list control system _ anti-heeling
TRANSCRIPT
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SNAME
T rans ac t i ons ,
Vol. 105, 1997, pp. 171-189
List Contro l Sy stem s
Fr an k H. Se l lars, M emb er, Flume Stabil ization Systems, J oh n P. Ma rt in , Mem ber, Flume Stabil ization
Systems, Wi l l iam L. Sehul tz, V is i tor , Hyde Marine Systems
A B S T R A C T
The types of ships that normally apply automatic l is t control systems are revie wed and the
reasons fo r the application are summarized. Various types o f l is t control system s that are
comm ercial ly available are enumerated in detai l and the dif ferences betw een th ese system s
are explained. De sign C onsiderations that couM define select ion cri ter ia are l is ted,
inc lud ing som e po ten t ia l p rob lem areas .
N O M E N C L A T U R E
L C S = l is t c o n t r o l s y s t e m
H P U = h y d r a u li c p o w e r u n it
T D H = t ot al d y n a m i c h e ad
B = d i s t a n c e b e t w e e n l i st c o n t r o l t a n k s
H p = p o r t t a n k l e v e l
H s = s t a r b o a r d t a n k l e v e l
H A = h e a d a d d ed b y p u m p
H L = h e a d l o s s d u e to f r i c t io n
law = b l o w e r d i s c h a r g e h e a d
A o = c r o s s d u c t a r e a
P a = a t m o s p h e r i c p r e s s u r e
p p = a i r p r e s s u r e i n p o r t t a n k
p , = a i r p r e s s u r e i n s t a r b o a r d t a n k
PB = b l o w e r d i s c h a r g e p r e s s u r e
Q~ = l i q u i d v o l u m e t r i c f l o w
Q a = a i r v o l u m e t r i c f l o w
= l i s t a n g le
V~ = t l u i d v e l o c it y
V , = a i r v e l o c i t y
K I = f l u i d f l o w l o s s c o e f f i c i e n t
I ~ = a i r f l o w l o s s c o e f f i c i e n t
t~ = l i q u id s p e c i f i c g ra v i ty
y ~ = a i r s p e c i f i c g r a v i t y
I N T R O D U C T I O N
E x c e s s l is t o f a s h i p d u r i n g c a r g o h a n d l i n g o p e r a t i o n s
c a n b e d e t r im e n t a l . F e r r i e s an d R O / R O v e s se l s u s e
l o a d i n g r a m p s t h a t a r e i n st a l le d o n e i t h e r th e b o w , s t e m , o r
t h e s i d e o f t h e sh i p . T h e s e r a m p s u s u a l l y c a n n o t s u s t a i n
l i st s o v e r 2 t o 3 d e g r e e s w i t h o u t d a m a g e . C o n t a i n e r s h i p
l o a d i n g a n d s h i p b o a r d c r a n e o p e r a t i o n s a r e d e l a y e d o r
i n t e r r u p t e d i f l i s t e x c e e d s 5 d e g r e e s .
M o d e m c a r g o l i ne r o p e r a ti o n s d e p e n d o n r a p i d l o a d in g
a n d o f f l o a d i n g t o m a i n t a i n s ch e d u l e . A u t o m a t i c li s t
c o n t r o l s y s t e m s h a v e b e e n d e v e l o p e d t o r a p i d l y t r a n s f e r
l a i 'g e q u a n ti t i e s o f w a t e r ( 2 , 0 0 0 t o 1 1 0 , 0 0 0 g a l l o n s p e r
m i n u t e ) f r o m s i d e t o s i d e f o r t h i s p u r p o s e . T h e s e l i st
c o n t r o l s y s t e m s c o n s i st o f p a i r s o f w i n g b a l l a st t a n k s a n d
c r o s s o v e r p i p i n g , a m e c h a n i c a l d e v i c e t o t r a n s f e r w a t e r a n d
a n a u t o m a t i c e l e c t r o n i c c o n t r o l s y s t e m t o s e n s e l i s t a n d
t a k e t h e n e c e s s a r y c o r r e c t i v e a c t i o n . S y s t e m s t h a t h a v e
b e e n d e v e l o p e d i n c l u d e c e n t r i f u g a l o r a x i a l f l o w p u m p
c o n f i g u r a t i o n s a s w e l l a s p r o c e s s e s t h a t u se c o m p r e s s e d a i r
t o m o v e t h e w a t e r .
T h e p u r p o s e o f th i s p a p e r i s t o r e v i e w t h e f a c t o r s
t h a t h a v e l e d t o t h e d e v e l o p m e n t o f m o d e r n , h i g h c a p a c i t y
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t h a t a r e in u s e a n d c o m m e r c i a l l y a v a i l a b l e .
S o m e a s p e c t s o f l is t c o n t r o l s y s t e m d e s i g n c r i t e r i a
a r e c o n s i d e r e d , i n c l u d i n g s i z i n g o f t a n k s a n d c r o s s o v e r
d u c t s , s e l e c ti o n o f a p r i m e m o v e r a n d s y s t e m
a r r a n g e m e n t . A u t o m a t i c c o n t r o l s y s t e m r e q u i r e m e n t s a r e
d i s c u s s e d .
L I S T C O N T R O L S Y S T E M A P P L I C A T I O N S
A l l s h i p s h a v e s o m e f o r m o f l is t c o n t r o l c a p a b i l i t y
u s i n g t h e b a l l a s t s y s t e m . H o w e v e r , i f t h e s y s t e m i s n o t
au to m ated , co n s id e rab l e t ime an d e f fo r t i s r eq u i r ed t o l i n e
u p v a l v e s a n d s t a r t a n d s t o p p u m p s . I n a d d i t i o n , b a l l a s t
p u m p s h a v e r e l a t i v e l y l o w f l o w r a t e s . S h i p s t h a t o f t e n
e m p l o y a u t o m a t i c l i s t c o n t r o l s y s t e m s i n c l u d e c o n t a i n e r
s h i p s , r o l l - o n / r o l l - o f f ( R O / R O ) c a r g o c a r r i e r s , f e r r ie s ,
h e a v y l i f t s h i p s a n d s i d e c a s t i n g d r e d g e s .
R O / R O v e s s e l s d e l i v e r w h e e l e d c a r g o s o f a v a r i e t y o f
s i ze s w i t h i n t i gh t s c h e d u l e s . T h e y o f t e n u s e j u m b o s t e m
r a m p s th a t a r e a s w i d e a s p r a c t i c a b l e ( A r g y r i a d i s 1 9 7 9 ).
Th e t o r s i o n a l f l ex ib il i t y o f t h e r am p l imi t s t h e l i s t t h a t ca n
b e a c c o m m o d a t e d w h e n i t i s d e p l o y e d t o 2 ~ o r 3 ~
A c c o r d i n g l y , l i s t c o n t r o l s y s t e m s ( L C S ) h a v e b e e n
d e s i g n e d t o k e e p l i st b e l o w t h i s , o r o n a n e v e n k e e l ,
w i t h o u t i n t e r r u p t i n g c a r g o h a n d l i n g .
C o n s i d e r i n g c o n t a i n e r s h i p s , l is t c o n t ro l s y s t e m s th a t
m a i n t a i n l i s t w i t h i n 2 ~ w i l l e x p e d i t e p l a c e m e n t o f
c o n t a i n e r s i n c e l l s . S o m e o p e r a t o r s w i l l a p p l y a n i n i t i a l
l i s t o f 1~ o u t b o a r d o f t h e p i e r t o a c c o m m o d a t e d e c k c a r g o .
H e a v y l i f t c r a n e v e s s e l s u s e t h e L C S t o k e e p l i s t
w i t h i n c r a n e o p e r a t i n g l i m i t s ( a p p r o x i m a t e l y 5 ~
e x p e r i e n c e d . I n t h e m a n u a l m o d e , t h e f lo w d ir e c t i o n is
s e l e c t e d b y h a n d a n d th e f l o w r a te m a y o r m a y n o t b e
c o n t r o l l e d e l e c t r o n i c a l l y , d e p e n d i n g o n t h e te c h n i c a l
c a p a b i l i ti e s o f t h e m e c h a n i c a l s y s t e m b e i n g c o n t r o l le d .
T h e m a n u a l m o d e w i l l a l lo w t h e s h i p t o b e p r e - h e e l e d t o
o n e s i d e i n an t i c i p a t i o n o f a l a rg e l o ad b e in g p l aced o n t h e
o p p o s i t e s i d e .
9 RGOWEIGHT
MOVING A WEIGHT
~ AR G O WEIGHT
C O U N T E R B A L A N C IN G
WITH
LIST CONTROL TANKS
E F F E C T O F C A R G O L O A D I N G
F I G U R E 1
D E S C R I P T I O N , P R I N C I P A L C O M P O N E N T S A N D
A P P L I C A B I L I T Y O F L I S T C O N T R O L S Y S T E M S
T h e li s t c o n t r o l s y s t e m m o v e s w a t e r b e t w e e n s e t s o f
w i n g b a l l a s t ta n k s t o c o u n t e r - b a la n c e a n o f f - c e n t e r l o a d o r
t he t r a n s ve r s e m o v e m e n t o f c a r g o . F i g u r e 1 s h o w s h o w
l o a d i n g c a r g o o n a s h i p w i l l c a u s e l i s t a n d h o w t h e
m o v e m e n t o f w a t e r b e t w e e n t h e l is t c on t r o l ta n k s w i l l
co r r ec t t h i s l i s t .
T h e h e a r t o f t h e s y s t e m i s a r e v e r s i b l e d e v i c e w h i c h
i s c o n t r o l l e d b y a n e l e c t r o n i c c o n t r o l s y s t e m f o r th e
p u r p o se o f m o v i n g w a t e r b e t w e e n w i n g t a n k s o n t h e p o r t
a n d st a r b o a r d s i de s . T h e r e a r e u s u a l l y o p t i o n s f o r u s e in
e i t h e r a n a u to m a t i c m o d e o r a m a n u a l o p e r a t in g m o d e . I n
th e au to mat i c mo d e , t h e f l o w an d i t s d i r ec t i o n a r e se l ec t ed
a n d c o n t r o l l e d e l e c t r o n i c a l l y t o c o r r e c t a n y l i s t a n g l e
A n a d d i t i o n a l f e a t u r e o f s o m e l i s t c o n t r o l s y s t e m s i s
t h a t t h e tr a n s v e rs e m e t a c e n t ri c h e i g h t ( G M ) , a m e a s u r e o f
t h e s t ab il i ty o f t h e sh ip , can b e m easu red i n p o r t , p r i o r t o
s a il in g . I f a c o m p u t e r b a s e d e l e c t r o n i c c o n t r o l s y s t e m i s
a v a i l a b l e , t h e h y d r o s t a t i c c h a r a c t e r i s t i c s o f t h e sh i p a n d
th e cap ac i t y cu rv es o f t h e li s t co n t ro l t an k s c an b e l o a d ed
i n t o t h e m e m o r y o f t h e e l e c t r o n i c c o n t r o l s y s t e m . T h e
f o r w a r d a n d a f t d r a f t m a r k s e i t h e r c a n b e e n t e r e d
m a n u a l l y o r o b t a i n e d v i a a l i n k w i t h a d r a f t i n d i c a t i o n
s y s t e m t o d e t e r m i n e t h e d i s p la c e m e n t . W a t e r i s
t r a n s f e r r e d b e t w e e n t h e l i s t c o n t r o l t a n k s u n t i l t h e s h i p
a s s u m e s a p r e - p r o g r a m m e d a n g le . T h e c o m p u t e r c a n
d e t e r m i n e t h e h e el in g m o m e n t f r o m t h e t a n k c a p a c it y
t a b l e s a n d t h e c h a n g e i n t a n k l e v e l s . S i n c e t h e
d i s p l a c e m e n t a n d h e e l a n g l e a r e k n o w n , t h e G M c a n b e
c a l c u l a t e d w i t h r e a s o n a b l e a c c u r a c y .
172 Design Session
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Several types of list control systems have been
deve loped. Those that have been installed recently
encompass ~';ystems that control an exis ting pump of
acceptable c,apacity as well as dedicated systems that
include axial flow pumps with either constant speed,
variable speed, or variable pitch drives oi" that use a
forced draft blower to transfer water by air pressure in
place of a pump. The type of LCS selected depends on
the vessel size and operational requirements. Electronic
control systems that use existing high capacity pumps
(such as a ballast or fire pump) have been developed to
minimize list. Such systems are generally used when
heeling moment correction requirements are relatively low
and ship operations do not require a short port stay.
Independent heeling pumps with variable speed/pitch
or blower systems are used when heeling moments are
large enough to cause uncorrected list of over 1.5
degrees. For longer cycle times and lower heel moment
requirements, but still requiring a higher capacity than can
be provided by an existing pump (such as occur on
passenger ferries or container feeder ships) a constant
speed axial f low or propelle r pump sys tem is often
selected, since its cost is relatively low. For ships with
short turn around requirements and rapid cargo handling
rates, or that handle heavier cargos, such as RO/RO
vessels that car ry trains or cassettes, and military Sealift
ships, either a variable speed/pitch propeller pump or a
compressed air system is selected. Operations that
require rapid application of a large heel control moment
such as heavy lift vessels with small allowable heeling
angles, would use a compressed air system or a large
capacity variable speed/pitch pump.
The system reaction time is the time required to react
to and correct a change in list. Variable speed/pitch
propeller pump systems and compressed air systems have
rea ctio n times as short as several seconds. Constant
speed, reversible propeller p ump control systems take
longer to react.
Selec t ion Cr i ter ia for Hee l ing Tanks
All of these systems transfer ballast between two
designated list control or heeling tanks. As the transverse
distance between the centers o f the tanks increases, the
amount of water that must be Iransferred to obtain the
neces sary heeling moment decreases. Therefore, these
tanks are u,,;ually narrow wing ballast tanks located
directly inboard of the shell in the parallel middle b ody of
the hull. Such tanks generally are readily available in
typical RO/ PO or container ship arrangements. The
height between the top and the bottom of the tank will
have an impact on the required power, and should be
minimized. In addition, the vertical distance between the
bottom of the tank and the pump will influence the suction
pressure and the onset of cavitation, so it should be
maximized. These considerations lead to the conclusion
that long, shallow, upper wing tanks should be selected as
the heeling tanks, when available.
The structure of the tanks should be checked to insure
good flow to the crossover duct. The vents or air
exchange ducts should be adequately sized to ensure that
the movement of large volumes of water will not draw a
vacuu m or increase the discharge head. At the start of
list control operations, each tank should be approximately
50% full to allow the maximum heel correction moment
to be realized when the entire contents of one of the wing
tanks is transferred to the other.
Lis t Control Us ing Avai lab le Pumps
Many container feeder ships, passenger ferries, or
smaller RO/RO ships will use an electronic heel control
system to operate an existing ballast or fire pump. When
operating as part of the heel control system, the pump
must be isolated from all other tanks except the heeling
tanks. The main components of the system are:
the electronic control system
an "ez~isting" centrifugal pump
remotely operated valves
level sensors
A schematic of the system is shown in Figure 2. When
operating as a list control system, the pump is always
running, recirculating water through the tank with the
highest level w]aen no heel correction is required. When
a preset "threshold" heel angle is exceeded , the valves in
the suction and discharge lines are positioned to allow
ballast transfer to offset the load. The transfer continues
until another preset ~mgle the "shut-of f" angle, which is
usually much lower than the threshold angle, is reached.
The electronic control system senses the list angle,
compares it to the threshold and shut-off values and
properly positions the valves to achieve either
recirculation (no transfer) or the movement of ballast to
cor rec t the heel angle. An example of a valve
arrangement that will meet these requirements is shown in
Figure 2. This can generally be done with reasonably
simple circuit boards and a computer based system is not
necessary.
The pump selected should be the pump that has the
highest capacity, usually the ballast or the fire pump. The
advantage of the system is economic since a dedicated
pump is not necessary . However, capacity is usually
limited when compared to a dedicated system.
List Control Systems 73
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F .W . F ILL - H O S E C O N N E C T I O N
r r ~
H E E L I N G
T A N K
( P O R T )
S.W . BALLAST F ILL / D R AIN
F R O M B A L L A S T M A N I FO L D
F . W . FI L L . H O S E C O N N E C T I O N
r l l
A B
c
v ~ v ~
H E E L I N G
T A N K
( S T B D )
S.W . BALLAST F ILL / D R AIN
F R O M B A L L A S T M A N IF O L D
S Y S T E M L O G I C
S T A N D B Y ( R E C I R C U L A T I O N ) V A L V E S A & C - O P E N
V A L V E S B & D - C L O S E D
S T A N D B Y ( R E C I R G U L A T I O N ) V A L V E S B S D - O P E N
V A L V E S A & C - C L O S E D
H E E L T O S T A R B O A R D V A L V E S A S D - O P E N
V A L V E S C & B - C L O S E D
H E E L T O P O R T V A L V E S C & B - O P E N
V A L V E S A & D - C L O S E D
C O N T R O L U S I N G A V A I L A B L E P U M P S
F I G U R E 2
T h e v a l v e s a r e s i ze d t o h an d l e t h e m a x i m u m f l o w
c a p a c i t y a n d a r e r e m o t e l y o p e r a te d e i t h e r p n e u m a t i c a l ly o r
e l e c t r ic a l l y . T h e s e s a m e v a l v e s c a n s o m e t i m e s b e
a r r a nge d t o fi ll a nd d r a i n t he t a nks i f t he ba l l a s t pum p i s
u s e d . I n o r d e r t o a v o i d w a t e r h a m m e r , i t i s i m p o r t a n t
t h a t t h e v a l v e s i n t h e s u c t i o n a n d d i s c h a r g e l i n e s a r e
o p e n e d a n d c l o s e d s i m u l t a n e o u s l y .
L e v e l s e n s o r s a r e n e c e s s a r y t o i n f o r m t h e c o n t r o l
s y s t e m w h e n e i t h e r o f t h e t a n k s is fu l l o r e m p t y a n d
w h i c h t a n k h a s th e h i g h e s t l e v e l. H i g h a n d l o w le v e l
a l a r m s w i l l no t a l l ow ope r a t i ons t o c on t i nue i n a d i r e c t i on
t h a t w i ll a g g r a v a t e t h e s i t u a ti o n . T h e t a n k w i t h t h e
h i g h e s t l e v e l w i ll b e s e l e c te d f o r r e c ir c u l a t io n w h e n
t r a n s f e r i s n o t r e q u i r e d .
Constant Speed Pump System
A l i s t c on t r o l s y s t e m us i ng a c ons t a n t s pe e d r e ve r s i b l e
p u m p i s s h o w n i n F i g u r e 3 . M a i n c o m p o n e n t s i n c lu d e ;
t h e r e v e r s i b le p r o p e l l e r p u m p ,
a he e l i nd i c a t o r a nd t a nk l e ve l s w i t c he s ,
a b u t t e r f l y v a l v e , a n d
a n e l e c t r o n i c c o n t r o l p a n e l .
T h e r e v e r s i b l e p r o p e l l e r p u m p i s d r i v e n t h r o u g h a
g e a r b o x b y a n A C e l e c t ri c m o t o r. T h e p u m p i s r e v e r s e d
b y r e v e r s i n g t h e m o t o r d i r e c t i o n o f r o t a t io n . A v a i l a b l e
p u m p d i a m e t e r s r a n g e b e t w e e n a b o u t 1 0 t o 1 6 a n d h a v e
a o n e o r t w o s t a g e d e s i g n d e p e n d i n g o n t h e r e q u i r e d
d i s c h a r g e h e a d .
T h e e l e c t r i c m o t o r i s c o n n e c t e d t o t h e p u m p b y a
c o u p l in g a n d g e a r b o x . T h e g e a r b o x i s l u b r i c a t e d b y a n
o i l b a t h w h i c h i s c o n n e c t e d t o a h e a d e r t a n k w h i c h i s
e q u i p p e d w i t h a l o w o i l l e v e l i n d i c a t i o n .
B e t w e e n o i l s e a l s a nd t he ba l l a s t w a t e r , t he r e i s
a n o t h e r s e a li n g c h a m b e r c o n n e c t e d t o a n o t h e r h e a d e r t a n k .
The f l u i d i n t h i s s e a l i ng c ha m be r p r o t e c t s t he ba l l a s t w a t e r
a g a i n s t o i l c o n t a m i n a t i o n a n d l u b r i c a t e s a n d c o o l s s h a f t
s e a l s .
The he e l i ng i nd i c a t o r i s m oun t e d i n a c a b i ne t s u i t a b l e
f o r s e p a r a t e b u l k h e a d m o u n t i n g . T h e l is t a n g l e c a n b e
m on i t o r e d on t he he e l i nd i c a t o r . Ta nk l e ve l s a r e ne i t he r
m e a s u r e d n o r u s e d to c o n t r o l t h e s y s t e m . T a n k l e v e l
s w i t c he s a r e i n s t a l l e d t o s t op t he pum p i f e i t he r o f t he l i s t
c o n t r o l t a n k s i s e m p t y .
The e l e c t r om a g ne t i c bu t t e r f l y va l v e i s i n s t a l l e d i n t he
c r o s s o v e r p i p e a n d is o p e r a t e d b y t h e c o n t r o l s y s t e m . T o
a v o i d w a t e r h a m m e r , t he v a l v e i s o p e n e d w h e n t h e p u m p
i s r u n n i n g a n d c l o s e d w h e n t h e p u m p s t o p s .
T h e e l e c t r o n i c c o n t r o l p a n e l c o n t a i n s a n a n a l o g
c on t r o l s y s t e m . A n e l e c t r on i c a m pl i f i e r r e c e i ve s a s i gna l
p r o p o r t i o n a l to h e e l f r o m t h e i n c l i n o m e t e r . T h e p r e -
a m p l i f i e r i s se t to ze r o a t ze r o l i st a nd a ny he e l w i l l o f f s e t
t he s i gna l f r om the p r e - a m p l i f ie r . A s t a r bo a r d o r po r t l i s t
i s s e n s e d b y a z e r o c o m p a r a t o r w h i c h i s u s e d t o c o n t r o l
t h e d i r e c t i o n o f r o t a t i o n o f t h e p u m p . A s w i tc h is
p r o v i d e d t o sh u t o f f t h e e l e c tr o n i c c o n t r o l a n d p e r m i t
m a n u a l c o n t ro l o f t h e p u m p . C h a n g e t o e i t h e r a u t o m a t i c
o r m a n u a l o p e r a t i o n s c a n b e d o n e a t a n y t i m e i n t h e
ope r a t i ng c yc l e . A l a r m s w i l l s oun d i f the l e ve l s i n e i t he r
t he he e l i ng t a nks o r t he l ub r i c a t i on a nd s e a l he a de r t a nks
a r e b e l o w m i n im u m s . I n a d d i ti o n , i f a c t u a l h e e l e x c e e d s
a p r e s e t m a x i m u m a n g l e , a n a l a r m i s g i v e n .
U s e o f a c o n s ta n t s p e e d a x i a l f l o w p u m p u s u a l l y
r e s u lt s i n a s low r e a c t i on t i m e . The pum p m us t be s t a r t e d
a n d t h e v a l v e o p e n e d e v e r y t i m e t h e h e e l a n g l e d i f f e r s
f r o m a n e v e n k e e l . I n a d d i t i o n , t h e r e i s a r e d u c t i o n i n
f l o w a s h e a d i n c r e a s e s f o r c o n s t a n t s p e e d o p e r a t i o n .
Variable Speed/Pitch Pu mp Systems
A l is t c on t r o l s y s t e m u t i li z ing a s ub m e r s i b l e , va r i a b l e
s p e e d p r o p e l l e r p u m p is s h o w n i n F i g u r e 4 . A s y s t e m
u s i n g a c o n s t a n t s p e e d , v a r i a b l e p i t c h p r o p e l l e r p u m p
w o u l d b e s i m i l a r, w i t h d i f f e r e n c e s a s n o t e d b e l o w . T h e
m a i n c o m p o n e n t s a r e :
- a p r o p e l l e r p u m p ,
- a bu t t e r f l y va l v e ,
a n e l e c t r on i c c on t r o l un i t w i t h he e l s e ns o r , a nd
- t a nk l e ve l t r a ns m i t t e r s . .
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T h e p u m p s h o w n i n F i g u r e 4 i s a s u b m e r s i b l e u n i t
i n s t a ll e d i n t h e c r o s s o v e r p i p e . T h e t u b e h o u s i n g t h e
p u m p f o r m s p a r t o f t h e c r o s s o v e r d u c t . P u m p s i z e s r a n g e
f r o m 1 8 t o 4 2 p r o p e l l e r d ia m e t e r . T h e p u m p i s d r i v e n
b y a h y d r a u l ic m o t o r t h a t i s m o u n t e d i n a p o d w i t h i n t h e
h o u s i n g . T h e d i r e c t i o n o f f l o w is f u l ly r e v e r s i b l e a nd
s p e e d i s a d j u s t a b l e w i t h i n th e s p e e d r a ng e . T h e r e i s a
f i x ed p i t ch , f i v e b l ad ed s t a in les s s t ee l p ro p e l l e r t h a t , a l o n g
wi th t h e p o d an d h y d rau l i c mo to r , i s su p p o r t ed b y a s t ru t .
T h e t u b e , p o d , s t r u t a n d h o u s i n g a r e f a b r i c a t e d o f
co r ro s io n r es i st an t ma t e r i a l s . Th e p o d an d s t ru t a r e f i l led
a f t e r a s s e m b l y w i t h a w a t e r s o lu b l e g r e a s e . T h e
p l a c e m e n t o f t he p u m p a n d m o t o r w i th i n t h e p ip e l in e
m i n i m i z e s th e s p a c e r e q u i r e d a n d r e d u c e s t h e c o m p l e x i t y
o f th e i n s ta l l a t io n . H y d r a u l i c c o n n e c t i o n s t o t h e m o t o r
a re ru n th ro u g h t h e s t rut . Th e y in c lu d e two h ig h p ressu re
l i n e s a n d a s i n g l e , l o w p r e s s u r e c a s e d r a i n .
W h e n t h e L C S is s h u t d o w n o r h a s b e e n i n a c t iv e f o r
s o m e t i m e w h i l e t he s y s t e m i s i n o p e r a t i o n , f l o w b e t w e e n
t h e l i s t c o n t r o l t a n k s i s p r e v e n t e d b y a w a f e r t y p e
b u t t e r f l y v a l v e in s t a l le d i n t h e c r o s s o v e r p i p i n g . T h e
v a lv e i s a s semb led as a u n i t w i th a h y d rau l i c ac tu a to r an d
p o s i t i o n i n d i c a t i n g s w i t c h e s . T h e h y d r a u l i c a c t u a t o r
a l l ow s r e m o t e o p e r a t i o n f r o m t h e e l e c t r o n i c c o n t r o l u n i t .
T h e p o s i t i o n o f th e v a l v e i s e it h e r c o m p l e t e l y o p e n o r
f u l l y c l o s e d , w h i c h i s c o n f i r m e d b y o n e o f t h e p o s i ti o n
s w i t c h e s . T h e m a t e r i a l s u s e d i n t h e v a l v e a n d p o s i t i o n
s w i t c h e s a r e b r o n z e a n d s t a i n le s s s t e e l t o p e r m i t
s u b m e r s i b l e o p e r a t i o n . A n a c c u m u l a t o r c a n b e a d d e d to
t h e h y d r a u l i c s y s t e m t o s h u t t h e v a l v e i n t h e e v e n t o f a n
e l e c tr i c o r h y d r a u l ic p o w e r l o s s . T h e c l o s i n g t i m e o f th e
v a lv e i s ad ju s t ed h y d rau l i ca l l y o r v i a t h e e l ec t ro n i c co n t ro l
s y s t e m t o a v o i d a n y w a t e r h a m m e r p r o b l e m s .
T h e p r o p e l l e r p u m p a n d b u t te r f ly v a l v e a r e d r iv e n b y
a hy d r a u li c p o w e r u n it ( H P U ) l o c a t e d n e a r t h e p u m p a n d
v a lv e . Th i s H P U i s co n n ec t ed t o an d i s co n t ro l l e d b y t h e
e l e c t r o n i c c o n t r o l u n i t . T h e r e i s a m a i n v a r i a b l e
d i s p l a c e m e n t h y d r a u l ic p u m p t o d r i v e t h e p r o p e l l e r p u m p
a n d a g e a r p u m p f o r th e b u t t e r f ly v a l v e a c t u a t o r , b o t h
m o u n t e d o n a c o m m o n s h a f t a n d d r i v e n b y a n A C e l e c t r i c
m o t o r .
T h e d i r e c t i o n a n d s p e e d o f ro t a t i o n o f t h e p r o p e l l e r
p u m p i s d e l e r m i n e d b y a c o n t ro l p l a t e i n t h e m a i n
h y d r a u l i c p u m p . T h e p o s i t i o n o f t h is p l a t e is s e t b y
s i g n a l s f r o m a n e l e c t r o n ic c o n t r o l u n i t. T h e o p e n / c l o s e
m o v e m e n t o f th e b u t t e r f l y v a l v e i s c o n t r o l l e d b y a
d i r e c t io n a l s o l e n oi d v a l v e m o u n t e d o n th e H P U .
C o n f i r m a t i o n o f t he v a l v e p o s i ti o n i s p ro v i d e d b y
m a g n e t i c p o s i t i o n in d i c a t i n g s w i t c h e s m o u n t e d o n t h e
v a l v e .
T h e e l e c t r o n i c c o n t r o l u n i t, w h i c h i s P C b a s e d ,
a l l o w s s e l e c t i o n o f t h e o p e r a t i n g m o d e a n d p r o v i d e s
c o n tr o l o f p u m p s p e e d a n d d i r e c t i o n a s w e l l a s o p e r a t i n g
in fo rmat io n an d sa f eg u ard s wh en ev er t h e sy s t em i s i n u se .
T h e L C S i s s t a r te d a n d o p e r a t e d f r o m t hi s u n i t . E i t h e r
a u t o m a t i c o r m a n u a l o p e r a t i n g m o d e s a r e a v a i l a b l e .
T h e e l e c t r o n i c c o n t r o l u n i t a l s o h o u s e s t h e l i s t
i n d i c a t i o n u n i t. T h i s un i t is a n a n a l o g ty p e e l e c t r o n ic
c l i n o mete r , wh ich is accu ra t e to wi th in 1 / 1 0 d eg ree . Th e
i n f o r m a t i o n s e n s e d a n d t r a n s m i t t e d b y t h i s c l i n o m e t e r i s
u s e d b y t h e c o n t r o l u n i t , a l o n g w i t h i n f o r m a t i o n o n t h e
t a n k l e v e l s , t o a d j u s t t h e s p e e d o f t h e p u m p w h e n t h e
s y s t e m is o p e r a t i n g i n t h e a u t o m a t i c m o d e . A l l v i t a l
o p e r a ti n g , s ta t us a n d a l a r m i n f o r m a t i o n f r o m o t h e r s y s t e m
c o m p o n e n t s s u c h a s th e H P U , t h e b u t t e r f l y v a l v e a n d t h e
t a n k l e v e l t r a n s d u c e r s a r e f e d i n t o a n d d i s p l a y e d o n t h e
co n t ro l u n i t t o p e rmi t co o rd in a t ed an d co n t ro l l ed o p era t i o n
o f t h e s y s t e m .
T h e c o n t r o l s o f t w a r e i s s t r u c t u r e d s o a s t o a p p e a r a s
a s i n g le D O S p r o g r a m . H o w e v e r , i t s i n t e r n a l o p e r a t i o n
i s s t r u c t u re d t o o p e r a t e a s s e v e r a l i n d e p e n d e n t t a s k s f o r
t h e p u r p o s e o f a c q u i r in g d a t a f r o m s e n s o r s a n d c o n t r o l s ;
u p d a t i n g d a t a t o d i sp l ay s , r e l ay s an d t h e h y d rau l i c sy s t em;
p r e s e n t i n g s t a t u s i n f o r m a t i o n t o t h e g r a p h i c s d i s p l a y ;
r e c e iv i n g o p e r a t o r in f o r m a t io n ; m o n i t o r i n g p o s s i b l e a l a r m
co n d i t i o n s an d ex ecu t i n g t h e co mman d ed co n t ro l f u n c t i o n .
E a c h a n d a l l o f t h e se f u n c t i o n s a r e a r r a n g e d t o o p e r a t e
i n d e p e n d e n t l y a n d s i m u l t a n e o u s l y .
A co n t i n u o u s , i n s t an t an eo u s , accu ra t e r ea d i n g o f t h e
l e v e l s i n e a c h o f t h e l i st c o n t r o l t a n k s i s a n i m p o r t a n t
c o n t r o l f e a t u r e . T h i s i s a c c o m p l i s h e d b y s u b m e r s i b l e
p r e s s u r e t r a n s d u c e r s i n s t a l l e d i n t h e t a n k s i n s i d e p i p e s
l ead in g t o t h e l o w p o in t i n th e t an k . Th i s p ip e i s l o ca t ed
f a r f r o m t h e p u m p t o a v o i d t u r b u l e n c e f r o m t h is s o u r c e .
Th e i n fo rmat io n f ro m th ese t r an smi t t e r s a l l o ws t h e co n t ro l
sy s t em to adju s t t h e p u m p sp eed t o o b t a in th e d es i r ed f l o w
r a t e w h i l e o p e r a t i n g i n e i t h e r t h e a u t o m a t i c o r m a n u a l
m o d e s .
A s i m i l a r s y s t e m u s i ng a v a r i a b l e p i t c h p r o p e l l e r h a s
a l s o b e e n d e v e l o p e d . T h e p u m p i s d r i v e n b y a c o n s t a n t
s p e e d A C m o t o r th r o u g h a g e a r b o x , s i m i l a r to t he
c o n s ta n t s p e e d s y s t e m d e s c r i b e d i n th e p r e v i o u s s e c t i o n .
Th e same f l o w ch arac t e r i s t i c s a s t h e v a r i ab l e sp eed sy s t em
c a n b e o b t a i n e d b y v a r y i n g t h e p r o p e l l e r p i t c h v i a t h e
h y d r a u l i c s y s t e m .
T h e s e s y s l e m s h a v e a n a d v a n t a g e o v e r t h e c o n s ta n t
sp eed sy s t ems d e ta i l ed p rev io u s ly in t h a t t h ey d o n o t h av e
t o c l o s e t h e v a l v e w h e n t h e r e a r e n o f l o w r e q u i r e m e n t s .
T h e s p e e d o r th e p i tc h c a n b e a d j u s t e d t o a d e a d h e a d
s e t t i n g , m a i n t a i n i n g a n y l e v e l d i f f e r e n c e b e t w e e n t h e
h e e l i n g t a n k s w i t h o u t a n y f l o w . T h i s d i f f e r e n c e a l l o w s
t he s y s t e m to r es p o n d i m m e d i a t e l y t o a n y h e e l c o r r e c t i o n
r e q u i r e m e n t s .
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~ c ~ coNr~c~ c~e~r
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F ~ M ~ H ~ A ~
rz~vI ~ v,,,0v*c
[Lec~*~,c
v ~ v ~
2,
oc
BALLAST TANK
STARBOARD
C O N S T A N T S P E E D PU M P S Y S T E M
F I G U R E 3
V A R I A B L E S P E E D P U M P S Y S T E M
F I G U R E 4
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C o m p r e s s e d A i r S ys t e m
A l i st c o n t r o l s y s t e m u s i n g c o m p r e s s e d a i r to m o v e
w a t e r b e t w e e n t a n k s i s s h o w n in F i g u r e 5 . P r i n c i p a l
c o m p o n e n t s i n c l u d e ;
a n e, e c t ro n ic c o n t ro l u n i t . (1 , 2 )
a n a i r v a l v e g r o u p w i t h p n e u m a t i c c o n t r o l s , ( 4 )
a f o r c e d d r a f t b l o w e r , ( 5 )
a t a n k l e v e l d i f f e r e n c e i n d i c a t o r , ( 6 )
a h i g h w a t e r l e v e l s e n s o r , ( 7 ) , a n d
a b u t t e r f l y v a l v e , ( 8 )
T h e l i s t c o n t r o l t a n k s a n d c r o s s o v e r p i p e a r e s i m i l a r
t o o t h e r s y s t e m s . T a n k l e v e l i n d i c a t o r s a r e n o t i n c l u d e d ,
h o w e v e r a t a n k l e v e l d i f f e r e n c e i n d i c a t i o n i s p r o v i d e d .
A n e l e c t r i c m o t o r d r i v e n f o r c e d d r a f t b l o w e r s u p p l i e s
c o m p r e s s e d a i r t o t h e a i r - v a l v e g r o u p . T h e v a l v e s a r e
c o n t r o l l e d b y t h e e l e c t r o n i c c o n t r o l u n i t a n d c a n r a p i d l y
p r e s s u r iz e o n e o f th e l is t c o n t r o l t a n k s a n d v e n t t h e o t h e r
t o c r e a t e a d i f f e re n t i a l p r es s u r e t h a t m o v e s w a t e r b e t w e e n
t h e t a n k s. F l o w r a t e s o f u p t o 1 1 0 , 0 0 0 G P M c a n b e
a c h i e v e d , w h i c h c o r r e s p o n d s t o th e o u t p u t o f a 4 2 i n c h
d i a m e t e r v a r i a b l e s p e e d p r o p e l l e r p u m p a t l o w d i s c h a r g e
h e a d s . .
T a n k w a t e r l e v e l m e a s u r e m e n t s a r e n o t a c c o m p l i s h e d
f o r t h i s s y s t e m . H o w e v e r , t h e p r e s s u r e d i f f e r e n c e
b e tw e e n t a n l , : s i s s e n s e d a n d u s e d to g iv e w a te r l e v e l
d i f f e r e n c e . M a x i m u m w a t e r l e v el s e n s o rs ar e p r o v i d e d t o
s t o p o p e r a t i o n i f t h e w a t e r l e v e l i s t o o h i g h .
A b u t t e r f l y v a l v e i s in s t a l l ed i n t h e c r o s s o v e r p i p e
b e t w e e n t a n k s . I t i s n o t c o n t r o l l e d b y t h e e l e c t r o n i c
c o n t r o l u n i t a n d i s o p e n w h e n e v e r t h e L C S i s o p e r a t e d .
C O M P R E S S E D A I R S Y S T E M
F I G U R E 5
T h e f o r c e d d r a f t b l o w e r r u n s c o n t i n u o u s l y
w h e n e v e r t h e L C S i s i n o p e r a t i o n a n d t h e e l e c t r o n i c
c o n t r o l u n i t o p e r a t e s th e a i r v a l v e s . M a n u a l a n d
a u t o m a t i c o p e r a ti n g m o d e s a r e p r o v i d e d . I n a u t o m a t i c
o p e ra t io n , l i s t a n g le m e a s u re m e n t s a re u s e d to s e l e c t f l o w
d i r e c t i o n t o c o r r e c t l i s t .
T h e e l e c t r o n i c c o n t r o l u n i t c o n t a i n s t h e r o l l s e n s i n g
u n i t an d l i st c o n t r o l a l g o r i t h m s . A u x i l i a r y c i r c u i t s a r e
p r o v i d e d t o d i s t r i b u t e c o n t r o l s i g n a ls a n d p e r f o r m
s u r v e i l l a n c e a n d a l a r m f u n c t i o n s .
T h i s t y p e o f h e e l c o n t r o l s y s t e m c a n a l s o b e u s e d i n
th e s a m e t a n k s th a t a re u s e d fo r a t s e a s t a b i l i z a t io n o f ro l l
a n g l e s . L a r g e r c r o s s o v e r d u c t s ar e r e q u i r e d f o r t h i s
p u r p o s e . I n a d d i t io n , s o m e o f t h e s a m e e q u i p m e n t c a n
b e u s e d t o c o n t r o l t h e m o v e m e n t o f t h e s t a b i l iz e r l i q u i d .
D E S I G N C O N S I D E R A T I O N S
L i s t c o n t r o l s y s t e m d e s i g n c o n s i d e r a t i o n s i n c l u d e
s e l e c t i o n o f t h e s i z e o f t h e h e e l i n g t a n k s a n d t h e w a t e r
t r a n sf e r r at e n e c e s s a ry t o c o u n t e r s p e c i f ie d c a r g o h a n d l i n g
r e q u i r em e n t s . S y s t e m h y d r a u l i c p e r f o r m a n c e m u s t t h e n
b e d e v e l o p e d a n d e l e c t r o n i c c o n t r o l s o p t i m i z e d t o s at i s f y
t h e se r e q u i re m e n t s . D e s i g n c r i te r i a f o r s y s t e m d e f i n i t i o n ,
h y d r a u l i c p e r f o r m a n c e , a n d p r o g r a m m i n g t h e c o n t r o l
s y s te m a re s u m m a r i ze d b e l o w . .
T h e l i s t c o r r e c t i o n r e q u i r e m e n t s f o r e a c h i n d i v i d u a l
s h ip w i ll d e p e n d o n w h a t is l o a d e d , h o w i t i s l o a d e d , a n d
h o w fa s t i t i s b e in g lo a d e d . S o m e e x a m p le s w i l l i l l u s t r a t e
t h e d i v e r s i t y o f c a r g o a n d s t o w a g e c o n s i d e r a t i o n s t h a t
h a v e b e e n f a c t o r s i n d e t e r m i n i n g t h e s p e c i f i c a t i o n s f o r a
l i s t c o n t ro l s y s ~c em . O n e o f t h e m o s t s e v e re c a s e s i s t h e
l o a d in g o f ra i l r o ad c a r s o n a t r a in f e r r y . T h e e f f e c t o f t h e
c a r s o n t h e s t e m r a m p a n d o n t h e c a r s i m m e d i a t e l y b e h i n d
th e m th a t a re s t i l l o n s h o re i s s u c h th a t t h e h e e l a n g le s
m u s t b e m i n i m i ze d . T h e m a x i m u m r e q u i r e m e n t d e v e l o p s
w h e n t h e o u t b o a r d l a n e i s b e i n g l o a d e d a n d i s q u a n t i f i e d
b y t h e t ra n s v e rs e l o c a t i o n o f t h e t r ac k , t h e a v e r a g e w e i g h t
o f t h e c a r s , a n d th e lo a d in g s p e e d . I f t h e t r a in i s 1 0 c a r s
l o n g , e a c h c a r w e i g h s 4 5 t o n s a n d i s 4 0 f t . l o n g , t h e
o u t b o a r d t r ac k is 3 5 f t. o f f c e n te r , a n d t h e l o a d i n g s p e e d
i s 5 f t . / s e c o n d , t h e to t a l m o m e n t r e q u i re d to k e e p th e s h ip
o n a n e v e n k e e l i s 1 8 , 0 0 0 f t . t o n s (4 5 to n s / c a r x 1 0 c a r s
X 35 f t .) . T h e c o r r e c t i o n r a t e n e c e s s a r y i s a b o u t 1 1 , 8 0 0
f t. t o n s / m i n u t e ( 4 5 t o n s / c a r d i v i d e d b y 4 0 f t . / c a r X 3 5 f t .
X 5 f t . / s e c o n d X 6 0 s e c o n d s / m i n u t e ) .
A c o n t a i n e r f e e d e r s h i p w i t h a s h i p b o a r d m o u n t e d
c ra n e m a y b e r e q u i re d to l i f t a 4 0 to n c o n ta in e r l o c a te d o n
t h e q u a y a b o u t 7 5 f t . o f f t h e c e n t e r l i n e o f th e s h i p .
C o n s i d e r i n g n o t o n l y t h e w e i g h t o f t h e c o n t a i n e r b u t a l s o
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t h e b o o m o f t h e c r a n e a n d t h e l if t i n g g e a r, t h e to t a l
t r a n s v e r s e m o m e n t c a n b e a s h i g h a s 6 , 0 0 0 f t . t o n s ,
r e l a ti v e l y h i g h f o r a sm a l l s h i p . T h e s l e w i n g r a t e o f t h e
c r a ne w i ll de t e r m i n e t he c o r r e c t i on r a t e . ( I n m a ny c a s e s ,
t h i s r a t e m us t be c ons i de r e d i n l i ne w i t h p r a c t i c a l l i m i t s
o n t h e s p e e d o f th e m o v e m e n t o f a p e n d a n t w e i g h t .) A
p r a c t i c a l r a t e w o u l d b e i n t h e r a n g e o f 6 , 0 0 0 f t . t o n s /
m i n u t e , a l l o w i n g th e m a x i m u m c a r g o m o v e m e n t to b e
c o m p l e t e d w i t h i n o n e m i n u t e .
T h e l i s t c o n t r o l s y s t e m s o n t h e S E A L I F T s h i p s f o r
t he U .S . Mi l i t a r y Se a l i f t C o t nm a nd w e r e s i ze d t o m e e t t he
h i g h e s t o f t w o r e q u i r e m e n t s :
m o v i n g a n M 1 A 1 t a n k a th w a r t s h i p f ro m s h el l to
s he l l i n 1 .5 m i nu t e s .
m o v i n g a s i d e -l o a d in g w a r p i n g t u g b y c r a n e f r o m t h e
c e n t e r l i n e t o 2 5 f t . o u t b o a r d w i t h i n 2 . 5 m i n u t e s .
Ge ne r a l l y , t he t ug i s c on t r o l l i ng , due t o t he w e i gh t o f
t h e c r a n e b o o m a n d l i ft i n g g e a r , a n d r e s u l t s i n a m o m e n t
o f a b o u t 1 0 , 3 0 0 f t. t o n s a n d a ra t e o f 4 , 1 2 0 f t.
t o n s / m i n u t e
T o p r o p e r l y c o u n t e r a c t t h e se t r a n s v e r s e m o m e n t s , t h e
l i s t c o n t r o l s y s t e m w i l l h a v e t o t r a n s f e r b a l l a s t b e t w e e n
t h e d e s i g n a t e d h e e l i n g t a n k s . S i z e s e l e c ti o n d e p e n d s o n
t w o i n t e r r e l a t e d m a i n f a c t o r s : t h e t r a n s v e r s e m o m e n t t h a t
c a n b e d e v e l o p e d a n d t h e t i m e r e q u i r e d t o t r a n s f e r t h e
ba l l a s t .
T h e m a x i m u m h e e l in g m o m e n t g e n e ra l ly c a n b e
d e v e l o p e d b y t h e t a nk s i f th e y a r e h a l f f u l l a n d t h e e n t i re
c on t e n t s o f one t a nk is t r a ns f e r r e d t o the o t he r . Se l e c t i ng
l o n g , n a r r o w t a n k s d i r e c t l y i n b o a r d o f t h e s h e l l w i l l
m a x i m i z e t h e d i st a n c e b e t w e e n t h e t r a n s v e r s e ce n t e r s o f
t h e t a n k s a n d m i n i m i z e r e q u i r e d c a p a c i t y . C o n s i d e r i n g
t h e S E A L I F T e x a m p l e g i v e n a b o v e , i f th e d is t an c e
b e t w e e n t a n k c e n t e rs o n t h e s e P A N A M A X s h i p s is a b o u t
1 0 0 fe e t , th e r e q u i re d c a p a c i t y o f e a c h t a n k i s a m i n i m u m
o f 2 0 6 t o n s ( h a l f c a p a c i t y , 1 0 3 t o n s , w i l l p r o v i d e th e
r e q u i re d m o m e n t o f 1 0 , 3 0 0 f o o t t o n s w h e n m o v e d t h e 1 0 0
f t. d i s ta n c e b e t w e e n c e n te r s ). T h e p u m p m u s t b e c a p a b l e
o f m o v i n g 4 1 . 2 t o n s / m i n u t e ( 4 1 2 0 / 1 0 0 ) o r a b o u t 1 0 , 80 0
G P M . I f t h e t a n k s w e r e w i d e r , w i th t he d is t a n c e b e t w e e n
t h e t r a n s v e r s e c e n t e r s d e c r e a s e d c o r r e s p o n d i n g l y , t h e
c a p a c i t y o f t h e t a n k s a n d t h e p u m p w o u l d i n c r e a s e i n
p r o p o r t i o n .
T h e s e l e ct i o n o f th e s i z e o f t h e c r o s s - o v e r c o n n e c t i o n
b e t w e e n t h e h e e l i n g t a n k s i s m a d e o n t h e s a m e f l o w
v e l o c i t y b a si s a s a n y p i p e l i n e . H o w e v e r , t h e r e m a y b e
s o m e a d d i t i o n a l c o n s i d e r a t i o n t o m i n i m i z e l o s s e s i n t h e
c onne c t i ng p i p i ng i f a n a x i a l f l ow pum p i s s e l e c t e d , a s t he
p o w e r r e q u i r e m e n t s f o r t h i s p u m p t y p e a r e v e r y s e n s i t i v e
t o t h e t o t a l d e s i g n h e a d .
Hydraulic Performance
Li s t c on t r o l s y s t e m hydr a u l i c ope r a t i ng c o nd i t i ons a r e
c o m p l e x w h e n v a r i a b le s p e e d / p i t c h p u m p s a r e a p p l ie d . .
A t t he s ta r t o f l oa d i ng , t he s h i p is on a n e ve n ke e l a nd t he
l i st c o n t r o l ta n k s a r e e q u a l i z e d . C o n s i d e r i n g a R O / R O
c a r g o l i n e r w i t h a v a r i a b l e s p e e d p r o p e l l e r p u m p a s a n
e x a m p l e , l o a d i n g s t a r ts o n o n e s i d e o f t h e s h i p a n d t h e
L C S a u t o m a t i c a ll y tr a n s f e r s w a t e r t o t h e o p p o s i t e s i d e t o
m i n i m i z e li s t . Th i s c a n c on t i nue un t i l a l l t he w a t e r ha s
b e e n t r a n s f e r r e d t o o n e s i d e . D u r i n g t h i s p h a s e o f t h e
ope r a t i on , t he pum p i s m ov i ng w a t e r a ga i ns t a n i nc r e a s i ng
d i s c h a r g e h e a d a n d R P M i s i n c r e a s i n g t o m a i n t a i n
r e q u i r e d f l o w .
T h e s e c o n d p h a s e o f lo a d i n g t h e n s t a r t s o n t he
o p p o s i t e s i d e . T h e p u m p i s r e v e r s e d a n d w a t e r is
t r a n s f e r r e d f r o m t h e t a n k w i t h a h ig h l e v e l t o t h e l o w
l e v e l t a nk . I n t h i s c a s e , t he r e i s a n in i t i a l g r a v i t y f l ow
t h a t a c ts in c o m b i n a t i o n w i t h th e p u m p . I n s o m e c a s e s
t h e c o n t r o l s y s t e m m a y b e r e q u i r e d t o re v e r s e t h e p u m p
d i r e c t i on t o r e t a r d t he g r a v i t y f l ow t o ob t a i n a c on t r o l l e d
f l o w a n d p r e v e n t o v e r c o r r e c ti o n o f t h e l is t a n g l e . T h e
s e c o n d p h a s e c a n c o n t i n u e u n t i l t h e t a n k l e v e l s e q u a l i z e
a n d g r a v i t y f l o w is n o l o n g e r p r e s e n t . A f t e r th i s t h e
hydr a u l i c s a r e s i m i l a r t o t he f i r s t pha s e . Th e s e op e r a t i ng
c y c l es m a y b e r e p e a te d n u m e r o u s t i m e s d u r i n g l o a d i n g o r
u n l o a d i n g o p e r a t i o n s , d e p e n d i n g o n t h e s i z e o f th e s h i p
a n d t h e h e e l i n g t a n k s a n d t h e l o a d i n g p a t t e r n .
The hyd r a u l i c de s i gn o f t he s ys t e m r e qu i r e s de f i n i t i on
o f a f o u r q u a d r a n t s e t o f p u m p h e a d f l o w c u r v e s f o r u s e
i n d e s i g n i n g th e c o n t r o l sy s t e m . T h e s i g n c o n v e n t i o n
u s e d i n t h i s p a p e r i s f l o w a n d R P M t o s t a r b o a r d a r e
p o s i t i v e . P o s i t i v e h e a d i s d e f i n e d a s t h e d i f f e r e n c e
b e t w e e n p o r t an d s t a r b o a r d t a n k l e v e l s ( c o r r e s p o n d i n g t o
a pos i t i ve g r a v i t y f l ow ) . A c c o r d i ng l y , s t a r boa r d ( pos i t i ve )
p u m p f l o w c o r r e s p o n d s t o n e g a t i v e h e a d a n d p o r t
( n e g a t i v e ) p u m p f l o w c o r r e s p o n d s t o p o s i t i v e h e a d .
A s e t o f fo u r q u a d r a n t h e a d f l o w c u r v e s d e v e l o p e d
f r o m s h i p b o a r d t e st s is s h o w n i n F i g u r e 6 . I n th e f i rs t
qua d r a n t he a d a nd f l ow a r e pos i t i ve a nd t he da t a r e p r e s e n t
c o m b i n e d g r a v i t y a n d p u m p f l o w t o s t a rb o a r d . T h e
s e c o n d q u a d r a n t w i t h p o s i t i v e f l o w a n d n e g a t i v e h e a d i s
d u e to t h e p u m p o n l y . T h e t h i r d q u a d r a n t h a s n e g a t i v e
f l o w a n d h e a d a nd c o r re s p o n d s t o a c o m b i n a t i o n o f p u m p
a nd g r a v i t y f l ow to po r t . The f ou r t h qua d r a n t ha s pos i t i ve
h e a d w i t h n e g a t i v e f l o w a n d r e p r e s e n t s p u m p i n g t o p o r t
w i t h o u t g r a v i t y f l o w .
I n a d d i t i o n t o d e v e l o p i n g t h e p u m p d e s i g n a n d
d e t e r m i n i n g p o w e r r e q u i r e m e n t s , t a n k v e n t s m u s t b e
d e s i g n ed f o r t h e h i g h f l o w s r e q u i r e d b y t h e s y s t e m . T h e
i n t e r n a l s t r u c t u re m u s t h a v e s u f f ic i e n t f l o w o p e n i n g s t o
a l l o w in t e r n a l a i r a n d w a t e r fl o w . F o r e x a m p l e ,
178 Design Session
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L I ST C O N T R O L SY S T E M P U M P C U R V E S
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t r a n s v e rs e w e b f r a m e s a t t h e b o t t o m o f t a n k s c a n o b s t r u c t
f l o w t o t h e p u m p s u c t i o n .
T h e s y s t e m d e s i g n m u s t b e s u c h t h a t w a t e r h a m m e r
i s a v o i d e d . T h i s is u s u a l l y a c c o m p l i s h e d b y e i t h e r
c o n t r o l l i n g t h e v a l v e c l o s i n g s p e e d o r th e s e q u e n c e o f
s t a r t i n g a n d s t o p p i n g t h e p u m p a n d o p e n i n g a n d c l o s i n g
t h e v a l v e .
Systems Using a P ropeller Pum p
F i g u r e 4 , s h o w s t h e s y s t e m s c h e m a t i c f o r a li st
c o n t r o l s y s t e m w i t h a p r o p e l l e r p u m p i n s t a ll e d i n t h e c r o s s
d u c t b e t w e e n t h e ta n k s . T h e t a n k s a r e v e n t e d t o t h e
a t m o s p h e r e a n d t h e l i q u i d s y s t e m e n e r g y b a l a n c e i s g i v e n
b y ;
(p p-p s)/3 '~ + H p - H s + B * s i n ( 4 0 + H A - H L = 0 ; (1)
Sy s t e m he a d l o s s ; Hr . = I ~*V~2 / 2g
T h e t o t a l d y n a m i c h e a d i s d e f i n e d a s
T D H = H ~ - H s + B * s i n( ~ b ) + I ~ , a n d
HA = -T D H -(Pe-1%)/~ ; (2)
T o t a l d y n a m i c h e a d a t z e r o l i s t p r o v i d e s a b a s i s f o r p u m p
d e s i g n w h e n v e n t h e a d l o s s i s l o w .
T h e e n e r g y b a l a n c e f o r t h e a i r v e n t i s g i v e n b y ;
Pa -PP = % ( 1 + K~) Va p2 / 2g ; ( f l O W n t o t a nk )
P ,-P ,, = -3' , ( l + K ~) V , s 2 / 2g ; ( f l ow ou t o f t a nk )
a n d t h e v e n t h e a d l o s s i s ;
Pp-Ps = - % (1 +Ka)*(Vap 2 + Vas2)/2g;
(3)
A v e n t a i r v e l o c i ty l i m i t o f 8 0 f t/ s e c i s r e c o m m e n d e d .
A s s u m i n g a 3 0 0 m m v e n t d i a m e t e r , a i r d e n s i t y o f . 0 76 4
l b s p e r c u b i c f o o t a n d a l o s s c o e f f i c i e n t o f 2 , t h e a i r f l o w
i n e a c h l i n e i s 2 8 , 1 8 0 G P M a n d th e v e n t h e a d lo s s i s
0 . 7 5 f t T h i s i s l e ss th a n 4 p e r c e n t o f t h e m a x i m u m
g r a v i t y h e a d f o r r e p r e s e n t a t i v e b a l l a s t ta n k s . I t i s n o t e d
t h a t h i g h e r a i r f l o w v e l o c i t i e s w i l l q u i c k l y c a u s e
a p p r e c i a b l e v e n t h e a d l o s s .
Gravity
F l o w
I n c a s e s o f w a t e r t r a n s fe r f r o m a ta n k w i t h a h i g h
wa t e r l e ve l t o t he l ow l e ve l t a nk , a g r a v i t y f l ow c a n a c t i n
c o m b i n a t i o n w i t h t h e p u m p o r b l o w e r . F o r a u t o m a t i c
c o n t r o l , t h i s w i l l p r o v i d e a h i g h f l o w r a t e a n d r e d u c e t h e
t i m e r e q u i r e d t o c o r r e c t a l i s t. F o r m a n u a l c o n t r o l
p r o g r a m m e d t o m a i n t a in a c o n s t a n t f lo w r a te , t h e p u m p
m a y h a v e t o b e r e v e r se d t o s lo w d o w n f l o w u nt il th e
g r a v i t y h e a d a n d g r a v i t y f l o w a r e r e d u c e d t o t h e d e s ir e d
p r o g r a m m e d f l o w .
F o r c o m b i n e d p u m p a n d g r a v i t y f l o w i t i s a s s u m e d
t h a t t h e t o t a l f l o w i s t h e s u m o f a g r a v i t y i n d u c e d
c o m p o n e n t an d th e p u m p f l o w a t c o n s ta n t R P M a n d z e r o
h e a d . T h e g r a v i ty f lo w c o m p o n e n t is a p p r o x i m a t e d b y ;
G F L O W = A r ~ * [ 2 g (H p - H s )/ K 2 ]l /2 ;
(4 )
w h e r e G F L O W i s i n c u b i c fe e t p e r s e c o n d a n d K 2 =
s y s t e m f l o w l o ss c o e f f i c i e n t t h a t in c l u d e s l o s s e s d u e t o
r e l a t i v e f l o w o v e r p u m p i n t e r n a l s .
S h i p b o a r d t es ts r e s ul ts h a v e b e e n u s e d t o e v a l u a t e t h e
g r a v i t y h e a d f lo w c h a r a c te r i st i c s. A v a r i a b l e s p e e d p u m p
w a s u s e d t o e s t a b l i s h a n i n i ti a l d i f f e r e n t i a l h e a d . F l o w
w a s t h e n r e v e r s e d a n d a c o n s t a n t p u m p R P M w a s
p r o g r a m m e d . G r a v it y h e a d - f lo w c u r v es f o r z e r o R P M
a r e s h o w n i n F i g u r e 7 . T e s t d a ta f o r fl o w t o p o r t a n d
s t a r b o a r d a r e s h o w n t o g e t h e r w i t h a c a lc u l a t io n o f
G F L O W i n G P M f o r a l o ss c o e f f i c ie n t K = 7 . I t s h o u l d
b e n o t e d t h a t e s t im a t e s o f th e l o s s c o e f f i c i e n t w i t h o u t t h e
p u m p a re K = 5 . 4 .
T e s t re s u lt s fo r g r a v i t y a s s i s t e d f l o w t o p o r t a t p u m p
s p e e d s o f 4 0 0 a n d 5 0 0 R P M a r e s h o w n i n F i g u r e 8 .
G r a v i t y fl o w w a s c a l c u la t e d a s t h e d i f f e r e n c e b e t w e e n t h e
t o ta l f l o w i n t h e t es t a n d p r e v i o u s t e s t r e s u l t s a t c o n s t a n t
R P M a n d z e r o h e a d . T e s t r e s u lt s f o r f l o w t o p o r t a t z e r o
R P M a r e a l s o i n c l u d e d o n t h e F i g u r e a n d s l i g h t l y l o w e r
g r a v i t y f l o w w a s o b s e r v e d w i t h t h e p u m p r u n n i n g .
Axial Flow Pump Des ign
T h e d e s i g n o f p r o p e l l e r p u m p s i s o ft e n b a s e d o n
m o d e l o r f u l l s c a l e t e s t d a t a w h i c h c o v e r v a r i a t i o n s o f
d e s i g n p a r a m e t e r s s u c h as p i t c h r a ti o , b l a d e a r e a , n u m b e r
o f b l a d e s a n d s e c t i o n s h a p e . P u m p d i a m e t e r i s s e l e c t e d
o n t h e b a s is o f s y s te m h e a d a n d f l o w r e q u i r e m e n t s .
P e r f o r m a n c e e n h a n c e m e n t c a n b e p r o v i d e d b y a d d i n g
s t a t o rs i n t h e c r o s s o v e r p i p e a n d f i t t in g a p r o p e l l e r h u b .
P u m p d e s i g n p e r f o r m a n c e i s c h e c k e d b y s h o p t e s t s o f
c o m p l e t e d u n i t s s e t u p a s c l o s e a s p o s s i b l e t o s h i p b o a r d
c o n d i t io n s . I t s h o u l d b e n o t e d th a t i t m a y n o t b e p o s s i b l e
t o s i m u l a te t h e h i g h e s t l e v e ls o f s h i p b o a r d d i s c h a r g e h e a d s
i n t h e s h o p .
Systems Using Compressed Air
F i g u r e 5 s h o w s t h e s y s t e m s c h e m a t i c f o r a L C S u s i n g
c o m p r e s s e d a ir t o m o v e w a t e r . C o n s i d e r t h e ca s e w h e r e
t h e b l o w e r i s d i s c h a r g i n g a i r i n t o t h e p o r t t a n k a n d t h e
s t a r b o a r d ta n k i s v e n t e d t o t h e a t m o s p h e r e . T h e r e i s n o
p u m p , a n d t h e l i q u i d s y s t e m e n e r g y b a l a n c e b e c o m e s ;
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(Pp-P~) = 3 '1 T D H ; (5 )
A s s u m e a n i s o t h e r m a l p e r f e c t g a s a n d c o n ti n u i t y , t h e n
t h e s p e c i f i c g r a v i t y ~ b o f t h e c o m p r e s s e d a i r o u t o f th e
b l o w e r i s ;
% = % * I ~ / l q ( 6 )
F o r a i r f l o w i n t o t h e p o r t t a n k ;
Pp = PB - % *(1 +Kap)V~2/2g
(7)
F o r a i r f l o w o u t o f th e s t a r b o a r d t a n k ;
, '}
P s = P a -F % (1 +K . p )V a ' /2 g
(8)
T h e a i r p r e s s u r e d i f f e r e n c e b e t w e e n t h e t a n k s i s
d e t e r m i n e d f r o m e q u at i o n s (7 ) a n d ( 8 ) a n d c o m b i n e d w i t h
t h e w a t e r s y s t e m e n e r g y b a l a n c e , e q u a t i o n ( 5 ) , t o o b t a i n
t h e b l o w e r d e s i g n d i s c h a r g e p r e s s u r e .
PB =
Y,*TDH.__+_+Pa[ 1 + _
(_% /B)(V.! /2g___)_(_I 47K_._~_ ] (9 )
[ 1 - (% /p a ) (V a 2 /2 g ) (1 + N a p )]
T h e b l o w e r t o t a l d i f f e r e n t i a l p r e s s u r e i s P B - P . a n d
e x p r e s s i n g t h i s i n f ee t o f s al t w a t e r , h w :
hw = TD .H + (I~/3~)*[X~ + X~ 1 (10 )
[ 1 - X p ]
w h e r e ;
Xp = ( % / p , ) ( V f / 2 g ) ( 1 + l ~ p )
Xs = ( % / P a ) ( V f / 2 g ) ( l + K .~ )
T h e b l o w e r a i r h o r s e p o w e r ( f o r s a l t w a t e r ) i s d e f i n e d a s ;
A H P = h w Q ~ / 5 1 6 . 9 1
A n d s i n c e Q t = Q ~
A H P = W H P + ( P a / 3 't ) ( X p + ; ~ ) ( Q / 5 1 6 . 9 1 ) ; ( 1 1 )
( 1 - X p )
P o w e r R e q u i r e m e n t s
T h e b a s i c s y s t e m p o w e r r e q u i r e d i s t h e w a t er
h o r s e p o w e r ( W H P ) , w h i c h i s d e f i n e d f o r sa lt w a t e r a s;
W H P = G P M * T D H / 3 8 4 8 ( 12 )
w h e r e : G P M = b a ll a st w a t e r f l o w
T D H = t o ta l d y n a m i c h e a d ( f t)
F o r s y s t e m s u s i n g a p r o p e l l e r p u m p , t h e r e q u i r e d
s h a f t h o r s e p o w e r i s ;
S H P = W H P / e p 13)
w h e r e : e p = p r o p e l l e r e f f i c i e n c y
T h e p r o p e l l e r e f f i c i e n c y v a r i e s b e t w e e n 5 0 % a n d
8 0 % d e p e n d i n g o n t h e f l o w . F i g u r e s 9 a n d 1 0 s h o w
e x a m p l e s o f p r o p e l l e r t e s t d a t a f o r t h e s a m e f i x e d p i t c h
p u m p a t v a r i o u s R P M v a l u es .
W h e n a f o r c e d d r a f t b l o w e r i s u s e d t o t r a n s f e r w a t e r ;
S H P = A H P / % ( 14 )
w h e r e : % = b l o w e r e f f i c i e n cy
A H P = a ir h o r s e p o w e r
T h e e ff i c i e n c y o f a x i a l f l o w b l o w e r s v a r i e s f r o m 4 0 % t o
6 5 % ( M a r i n e E n g i n e e r i n g , 1 9 9 2 ) .
T h e p u m p o r b l o w e r d r i v e s y s t e m e f f i c i en c y , e s ,
m u s t a l so b e c o n s i d e r e d . F o r e x a m p l e , t h e e f f i c i e n c y o f
a h y d r a u l i c d r i v e i s i n t h e r a n g e o f 9 0 % .
T h e r e q u i r ed m o t o r h o r s e p o w e r ( M H P ) i s t h e n
d e t e r m i n e d b y
M H P = S H P /e Me s (1 5 )
w h e r e : eM i s t h e m o t o r e f f i c i e n c y w h i c h , f o r a n
e l e ct r ic m o t o r , v a r i e s f o r m 8 5 % a t I / 4
lo a d to 9 3 % a t fu l l l o a d .
D e s i g n o f L i s t C o n t r o l Ta n k V e n t s
U n d e r s i z e d L C S t a n k v e n t l i n e s c a n s i g n i f i c a n t l y
r e d u c e t h e w a t e r t r a n s f er r a te a c h i e v e d a n d j e o p a r d i z e t h e
s y s t e m ' s ab i l i t y t o c o n t r o l l i st . A d e s i g n p r o c e d u r e t h a t
u s e s th e m a x im u m a i r v e lo c i ty in th e v e n t l i n e a s a d e s ig n
c r i t e r i a h a s b e e n u s e d to s i z e v e n t l i ne s . T h e p r e s s u r e
d r o p i n t h e v e n t l i n e m u s t b e c h e c k e d t o a s s u r e t h a t t h e
s i z e s e l e c t e d d o e s n o t p r o d u c e a v a c u u m o n t h e s u c t i o n
s i d e d u r i n g m a x i m u m e x p e c t e d f l o w ( C r a n e T e c h n i c a l
P a p e r 4 1 0 ) . F o r c o m m e r c i a l c o n s t r u c t i o n , a d e s i g n li m i t
a i r v e l o c i t y o f 8 0 f t / s e c o n d i s u se d , t h e l i m i t u s e d i f b a l l
v a l v e s a r e i n s t a l l e d i n t h e v e n t l i n e is 5 0 f t / s e c . U s i n g
th is a p p ro a c h , 1 2 in c h d i a m e te r v e n t s w e re s e l e c t e d fo r a
L C S t h a t ha d a m a x i m u m f l o w r a t e o f 2 7 , 0 0 0 G P M .
T h i s v e n t s iz e w a s s e l e c t ed o n t h e b a s i s o f n o b a l l v a l v e s
in th e l i n e .
I f b a l l v a l v e s a r e i n s t a l l e d i n a v e n t l i n e w i t h f l o w
v e l o c i t i e s o v e r 5 0 f t / se c , a e r o d y n a m i c d r a g o n t h e b a l l
c a n c a s e i t t o s l a m sh u t . T h i s w a s o b s e r v e d d u r i n g
s h i p b o a rd t e st s o f a L C S a n d i t w a s n e c e s s a r y t o r e m o v e
t h e b a l l s t o a c h i e v e p r o p e r o p e r a t i o n .
In s y s t e m s th a t u s e a i r b lo w e rs , t h e a i r i s t r a n s fe r re d
w i t h i n t h e s y s t e m r a t h e r t h a n t o t h e a t m o s p h e r e . V e n t
l in e s n e c e s s a r y l o m e e t t h e r e q u ir e m e n t s o f t h e r e g u l a t o r y
b o d i e s a r e in s ta l le d b e l o w t h e m i n i m u m w a t e r l e v e l to
a v o i d t h e p o s s ib i l it y o f p r e s s u r i z i n g t h e t a n k s b e y o n d t h e
d e s ig n c r i t e r i a .
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1.0 . . . .
0 .9
i 9
0 . 8 A ' A _ A _ A
u ' F Ato l l 9 I 9 ' [
I ~ , ~
. 6 - - - - - - - ~ - - - - -
w : i i ,
0 4 ~ - - - - ~ 1 [ ; -~ ~ . . .
~ "
o.3 ~ .
o , [ I ~ =
1 i I i
C O M P A R I S O N O F SH I P A N D S H O P T E ST D A T A
P U M P S TA R B O A R D A T 2 0 0 R P M
]
SHOP
T E S T
SHIP
E S T
i
0 . 0
0 20 0 0 40 0 0 60 0 0 8 0 0 0 1 0 0 0 0 1 20 0 0 1 40 0 0
F L O W
(GPM)
P U M P P R O P E L L E R E F F I C I E N C Y
F I G U R E 9
C O M P A R I S O N O F SH IP A N D S H O P T E ST D A T A
P U M P S TA R B O A R D A T 5 0 0 R P M
1.0 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
0 .8 ! . . . . ~ - - 9 9 1 4 9
0.7 : . . . . . . . . . 9 - -
0 . 6
I
0 . 4 - - - -
~ - - - J - 9 SHOP TEST 1
0.3 ~ / 9 SHIP TEST
0.10.2
D E S I G N
0.0
0 1 0 0 0 0 20 0 0 0 30 0 0 0
F L O W
(GPM)
P U M P P R O P E L L E R E F F I C I E N C Y
F I G U R E 1 0
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Water Hammer
Systems that use a remotely operated butterfly valve
in the crossow~r pipe can experience water hammer if the
valve is closed too rapidly. Valve closing in the presence
of flow could occur, for example, during emergency shut
down procedures.
Valve operators are available that can close the valve
as quickly as 0.70 seconds and water hammer has been
exper ienced using this closing rate. When the valve
closing period was increased to 3 seconds, water hammer
did not occur.
Care should be taken during the overall system
design, either in the control system or in the selection of
the valve actuator, to be sure a valve is not closed rapidly
unless a corresponding valve is being opened
simultaneously or pump start and stop sequence is timed..
Cavi tat ion
List control systems that use a variable speed or
variable pitch pump to transfer water can maintain a
differential head between the list control tanks at zero
flow with the valve open. This configuration can occur
during either automatic or manual operation and is termed
a dead head condition. Dead head increases as RPM
increases and the maxim um differential head that can be
maintained by a variable speed pump is limited by
propel ler cavitation. Cavitation occurs when the local
pressure on propeller pump blades falls below the water
vapor pressure. Cavitation will reduce the propeller
efficiency and more torque must be applied to maintain a
desired head. At some point, the available horsepower
limit is reached and the available dead head and RPM
reach an upper limit. Cavitation limits on head can also
affect constant speed pumps.
The cavitation number is defined as the ratio of the
difference between the static pressure and the water vapor
pressure to the dynamic pressure (Principles of Naval
Architecture, Vol II 1985).. When the cavitation number
falls below 1, the total pressure on the blades is less than
the vapor pres,;ure. This definition of cavi tation inception
has been observed to provide good correlation for dead
head test data limits.
Cavitation No.
=
(po-Pv)/(0.5pV, 2)
where:
Po = Palm +pgh
Pat,n = atmospher ic pressure
Pv = vap or pressure of water
h = suction head
0 = mass density of water
g = gravitational acceleration
V r = velocity at blade 0.7 radius
The local cavitation number at 0.7R has been
used to predict cavitation inception. For dead head
conditions axial velocity is zero and at the 0.7 blade
radius position;
V r = 0.7riD
D = propeller diameter
n = propeller rotational speed, rps
Figur e 11 shows propeller dead head test data. Total
dynamic head and cavitation number at 0.7 radius are
shown as a function of propeller RPM. Cavitation and
power limits are observed between 500 and 600 RPM.
DEAD HEAD SHOP TEST RESULTS
p I0
F I t
. . 'B
., . , ' ,
..;::"
,.;"
I ; . . . . .
I
1
4
" 100 2 f ~ . ~00 4 ~ ] 500 6 00
7 0 0
R P M
0
6 0 0
CAVITATION NUMBER
FIGURE 11
Electron ic Control Sys tem
The electroztic control system discussed below is that
used for a variable speed propeller pump system. It
represents the state-of-the-art and is more extensive in
terms of variables monitored and controlled than other
systems presently available. Control of the LCS is
provided by a raicroprocessor based real time computer
system. This type of system, often called an embedded
system, uses the computer to acquire data from tank level
transducers, li,:t angle sensor output, butterfly valve
position sensor and other data for the hydraulic system
which is monitored for safety and alarm purposes.
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T h e m o v e m e n t o f b a ll a s t w h i l e c h a n g i n g e i t h e r h e a d
o r f l o w w h e n u s i n g a v a r i a b l e s p e e d p u m p r e q u i r e s r e a l
t i m e c o m p u t e r c o n t r o l. C o n t r o l is e f f e c t e d u s i n g d a t a
f ro m t an k wa t e r lev e l t r an sd u cer s an d a l i s t an g l e sen so r .
A s i g n a l f o r c o n t r o l o f p u m p s p e e d i s c o m p u t e d a t
i n t e r v a l s b a s e d u p o n t r a n s d u c e r d a t a a n d t h e m o d e o f
s y s t e m o p e r a t i o n . I n m a n u a l c o n t r o l , c o n s t a n t b a l l a s t
f l o w i n t h e d e s i r e d d i r e c t io n i s p r o v i d e d . I n a u t o m a t i c
co n t ro l , b a l l a s t i s t r an s fe r r ed i n t h e d i r ec t i o n r eq u i r ed t o
r es to re z e ro l i s t a t a f l o w r a t e p ro p o r t i o n a l t o t h e an g l e o f
list .
T h e p r o p e l l e r p u m p a n d a s s o c ia t e d b u tt e r fl y v a l v e a r e
p o w e r e d h y d r a u l i c a l l y . I n a d d i t i o n t o t h e c o n t r o l
a l g o r i t h m n e c e s s a r y t o m a i n t a i n r e q u i r e d b a l l a s t f l o w ,
s y n c h r o n o u s c on t r o l o f t h e b u t t e r f l y v a l v e i s n e c e s s a r y t o
a v o i d w a t e r h a m m e r . F u r t h e r, t h e h y d r a u l i c s y s t e m m u s t
b e m o n i t o r e d t o a s s u r e s a f e o p e r a t i o n .
T h e e l e c t r o n i c c o m p o n e n t s i n c l u d e a P C - l i k e
c o m p u t e r o n a P C / 1 0 4 s t a n d a r d m o d u l e . T h e P C / 1 0 4
c o m p o n e n t s i n c l u d e t h e c o m p u t e r , a d i s p la y c o n t r o l , a
t o u c h p a n e l c o n t r o l , A / D a n d D / A c o n v e n e r m o d u l e s w i t h
b i n a r y I / O a nd a P C M C I A m e m o r y c ar d m o d u le t o
co n t a in t h e co n t ro l p ro g ram an d sh ip ch a rac t e r i s t i c s d a t a .
A l l t o g e t h e r, t h e s y s t e m i s c o m p o s e d o f t h e c o m p u t e r
a s s e m b l y , a g r a p h i c s d i s p l a y w i t h t o u c h p a n e l , a n d a n
e l ec t r i ca l l y i so l a t ed i n t e r f ac e t o t h e t r an sd u c er s an d sh ip s
m a c h i n e r y c o n t r o l s y s t e m .
T h e o p e r a t o r i n t e r a c t s w i t h t h e s y s t e m u s i n g th e
d i sp l ay , t o u ch p an e l an d p an e l swi t ch es . L i s t co n t ro l t an k
l e v e l s , l i s t a n g l e , b a l l a s t f l o w ra t e , p u m p R P M , a n d
h y d r a u l i c s y s t e m p r e s s u r e , t o g e t h e r w i t h b u t te r f l y v a l v e
p o s i t i o n a n d a l a r m i n d i c a t i o n s , a r e p r e s e n t e d b o t h
n u m e r i c a l l y a n d g r a p h i c a l l y o n t h e d i s p l a y .
W i th t h e ad d i t i o n o f sh ip h y d ro s t a t i c ch arac t e r i s t i c s t o
t h e c o n t ro l so f t w a r e , t h e m e t a c e n t r i c h e i g h t , G M , c a n b e
o b t a i n e d b y a p p l y i n g a m e a s u r e d a m o u n t o f b a ll a s t
t r a n s f e r to a f fe c t a n o r d e r e d c h a n g e i n l is t a n g l e . T h i s
f e a t u r e i s p r o v i d e d b y m a n y L C S c o n t r o l u n i t s a s a n a i d
to sh ip o p era to r s i n d e t e rmin in g s t ab i l i t y p r i o r t o l eav in g
p o r t .
A l a r m s p r o v i d e d b y t h e e l e c t r o n i c c o n t r o l s y s t e m
i n c l u d e h y d r a u l i c s y s t e m t e m p e r a t u r e , p r e s s u r e , a n d o il
l e v e l, h ig h a n d l o w l is t c o n t r o l t a n k l e v e l s a n d m a x i m u m
l is t a n g le . T h e c o n tr o l s y s t e m p r e v e n t s c o m p o n e n t s f r o m
o p era t i n g i n a d i r ec t i o n t h a t in c rease s an a l a rm e r ro r . I t
a l l o ws o p era t i o n i n a d i r ec t i o n t h a t wi l l cau se an a l a rm to
b e c l e a r e d . A s a r e s u l t, n o o v e r r i d e o r e m e r g e n c y
m o d e s a r e u s e d t o r e s e t o r r e p o s i t i o n e q u i p m e n t a f t e r a n
a l a r m .
P o w e r i n g o n a n d o f f i s a n i m p o r t a n t i s s ue t o t h e s a f e
c o n t ro l o f r e a l t i m e e m b e d d e d s y s t e m s . F o r b o t h t u r n o n
a n d o f f s i t u a t i o n s , i t i s d e s i r a b l e t h a t t r a n s i e n t a l a r m
c o n d i t i o n s b e s u p p r e s s e d . T o e n s u r e t h at t h e s y s t e m ' s
c o m p o n e n t s a r e p r o p e r l y s e c u r e d b e f o r e t h e c o m p u t e r i s
t u r ne d o ff , th e e l e c t r o n i c c o n t r o l s y s t e m t a k e s c o n t r o l o f
s y s t e m p o w e r u p o n t u rn o n , b y p a s s i n g t h e s y s t e m p o w e r
o n / o f f s w i tc h . I t t h e n m o n i t o r s t h e s w i t c h i n s u c h a w a y
t h a t t h e o p e r a t o r ' s i n t e n t io n s a r e k n o w n . S h o u l d t h e
o p e r a t o r r e q u e s t t h a t th e p o w e r b e s h u t o f f w h i l e b a l l a s t
i s i n mo t io n , t h e b u t t e r f l y v a lv e i s o p en , o r t h e h y d rau l i c
p o w e r u n i t i s i n o p e r a t i o n , t h e s y s t e m f i r s t a t t e m p t s t o
s to p b a l l a s t f l o w, c lo se t h e b u t t e r f l y v a lv e , s t o p t h e p u m p
a n d s h u t d o w n t h e H P U b e f o r e d i s c o n n e c t i n g i t s e l f f r o m
p o w er . I n t h e wo r s t case t h i s p ro ces s wi l l t ak e l e s s t h an
5 s e c o n d s a n d t h e o p e r a t o r i s s e l d o m a w a r e t h a t t h i s
s h u t do w n s e q u e n c e c h e c k h a s o c c u r r e d . B y s u p p r e s s i n g
t ra n s ie n t a l a r m s d u r i n g t h e s t a r t u p / s h u t d o w n p r o c e s s , t h e
a n n o y a n c e o f th e a l a r m s o u n d i s a v o i d e d .
C O M P A R I S O N O F A V A I L A B L E S Y S T E M S
T h e c o m p a r i s o n o f t h e v a r i o u s s y s t e m s t h a t a r e
a v a i l a b le c a n o n l y b e m a d e o n t h e a b i l i t y o f t h e s y s t e m s
t o r e s p o n d i m m e d i a t e l y t o a c i r c u m s t a n c e t h a t r e q u i r e s
h e e l c o r r e c t i o n . T h e a m o u n t o f h e e l c o r r e c t i o n t h a t c a n
b e p r o v i d e d w i t h i n a s h o r t p e r i o d o f t i m e i s p a r t o f th i s
co n s id e ra t io n . Th i s q u ick resp o n se ab i li t y , wh ich m ay n o t
b e n e c e s s a r y f o r a l l ty p e s o f s h i p s , d e p e n d s n o t o n l y o n
t h e c o n t r o l s y s t e m b u t a l s o o n t h e c a p a c i t y a n d t y p e o f
m e c h a n i c a l s y s t e m u s e d t o p h y s i c a l l y t r a n s f e r b a l l a s t.
C o n s i d e ri n g t h e ty p e s o f s y s t e m s t h a t a r e c o m m o n l y u s e d
to d ay , a b r i e f ev a lu a t i o n o f t h e r eac t i o n times sh o w s so m e
s ig n i f i can t d i f f e r en ces .
T h e m o s t b a s i c s y s t e m , u s i n g a n e x i s t i n g c e n t r i f u g a l
p u m p w i th a s y s t e m o f a u t o m a t i c a l l y c o n t r o l l e d v a l v e s t o
p r o v i d e h e e l c o r r e c t i o n , a s s h o w n i s F i g u r e 2 , a l s o h a s
t h e s l o w e s t r e a c t i o n t i m e s . T h e f a s t e s t re a c t i o n t i m e f o r
t h is s y s t e m c a n b e o b t a i n e d b y h a v i n g t h e p u m p r u n n i n g
co n t i n u o u s ly d u r in g h ee l co r r ec t i o n p ro ce d u res . W h en n o
h e e l c o r r e c t i o n i s n e c e s s a r y , t h e p u m p w i l l b e
r e c i r c u l a t i n g b a l l a s t t h r o u g h t h e t a n k w i t h t h e h i g h e s t
l e v e l . T h e c o n t r o l s y s t e m i s g e n e r a l l y s e t t o r e c o g n i z e
two l is t an g l e se t p o in t s . Th e t h r esh o ld an g l e i s d e f i n ed
as t h e li st a t wh ich h ee l co r r ec t i o n wi l l b e i n i t i a ted wh i l e
th e sh u t -o f f v a lu e is d e f i n ed as t h e an g l e a t wh ich h ee l
c o r r e c t i o n p r o c e d u r e s w i ll s t o p . W h e n t h e t h r e s h o l d
v a l u e i s r e a c h e d , t h e c o n t r o l s y s t e m w i l l c o n f i g u r e t h e
v a lv es t o t r an s fe r b a l l a s t b e tw een t h e a p p ro p r i a t e t an k s t o
d ecrease t h e an g l e u n t i l t h e h ee l an g l e r each es t h e sh u t -
o f f v a l u e , w h e n t h e v a l v e s w i l l b e r e c o n f i g u r e d t o
r e s u m e c i r c ul a ti o n w i t h n o b a l l a s t tr a n s f e r . T h e r e a c t i o n
t ime is d ep en d en t o n t h e v a lu e as s ig n ed t o t h e t h r esh o l d
184 Design Session
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7/26/2019 List Control System _ Anti-heeling
15/19
a n g l e , w h i c h m u s t b e l a r g e e n o u g h t o a v o i d c o n s t a n t l y
o p e n i n g a n d c lo s i n g th e v a l v e s . T h e t im e n e c e s s a r y to
a v o i d p o s s i b i l i t y o f w a t e r h a m m e r w h i l e c l o s in g t h e
v a l v e s m u s t a l s o b e c o n s i d e r e d . H o w e v e r , s in c e o n e
v a l v e o n e i t h e r t h e s u c t io n o r d i s c h a r g e s i d e o f t h e p u m p
i s ope n i ng w h i l e t he o t he r i s c l o s i ng , t h i s a s pe c t i s no t a s
p r o b l e m a t i c a s it is w i t h a x i a l fl o w p u m p s y s t e m s .
G e n e r a l l y , t h is t y p e o f s y s t e m a l s o h a s t h e lo w e s t p u m p
c a p a c i t y , s i n c e t h e p r i m a r y p u r p o s e o f th e p u m p i s
s e l e c t e d t o f i l l b a l l a s t t a n k s i n a p p r o x i m a t e l y 3 0 r a t h e r
t h a n 3 m i n u t e s o r m o v e w a t e r th r o u g h a 3 f i r e h o s e
r a t h e r t h a n .an 1 8 t r a n s f e r p i p e . R e g u l a t o r y b o d y
r e q u i r e m e n t s f o r f i r e p u m p s , f o r e x a m p l e , a r e le s s t h a n
1 , 0 0 0 G P M .
T h e s y s t e m u s i n g a c o n s t a n t s p e e d a x i al f l o w p u m p
ha s s om e o f t he s a m e ope r a t i ng l i m i t s a s the s ys t e m us i ng
a n e x i s t i ng c e n t r i f uga l pum p . The s ys t e m i s i na c t i ve un t i l
t h e h e e l a n g l e c h a n g e s f r o m 0 ~ o r s o m e o t h e r v a l u e
r e c o g n i z e d b y t h e c o n t r o l s y s t e m , a t w h i c h p o i n t t h e
p u m p is s t a r te d a n d t h e v a l v e is o p e n e d . P u m p i n g
c o n t i n u e s u n t i l a n e v e n k e e l i s r e g a i n e d , w h e n t h e p u m p
i s s hu t dow n a nd t he va l v e is c l o s e d . S i nc e t h is is a n
a x i a l f l o w p u m p , t h e d e l i v e r y c a n d e c r e a s e a s t h e
d i s c ha r ge he a d i nc r e a s e s , de pe nd i ng on t he s e l e c t e d s pe e d
a n d p o w e r . G r a v i t y f l o w i s n o t c o n t r o l l e d . T h e
a d v a n t a g e t h is s y s t e m h a s o v e r t h e e x i s t in g p u m p
s ys t e m i s t ha t t he pum p i s s pe c i f i c a l l y s e l e c t e d f o r l i s t
c on t r o l a nd us ua l l y ha s h i ghe r c a pa c i t i e s , i n t he r a nge o f
3 , 0 0 0 t o 6 , 0 0 0 G P M .
T h e r e m a i n i n g t y p e s o f s y s te m s , t h e v a r i a b le s p e e d o r
v a r i a b le p it c h a x ia l f lo w p u m p a s w e l l a s t h e c o m p r e s s e d
a i r s y s t e m s , a l l c a n m a i n t a i n a h e a d d i f f e r e n c e b e t w e e n
t h e t w o h e e l i n g t a n k s w i t h o u t c l o s i n g th e v a l v e . T h i s i s
a c c o m p l i s h e d , b y a d j u s t i n g e i t h e r t h e p r o p e l l e r s p e e d o r
p i t c h o r t h e a i r p r e s s u r e t o m a i n t a i n t h e h e a d d i f f e r e n c e
w i t h o u t a n y f lo w . T h i s a l l o w s t h e s y s t e m t o r e a c t
i m m e d i a t e l y t o a n y c h a n g e i n h e e l a n g le . T h e a b i l i ty t o
r e a c t w i ll de pe nd m o r e on t he s oph i s t i c a ti on o f t he c on t r o l
s y s t e m r a t h e r t h a n m e c h a n i c a l r e s p o n s e c h a r a c t e r i s ti c s .
F i g u r e 1 2 s h o w s a n e x a m p l e o f th e r e a c t i o n t i m e t h a t
c a n b e o b t ai n e d w i t h a v a r i a b l e s p e e d p u m p . T h e s y s t e m
i s r e a c t i ng t o r e ve r s e s i n t he s l e w i ng o f a c r a ne w i t h a l i f t
o f a bou t 100 t ons . A s the c r a ne c ha nge s d i r e c t i on a nd t he
l i s t a n g l e p a , ; se s t h r o u g h 0 % th e d i r e c t i o n o f t h e p u m p
c h a n g e s a lm o , ;t i m m e d i a t e l y , a s e v i d e n c e d b y t h e c h a n g e
i n t he l e ve l s i n t he he e l c on t r o l t a nks .
G i v e n t h e c a p a b i l it y o f th e m e c h a n i c a l c o m p o n e n t s t o
p r o v i d e a n i m m e d i a t e r e s p o n s e , t h e t o t a l r e a c t i o n t i m e
w i l l de pe nd o n t he a b i l it y o f the e l e c tr on i c c on t r o l s y s t e m
t o r e c ogn i ze t he ne e d f o r he e l c on t r o l , c on t i nua l l y a na l yze
t he inpu t i n f o r m a t i on r e c e i ve d f r om va r i ou s s e ns o r s ( e . g . ,
t he i nc l inom e t e r a nd t he t a nk l e ve l s e ns o r s ) a n d i s s ue t he
p r o p e r i n s t r u c t i o n s t o c h a n g e t h e r e s p o n s e c h a r a c t e r i s t i c
o f th e p r i m e m o v e r ( a i r p r e s s u r e , p r o p e l l e r p i t c h , e t c . ) .
T h i s o f t e n i n v o l v e s a f e e d b a c k l o o p t o d e t e r m i n e i f t h e
de s i r e d e f f e c t i s be i ng a c h i e ve d a nd c on t i nu i ng c o r r e c t i ons
t o s t ri v e f o r t h