Indian Journa l o r Engineeri ng & Materi als Sciences Vo l. 9, December 2002, pp. 440-447

Design and development of water hydraulic pressure compensated flow control valve t

Saurabh Pandharikar, Abhijit Khuperkar, N L Soni & R G Agrawal

Rcruc lling Techno logy Division. Bhabha A tomic Research Centre , T rombay, Mumbai 400085 , India

Received / 3 December 200 / ; accepted 2 September 2002

T he Prcssure Compensatcd Flow Contro l Va lves (PCFCVs) are req uired fo r main ta ini ng constant fl ow in a hydra ulic c ircu it as there is fl uc tuati on in supply or re turn pressure and o ther res istance on ac tuato rs. T he water hydraul ic PCFCV has been designcd which can cont ro l the load fl ow as well as pump pressure. For achi ev ing constant tlow require ment , a hydrostate has been des igned which maintai ns a constant d ifferentia l pressure across a manu a ll y sellable va lve and hencc ma in ta in con stan t fl ow across the va lve. T he pump pressure cont rol is achi eved by cont ro lling the sensing line pressure of hyd ros tate w ith the help o r an a ir pi ston actu ated pilo t operated re lie f val ve. T he paper disc usses concept ual des ig n. mathcm3tica lmodc lling. parameter opti mizatio n and design of PCFCV.

Flow contro l is important in hydraulic circuits l•4 since

it direc tl y deals with rate of energy transfer. To con tro l the tlow, various types of valves are used like simp le ori fice , pressure compensated res tricted type, pressure co mpensated bypass type, press ure and temperature compensated type, priority type, flow dividers, etc. Most of the va lves ava il able in market are developed fo r oil hydraulic applicati onss. Successive development in hydraulic components and flui ds, parti cularly over las t 15 years, has led us to use mix ture of' oil and water, and now pure water as working fl uid fo r various applications like food industries, stee l industri es, power sector, ag ri cultural equipments, etc.

The water hydrauli c cont ro ls are being used extensively in Indian nuclear power pl ants6

. A distingui shing fea ture of Indian Pressuri zed Heavy Water Reactors (PHWR) is the capability to refu el the reactor even at full power operation, i.e. at 100 bar and 300°C of heavy water (0 20 ) working as coolant fl uid. It eliminates long and cos tly outage and also permi ts reactor to be operated with less excess reacti vity. The on power refu elling is carried out with the help of robo tized Fuelling Machines (FMs), located at each end of one hori zontal coolant channel, which is to be refuelled. These FMs operate remotely and handle hi ghly radioactive fuel bundles lying in coolant channel. During refillin g, the closure plugs

t presented at the 281h Na tional Confe rence o n Flui d Mechani cs & Fluid Power. he ld at Punj ab Engineering College, C hand igarh , during 13- 15 December 200 I

are to be removed and FM becomes a part of reactor as far as water environment is considered. It is a must to protect FM and its mechani sm fro m getting over heated by hi gh temperature coolant of the channel. So, a positi ve di fferenti al press ure is maintained between fuelling machine and coo lant cha nnel so that heavy water will fl ow from FM to coo lant channe l. The refuelling operati on is not continuous and also during the refuelling operation, the pressure req uired by the FMs is not constant all the time, whi Ie the fl ow requirements are constant. There are many situati ons li ke thi s where the pressure may fluctu ate but the fl ow requirement is constant. The PCFCVs are required for maintaining constant flow in a hydraulic circui t even if there is fluctuati on in supply or return pressure and other resistance on actuators. The PCFCV descri bed in thi s paper has been developed fo r Water Hydraul ic Supply System (WHSS) of PHWR fuelling machi ne.

This paper deals with the conceptual design and mathemati cal modelli ng for understanding behav iour with respect to variolls parameters, optimizati on of variolls parameters with the help of mathemat ical model, design problems and solution fo r water as working flui d and detailed design of th e valve.

Theory Conceptual design of PCFCy 7

The PCFCV is required fo r main taining constant fl ow in a hydrauli c circuit even if there is flu ctuat ion in supply or return pressure and other res istance on actuators. It has to control load fl ow as well as pump pressure. The constant fl ow can be maintained by

-

PANDHARIKAR el of. : DESIGN AND DEVELOPMENT OF WATER FLOW CONTROL VALVE 44 1

maintain ing a constant differentia l pressure across an orifice with the help of a hydrostate. The orifice is a Manually Settable Valve (MSV) so as to set the required load tlow. The pump pressure control is ach ieved by controll ing the sensing line of hydrostate with the help of an Air Piston Actuated Relief Valve (A PARV). APARV setting is achieved by maintaining air pressure at the piston. The air pressure is controlled by Electrical-to-Air (EI A) converter. The E/A converter gives an air pressure output proportional to electric input signal. So, ultimately the pump pressure can be set with the help of an electric setting.

Mathematical modell ing

The valve function can be modeled by generating dynamic differential equations by considering momentum balance equations, force balance equations and volume continuity equations. By solving these simultaneous differentia l equations, the va lve characteri stics can be obtained.

The mathematical models of thi s type of physical system are characterized by differentia l equations. A model is li near if the coefficients are either function of independent variable or constants. In thi s model, following non-linear parameters are considered:

(i) The coulomb friction (the funct ion of velocity);

(i i) (i i) The flow pressure relationships of ori fices, and

(i i i) (i i i) The limiting di splacement. (Sign of conventions is as show n in Fi g. 3).

Force balall ce of spool alld piS/OIl.'

- F - K X - 11/)( - Fv X - Fc (Sig ll of' X ) S i' fJ P {) fJ P . P

- P, A" + ~. Ai) + Conf, = 0

. .. ( I )

.. . - III X - FI! X + (P - P )(A - A )

.\ ." ,\ .\ (J \a ll pop r(}d ... (2)

- Fc , (Sign of X , ) - COII/" = 0

Flo w eqlla/ioll for sellsillg challlber:

v P . . K (P - P ) - - '-' _c + A X - A X

I () c' - 8 P fJ rod s . . . (3)

Flow equatioll for ref erell ce sensillg chamber:

= V, P, _ A X K 2 ( ~IIIII - P, ) B "" ... (4)

Flow contilluity equa/ioll :

... (5)

Sensillg challlber.f7ow equa/ion:

Q ( P _ P ) K = VolIlI Pow I mil l = 0111 o lll l :l B

+ K 2 ( PoWI - P,)+ Kv ,l'( PolIII - PIIIII )+ Kq ll' X ,l'

... (6)

Flow equations for spool a/ various pressure drop slOges:

v P "I {' I

+K'I2 X ., +-­B

KV4( Pp:l -Pp~)+ Kq4X , = KV S (P,,4 - Ptan )

. .. (7)

. .. (8)

... (9)

V P . .. ( 10) p~ ,,~

+ Kq< X \ + ----'----'--, . B

Force balance equa /iull for relief valve:

- lI l n , X 11' - Fs n, - K n··X IV + A{/ir~'ir + ~IIII I A p O,,'1'

- F,. X n ' - FJ S ign of X 11' )

+ ~(/IIA"o"rl'i = 0

... ( II )

442 INDIAN J. ENG. MATER. SCI., DECEMBER 2002

Out/et.f7ow equatioll:

+ Kv (P _ P ) Vow POIII I OUI ta n + B ... (12)

+ K} (r~1II1 - P'1II1 I )

The solution of these eq uati ons will describe the performance of the valve. The solution is obtained by usi ng state space technique. Following parameters has been taken as state va riables.

Swte variahles:

XI = XI)' X] = X I" Xl = )(., X.J = X "

X, = Pc , Xo = PI" X7 = Po, Xs = POIIII ' X Y = Pf!l , X /IJ = Pf!2 , X /I = Pp3 , X 12 = Pp.J,

Xf./ = XI.,.' Xu = X r v ' X I5 = Pollt

Input parametersJor tli e systelll:

V I = F" V2 = Fe" = Fc" U; = Cal!!.;, V.J = CO/!~) ,

V5 = p /(/II' V6 = Q.I·' V 7 = A "11 , Vs = A loo.!,

Vy = Fsn · , Uf() = P ail'

Rewriting Eqs ( I) to ( 12),

... ( 13)

... ( 14)

... ( IS )

. (BK2) (K2 B] (BAI)]. X = -- X -l-- X + -- X (, V 8 V (, V 2

r ,. ,.

... ( 16)

( 17)

... ( 19)

... (20)

.. . (21)

... (22)

PANDHARIKAR e f il l. : DESIGN AND DEVELOPMENT OF WATER FLOW CONTROL VALVE 443

,. ,

X =(_1 IV - [~)x +(~ IV 14 9 13 10 1Il,-" ) mrv 1'1/, ,.,. )

+[ AIWI'IT IX 8 _[~ IX 14 __ ( (Sign of X,.,.) Iv 2

111 1"' ) IIl 'T ) III " . )

[

AI'O"'''' ] + -- V s IIl n ·

... (23)

. ( KV6B ] [KV6B KV7 B K,B: X - = -- X - --+--+--- X -I ) V 7 I I I )

(Jul , \ 01/1 VOIll \ (Jut

- -- V + -- V + -- V -( KCf7Bl [Kq6B] [KV7 B; l V'lli! S VOII! 7 VII/" )

+ _3_ X ( K 8] V",,, 8

and, X2= XI

X4 = X ,

X 14 = X 13

(24)

(25)

(26)

(27)

Abovc equations further con verted 111 state space formulation as below8

:

{x-}= [A]{x) + rB]{u} al/d

{y}= [c ]{ x } + [D]{u}

where {x} is state vector and ! u} is input vector. These state eq uati ons are so lved using MATLAB_

Design problems and solutions for water hydraulic controls'

The design problems with use of plain water as work ing fluid are: (i) Water has much lower viscosity than mineral oil and wi ll therefore leak at increased ratc through any av;:: il ab le clearances. For the same reason, it will al so be less effcctive in generating films between rubbing components; (ii ) Water does not contain constituents similar to those in mineral oils, whi ch reduce stat ic friction between metal components and reduce their wear rate when operating in rubbing contact. If conventional metal components are used through out, problem can be 'xpected to arise from stati c fric ti on and also from igh wear rate and possible ri sk of component

If'fing the seizure; (iii ) Due to low viscosity, the ')City of leakage from small clearances and

controlled orifices will be hi gh; water has high vapour pressure compared to mineral oils, so cavitation is prominent. As water is very corrosive in nature, susceptible to cav itation, plus hi gh velocity at control orifi ces cause high erosion-corrosion problem and wear of orifice will be fast. This phenomenon is called wire drawing. Because of thi s reason, control orifi ce or clements of poppet and seat types are used instead of spool and sleeve type as in case of mineral oil hydraulic in most of the applications; (iv) The water has one advantage that its viscosity does not change with temperature drastically as compared to mineral oil, i.e. its viscosity index is very hi gh. Since viscosity fluctu ation is very small with respect to change in temperature, the water hydraulic flow control valves are normally having temperature compensation feature.

Considering the above problems of plain water interacting with working components or wetted parts , some of the following considerations have to be followed for satisfac tory operation and long li fe of water hydraulic components: (i) Materials- There will be a need for built in improved friction and wear characteri stics by use of non-metallic material and/or speci al surface treatment or coating on metal components. Preci pitation hardened 17-4 PH and 13-8 MO Stainless Steel (SS) are normally used for seat and poppet. Spring material is normally SS-302 or 17-4 PH ; (ii ) Valve design- Instead of spool design, poppet and seat type trim design is adopted for reducing problem of wire drawing due to excessive hi gh velocity of corros ive water through small clearance between trim components. It also reduces leakage losses. For maintaining a positi ve seal, poppet is normally loaded by spring for maintaining a contact force with seat; (iii ) Clearances are maintained small between mating parts. The bearing area of movi ng parts and load carryi ng components are over designed to reduce contact fo rces and since no fi 1m is being made by water at mating parts. Therefore, overall size of components increases; and, (iv) Since, viscosi ty index is very high for water, so no temperature compensation feature is added in water hydraulic valves.

Existing pressure control circuit of water hydraulic supply system (WHSS)

Existing set up is shown in Fig. I. Water Hydraulic Supply Pump (WHSF) is a positi ve di splacement tripl ex plunger pump, which can develop pressure up

444 INDIAN J. EN('. MATER. SCI., DECEMBER 2002

TA"'lK

I .. : ..

l j

li'.I·I·, \/2 VI

I'IJ!>.IP

--l- - TO LOAD

Fig. I-Ex isting fl ow she.: t () f WI-ISS lIsing ked back con tro l systelll

to 200 bar at 200Llmin fl ow. It suppli es water to FMs and other hyd raulic circuits.

All the time, FM and tes t facility do not require such hi gh fl ow and pressures. So, the arrangement as shown in Fig. I is made to control the fl ow and pressure. Press ure Trans mitter (PT) senses the pressure and sends it to PID contro ll er, PID controller is se t at a set point. The pressure is compared with set poi nt and error signa l is generated . PID setting and controll er further mod ify the error signal and its output is in form of 4 to 20 mA electri ca l signal. The signa l is given to EI A converter, which generates appropriate air signal, which is fed to fl ow control valves VI and V2 to avo id cav itation. Two va lves are used since press ure drop across va lves is very hi gh.

The circ uit req uires large size of accumulators for better and bumpless co ntrol. The pump is normally req uired for two pressure settings, i.e., Hi gh and Low fo r 145 bar and 76 bar respectively whi ch is achi eved by changing the set print of PID co ntro ll er. Similar control action can be carri ed out with the help of PCFCV.

Valve requirements

The water hydrauli c fl ow coniro l valve is hav ing fo ll owing requirements: (i) Flow capac ity and range-Depending upon the various applications , the fl ow co nt ro l range is different. For thi s spec ific appl ica ti on, the fl ow capacity is 200 Llmin and fl ow co ntro l range is 0-200 Llmin; (ii ) Pressure fluctuation- Load may flu ctuate from 40 bar to 200 bar; (iii ) Differential pressure sett ing range­Di fferential pressure can be adjusted fro m 0 bar to 7 bar across ori fice ; (i v) Temperature compensa ti on-

The viscosity of water changes margina lly with respect to temperature; compensation is not required for this appli cati on; and, (v) Physical requirements­The pressure drop should not be very high at cont ro l orifice. It is achieved by di viding the complete pressure drop in number of steps. For leak tight clos ure, first control orifice trim is selec ted as poppet type, which is not much affected by high velocity corros ive-eros ive water. The material of const ructi on of valve components shall be SS-3 16, SS-17 -4 PH, SS-13-SMo.

Design and working of PCFCV

The modifi ed circuit with PCFCV is shown In Fig. 2. The sec tional view of assembly is shown In Fig. 3.

PCFCV consists of two va lves , one Main Control Valve (MCV) and another APARV. To control the fl ow, we are required to maintain a constant differential pressure across one orifi ce, which is MSV. To achi eve thi s, a hydrostate i .~ designed . It cons ists of a balance piston in the pressure chamber. The inl et press ure acts on one side of Ba lance Pi ston (BP), whereas on the other side, the pressure at outl et of MSV ac ts and is suppl emented by a spring force in the same side of BP. Thus, for BP to be in equilibrium:

Force due to inlet pressure =

Force due to out let pressure of MSV

Sprino + '" Force

Since the spring force is almost constant , the differential pressure ac ross MS V will be almost constant. Because of thi s, the fl ow pass ing th rough MSV will be proponional to manua ll y set va lue irres pective of pump pressure or load fl uctu ation.

The excess of water, wh ich is not requi red by load, will be bypassed to lank line through MCV . The pressure drop across MCV is very high and may be up to 200 bar. Such hi gh pressure across single spool may cause cavitation problem. Therefore, main orifice is des igned sLlch that total pressure drop occurs ac ross 4 spools and I poppet. This will avoid the cavitat ion prob lem.

The pump has to develop a pressLlre eq ual to load pressure plus the differential pressure across the MSV ; thus the energy required by the pump will be saved. The pump can even be unloaded by setting the APARV to zero.

In case the supply line to load is closed or no f1 0~

is required by main valve, rhe pressure at inlet of ma' va lve will shoot up. Th is will open the APARV a'

PANDHARIKAR el at.: DESIGN AND DEVELOPMENT OF WATER FLOW CONTROL VALVE

TANK

--(\ ! I ' i I I ! I

ii i ! I I

\~ J

r I

Fig. 2- Proposed now sheet of WHSS using PCFCV

TOREU EFVALVE-

TO LOAD _ ----' ,/ /". I'bul

Vou!

- (+)

o (i) ~0

I'b Vo

I BODY 2 SI'IUNG I 3 Sl'lUNG2 4 FU\NGE j SLEEVE 6 REllEF VALVE 7 POPPET I~PISfON

<) WASHER 10 BAU.. II aJVEII. PLATE 12 SIoTSCREW

130000CE

Fig. 3- Sectional view of PCFCV

PUMP FLOWWTH FLUCruATlON 220.----,---,----.,------.-- --,

210

180

;

-~--~---_T-----+_---1

----1 ____ -----~---+---~ i

170~--~---_t_----~--_+--~

160 i , --.--- .-----+---+------1

1~L----L--~~---L--~--~ o 2 3 4 5

TlMENSEC.

Fig. 4----Pump outlet now

LOAD FLOW 10.----.--- .----.---.----,

8 : ' I

7~----~----~------+_----~----_1

6f-----+----~----1-~·--·- -~ 5~--_+---~---+---~r---~

4~----+_----+-----~----~----~

3~----+_----+-----~----~-----

2~----~----~------+_----_+------4

0·L------L--__ ~~ ____ ~ ____ ~ ____ ~ o 2 3

aENSEC.

Fig. 5- Flow to load

4 5

445

446 INDIAN J. ENG. MATER. SCI. , DECEMBER 2002

will leak water to tank line. The orifice (OR) is designed such that the flow from the downstream of MSV will be less than the leak rate of APARV when it opens. This will lead to flow of water from spring side of BP to tank line. This action will create a unbalance force in balance piston and will lead to opening of MCV and re li eve full flow to the tank line. The pilot operated valve can be set for a range from 0 to 210 bar with the help of e lectrical signal to EI A converter.

In case the MSV is completely closed, the full supply pressure will act at the inlet side of balance pi ston, the MCV will open and bypass the flow to tank . The pump can be unloaded by just closing the MSV.

By varying the spring force on BP, differential pressure across the MSV can be set. The APARV is designed such that in case of air failure, it will set the system pressure to max imum pump pressure. This ensures that the hot heavy water from reactor wi II never flow to fuelling machine.

Flow and pressure controls for ram assembly test set-up using PCFCV

Ram Assembly Test Facility (RATF) is simulating pressure and flow in a subassembly of fuelling machine. The hi gh-pressure water is obtained from main triplex plunger pump of Fuelling Machine Test Facility (FMTF) . The flow requirement in RATF is less co mpared to FMTF. For utili z ing the WHSS of FMTF optimum way, the PCFCV has been used as shown in Fig. 2. The pump output flo w is 200 Llmin and flow required to operate RATF is 90 Llmin . The PCFCV is being used as bypass flow regulator. It will limit the flow going to RATF by 90 Llmin and rest of flow is bypassed through itse lf. By adopting such arrangement, the pump pressure will remain equal to the RATF pressure requirement, i.e ., if RATF pressu re requirement is hi gh, the pump will require hi gh pressure, and if RATF requires low pressure, the pump will develop less pressure (Figs 4 and 5). This is load sensing pressure control scheme, i.e. pump will develop the pressure what is demanded by load. The maximum pressure setting of pump or system limiting pressure can be carried out by setting APARV pressure. This scheme eliminates pump pressure control circuit.

Conclusions The theore tical analysis of PCFCV was carried out

using state space technique. Using the mathematical

model, its performance was predicted and certain parameters were optimized. Its predicted performance was found satisfactory. The problems associated with water as working fluid were discussed and special features were added in the prototype des ig n for be tter performance of PCFCV. The theoretical results show that the pump pressure fluctuations are not affecting the control flow of RATF and PCFCV is a lso reducing the pressure fl llctuations generated by multi pi ston pump. The PCFCV is al so controlling the pump pressure proportional to input a ir signal to APRV. Prototype is being fabricated and its performance will be compared with theoretical results.

Nomenclature Aair

Atom/

1\1' A pop

Apopn l

Apopn'l A m d

Am B COli!" COli!, Difa

Fe I; Fe ., F ., Fsn ,

F vp

Fv,

KI

Kz K 3

K, Kq l Kqz Kq./ Kq4 K(/., Klf6

Kq7 Kqn· Kn· K VI

K Vl

K V3

K V4

Kv, K V6

K V7

Kvn ·

11/

III tv

Area of diaphragm, m2

Area of load orifice, m2

. J pI ston area, m-Area of poppet, m2

Area of poppet on which supply pressure to relief valve act, m2

Area of poppet on which tank pressure act, 1112

Area of rod, m2

Area of variable orifice, m2

Bulk modulus of water, Pa = 6.9 x 107

Total force of pi ston acting on spool, N Total force of spool acting on piston, N (Apop-Arod) - Differential area of spnol , m2

Columb fri ction of pi ston, m2

Columb friction of spool Initia l spring setting, N Initi al spring setting of re lie f valve, N viscous coeflic ient of pi ston, N/(m/sec) viscous coefficient of spool, N/(m/sec ) orifice constant of sensing chamber orifice constant of reference sensi ng chaillber orifice (3) constant as shown in figure spring constant, N/mlll Flow ga in coefficient for I st stage Flow gain coeffi cient for 2nd stage Flow gain coeffici ent for 3rd stage Flow gain coeffi c ient for 4th stage Flow gain coefficient for 5th stage Flow gai n coeftlci ent for variable orifice Flow gain coeffic ient for load orifice Flow gain coefficient for relief va lve Spring constant of relief valve, N/mm Flow pressure coefficient for 1st stage Flow pressure coefficient for 2nd stage Flow pressure coefficient for 3rd stage Flow pressure coefficient for 4th stage Flow pressure coefficient for 5th stage Flow pressure coeftlcient for variable orifice Flow pressure coefficient load orifice Flow pressure coefficient for relief valve mass of piston , kg Mass of relief valve, kg

P,

Q, II

v .. V"

PANDHARIKAR et al.: DESIGN AND DEVELOPMENT OF WATER FLOW CONTROL VALVE 447

mass o f spool, kg

Re li ef valve air pressure setting, Pa

Sensi ng chamber pressure, Pa Supply pressure. Pa Pressure at downstream of MSV , Pa Pressure in relie f valve line, Pa

Pressure in Cavity between spool and valve for 1st stage pressure drop, Pa Pressure in Cavity between spool and valve for 2nd stage pressure drop, Pa Pressure in Cavity between spooi and valve for 3rdstage pressure drop, Pa Pressure in Cavity between spool and valve for 4th stage: pressure drop, Pa Referene:e sensing chambe:r pressure, Pa

Tank pressure, Pa Supply fl ow

Input vector rU/ ... U/IJ] U1 ... U IO= Input parameters Volume of sensing chamber, m' Volume of supply line, m3

Volume at downstream of MSV , m' Volume of relie f valve line, m'

Cavity between spool and valve for I st stage pressure drop, m'

Cavity betwecn spool and valve for 2nd stage pressure drop, ny' Cav ity between spool and valve for 3rdstage pressure drop, m'

Vp~ Cavity between spool and valve for 4th stage pressure drop, m'

Vr Volume of reference sensing chamber, m} x State vector IX/ ... X/5 1

X/ ... X/5= State variables XI' Piston displacement, m Xn . Relie f valve spool displacement, m X, Spool di splacement, m Y Output vector

References I Ma C A, Trans ASM £ Eng Ind, (May 1967) 301-J08. 2 Marrit H E, Hydraulic control system (Witey, New York).

1967. 3 Me C loy D & Martin H R, COlltrol of fluid pOll'er: Analn'is

and design (Ellis Horwood Limited, New York), 1990. 4 Soni N L & Agrawal R G, Development of water hydraulic

pressure compensated flo w cOlltrol valve, G!obal Conference on Fluid Metering and Control in New Millennium. FCRI , Pa lghat, Sept 20-22, 2000.

5 Sawan S, Fluid Power 1,3 (2) (2000) 8-32. Ii Soni N L, Alternate Design of Water Hydraulic Pressure

control System for Actuators of Fuelling Machine of Indian PHWRs, M . Tech. di ssertat ion, Electrical Engineering Dept, liT, Bombay, 1992.

7 Pandharikar S B & Soni N L, Water Hydraulic Pressure Compensated Flo w Control Valve, BARC, RTD Repon No. 38,2001.

8 Friendland B, Control system design - An introduction to state space methods (Me-Graw- Hill , New York), 1986.