure drop is corrected by a backflush
of the offending tank. If, however, the cooling tower water
level is low, refilling to a specified level would take priority
and the backwash itself would be put on “hold.”
In this application, several different conditions enable
various processes. There are 32 inputs, 64 outputs, 114
internal logic relays, and 77 storage registers. Only 44 of the
77 storage registers are used for time delays and time-outs.
To implement this design in discrete components would
require a relay panel that was cost prohibitive, not to mention
labor intensive. Moreover, should a slight change in process
requirements occur, the task of rewiring would be time con-
suming and costly.
From an extensive survey of commercial programmable
controllers available for this application, the Square D SY/
MAX-20 (Fig. 6) was selected. Following are two of the
features that determined the selection.
A. Dual Memory
The dual memory fits the reliability philosophy of this
subsystem, which must support a very critical and probably
unrepeatable mission. The application memory combines the
advantages of random access memory (RAM) and electrically
alterable read-only memory (EAROM) to ensure both the
program flexibility offered by the RAM and the memory-
saving characteristic of the EAROM. Program information
stored in the EAROM is retained in the event of power failure,
and even if the memory module is removed from the processor.
The memory can be reprogrammed as often as desired, the
change being as small as one contact or as large as the entire
program .
These memory modules had several features unlike those of
other manufacturers, at the time of purchase. The memory
was programmed, when installed in the PC, with a command
from the program to transfer the contents of the RAM to the
EAROM. The memory was encased in a steel box with a
rugged connector on the back. The connector configuration
prevented the memory module from being inserted incorrectly
into the PC.
B. Computer Interface Module
Another important feature is the computer interface mod-
ule, which permits remote control and monitoring, as well as
communication with the programmer. In this application all
inputs, outputs, internal relays, and storage registers are
accessible to a computer at a remote location. A modem in the
PC cabinet is used to transmit this data to the computer
terminal 14 miles from the pure water system installation. The
computer can request any data that is stored in the PC at any
time; and the PC can be called up as desired. This allows a
historical record of how the system has been performing, and
can indicate the development of trends in the process, such as
gradually decreasing time between carbon filter backwashes,
indicating the approach of the need to replace the carbon.
The computer interface allows access to information in the
PC through a non-proprietary serial port. Any computer with
a serial port could be programmed to communicate with the
Square D processor.
X. Summary
The reverse osmosis water purification system for the
NASA/JPL 64-meter antenna has been designed and installed.
Utilization of a programmable controller made it possible to
meet stringent performance requirements. The system has
operated successfully since July 1982, with little or no pre-
ventive maintenance. Due to this increased reliability perfor-
mance, the cooling system of the antenna has been much more
reliable and has required significantly less maintenance.
One advantage of the programmable controller was the
ability to make small program changes quickly during system
startup: RAM could be modified and tested operationally
before it was transferred to EAROM.
The successful experience gained by this PC installation
led to the selection of PCs for control of other facilities at
the Deep Space Communications Complex. Examples are
as follows:
(1) A 15 -foot diameter mobile antenna used in the Orion
earthquake research program: The SY/MAX-20 PC
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controls antenna deployment, storage, and leveling
of the trailer and antenna, making it possible to assem-
ble and disassemble the system in 20 to 30 minutes.
(2) The electrical power generating station at the Echo
Station (DSS 12): This installation, with five 500-kW
diesel generators, is also controlled by a SY/MAX-20.
(3) Air conditioning for three buildings at DSS 12, build-
ings G26 (control room), G34 (hydromechanical build-
ing), and G35 (antenna): This complex system, designed
to meet the varying needs of three very different
structures, utilizes a Square D Model 500 program-
mable controller, which simplifies' control of a highly
automated and energy efficient system.
The reverse osmosis water purification system, under pro-
grammable controller management, has met all of its principal
design objectives: a cost-effective, high reliability, maintenance-
free plant that incorporates fully automated operation, process
error checking, automatic performance of routine housekeep-
ing functions, and capability of remotely controlled operation.
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ORIGINAL 'PAGE iZ
OF POOR QUALITY
Fig. 1. The 64-m antenna at Goldstone, California
Fig. 2. Reverse osmosis water purification system
TRANSMITTER
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Fig. 3. Simplified flow diagram of the R.O. system
ORIGINAL PAGE 13
OF POOR QUALITY
Fig. 4. R.O. instrument panel
Fig. 5. R.O. control panel
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